Performance of strategically sandwiched continuous sisal fiber core 3D printed PLA composites for engineering applications

Performance of strategically sandwiched continuous sisal fiber core 3D printed PLA composites for engineering applications | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Performance of strategically sandwiched continuous sisal fiber core 3D printed PLA composites for engineering applications Mohit Kumar, Vinod Ayyapan, Ranvijay Kumar, Manoj Kumar Singh, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7575389/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 18 Feb, 2026 Read the published version in The International Journal of Advanced Manufacturing Technology → Version 1 posted 5 You are reading this latest preprint version Abstract The study presents an innovative method to enhance the structural performance of 3D-printed polylactic acid (PLA) composites by strategically incorporating continuous Agave sisalana fibers as a sandwiched core. This architecture aims to overcome existing limitations by enhancing fiber alignment, interfacial bonding, and mechanical performance, while maintaining the lightweight benefits of PLA. Alkaline treatment enhanced fiber/matrix interaction, confirmed via SEM analysis. Compared to neat 3D printed PLA, the treated fiber composite showed notable enhancement of 36.19% in tensile strength and 46.25% in interlaminar shear strength. The treated configuration showed tensile strength of 40.87 MPa and tensile modulus of 2420.88 MPa. While neat 3D printed PLA retained higher flexural strength and modulus, the treated fiber composite excelled in toughness (938.9 kJ/m³) and energy resilience (307 kJ/m³). Dynamic mechanical analysis revealed better thermal stability in the treated fiber composite (Tan δ = 0.53 vs. 1.49 for neat 3D printed PLA). Impact testing showed that untreated fiber layers absorbed more energy, evidenced by the highest puncture force (349.78 N) and maximum deformation. Overall, the study confirms the benefits of a continuous fiber core in optimizing 3D-printed PLA composites and suggests future work on fiber arrangement and volume fraction for enhanced performance. 3D printing polylactic acid natural fibers sandwiched structure biocomposites NaOH treatment Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Surface treated continuous natural fibers fibers integrated into PLA with the help of additive manufacturing offers susitanable composite structures for engineering applications. In engineering applications, lightweight and good mechanical properties of the manufactured parts are highly recommended, and can be accommodated by using continuous natural fibers into thermoplastics. The sdudies suggest that the fibers reinforced with polymers can greatly enhance the mechanical, thermal, chemical, surface, durability, and tribological properties [ 1 , 2 ]. A number of studies have been reported in the recent past for the development of the natural fibers reinforced thermoplastics [ 3 , 4 ]. PLA-based composites are known for good biocompatibility, degradability, good stiffness, and mechanical properties [ 5 ]. These excellent material properties make PLA as one of the acceptable thermoplastics for applications in tissue engineering, sensors, smart textiles, automobile, and aerospace applications [ 6 ]. The previous studies have explored the different materials performance of the natural fiber reinforced PLA composites. The studies conducted to investigate the effect of jute, sisal and elephant grass-based fibers in PLA matrix have suggested that impact strength was increased by 22.3%, 111.5% and 129.5% respectively as compared to plain PLA [ 7 ]. The addition of natural fibers is one of the ways to restore the material's properties, which can be ensured or established by mechanical recycling. However, the number of thermal cycles affects the mechanical properties of the composites. The study revealed that when basalt fibers and halloysite nanotubes were reinforced in PLA matrix by a combination of twin-screw extrusion and injection molding, the tensile strength was significantly affected after three recycling steps [ 8 ]. The reinforcement of pre-stressed natural fibers in PLA have ensured the 116% and 62% increase in tensile strength and stiffness respectively along with 14% and 10% increases in flexural strength and rigidity, respectively [ 9 ]. 3D printing is becoming a promising tool for the manufacturing of natural fiber-based composite materials [ 10 , 11 ]. The flour extract (90–250µm) from henequen fibers reinforced with PLA shows a potential for 3D printing of crack-resistant structures [ 12 ]. Fused deposition modeling (FDM) is a materials extrusion (MEX) based 3D printing process that is largely reported for the manufacturing of natural fiber reinforced polymer composite structures [ 13 , 14 ]. The study has reported for incorporation of different natural fibers of oil palm, pineapple leaf, coir, and bamboo (size: 250–500µm) into PLA for 3D printing. A higher tensile strength was observed for the combination of fibers (hybrid) as compared to pure biocomposites or pure PLA [ 15 ]. It was reported los decrease in mechanical properties of 3D printed natural fiber reinforced PLA composites after hydrolytic degradation, a potential candidate for furniture, decorations, and the automotive industry [ 16 ]. The manufacturing of micro-perforated panels is possible by 3D printing using cork fiber reinforced PLA for engineering noise control and the effectiveness of hearing conversation [ 17 ]. In 3D printing applications, PLA is often considered as a brittle and low impact-resistant material, which results in limited use. However, the addition of continuous fibers can improve the strength, load transfer efficiency, thermal stability, and crack resistance of PLA towards manufacturing lightweight components [ 18 – 20 ]. Cersoli et al. (2021) have outlined that continuous Kevlar fiber reinforcement has increased the tensile strength by two times and impact energy by a factor of four [ 21 ]. Similar observations have been reported in previous studies [ 14 ]. However, direct reinforcement of the fibers with PLA matrix is associated with many challenges, including poor fiber-matrix adhesion, non-uniform distribution, alignment, and formation of voids/porosity, which contributes to defects in the final manufactured parts [ 22 , 23 ]. In some cases, reinforcement can create the problem of nozzle clogging in materials extrusion-based 3D printing [ 24 , 25 ]. To solve these issues, the sandwiching of the continuous fibers within regular additive layers can be implemented. Sandwiching of the fibers is one of the alternative methods to incorporate the continuous fibers, especially for increasing the mechanical strength [ 26 ]. The process may also be named as the ‘interleaving’ process. A 44% higher elastic modulus and 119% higher strength were observed for a sandwich structure made by 3D printing of carbon fiber reinforced composites [ 27 ]. A higher stiffness (up to 4.1 times) could be ensured if 3D printing is performed for the manufacturing of a sandwich structure with a corrugated core [ 28 ]. Also, the bio-inspired sandwich structures prepared by 3D printing (of carbon fiber reinforced composites) can be used for energy absorption applications [ 29 ]. The surface treatment is one of the best approaches for tuning the properties. This can result in improved adhesion between the layers and mechanical strength. The study reveals that the 3D printed rice husk and rice straw (surface treated with NaOH) reinforced (5–20%) to PLA can be used for automotive and construction applications [ 30 ]. Along with this, the surface treatment can be helpful for controlling the hydrophobic behavior of the polymer composites [ 31 ]. The strength retention of surface treated sisal-polyester composites was observed higher as compared to untreated composites [ 32 ]. The reported literature is evident that reinforcement of the natural fibers as filler with PLA matrix has significantly improved the mechanical and physical properties [ 7 , 9 , 33 ]. Reinforcement of continuous fibers in the PLA matrix has also been reported for the improvement of the mechanical properties [ 14 , 21 ]. 3D printing has become a crucial technology for the fabrication of PLA-natural fiber-based composite structures with suitable mechanical properties [ 13 , 14 , 16 , 17 ]. In fact, surface treatment of the natural fibers can boost the mechanical properties by controlling the hydrophobic nature [ 30 – 32 ]. A very few studies have been reported on 3D printed sandwiched strcutures of continuous fibers configuration for tunable mechanical properties [ 27 , 28 ]. Hitherto, fewer have reported on the manufacturing of sandwiched continuous fiber core structured composites using 3D printing. This study highlights a strategical approach for sandwiching the continuous fibers as a core in PLA composites using 3D printing technology. The manufactured 3D printed biocomposite plate was further characterized for mechanical performance, thermal behaviour, morphological analysis, viscoelastic behavior and low velocity impact evaluation. This study seeks to address current challenges by improving fiber orientation, strengthening interfacial adhesion, and boosting mechanical properties, all while preserving the lightweight nature of PLA for engineering applications. 2. Materials and methods 2.1. Materials In the present study, PLA 4043D grade was procured from NatureWorks LLC. This PLA grade is biodegradable and suitable for making 3D printed filaments. The density of the material is 1.25 g/cm 3 , glass transition temperature of ⁓63 o C and melting point of ⁓155 o C. The tensile strength and elongation at break of the material are 55–60 MPa and 2–4% respectively. Natural sisal fibers derived from Agave sisalana were utilized as reinforcement in this study. These fibers had an average diameter of approximately 0.18 mm, a tensile strength of 298 MPa, and a tensile modulus of 11.4 GPa. The fibers were sourced from Metro Composites, Chennai, India. To facilitate the application of PLA as an adhesive, chloroform was used as a solvent. The chloroform, with a stated purity of 99.98%, was procured from RCI Labscan Limited, located in Bangkok, Thailand. 2.2. Manufacturing process 2.2.1. Surface treatment of Agave sisalana (Sisal) fiber The concentrated NaOH solution (5%) was prepared by dissolving 63.23 g of 98% NaOH pellets in 1200 mL distilled water and stirred till the pellets were completely dissolved. The actual density of NaOH pellets was 2.13 g/cc, and the final density for 1200 mL of 5% NaOH solution was 1.054 g/cc. The alkali surface treatment was done by soaking 200 g of derived fibers in 1200 mL of 5% NaOH solution using a magnetic stirrer at room temperature for 8 h. Later, the treated fibers were washed completely with distilled water until the pH was neutralized. Further, these treated fibers were dried in a hot oven at 65°C for 24 h. The chemical reaction of fiber with NaOH is given in Eq. \(\:\left(1\right)\) . $$\:Fiber-OH+NaOH\:\:\to\:\:\:Fiber-ON{a}^{+}+{H}_{2}O\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(1\right)$$ The hydroxyl (OH) groups on the fiber surface were disintegrated, resulting in the removal of excess amorphous constituents like hemicellulose, wax, lignin, pectin, and impurities. NaOH treatment infiltrates the amorphous zones of cellulose, swelling the fibres and expanding their cell walls. The rougher surface and enlarged pores give the resin more room to penetrate, nurturing stronger fiber/matrix adhesion through mechanical interlocking. Meanwhile, NaOH cuts the original, moisture-prone hydroxyl linkages subsequent re-arrangement of the cellulose chains creates new hydroxyl bonds. With many of the moisture-prone hydroxyl groups removed, the treated fibres exhibit better resistance to moisture [ 34 ]. 2.2.2. Manufacturing of PLA/Sisal fiber biocomposites laminate The manufacturing procedure starts with preparing a PLA solution. For this, PLA pellets of 4043D grade are weighed and placed into a beaker, followed by the addition of chloroform as a solvent. A 1:10 weight-to-volume ratio is used specifically; 25 grams of PLA is dissolved in approximately 150 mL (around 222 grams) of chloroform. To facilitate complete dissolution, the mixture is stirred using a magnetic stirrer set to 60°C. Once a homogeneous PLA solution is obtained, it is uniformly spread over both untreated and chemically treated agave sisalana fibers that are aligned in a single direction. The coated fibers are then left undisturbed for about two hours, allowing the chloroform to evaporate completely. The dried composite is subsequently processed through compression molding at 150°C, under a pressure of 4000 kgf, for a duration of 15 minutes using a mold with a thickness of 0.4 mm. This step is essential for minimizing void content and ensuring a consistent final thickness in the fiber-sandwiched PLA composite. In this process, two types of biocomposite laminates were developed, one using untreated sisal fibers and the other incorporating treated sisal fibers. 2.2.3. Filament manufacturing and 3D printing of composites To prepare the PLA filament, PLA pellets were initially dried in an oven at 45°C for 24 hours to eliminate any residual moisture. Following this drying step, the pellets were processed using a single-screw extruder (Noztek Xcalibur) to produce filaments with a target diameter of 1.75 ± 0.2 mm. The extruder featured three distinct heating zones: the nozzle (T1) was maintained at 200°C, the intermediate (T-2) at 190°C, and the feed zone near the hopper (T-3) at 185°C. The extrusion was conducted at a screw rotation speed ranging from 17 to 22 revolutions per minute (rpm). In the subsequent step, samples were prepared using a 3D printer, model BBC3040 manufactured by Bangkok Blue Ocean Co. Ltd, Bangkok, Thailand. The 3D printing parameter used in this work is presented in Table 1 . The CAD design was created and saved in .stl format, further imported into the simply3D software for printing. The desired 3D printed composite specimen thickness was 3mm, which comprises of 3 stacks (stack 1- 1.3 mm, stack 2- 0.4 mm and stack 3- 1.3 mm) that were printed layer by layer. The composite sample descriptions were given in Table 2 . Stack 2 is filled with PLA/sisal fiber laminate in case of untreated and treated samples. Table 1 3D printing parameters Parameters Values (units) Infill density 80% Extruder temperature 210 o C Bed temperature 55 o C Printing speed 0.40 mm/s Nozzle size 0.6 mm Layer height 0.20 mm Filling pattern Rectilinear Extrusion width 0.4 mm Raster angle 45 Table 2 Description of composite samples Composite samples Designation 3D printed PLA biocomposite Neat 3D printed PLA biocomposite sandwiched with untreated sisal fiber Untreated 3D printed PLA biocomposite sandwiched with treated sisal fiber Treated To sandwich the biocomposites laminate, the 3D printing process was programmed to pause layer deposition at a height of 1.3 mm. After inserting the biocomposites laminate, the layer-by-layer deposition resumed from a height of 1.7 mm, ensuring a total sample thickness of 3 mm. To ensure proper adhesion of the biocomposites laminate after the stack 1, the prepared PLA solution was uniformly spread across the surface layer to bond with the deposited layers. Subsequently, the layer-by-layer deposition process was continued on top of the laminate, resulting in a final sample with a thickness of 3 mm. The complete schematic process for fabrication is shown in Fig. 1 . Before conducting the experiments, all 3D-printed composite samples were placed in a hot air oven at 50°C to eliminate any moisture absorbed by the PLA materials. 2.3. Characterization techniques 2.3.1. Mechanical characterization – Tensile, Flexural, and Interlaminar shear strength (ILSS) The mechanical characterization, such as tensile strength and modulus, flexural strength and modulus, and ILSS was measured via a Universal testing machine model, Comtech M1-type of 10 kN loadcell. The tensile test, flexural test, and ILSS test were performed using ASTM 3039D (100×10×3 mm 3 ), ASTM D790 (90×10×3 mm 3 ) and ASTM D2344 (60×10×3 mm 3 ), respectively. Five samples for each 3D printed composite sample were tested with a test speed of 2 mm/min, and average values were reported in the results. 2.3.2. Scanning Electron Microscopy (SEM) The surface morphology of tensile fractured samples was analyzed using Thermo Fisher Scientific-SEM, Ireland. Before SEM imaging, the fractured samples were gold-coated using an MP-19020NCTR Neocoater to ensure conductivity, as the materials were inherently non-conductive. 2.3.3. Dynamic Mechanical Analysis The dynamic mechanical analysis (DMA) was done to evaluate the effect of sandwiched untreated and treated fibers in the 3D-printed composite samples on their viscoelastic behaviour. The loss modulus (Eʺ), storage modulus (Eʹ) and Tan δ were evaluated as per ASTM D4065 standard, in dual cantilever mode with temperature range from 25°C to 125 o C at heating rate of 5 o C/min. The frequency mode and load were set at 1Hz and 0.8 N, respectively. The loss modulus signifies the energy dissipated during deformation, the storage modulus signifies the material's capacity to store energy under deformation, and Tan δ presents the material's damping properties. 2.3.4. Thermomechanical Analysis (TMA) TMA was conducted to evaluate the temperature effect on the thermal expansion behavior and dimensional stability of the 3D printed composite samples. A thermomechanical analyzer, model TMA/SDTA 1+, Mettler Toledo, Switzerland, was used to study samples as per ASTM E831-03. The test was performed by putting the sample between flat silica plates under a quartz probe inside the furnace, at temperature ranges between 30–100°C, at a rate of 5°C/min in a nitrogen environment. 2.3.5. Low Velocity Impact The low impact velocity was used to study the energy absorption behaviour, impact resistance, and damage tolerance of the 3D printed composite samples as per ASTM D7136 standard. The experiment was conducted on a 100×100 mm 2 plate using a Zwick Roell HIT 600F impact machine with an impact energy of 10J. The purpose of this study is to evaluate the real-time scenario where composites made from present materials get hit by any objects. 3. Results and Discussion 3.1. Tensile properties The tensile results of neat, untreated, and treated fiber sandwiched 3D printed composite samples were investigated and shown in Fig. 2 and Table 3 . It was observed that neat 3D printed PLA composite exhibited the lowest tensile performance among all tested samples, with a tensile strength of 30.01 MPa and a tensile modulus of 1750.91 MPa. Incorporating untreated fibers in a sandwiched structure improved the overall strength, resulting in a tensile strength of 33.17 MPa and a tensile modulus of 2316.55 MPa, reflecting a 10.20% increase in strength compared to the neat composite. Notably, the 3D printed composites sandwiched with treated fibers demonstrated the most significant enhancement, achieving a tensile strength of 40.87 MPa and a tensile modulus of 2420.88 MPa. This represents an overall improvement of 36.19% over the neat composite and 23.21% compared to the untreated fiber composite. It was attributed to the excellent stress transfer with the matrix and the fibers due to the increased surface area and improved interfacial adhesion resulting from the surface treatment [ 34 ]. Table 3 Tensile test results of 3D printed composite samples Sample Maximum tensile strength (MPa) Tensile modulus (MPa) Strain at break Modulus of toughness (kJ/m 3 ) Modulus of resilience (kJ/m 3 ) Neat 30.01 ± 1.8 1750.91 ± 46.12 0.028 ± 0.003 530 ± 10 103 ± 9 Untreated 33.17 ± 3.4 2316.55 ± 58.95 0.030 ± 0.002 666.8 ± 15 230 ± 10 Treated 40.87 ± 2.3 2420.88 ± 50.02 0.035 ± 0.002 938.9 ± 10 307 ± 7 The strain at break for all the composites was shown in Fig. 2 (a) and Table 3 . It was observed that the 3D printed composites sandwiched with treated fibers exhibited a maximum strain at break of 0.035, representing a 25% improvement compared to the neat composites. The observed increase in strain at break can be attributed to the improved interfacial bonding and enhanced tensile behavior. NaOH treatment of sisal fibers effectively eliminates considerable amorphous constituents such as lignin and hemicellulose, exposing a rougher and more reactive fiber surface [ 35 ]. This modification significantly strengthens the interaction between the fiber and the surrounding matrix, allowing for better mechanical interlocking and more efficient stress transfer [ 36 ]. As a result, the fiber sandwiched composite was able to endure greater deformation before fracturing. Moreover, the presence of continuous fibers acts as a mechanical bridge between cracks and redistributes localized stress, which helps to minimize stress concentrations. The stronger interface also contributes to improved energy absorption and dissipation during loading. Together, these mechanisms account for the increased strain at break observed in the treated fiber composites as compared to other composite sample configurations. The strain at break of composites significantly affects their modulus of toughness and resilience properties. The modulus of toughness represents how much energy a material can absorb per unit area before it breaks, covering both its elastic and plastic deformation stages. In simpler terms, it reflects the material’s capacity to endure stress and strain up to the point of fracture. On the other hand, resilience refers to a material's ability to absorb energy during elastic deformation and then recover its original shape once the applied stress is removed. This indicates the material’s resistance to temporary deformation without undergoing lasting damage. Generally, materials that exhibit both high toughness and high resilience are better equipped to handle significant forces and impacts, making them ideal for use in structural and protective applications [ 37 ]. The results of all composite configurations in terms of modulus of toughness and resilience are shown in Fig. 2 (c-d) and Table 3 . The lowest modulus of toughness (530 kJ/m 3 ) and resilience (103 kJ/m 3 ) were observed in the neat 3D printed PLA composite. This is attributed to brittle behaviour with limited energy absorption during both elastic and plastic deformation [ 38 ]. The increase in both modulus of toughness (666.8 kJ/m 3 ) and resilience (230 kJ/m 3 ) was obtained for untreated composite corresponds to improved performance with fiber reinforcement. Although these values were less than treated values of modulus of toughness (938.9 kJ/m 3 ) and resilience (307 kJ/m 3 ), because of poor interfacial bonding in untreated composites. The higher toughness properties in treated composites attributes to superior energy dissipation, better stress transfer and plastic deformation making it more suitable for applications requiring improved toughness and ductility. 3.2. Flexural properties The flexural results of neat, untreated, and treated fiber sandwiched 3D printed composite samples are shown in Fig. 3 . It was noted that neat sample exhibited the highest flexural strength and flexural modulus of 61.73 MPa and 2560.11 MPa, respectively. Incorporating sandwiched core fiber layer between 3D printed PLA composite did not result in any notable enhancement in flexural strength and flexural modulus. For the untreated composite, the recorded flexural strength and modulus were 51.10 MPa and 2220.34 MPa, respectively. The reason was analysed from Fig. 3 (a), where the strain values for sandwiched core fiber composites were significantly high (⁓50%) as compared to neat composite sample. This increase is likely due to the enhanced deformability of the sandwiched structure, which results in lower stiffness and improved flexibility (include Core is restricting crack propagation) [ 39 ]. However, after NaOH treatment of fibers, these values increased to 55.79 MPa for flexural strength and 2413.37 MPa for flexural modulus, indicating improvements of approximately 9.17% and 8.69%, respectively, as compared to untreated fiber core samples. This may be due to the increased surface roughness correspond to better interfacial bonding and better stress transfer between fibers and matrix [ 40 ]. This enables the material to absorb more energy and undergo greater deformation prior to failure. 3.3. Interlaminar shear strength (ILSS) properties ILSS is one of the key properties to evaluate the delamination behavior of layered composites, particularly in sandwich structures. The ILSS of neat, untreated, and treated fiber sandwiched 3D printed composite samples were evaluated and presented in Fig. 4 . The results showed that neat composite sample had interlaminar shear strength of 3.2 MPa, while untreated and treated fiber core composite samples had interlaminar shear strength of 4.13 MPa and 4.68 MPa, respectively. The lowest interlaminar shear strength was observed in neat composite sample, and the significant improvement of 29.70% was obtained for untreated composite sample This was attributed to the presence of a continuous longitudinal sisal fiber core, where higher deformation limits crack propagation between the intermediate layers. Another phenomenon observed is that reduced thickness increases flexibility, with the sisal fibers in the core acting as a barrier across the overall thickness. As a result, the total thickness is effectively halved and supported by the sisal core, which restricts crack propagation and enhances flexibility and stress transfer [ 41 ]. Fibers in the sandwich layer hinder crack growth and sliding, enhancing shear resistance and boosting ILSS despite bonding issues [ 42 ]. The highest interlaminar shear strength was observed in the treated composite samples, exhibiting improvements of 46.25% and 13.31% compared to neat PLA and untreated composites, respectively. This enhancement is attributed to the increased stiffness of the fiber core resulting from the removal of amorphous constituents, thereby providing greater resistance to interlaminar shear loads. However, the overall shear strength was slightly compromised by the increased flexibility associated with the reduced laminate thickness. Thus, this type of composies can be used for vibrational damping panels where high interlaminar shear strength resist delamination under cyclic loads and can absorb vibrations. 3.4. SEM analysis 3.4.1. Surface morphology of sisal fibers The SEM images showing the surface morphology of untreated and NaOH treated fibers are shown in Fig. 5 (a-b) . From Fig. 5 (a-b) , it can be seen that the surface texture of the sisal fiber changed from smooth to rough after the NaOH treatment. The diameter of the untreated and treated sisal fibers was measured using Image J (open-source software). The diameter of the untreated sisal fiber was 183.818 µm, but after NaOH treatment, the fiber diameter reduced to 164.047 µm. It was observed that NaOH treatment reduced the fiber diameter by 12.05%. The NaOH solution was strong enough to disintegrate more amorphous materials (Lignin/Hemicellulose), which led to a reduction in fiber diameter and an increase in surface roughness [ 43 ]. 3.4.2. Morphology of fractured fiber sandwiched 3D printed composite samples. The SEM images showing the morphology of tensile test fractured fiber sandwiched 3D printed composite samples are presented in Fig. 5 (c-e) . Figure 5 (c) presents the SEM image of the neat PLA composite printed with an 80% infill density, exhibiting a raster angle of 45° and a layer height of 0.20 mm. Figure 5 (d-e) reports the fractured morphology with fiber-matrix interaction and the stacking of sandwiched fiber layer in-between the 3D printed PLA layers. It was observed that the fracture morphology of the neat composite is different from sandwiched fiber composites, as fiber sandwiching increases the load-carrying capacity of the composites, as justified through tensile test results [ 44 ]. The fracture surface of the composite sandwiched with untreated fibers (Fig. 5 (d) ) exhibits evident fiber pull-outs, suggesting weak interfacial bonding. Conversely, the treated fiber sandwiched composite (Fig. 5 (e) ) demonstrates improved matrix-fiber adhesion, with minimal pull-out and enhanced fiber adhesion. As discussed in Section 3.4.1, this improvement is attributed to the NaOH surface treatment, which increased the fiber surface roughness and surface area, thereby introducing more active sites for matrix interaction. This enhanced interfacial adhesion was directly linked to the improved ultimate tensile strength of the treated fiber sandwiched composite. In contrast, the untreated samples exhibit lower tensile strength due to poor stress transfer resulting from inadequate fiber-matrix bonding and prevalent fiber pull-outs. 3.5. DMA analysis The DMA was conducted on neat, untreated, and treated 3D printed composites samples, and the results were shown in Fig. 6 . The following section discusses the results obtained through storage modulus and Tan δ. 3.5.1. Storage Modulus The storage modulus represents the amount of energy retained in a material as a function of temperature. Figure 6 (a) shows the storage modulus curves of the 3D printed composite samples. The results indicate that modulus values decreased slowly before the glass transition region, but after this region, samples exhibited a plastic behavior as the PLA matrix began to soften due to increase in temperaure. This softening facilitated increased polymer chain mobility, leading to a noticeable decrease in the stiffness of the composites [ 45 ]. From storage modulus curves, it can be seen that the neat composite sample has a steep curve after the transition phase as compared to fiber sandwiched composite samples. This was due to more polymeric mobility in neat composite samples due to the absence of fibers. Sandwiching continuous fibers aids in carrying some of the mechanical load, preventing a sharp decline [ 46 ]. The storage modulus of all 3D printed composite samples lies between 1130 MPa and 785 MPa, with noticeable differences between neat and fiber-sandwiched composite samples. The neat composite samples showed a higher modulus of 1130 MPa, and the fiber sandwiched composite (untreated) showed the lowest modulus of 785 MPa. It is evident that the storage modulus exhibits a similar trend to the flexural modulus. The reduction in storage modulus is attributed to the composite architecture, wherein both the infill and overall composite thickness are effectively halved due to the incorporation of fibers as a sandwiched core, resulting in increased flexibility. However, composites with chemically treated fibers as the core demonstrated an enhanced storage modulus, owing to the increased stiffness imparted by the treatment. For the untreated fiber sandwiched composite, The decrease in modulus value may be due to the weak fiber/PLA interface the reduces the effective stress transfer especially under oscillatory loading. Another reason may be the local compliance due sandwiched structure, which means sandwiched fiber layer does not constraint deformation (as fiber layer is thin) and behave flexible (as reported in section 3.2), reducing apparent modulus. Although the treated composite, having a storage modulus of 925 MPa, showed an increase in modulus by 17.83% compared to the untreated composite sample. 3.5.2. Damping factor (Tan δ) The damping factor (Tan δ) represents a material's ability to dissipate mechanical energy as heat. Tan δ curves for the 3D-printed composites are presented in Fig. 6 . The damping behavior of the fiber-sandwiched composites is influenced by several factors, including the composite architecture, fiber–matrix interaction, the chemical characteristics of the fibers, and the softening behavior of the matrix under thermal loading. Figure 6 (b) shows that the glass transition temperatures (Tg), as indicated by the Tan δ peaks, exhibited minimal variation among all fiber-sandwiched 3D-printed composites. However, a notable difference in peak Tan δ values was observed across neat PLA, untreated fiber, and treated fiber sandwiched composites. The neat PLA 3D printed composite exhibited the highest Tan δ value (1.49), followed by the untreated fiber sandwiched composite (0.66) and the treated fiber sandwiched composite (0.53). The higher Tan δ value of neat 3D printed PLA reflects its greater molecular mobility and superior energy dissipation capacity at elevated temperatures [ 47 ]. In contrast, the treated and untreated fiber sandwiched composite demonstrated the lowest Tan δ value, indicating restricted molecular mobility due to the reinforcing effect of the fiber core. This was further supported by the E′ curves, which revealed a gradual decline in storage modulus for the fiber-sandwiched composites. Although these composites entered the rubbery region, the presence of a stiff fiber core limited the softening behavior, enabling the composite to retain a significant level of stiffness (E′) even at higher temperatures. This constraint reduced chain mobility, thereby lowering the damping capacity. Additionally, the steep drop in E′ observed in the neat PLA sample suggests a quicker transition into the rubbery state compared to the fiber-reinforced composites. The inherent thermal response of neat PLA allows for greater stiffness at ambient conditions and enhanced energy dissipation near Tg. Therefore, the damping behavior, as reflected by the Tan δ profile, is governed by the composite's ability to balance stiffness retention and molecular mobility across the temperature range. Mostly, lower Tan δ values are desirable for structural applications, which means treated composite (PLA composite with NaOH-treated sisal fibers) are favorable for structural and automobile applications. 3.6. Thermomechanical analysis (TMA) TMA of 3D-printed composites was performed to assess the material expands or contracts in response to varying temperatures. The Fig. 7 (a-b) shows the variation in thermal expansion and expansion coefficient of neat, untreated and treated 3D printed composites with change in temperature. The thermal shrinkage results in Fig. 7 (a) indicate that all composite samples maintained dimensional stability around 52°C. This stability could be attributed to the fact that fiber reinforcement does not significantly influence the glass transition temperature (T g ), which, as shown in Table 4 , falls within the range of 50–55°C. Although after T g region, a drastic change was noticed in thermal expansion. The addition of fibers reduces the thermal expansion % in untreated and treated fiber composites samples as compared to neat composite sample showing the steep curve (around 55–60°C) after the T g . The neat composite shows the maximum thermal expansion (⁓3.5%) whereas the treated composite sample shows the minimum thermal expansion (⁓0.7%) during entire temperature range. This attributes that neat PLA composite was highly susceptible to dimensional instability near its T g and NaOH treatment of sisal fibers likely improved fiber-matrix adhesion, limiting thermal deformation and enhancing dimensional stability. It was noted that thermal expansion of the composite samples is directly linked with the results of thermal expansion coefficient as shown in Fig. 7 (b) and Table 4 . At 40 o C, no significant changes were observed in the thermal expansion coefficient for all the composites. Similar trends were observed in the thermal expansion coefficient till T g . Beyond the glass transition temperature (Tg), the neat composite exhibits a sharp rise in thermal expansion coefficient, reaching up to 850 ppm/°C, which reflects its limited dimensional stability caused by increased polymer chain mobility. In contrast, the composite containing untreated fibers shows a reduced expansion coefficient of approximately 460 ppm/°C, indicating some restriction in thermal expansion. However, the treated fiber composite displays minimal variation in thermal expansion across the temperature range. This difference arises because untreated fibers offer limited resistance to thermal movement due to weak interfacial bonding, whereas the enhanced adhesion in treated fiber composites effectively suppresses thermal deformation, resulting in improved dimensional stability. Table 4 Thermomechanical analysis and thermal expansion coefficient Temperature ( o C) Thermal expansion coefficient (ppm/ o C) Neat Untreated Treated 40 20.07 36.14 38.96 50 51.64 78.34 63.65 60 856.91 363.16 104.78 70 860.2 430.89 142.6 Glass transition temperature, T g ( o C) 53.21 56.12 57.65 3.7. Low velocity impact evaluation The peak force data obtained during the low velocity impact test was analyzed and is presented in the Table 5 . The force vs. displacement and force vs. time graphs are shown in the Fig. 8 (a-b) The work done data was presented in Fig. 8 (c-d). These graphs were used to perform peak force analysis [ 48 ]. From the figures, it was observed that when the impact load was applied, initial deformation occurred, which progressed to a peak load as the energy was dissipated and absorbed [ 49 ]. After reaching the peak, crack initiation and fracture took place, extending the deformation. For the pure PLA composite, a total deformation of 12.20 mm was observed. A sudden drop in force at the peak indicated a sudden failure, as cracking initiated without further propagation, resulting in a large puncture hole. This implies higher energy dissipation, with a total work done of 4.19 J. In the case of the treated fiber-sandwiched composite, the peak force absorbed was 535.08 N, the lowest among the tested samples. This was attributed to the brittle nature of the sisal fiber after chemical treatment. The energy transferred from the matrix to the fiber layers, but the crack initiated and propagated gradually, as reflected by the smooth drop in force and increased deformation [ 50 ]. The presence of the intermediate sisal fiber layer helped reduce the damage area, leading to lower energy dissipation of 3.01 J. For the composite with untreated fiber reinforcement, a significantly higher force was required to cause damage, as the untreated fibers were less brittle and resisted deformation more effectively. Although crack propagation eventually led to failure, the lower brittleness of sisal allowed the composite to absorb more energy before failing, compared to composites sandwiched with stiffer fibers. The peak deformation values were lower, indicating that the composite restricted deformation more effectively, while still reaching peak load, with a work done of 3.34 J. Table 5 Peak force analysis Composite Designation Peak analysis Force (N) Deformation (mm) Work done (J) Neat 602.66 12.20 4.19 Untreated 699.57 8.11 3.34 Treated 535.08 9.56 3.01 The velocity and puncture analysis of the developed composites are presented in the Table 6 and Fig. 9 . It was observed that the behavior of puncture force, displacement and the change in impactor velocity differ from that obeserved in the peak force analysis [ 51 ]. This difference is attributed to composite architecture and the force distribution characteristics during impact loading. From the Table 6 , it was noted that the initial impact velocity for all composites was 2.15 m/s, but the end velocity decreased post-impact. The end velocity is a measure of how much the velocity was reduced during the interaction between the impactor and the composite [ 52 ]. The lower end velocity observed in both the Fig. 9 and the Table 6 indicates that a sudden failure occurred at the peak force, with no significant deformation beyond that point. Table 6 Velocity and Puncture analysis Composite Designation Velocity (m/s) Puncture Begin End Displacement (mm) Force (N) Neat 2.15 1.64 11.59 267.54 Untreated 2.15 1.43 12.50 349.78 Treated 2.15 1.71 12.49 301.33 The puncture displacement for the neat PLA composite was 12.49 mm, which was comparable to the displacement observed in the composite sandwiched with untreated sisal fibers. However, the untreated fiber-sandwiched composite required a higher puncture force than neat PLA to allow the impactor to penetrate. This was due to effective stress distribution and the ability of the interlaid sisal fibers to resist puncture, owing to their non-brittle nature [ 53 ]. The puncture force for the untreated sisal fiber-sandwiched 3D-printed composite was 349.78 N, the highest among the tested samples. This composite also exhibited the highest energy dissipation, evident from the greater puncture deformation (12.50 mm) and the lower end velocity of the impactor (1.43 m/s), indicating effective energy absorption. In contrast, the treated sisal fiber-sandwiched composite displayed lower puncture displacement due to the higher stiffness and brittleness of the treated fibers. As a result, the composite was unable to significantly reduce the impactor’s velocity, since cracking occurred rapidly [ 54 ]. Additionally, the treated composite required a lower puncture force, again due to the brittle nature of the chemically treated fibers. However, good stress transfer from the matrix phase to the treated sisal fiber phase was observed during impact, which was evident in the work vs. displacement curves. These curves exhibited a smooth transition, indicating consistent energy transfer throughout the puncture event. 4. Conclusions This study demonstrates the significant potential of 3D printing technology in fabricating continuous fiber-reinforced biocomposites using a strategically sandwiched architecture. The strategic placement of NaOH-treated continuous Agave sisalana sisal fibers within a PLA matrix resulted in significant improvements in mechanical and thermal performance, overcoming the common limitations of short or randomly oriented natural fiber composites. The mechanical performance of treated composite is impactful, showing the improvement of 36.19% in tensile strength and 46.25% in ILSS strength as compared to neat composite. In addition, the tensile modulus showed enhancements of 38.26% indicating a significant boost in stiffness and load-bearing capacity for automotive applications. Flexural strength and modulus of treated composites enhanced by 9.17% and 8.69%, respectively, as compared to untreated composites. SEM analysis confirmed improved fiber–matrix adhesion and a 12.05% reduction in fiber diameter post-treatment, contributing to better stress transfer. DMA analysis revealed a lower damping factor (Tan δ = 0.53) in treated composites, highlighting their suitability for semi-structural and vibration-damping applications. TMA results indicated a low thermal expansion of 0.7% and a coefficient of thermal expansion of 142.6 ppm/°C at 70°C, pointing to enhanced dimensional stability. Low-velocity impact tests provided additional insights into energy absorption mechanisms. The untreated fiber composite exhibited the highest puncture force (349.78 N), greatest deformation (12.50 mm), and lowest end velocity (1.43 m/s), indicating superior energy dissipation. In contrast, the treated fiber composite showed reduced puncture displacement and lower energy absorption (3.01 J), attributed to increased brittleness but also exhibited smoother force-displacement behavior, reflecting consistent stress transfer. The findings validate the effectiveness of the strategically sandwiched continuous fiber approach in producing lightweight, high-performance 3D-printed composites. Future work should explore optimizing stacking sequences and fiber volume fractions to further tailor the mechanical and thermal properties for specific end-use applications. 5. Future scope The present work offers the strong technique for producing sustainable, high performance biocomposites through strategic sandwiching of treated sisal fibers through 3D printing. This work opens up the possibilities for developing next generation sustainable composites to meet diverse engineering applications. Based on this future work can be done in following areas: Use of hybrid natural fibers and natural filler loaded biopolymers for manufacturing of composite with similar technique. Incorporation of nano-reinforcements such as nanoclay, graphene, or cellulose nanocrystals alongside the sisal fibers could offer multifunctionality including improved barrier, thermal, or electrical properties. Exploration of shape memory or self-healing functionalities in manufactured biocomposites using 4D printing for intelligent material systems. Declarations Acknowledgement The authors are highly thankful to Natural Composites Research Group Lab, Department of Materials and Production Engineering, The Sirindhorn International Thai-German Graduate School of Engineering (TGGS) and the COIDEM-STRI, King Mongkut’s University of Technology Thonburi, Thailand for their instruments and continuous support. Author Contributions Mohit Kumar : Conceptualization; methodology; visualization; software; validation; writing—original draft; writing—review and editing. Ranvijay Kumar : Conceptualization; visualization; writing—review and editing. Manoj Kumar Singh : Conceptualization; visualization; writing—review and editing. Vinod Ayyapan : Conceptualization; validation; visualization; writing—review and editing. Sanjay Mavinkere Rangappa : Supervision; conceptualization; methodology; visualization; writing—review and editing. Suchart Siengchin : Supervision; conceptualization; methodology; visualization; writing—review and editing. Declaration of Generative AI and AI-assisted technologies in the writing process The authors used ChatGPT-4o to assist with proofreading and refining certain sections of the manuscript. After using this tool, the author carefully reviewed and revised the manuscript to ensure accuracy and clarity. The authors accept full responsibility for the final content presented in this publication. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability statement Data will be made available on request. References S. Prashanth, K. Subbaya, K. Nithin, S. Sachhidananda, Fiber reinforced composites-a review, J. Mater. Sci. Eng 6(03) (2017) 2-6. D.K. Rajak, D.D. Pagar, P.L. Menezes, E. Linul, Fiber-reinforced polymer composites: Manufacturing, properties, and applications, Polymers 11(10) (2019) 1667. J. Proy, F. Massa, D. 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Cite Share Download PDF Status: Published Journal Publication published 18 Feb, 2026 Read the published version in The International Journal of Advanced Manufacturing Technology → Version 1 posted Editorial decision: Major Revisions Needed 05 Nov, 2025 Reviewers agreed at journal 21 Sep, 2025 Reviewers invited by journal 20 Sep, 2025 Editor assigned by journal 11 Sep, 2025 First submitted to journal 09 Sep, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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07:51:33","extension":"xml","order_by":23,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":149909,"visible":true,"origin":"","legend":"","description":"","filename":"JAMTD25051950structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/07f647a017cbb937abe99b08.xml"},{"id":92571379,"identity":"a14c5575-3903-4bb3-bb6c-bdea5614676e","added_by":"auto","created_at":"2025-10-01 07:51:33","extension":"html","order_by":24,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":158000,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/e655a1d884a9c8adecfd6813.html"},{"id":92571351,"identity":"04445a7f-bc64-4227-9b42-9bcffd5cc522","added_by":"auto","created_at":"2025-10-01 07:51:32","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":124863,"visible":true,"origin":"","legend":"\u003cp\u003eSchematics of the manufacturing of 3D printed sandwiched composite plate\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/5afc872535df04adaefa5b50.jpg"},{"id":92571350,"identity":"071c4b51-1bc4-4eff-b0f5-fcf68229a1af","added_by":"auto","created_at":"2025-10-01 07:51:32","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":165429,"visible":true,"origin":"","legend":"\u003cp\u003eTensile test results of 3D printed composite samples of neat, untreated and treated configuration (a) Tensile stress vs strain plots, (b) Maximum tensile strength and modulus, (c) representation of modulus of resilience and toughness and (d) Modulus of toughness and resilience plots.\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/2aa9ba3b0982169f6112bc44.jpg"},{"id":92572331,"identity":"177530f6-9289-4cf3-a99e-398f86185842","added_by":"auto","created_at":"2025-10-01 07:59:32","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":117033,"visible":true,"origin":"","legend":"\u003cp\u003eFlexural test results of 3D printed composite samples of neat, untreated and treated configurations (a) Flexural stress vs strain plots and (b) Maximum flexural strength and modulus\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/30ea05df06e02ac15516042f.jpg"},{"id":92571352,"identity":"73d0c4f0-ab8a-46cc-9ee4-3374367f0b16","added_by":"auto","created_at":"2025-10-01 07:51:32","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":92177,"visible":true,"origin":"","legend":"\u003cp\u003eInterlaminar shear strength of 3D printed composite samples of neat, untreated and treated configurations\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/a45a4dd65990a2fc2f7d4ba8.jpg"},{"id":92572329,"identity":"66129f54-c997-4a94-9f97-cfbe3708e3f4","added_by":"auto","created_at":"2025-10-01 07:59:32","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":143250,"visible":true,"origin":"","legend":"\u003cp\u003eSurface Morphology of the (a) untreated sisal fiber, (b) NaOH treated sisal fiber and fractured samples from tensile test for (c) Neat 3D printed composite sample, (d) 3D printed sandwiched with untreated fibers composite and (e) 3D printed sandwiched with untreated fibers composite samples\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/bbe2a39b4e1bb2ac54df9dd3.jpg"},{"id":92571353,"identity":"6188d468-7000-41fd-91b1-5b6501d5fcfb","added_by":"auto","created_at":"2025-10-01 07:51:32","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":128491,"visible":true,"origin":"","legend":"\u003cp\u003eDMA test results in terms of viscoelastic properties as (a) Storage modulus (Eʹ) and (b) Damping factor (Tan delta (δ))\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/a72898e5404896cf820d47fe.jpg"},{"id":92571355,"identity":"7943f7ce-affb-4a16-a5d7-039614c4a92e","added_by":"auto","created_at":"2025-10-01 07:51:32","extension":"jpg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":114133,"visible":true,"origin":"","legend":"\u003cp\u003eTMA of neat, untreated and treated 3D printed composites via (a) thermal shrinkage and (b) thermal expansion coefficient with temperature\u003c/p\u003e","description":"","filename":"7.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/4223c66caea568b24c56bfad.jpg"},{"id":92573117,"identity":"30771560-a854-405f-96d0-389790cb4da0","added_by":"auto","created_at":"2025-10-01 08:07:32","extension":"jpg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":131185,"visible":true,"origin":"","legend":"\u003cp\u003ePeak force results (a) force vs displacement, (b) force vs time, and work done (c) work vs displacement, (d) work vs time during low velocity impact\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/dd70fa3d17d11b05c6beedc6.jpg"},{"id":92571362,"identity":"66792a87-6af2-4247-9df6-d32280981d37","added_by":"auto","created_at":"2025-10-01 07:51:33","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":99618,"visible":true,"origin":"","legend":"\u003cp\u003eVelocity and puncture results (a) velocity vs displacement and (b) velocity vs time during low velocity impact\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/1cda82f2350e85fb014b8742.jpg"},{"id":103251228,"identity":"aac6adee-4c48-451e-9acd-5e9911e54186","added_by":"auto","created_at":"2026-02-23 16:06:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2362565,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7575389/v1/51ea915a-8ee3-4260-914f-f6a31ed45cf2.pdf"}],"financialInterests":"","formattedTitle":"Performance of strategically sandwiched continuous sisal fiber core 3D printed PLA composites for engineering applications","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSurface treated continuous natural fibers fibers integrated into PLA with the help of additive manufacturing offers susitanable composite structures for engineering applications. In engineering applications, lightweight and good mechanical properties of the manufactured parts are highly recommended, and can be accommodated by using continuous natural fibers into thermoplastics. The sdudies suggest that the fibers reinforced with polymers can greatly enhance the mechanical, thermal, chemical, surface, durability, and tribological properties [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. A number of studies have been reported in the recent past for the development of the natural fibers reinforced thermoplastics [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. PLA-based composites are known for good biocompatibility, degradability, good stiffness, and mechanical properties [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. These excellent material properties make PLA as one of the acceptable thermoplastics for applications in tissue engineering, sensors, smart textiles, automobile, and aerospace applications [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. The previous studies have explored the different materials performance of the natural fiber reinforced PLA composites. The studies conducted to investigate the effect of jute, sisal and elephant grass-based fibers in PLA matrix have suggested that impact strength was increased by 22.3%, 111.5% and 129.5% respectively as compared to plain PLA [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The addition of natural fibers is one of the ways to restore the material's properties, which can be ensured or established by mechanical recycling. However, the number of thermal cycles affects the mechanical properties of the composites. The study revealed that when basalt fibers and halloysite nanotubes were reinforced in PLA matrix by a combination of twin-screw extrusion and injection molding, the tensile strength was significantly affected after three recycling steps [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The reinforcement of pre-stressed natural fibers in PLA have ensured the 116% and 62% increase in tensile strength and stiffness respectively along with 14% and 10% increases in flexural strength and rigidity, respectively [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e3D printing is becoming a promising tool for the manufacturing of natural fiber-based composite materials [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. The flour extract (90\u0026ndash;250\u0026micro;m) from henequen fibers reinforced with PLA shows a potential for 3D printing of crack-resistant structures [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Fused deposition modeling (FDM) is a materials extrusion (MEX) based 3D printing process that is largely reported for the manufacturing of natural fiber reinforced polymer composite structures [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The study has reported for incorporation of different natural fibers of oil palm, pineapple leaf, coir, and bamboo (size: 250\u0026ndash;500\u0026micro;m) into PLA for 3D printing. A higher tensile strength was observed for the combination of fibers (hybrid) as compared to pure biocomposites or pure PLA [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. It was reported los decrease in mechanical properties of 3D printed natural fiber reinforced PLA composites after hydrolytic degradation, a potential candidate for furniture, decorations, and the automotive industry [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The manufacturing of micro-perforated panels is possible by 3D printing using cork fiber reinforced PLA for engineering noise control and the effectiveness of hearing conversation [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn 3D printing applications, PLA is often considered as a brittle and low impact-resistant material, which results in limited use. However, the addition of continuous fibers can improve the strength, load transfer efficiency, thermal stability, and crack resistance of PLA towards manufacturing lightweight components [\u003cspan additionalcitationids=\"CR19\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Cersoli et al. (2021) have outlined that continuous Kevlar fiber reinforcement has increased the tensile strength by two times and impact energy by a factor of four [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Similar observations have been reported in previous studies [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. However, direct reinforcement of the fibers with PLA matrix is associated with many challenges, including poor fiber-matrix adhesion, non-uniform distribution, alignment, and formation of voids/porosity, which contributes to defects in the final manufactured parts [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. In some cases, reinforcement can create the problem of nozzle clogging in materials extrusion-based 3D printing [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. To solve these issues, the sandwiching of the continuous fibers within regular additive layers can be implemented. Sandwiching of the fibers is one of the alternative methods to incorporate the continuous fibers, especially for increasing the mechanical strength [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. The process may also be named as the \u0026lsquo;interleaving\u0026rsquo; process. A 44% higher elastic modulus and 119% higher strength were observed for a sandwich structure made by 3D printing of carbon fiber reinforced composites [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. A higher stiffness (up to 4.1 times) could be ensured if 3D printing is performed for the manufacturing of a sandwich structure with a corrugated core [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Also, the bio-inspired sandwich structures prepared by 3D printing (of carbon fiber reinforced composites) can be used for energy absorption applications [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. The surface treatment is one of the best approaches for tuning the properties. This can result in improved adhesion between the layers and mechanical strength. The study reveals that the 3D printed rice husk and rice straw (surface treated with NaOH) reinforced (5\u0026ndash;20%) to PLA can be used for automotive and construction applications [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Along with this, the surface treatment can be helpful for controlling the hydrophobic behavior of the polymer composites [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. The strength retention of surface treated sisal-polyester composites was observed higher as compared to untreated composites [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe reported literature is evident that reinforcement of the natural fibers as filler with PLA matrix has significantly improved the mechanical and physical properties [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Reinforcement of continuous fibers in the PLA matrix has also been reported for the improvement of the mechanical properties [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. 3D printing has become a crucial technology for the fabrication of PLA-natural fiber-based composite structures with suitable mechanical properties [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. In fact, surface treatment of the natural fibers can boost the mechanical properties by controlling the hydrophobic nature [\u003cspan additionalcitationids=\"CR31\" citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. A very few studies have been reported on 3D printed sandwiched strcutures of continuous fibers configuration for tunable mechanical properties [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. Hitherto, fewer have reported on the manufacturing of sandwiched continuous fiber core structured composites using 3D printing. This study highlights a strategical approach for sandwiching the continuous fibers as a core in PLA composites using 3D printing technology. The manufactured 3D printed biocomposite plate was further characterized for mechanical performance, thermal behaviour, morphological analysis, viscoelastic behavior and low velocity impact evaluation. This study seeks to address current challenges by improving fiber orientation, strengthening interfacial adhesion, and boosting mechanical properties, all while preserving the lightweight nature of PLA for engineering applications.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1. Materials\u003c/h2\u003e\u003cp\u003eIn the present study, PLA 4043D grade was procured from NatureWorks LLC. This PLA grade is biodegradable and suitable for making 3D printed filaments. The density of the material is 1.25 g/cm\u003csup\u003e3\u003c/sup\u003e, glass transition temperature of ⁓63\u003csup\u003eo\u003c/sup\u003eC and melting point of ⁓155\u003csup\u003eo\u003c/sup\u003eC. The tensile strength and elongation at break of the material are 55\u0026ndash;60 MPa and 2\u0026ndash;4% respectively. Natural sisal fibers derived from \u003cem\u003eAgave sisalana\u003c/em\u003e were utilized as reinforcement in this study. These fibers had an average diameter of approximately 0.18 mm, a tensile strength of 298 MPa, and a tensile modulus of 11.4 GPa. The fibers were sourced from Metro Composites, Chennai, India. To facilitate the application of PLA as an adhesive, chloroform was used as a solvent. The chloroform, with a stated purity of 99.98%, was procured from RCI Labscan Limited, located in Bangkok, Thailand.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2. Manufacturing process\u003c/h2\u003e\u003cdiv id=\"Sec5\" class=\"Section3\"\u003e\u003ch2\u003e2.2.1. Surface treatment of Agave sisalana (Sisal) fiber\u003c/h2\u003e\u003cp\u003eThe concentrated NaOH solution (5%) was prepared by dissolving 63.23 g of 98% NaOH pellets in 1200 mL distilled water and stirred till the pellets were completely dissolved. The actual density of NaOH pellets was 2.13 g/cc, and the final density for 1200 mL of 5% NaOH solution was 1.054 g/cc. The alkali surface treatment was done by soaking 200 g of derived fibers in 1200 mL of 5% NaOH solution using a magnetic stirrer at room temperature for 8 h. Later, the treated fibers were washed completely with distilled water until the pH was neutralized. Further, these treated fibers were dried in a hot oven at 65\u0026deg;C for 24 h. The chemical reaction of fiber with NaOH is given in Eq. \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\left(1\\right)\\)\u003c/span\u003e\u003c/span\u003e.\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:Fiber-OH+NaOH\\:\\:\\to\\:\\:\\:Fiber-ON{a}^{+}+{H}_{2}O\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe hydroxyl (OH) groups on the fiber surface were disintegrated, resulting in the removal of excess amorphous constituents like hemicellulose, wax, lignin, pectin, and impurities. NaOH treatment infiltrates the amorphous zones of cellulose, swelling the fibres and expanding their cell walls. The rougher surface and enlarged pores give the resin more room to penetrate, nurturing stronger fiber/matrix adhesion through mechanical interlocking. Meanwhile, NaOH cuts the original, moisture-prone hydroxyl linkages subsequent re-arrangement of the cellulose chains creates new hydroxyl bonds. With many of the moisture-prone hydroxyl groups removed, the treated fibres exhibit better resistance to moisture [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section3\"\u003e\u003ch2\u003e2.2.2. Manufacturing of PLA/Sisal fiber biocomposites laminate\u003c/h2\u003e\u003cp\u003eThe manufacturing procedure starts with preparing a PLA solution. For this, PLA pellets of 4043D grade are weighed and placed into a beaker, followed by the addition of chloroform as a solvent. A 1:10 weight-to-volume ratio is used specifically; 25 grams of PLA is dissolved in approximately 150 mL (around 222 grams) of chloroform. To facilitate complete dissolution, the mixture is stirred using a magnetic stirrer set to 60\u0026deg;C. Once a homogeneous PLA solution is obtained, it is uniformly spread over both untreated and chemically treated agave sisalana fibers that are aligned in a single direction. The coated fibers are then left undisturbed for about two hours, allowing the chloroform to evaporate completely. The dried composite is subsequently processed through compression molding at 150\u0026deg;C, under a pressure of 4000 kgf, for a duration of 15 minutes using a mold with a thickness of 0.4 mm. This step is essential for minimizing void content and ensuring a consistent final thickness in the fiber-sandwiched PLA composite. In this process, two types of biocomposite laminates were developed, one using untreated sisal fibers and the other incorporating treated sisal fibers.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section3\"\u003e\u003ch2\u003e2.2.3. Filament manufacturing and 3D printing of composites\u003c/h2\u003e\u003cp\u003eTo prepare the PLA filament, PLA pellets were initially dried in an oven at 45\u0026deg;C for 24 hours to eliminate any residual moisture. Following this drying step, the pellets were processed using a single-screw extruder (Noztek Xcalibur) to produce filaments with a target diameter of 1.75\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 mm. The extruder featured three distinct heating zones: the nozzle (T1) was maintained at 200\u0026deg;C, the intermediate (T-2) at 190\u0026deg;C, and the feed zone near the hopper (T-3) at 185\u0026deg;C. The extrusion was conducted at a screw rotation speed ranging from 17 to 22 revolutions per minute (rpm). In the subsequent step, samples were prepared using a 3D printer, model BBC3040 manufactured by Bangkok Blue Ocean Co. Ltd, Bangkok, Thailand. The 3D printing parameter used in this work is presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The CAD design was created and saved in .stl format, further imported into the simply3D software for printing. The desired 3D printed composite specimen thickness was 3mm, which comprises of 3 stacks (stack 1- 1.3 mm, stack 2- 0.4 mm and stack 3- 1.3 mm) that were printed layer by layer. The composite sample descriptions were given in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Stack 2 is filled with PLA/sisal fiber laminate in case of untreated and treated samples.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003e3D printing parameters\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParameters\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eValues (units)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eInfill density\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e80%\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExtruder temperature\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e210 \u003csup\u003eo\u003c/sup\u003eC\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eBed temperature\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e55 \u003csup\u003eo\u003c/sup\u003eC\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003ePrinting speed\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.40 mm/s\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNozzle size\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.6 mm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLayer height\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.20 mm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFilling pattern\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eRectilinear\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExtrusion width\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e0.4 mm\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eRaster angle\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e45\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eDescription of composite samples\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eComposite samples\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eDesignation\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3D printed PLA biocomposite\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNeat\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3D printed PLA biocomposite sandwiched with untreated sisal fiber\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eUntreated\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e3D printed PLA biocomposite sandwiched with treated sisal fiber\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTreated\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eTo sandwich the biocomposites laminate, the 3D printing process was programmed to pause layer deposition at a height of 1.3 mm. After inserting the biocomposites laminate, the layer-by-layer deposition resumed from a height of 1.7 mm, ensuring a total sample thickness of 3 mm. To ensure proper adhesion of the biocomposites laminate after the stack 1, the prepared PLA solution was uniformly spread across the surface layer to bond with the deposited layers. Subsequently, the layer-by-layer deposition process was continued on top of the laminate, resulting in a final sample with a thickness of 3 mm. The complete schematic process for fabrication is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Before conducting the experiments, all 3D-printed composite samples were placed in a hot air oven at 50\u0026deg;C to eliminate any moisture absorbed by the PLA materials.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.3. Characterization techniques\u003c/h2\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003ch2\u003e2.3.1. Mechanical characterization \u0026ndash; Tensile, Flexural, and Interlaminar shear strength (ILSS)\u003c/h2\u003e\u003cp\u003eThe mechanical characterization, such as tensile strength and modulus, flexural strength and modulus, and ILSS was measured via a Universal testing machine model, Comtech M1-type of 10 kN loadcell. The tensile test, flexural test, and ILSS test were performed using ASTM 3039D (100\u0026times;10\u0026times;3 mm\u003csup\u003e3\u003c/sup\u003e), ASTM D790 (90\u0026times;10\u0026times;3 mm\u003csup\u003e3\u003c/sup\u003e) and ASTM D2344 (60\u0026times;10\u0026times;3 mm\u003csup\u003e3\u003c/sup\u003e), respectively. Five samples for each 3D printed composite sample were tested with a test speed of 2 mm/min, and average values were reported in the results.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\u003ch2\u003e2.3.2. Scanning Electron Microscopy (SEM)\u003c/h2\u003e\u003cp\u003eThe surface morphology of tensile fractured samples was analyzed using Thermo Fisher Scientific-SEM, Ireland. Before SEM imaging, the fractured samples were gold-coated using an MP-19020NCTR Neocoater to ensure conductivity, as the materials were inherently non-conductive.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\u003ch2\u003e2.3.3. Dynamic Mechanical Analysis\u003c/h2\u003e\u003cp\u003eThe dynamic mechanical analysis (DMA) was done to evaluate the effect of sandwiched untreated and treated fibers in the 3D-printed composite samples on their viscoelastic behaviour. The loss modulus (Eʺ), storage modulus (Eʹ) and Tan δ were evaluated as per ASTM D4065 standard, in dual cantilever mode with temperature range from 25\u0026deg;C to 125\u003csup\u003eo\u003c/sup\u003eC at heating rate of 5\u003csup\u003eo\u003c/sup\u003eC/min. The frequency mode and load were set at 1Hz and 0.8 N, respectively. The loss modulus signifies the energy dissipated during deformation, the storage modulus signifies the material's capacity to store energy under deformation, and Tan δ presents the material's damping properties.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\u003ch2\u003e2.3.4. Thermomechanical Analysis (TMA)\u003c/h2\u003e\u003cp\u003eTMA was conducted to evaluate the temperature effect on the thermal expansion behavior and dimensional stability of the 3D printed composite samples. A thermomechanical analyzer, model TMA/SDTA 1+, Mettler Toledo, Switzerland, was used to study samples as per ASTM E831-03. The test was performed by putting the sample between flat silica plates under a quartz probe inside the furnace, at temperature ranges between 30\u0026ndash;100\u0026deg;C, at a rate of 5\u0026deg;C/min in a nitrogen environment.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section3\"\u003e\u003ch2\u003e2.3.5. Low Velocity Impact\u003c/h2\u003e\u003cp\u003eThe low impact velocity was used to study the energy absorption behaviour, impact resistance, and damage tolerance of the 3D printed composite samples as per ASTM D7136 standard. The experiment was conducted on a 100\u0026times;100 mm\u003csup\u003e2\u003c/sup\u003e plate using a Zwick Roell HIT 600F impact machine with an impact energy of 10J. The purpose of this study is to evaluate the real-time scenario where composites made from present materials get hit by any objects.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003e3.1. Tensile properties\u003c/h2\u003e\u003cp\u003eThe tensile results of neat, untreated, and treated fiber sandwiched 3D printed composite samples were investigated and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e \u003cb\u003eand\u003c/b\u003e Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. It was observed that neat 3D printed PLA composite exhibited the lowest tensile performance among all tested samples, with a tensile strength of 30.01 MPa and a tensile modulus of 1750.91 MPa. Incorporating untreated fibers in a sandwiched structure improved the overall strength, resulting in a tensile strength of 33.17 MPa and a tensile modulus of 2316.55 MPa, reflecting a 10.20% increase in strength compared to the neat composite. Notably, the 3D printed composites sandwiched with treated fibers demonstrated the most significant enhancement, achieving a tensile strength of 40.87 MPa and a tensile modulus of 2420.88 MPa. This represents an overall improvement of 36.19% over the neat composite and 23.21% compared to the untreated fiber composite. It was attributed to the excellent stress transfer with the matrix and the fibers due to the increased surface area and improved interfacial adhesion resulting from the surface treatment [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eTensile test results of 3D printed composite samples\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"6\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSample\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eMaximum tensile strength (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eTensile modulus (MPa)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eStrain at break\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eModulus of toughness (kJ/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c6\"\u003e\u003cp\u003eModulus of resilience (kJ/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNeat\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e30.01\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e1750.91\u0026thinsp;\u0026plusmn;\u0026thinsp;46.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e0.028\u0026thinsp;\u0026plusmn;\u0026thinsp;0.003\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e530\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e103\u0026thinsp;\u0026plusmn;\u0026thinsp;9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eUntreated\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e33.17\u0026thinsp;\u0026plusmn;\u0026thinsp;3.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e2316.55\u0026thinsp;\u0026plusmn;\u0026thinsp;58.95\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e0.030\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e666.8\u0026thinsp;\u0026plusmn;\u0026thinsp;15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e230\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTreated\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e40.87\u0026thinsp;\u0026plusmn;\u0026thinsp;2.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e2420.88\u0026thinsp;\u0026plusmn;\u0026thinsp;50.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e0.035\u0026thinsp;\u0026plusmn;\u0026thinsp;0.002\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e938.9\u0026thinsp;\u0026plusmn;\u0026thinsp;10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e\u003cp\u003e307\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe strain at break for all the composites was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(a) and\u003c/b\u003e Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. It was observed that the 3D printed composites sandwiched with treated fibers exhibited a maximum strain at break of 0.035, representing a 25% improvement compared to the neat composites. The observed increase in strain at break can be attributed to the improved interfacial bonding and enhanced tensile behavior. NaOH treatment of sisal fibers effectively eliminates considerable amorphous constituents such as lignin and hemicellulose, exposing a rougher and more reactive fiber surface [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. This modification significantly strengthens the interaction between the fiber and the surrounding matrix, allowing for better mechanical interlocking and more efficient stress transfer [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. As a result, the fiber sandwiched composite was able to endure greater deformation before fracturing. Moreover, the presence of continuous fibers acts as a mechanical bridge between cracks and redistributes localized stress, which helps to minimize stress concentrations. The stronger interface also contributes to improved energy absorption and dissipation during loading. Together, these mechanisms account for the increased strain at break observed in the treated fiber composites as compared to other composite sample configurations. The strain at break of composites significantly affects their modulus of toughness and resilience properties.\u003c/p\u003e\u003cp\u003eThe modulus of toughness represents how much energy a material can absorb per unit area before it breaks, covering both its elastic and plastic deformation stages. In simpler terms, it reflects the material\u0026rsquo;s capacity to endure stress and strain up to the point of fracture. On the other hand, resilience refers to a material's ability to absorb energy during elastic deformation and then recover its original shape once the applied stress is removed. This indicates the material\u0026rsquo;s resistance to temporary deformation without undergoing lasting damage. Generally, materials that exhibit both high toughness and high resilience are better equipped to handle significant forces and impacts, making them ideal for use in structural and protective applications [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. The results of all composite configurations in terms of modulus of toughness and resilience are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e(c-d) and\u003c/b\u003e Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The lowest modulus of toughness (530 kJ/m\u003csup\u003e3\u003c/sup\u003e) and resilience (103 kJ/m\u003csup\u003e3\u003c/sup\u003e) were observed in the neat 3D printed PLA composite. This is attributed to brittle behaviour with limited energy absorption during both elastic and plastic deformation [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. The increase in both modulus of toughness (666.8 kJ/m\u003csup\u003e3\u003c/sup\u003e) and resilience (230 kJ/m\u003csup\u003e3\u003c/sup\u003e) was obtained for untreated composite corresponds to improved performance with fiber reinforcement. Although these values were less than treated values of modulus of toughness (938.9 kJ/m\u003csup\u003e3\u003c/sup\u003e) and resilience (307 kJ/m\u003csup\u003e3\u003c/sup\u003e), because of poor interfacial bonding in untreated composites. The higher toughness properties in treated composites attributes to superior energy dissipation, better stress transfer and plastic deformation making it more suitable for applications requiring improved toughness and ductility.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003e3.2. Flexural properties\u003c/h2\u003e\u003cp\u003eThe flexural results of neat, untreated, and treated fiber sandwiched 3D printed composite samples are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. It was noted that neat sample exhibited the highest flexural strength and flexural modulus of 61.73 MPa and 2560.11 MPa, respectively. Incorporating sandwiched core fiber layer between 3D printed PLA composite did not result in any notable enhancement in flexural strength and flexural modulus. For the untreated composite, the recorded flexural strength and modulus were 51.10 MPa and 2220.34 MPa, respectively. The reason was analysed from Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), where the strain values for sandwiched core fiber composites were significantly high (⁓50%) as compared to neat composite sample. This increase is likely due to the enhanced deformability of the sandwiched structure, which results in lower stiffness and improved flexibility (include Core is restricting crack propagation) [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. However, after NaOH treatment of fibers, these values increased to 55.79 MPa for flexural strength and 2413.37 MPa for flexural modulus, indicating improvements of approximately 9.17% and 8.69%, respectively, as compared to untreated fiber core samples. This may be due to the increased surface roughness correspond to better interfacial bonding and better stress transfer between fibers and matrix [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. This enables the material to absorb more energy and undergo greater deformation prior to failure.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003e3.3. Interlaminar shear strength (ILSS) properties\u003c/h2\u003e\u003cp\u003eILSS is one of the key properties to evaluate the delamination behavior of layered composites, particularly in sandwich structures. The ILSS of neat, untreated, and treated fiber sandwiched 3D printed composite samples were evaluated and presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The results showed that neat composite sample had interlaminar shear strength of 3.2 MPa, while untreated and treated fiber core composite samples had interlaminar shear strength of 4.13 MPa and 4.68 MPa, respectively. The lowest interlaminar shear strength was observed in neat composite sample, and the significant improvement of 29.70% was obtained for untreated composite sample This was attributed to the presence of a continuous longitudinal sisal fiber core, where higher deformation limits crack propagation between the intermediate layers. Another phenomenon observed is that reduced thickness increases flexibility, with the sisal fibers in the core acting as a barrier across the overall thickness. As a result, the total thickness is effectively halved and supported by the sisal core, which restricts crack propagation and enhances flexibility and stress transfer [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Fibers in the sandwich layer hinder crack growth and sliding, enhancing shear resistance and boosting ILSS despite bonding issues [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. The highest interlaminar shear strength was observed in the treated composite samples, exhibiting improvements of 46.25% and 13.31% compared to neat PLA and untreated composites, respectively. This enhancement is attributed to the increased stiffness of the fiber core resulting from the removal of amorphous constituents, thereby providing greater resistance to interlaminar shear loads. However, the overall shear strength was slightly compromised by the increased flexibility associated with the reduced laminate thickness. Thus, this type of composies can be used for vibrational damping panels where high interlaminar shear strength resist delamination under cyclic loads and can absorb vibrations.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec18\" class=\"Section2\"\u003e\u003ch2\u003e3.4. SEM analysis\u003c/h2\u003e\u003cdiv id=\"Sec19\" class=\"Section3\"\u003e\u003ch2\u003e3.4.1. Surface morphology of sisal fibers\u003c/h2\u003e\u003cp\u003eThe SEM images showing the surface morphology of untreated and NaOH treated fibers are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(a-b)\u003c/b\u003e. From Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(a-b)\u003c/b\u003e, it can be seen that the surface texture of the sisal fiber changed from smooth to rough after the NaOH treatment. The diameter of the untreated and treated sisal fibers was measured using Image J (open-source software). The diameter of the untreated sisal fiber was 183.818 \u0026micro;m, but after NaOH treatment, the fiber diameter reduced to 164.047 \u0026micro;m. It was observed that NaOH treatment reduced the fiber diameter by 12.05%. The NaOH solution was strong enough to disintegrate more amorphous materials (Lignin/Hemicellulose), which led to a reduction in fiber diameter and an increase in surface roughness [\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec20\" class=\"Section3\"\u003e\u003ch2\u003e3.4.2. Morphology of fractured fiber sandwiched 3D printed composite samples.\u003c/h2\u003e\u003cp\u003eThe SEM images showing the morphology of tensile test fractured fiber sandwiched 3D printed composite samples are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(c-e)\u003c/b\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(c)\u003c/b\u003e presents the SEM image of the neat PLA composite printed with an 80% infill density, exhibiting a raster angle of 45\u0026deg; and a layer height of 0.20 mm. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e \u003cb\u003e(d-e)\u003c/b\u003e reports the fractured morphology with fiber-matrix interaction and the stacking of sandwiched fiber layer in-between the 3D printed PLA layers. It was observed that the fracture morphology of the neat composite is different from sandwiched fiber composites, as fiber sandwiching increases the load-carrying capacity of the composites, as justified through tensile test results [\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e]. The fracture surface of the composite sandwiched with untreated fibers (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(d)\u003c/b\u003e) exhibits evident fiber pull-outs, suggesting weak interfacial bonding. Conversely, the treated fiber sandwiched composite (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003cb\u003e(e)\u003c/b\u003e) demonstrates improved matrix-fiber adhesion, with minimal pull-out and enhanced fiber adhesion. As discussed in Section 3.4.1, this improvement is attributed to the NaOH surface treatment, which increased the fiber surface roughness and surface area, thereby introducing more active sites for matrix interaction. This enhanced interfacial adhesion was directly linked to the improved ultimate tensile strength of the treated fiber sandwiched composite. In contrast, the untreated samples exhibit lower tensile strength due to poor stress transfer resulting from inadequate fiber-matrix bonding and prevalent fiber pull-outs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec21\" class=\"Section2\"\u003e\u003ch2\u003e3.5. DMA analysis\u003c/h2\u003e\u003cp\u003eThe DMA was conducted on neat, untreated, and treated 3D printed composites samples, and the results were shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The following section discusses the results obtained through storage modulus and Tan δ.\u003c/p\u003e\u003cdiv id=\"Sec22\" class=\"Section3\"\u003e\u003ch2\u003e3.5.1. Storage Modulus\u003c/h2\u003e\u003cp\u003eThe storage modulus represents the amount of energy retained in a material as a function of temperature. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e \u003cb\u003e(a)\u003c/b\u003e shows the storage modulus curves of the 3D printed composite samples. The results indicate that modulus values decreased slowly before the glass transition region, but after this region, samples exhibited a plastic behavior as the PLA matrix began to soften due to increase in temperaure. This softening facilitated increased polymer chain mobility, leading to a noticeable decrease in the stiffness of the composites [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. From storage modulus curves, it can be seen that the neat composite sample has a steep curve after the transition phase as compared to fiber sandwiched composite samples. This was due to more polymeric mobility in neat composite samples due to the absence of fibers. Sandwiching continuous fibers aids in carrying some of the mechanical load, preventing a sharp decline [\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. The storage modulus of all 3D printed composite samples lies between 1130 MPa and 785 MPa, with noticeable differences between neat and fiber-sandwiched composite samples. The neat composite samples showed a higher modulus of 1130 MPa, and the fiber sandwiched composite (untreated) showed the lowest modulus of 785 MPa. It is evident that the storage modulus exhibits a similar trend to the flexural modulus. The reduction in storage modulus is attributed to the composite architecture, wherein both the infill and overall composite thickness are effectively halved due to the incorporation of fibers as a sandwiched core, resulting in increased flexibility. However, composites with chemically treated fibers as the core demonstrated an enhanced storage modulus, owing to the increased stiffness imparted by the treatment. For the untreated fiber sandwiched composite, The decrease in modulus value may be due to the weak fiber/PLA interface the reduces the effective stress transfer especially under oscillatory loading. Another reason may be the local compliance due sandwiched structure, which means sandwiched fiber layer does not constraint deformation (as fiber layer is thin) and behave flexible (as reported in section 3.2), reducing apparent modulus. Although the treated composite, having a storage modulus of 925 MPa, showed an increase in modulus by 17.83% compared to the untreated composite sample.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec23\" class=\"Section3\"\u003e\u003ch2\u003e3.5.2. Damping factor (Tan δ)\u003c/h2\u003e\u003cp\u003eThe damping factor (Tan δ) represents a material's ability to dissipate mechanical energy as heat. Tan δ curves for the 3D-printed composites are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. The damping behavior of the fiber-sandwiched composites is influenced by several factors, including the composite architecture, fiber\u0026ndash;matrix interaction, the chemical characteristics of the fibers, and the softening behavior of the matrix under thermal loading. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e(b) shows that the glass transition temperatures (Tg), as indicated by the Tan δ peaks, exhibited minimal variation among all fiber-sandwiched 3D-printed composites. However, a notable difference in peak Tan δ values was observed across neat PLA, untreated fiber, and treated fiber sandwiched composites. The neat PLA 3D printed composite exhibited the highest Tan δ value (1.49), followed by the untreated fiber sandwiched composite (0.66) and the treated fiber sandwiched composite (0.53). The higher Tan δ value of neat 3D printed PLA reflects its greater molecular mobility and superior energy dissipation capacity at elevated temperatures [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. In contrast, the treated and untreated fiber sandwiched composite demonstrated the lowest Tan δ value, indicating restricted molecular mobility due to the reinforcing effect of the fiber core. This was further supported by the E\u0026prime; curves, which revealed a gradual decline in storage modulus for the fiber-sandwiched composites. Although these composites entered the rubbery region, the presence of a stiff fiber core limited the softening behavior, enabling the composite to retain a significant level of stiffness (E\u0026prime;) even at higher temperatures. This constraint reduced chain mobility, thereby lowering the damping capacity. Additionally, the steep drop in E\u0026prime; observed in the neat PLA sample suggests a quicker transition into the rubbery state compared to the fiber-reinforced composites. The inherent thermal response of neat PLA allows for greater stiffness at ambient conditions and enhanced energy dissipation near Tg. Therefore, the damping behavior, as reflected by the Tan δ profile, is governed by the composite's ability to balance stiffness retention and molecular mobility across the temperature range. Mostly, lower Tan δ values are desirable for structural applications, which means treated composite (PLA composite with NaOH-treated sisal fibers) are favorable for structural and automobile applications.\u003c/p\u003e\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Sec24\" class=\"Section2\"\u003e\u003ch2\u003e3.6. Thermomechanical analysis (TMA)\u003c/h2\u003e\u003cp\u003eTMA of 3D-printed composites was performed to assess the material expands or contracts in response to varying temperatures. The Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e \u003cb\u003e(a-b)\u003c/b\u003e shows the variation in thermal expansion and expansion coefficient of neat, untreated and treated 3D printed composites with change in temperature. The thermal shrinkage results in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e \u003cb\u003e(a)\u003c/b\u003e indicate that all composite samples maintained dimensional stability around 52\u0026deg;C. This stability could be attributed to the fact that fiber reinforcement does not significantly influence the glass transition temperature (T\u003csub\u003eg\u003c/sub\u003e), which, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, falls within the range of 50\u0026ndash;55\u0026deg;C. Although after T\u003csub\u003eg\u003c/sub\u003e region, a drastic change was noticed in thermal expansion. The addition of fibers reduces the thermal expansion % in untreated and treated fiber composites samples as compared to neat composite sample showing the steep curve (around 55\u0026ndash;60\u0026deg;C) after the T\u003csub\u003eg\u003c/sub\u003e. The neat composite shows the maximum thermal expansion (⁓3.5%) whereas the treated composite sample shows the minimum thermal expansion (⁓0.7%) during entire temperature range. This attributes that neat PLA composite was highly susceptible to dimensional instability near its T\u003csub\u003eg\u003c/sub\u003e and NaOH treatment of sisal fibers likely improved fiber-matrix adhesion, limiting thermal deformation and enhancing dimensional stability.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIt was noted that thermal expansion of the composite samples is directly linked with the results of thermal expansion coefficient as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e \u003cb\u003e(b)\u003c/b\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. At 40\u003csup\u003eo\u003c/sup\u003eC, no significant changes were observed in the thermal expansion coefficient for all the composites. Similar trends were observed in the thermal expansion coefficient till T\u003csub\u003eg\u003c/sub\u003e. Beyond the glass transition temperature (Tg), the neat composite exhibits a sharp rise in thermal expansion coefficient, reaching up to 850 ppm/\u0026deg;C, which reflects its limited dimensional stability caused by increased polymer chain mobility. In contrast, the composite containing untreated fibers shows a reduced expansion coefficient of approximately 460 ppm/\u0026deg;C, indicating some restriction in thermal expansion. However, the treated fiber composite displays minimal variation in thermal expansion across the temperature range. This difference arises because untreated fibers offer limited resistance to thermal movement due to weak interfacial bonding, whereas the enhanced adhesion in treated fiber composites effectively suppresses thermal deformation, resulting in improved dimensional stability.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eThermomechanical analysis and thermal expansion coefficient\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eTemperature (\u003csup\u003eo\u003c/sup\u003eC)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003eThermal expansion coefficient (ppm/\u003csup\u003eo\u003c/sup\u003eC)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNeat\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eUntreated\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eTreated\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e20.07\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e36.14\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e38.96\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e51.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e78.34\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e63.65\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e60\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e856.91\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e363.16\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e104.78\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e70\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e860.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e430.89\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e142.6\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cb\u003eGlass transition temperature, T\u003c/b\u003e\u003csub\u003e\u003cb\u003eg\u003c/b\u003e\u003c/sub\u003e \u003cb\u003e(\u003c/b\u003e\u003csup\u003e\u003cb\u003eo\u003c/b\u003e\u003c/sup\u003e\u003cb\u003eC)\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e53.21\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e56.12\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e57.65\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec25\" class=\"Section2\"\u003e\u003ch2\u003e3.7. Low velocity impact evaluation\u003c/h2\u003e\u003cp\u003eThe peak force data obtained during the low velocity impact test was analyzed and is presented in the Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The force vs. displacement and force vs. time graphs are shown in the Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e \u003cb\u003e(a-b)\u003c/b\u003e The work done data was presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e \u003cb\u003e(c-d).\u003c/b\u003e These graphs were used to perform peak force analysis [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. From the figures, it was observed that when the impact load was applied, initial deformation occurred, which progressed to a peak load as the energy was dissipated and absorbed [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e]. After reaching the peak, crack initiation and fracture took place, extending the deformation. For the pure PLA composite, a total deformation of 12.20 mm was observed. A sudden drop in force at the peak indicated a sudden failure, as cracking initiated without further propagation, resulting in a large puncture hole. This implies higher energy dissipation, with a total work done of 4.19 J. In the case of the treated fiber-sandwiched composite, the peak force absorbed was 535.08 N, the lowest among the tested samples. This was attributed to the brittle nature of the sisal fiber after chemical treatment. The energy transferred from the matrix to the fiber layers, but the crack initiated and propagated gradually, as reflected by the smooth drop in force and increased deformation [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e]. The presence of the intermediate sisal fiber layer helped reduce the damage area, leading to lower energy dissipation of 3.01 J. For the composite with untreated fiber reinforcement, a significantly higher force was required to cause damage, as the untreated fibers were less brittle and resisted deformation more effectively. Although crack propagation eventually led to failure, the lower brittleness of sisal allowed the composite to absorb more energy before failing, compared to composites sandwiched with stiffer fibers. The peak deformation values were lower, indicating that the composite restricted deformation more effectively, while still reaching peak load, with a work done of 3.34 J.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003ePeak force analysis\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eComposite Designation\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\u003cp\u003ePeak analysis\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eForce (N)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eDeformation (mm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eWork done (J)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNeat\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e602.66\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e12.20\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e4.19\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eUntreated\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e699.57\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e8.11\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.34\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTreated\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e535.08\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e9.56\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e3.01\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe velocity and puncture analysis of the developed composites are presented in the Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e. It was observed that the behavior of puncture force, displacement and the change in impactor velocity differ from that obeserved in the peak force analysis [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. This difference is attributed to composite architecture and the force distribution characteristics during impact loading. From the Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, it was noted that the initial impact velocity for all composites was 2.15 m/s, but the end velocity decreased post-impact. The end velocity is a measure of how much the velocity was reduced during the interaction between the impactor and the composite [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. The lower end velocity observed in both the Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e and the Table\u0026nbsp;\u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e indicates that a sudden failure occurred at the peak force, with no significant deformation beyond that point.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eVelocity and Puncture analysis\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eComposite Designation\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c3\" namest=\"c2\"\u003e\u003cp\u003eVelocity (m/s)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e\u003cp\u003ePuncture\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBegin\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eEnd\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eDisplacement (mm)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eForce (N)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNeat\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.64\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e11.59\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e267.54\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eUntreated\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.43\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e12.50\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e349.78\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTreated\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2.15\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.71\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e12.49\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e\u003cp\u003e301.33\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe puncture displacement for the neat PLA composite was 12.49 mm, which was comparable to the displacement observed in the composite sandwiched with untreated sisal fibers. However, the untreated fiber-sandwiched composite required a higher puncture force than neat PLA to allow the impactor to penetrate. This was due to effective stress distribution and the ability of the interlaid sisal fibers to resist puncture, owing to their non-brittle nature [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. The puncture force for the untreated sisal fiber-sandwiched 3D-printed composite was 349.78 N, the highest among the tested samples. This composite also exhibited the highest energy dissipation, evident from the greater puncture deformation (12.50 mm) and the lower end velocity of the impactor (1.43 m/s), indicating effective energy absorption. In contrast, the treated sisal fiber-sandwiched composite displayed lower puncture displacement due to the higher stiffness and brittleness of the treated fibers. As a result, the composite was unable to significantly reduce the impactor\u0026rsquo;s velocity, since cracking occurred rapidly [\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e]. Additionally, the treated composite required a lower puncture force, again due to the brittle nature of the chemically treated fibers. However, good stress transfer from the matrix phase to the treated sisal fiber phase was observed during impact, which was evident in the work vs. displacement curves. These curves exhibited a smooth transition, indicating consistent energy transfer throughout the puncture event.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eThis study demonstrates the significant potential of 3D printing technology in fabricating continuous fiber-reinforced biocomposites using a strategically sandwiched architecture. The strategic placement of NaOH-treated continuous \u003cem\u003eAgave sisalana\u003c/em\u003e sisal fibers within a PLA matrix resulted in significant improvements in mechanical and thermal performance, overcoming the common limitations of short or randomly oriented natural fiber composites. The mechanical performance of treated composite is impactful, showing the improvement of 36.19% in tensile strength and 46.25% in ILSS strength as compared to neat composite. In addition, the tensile modulus showed enhancements of 38.26% indicating a significant boost in stiffness and load-bearing capacity for automotive applications. Flexural strength and modulus of treated composites enhanced by 9.17% and 8.69%, respectively, as compared to untreated composites. SEM analysis confirmed improved fiber\u0026ndash;matrix adhesion and a 12.05% reduction in fiber diameter post-treatment, contributing to better stress transfer. DMA analysis revealed a lower damping factor (Tan δ\u0026thinsp;=\u0026thinsp;0.53) in treated composites, highlighting their suitability for semi-structural and vibration-damping applications. TMA results indicated a low thermal expansion of 0.7% and a coefficient of thermal expansion of 142.6 ppm/\u0026deg;C at 70\u0026deg;C, pointing to enhanced dimensional stability. Low-velocity impact tests provided additional insights into energy absorption mechanisms. The untreated fiber composite exhibited the highest puncture force (349.78 N), greatest deformation (12.50 mm), and lowest end velocity (1.43 m/s), indicating superior energy dissipation. In contrast, the treated fiber composite showed reduced puncture displacement and lower energy absorption (3.01 J), attributed to increased brittleness but also exhibited smoother force-displacement behavior, reflecting consistent stress transfer. The findings validate the effectiveness of the strategically sandwiched continuous fiber approach in producing lightweight, high-performance 3D-printed composites. Future work should explore optimizing stacking sequences and fiber volume fractions to further tailor the mechanical and thermal properties for specific end-use applications.\u003c/p\u003e"},{"header":"5. Future scope","content":"\u003cp\u003eThe present work offers the strong technique for producing sustainable, high performance biocomposites through strategic sandwiching of treated sisal fibers through 3D printing. This work opens up the possibilities for developing next generation sustainable composites to meet diverse engineering applications. Based on this future work can be done in following areas:\u003c/p\u003e\u003cp\u003e\u003cul\u003e\u003cli\u003e\u003cp\u003eUse of hybrid natural fibers and natural filler loaded biopolymers for manufacturing of composite with similar technique.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eIncorporation of nano-reinforcements such as nanoclay, graphene, or cellulose nanocrystals alongside the sisal fibers could offer multifunctionality including improved barrier, thermal, or electrical properties.\u003c/p\u003e\u003c/li\u003e\u003cli\u003e\u003cp\u003eExploration of shape memory or self-healing functionalities in manufactured biocomposites using 4D printing for intelligent material systems.\u003c/p\u003e\u003c/li\u003e\u003c/ul\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eAcknowledgement\u003c/p\u003e\n\u003cp\u003eThe authors are highly thankful to Natural Composites Research Group Lab, Department of Materials and Production Engineering, The Sirindhorn International Thai-German Graduate School of Engineering (TGGS) and the COIDEM-STRI, King Mongkut\u0026rsquo;s University of Technology Thonburi, Thailand for their instruments and continuous support.\u003c/p\u003e\n\u003cp\u003eAuthor Contributions\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMohit Kumar\u003c/strong\u003e: Conceptualization; methodology; visualization; software; validation; writing\u0026mdash;original draft; writing\u0026mdash;review and editing. \u003cstrong\u003eRanvijay Kumar\u003c/strong\u003e: Conceptualization; visualization; writing\u0026mdash;review and editing. \u003cstrong\u003eManoj Kumar Singh\u003c/strong\u003e: Conceptualization; visualization; writing\u0026mdash;review and editing. \u003cstrong\u003eVinod Ayyapan\u003c/strong\u003e: Conceptualization; validation; visualization; writing\u0026mdash;review and editing. \u003cstrong\u003eSanjay Mavinkere Rangappa\u003c/strong\u003e: Supervision; conceptualization; methodology; visualization; writing\u0026mdash;review and editing. \u003cstrong\u003eSuchart Siengchin\u003c/strong\u003e: Supervision; conceptualization; methodology; visualization; writing\u0026mdash;review and editing.\u003c/p\u003e\n\u003cp\u003eDeclaration of Generative AI and AI-assisted technologies in the writing process\u003c/p\u003e\n\u003cp\u003eThe authors used ChatGPT-4o to assist with proofreading and refining certain sections of the manuscript. After using this tool, the author carefully reviewed and revised the manuscript to ensure accuracy and clarity.\u0026nbsp;The authors accept full responsibility for the final content presented in this publication.\u003c/p\u003e\n\u003cp\u003eDeclaration of competing interest\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e\n\u003cp\u003eData availability statement\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eS. Prashanth, K. Subbaya, K. Nithin, S. Sachhidananda, Fiber reinforced composites-a review, J. Mater. Sci. Eng 6(03) (2017) 2-6.\u003c/li\u003e\n\u003cli\u003eD.K. Rajak, D.D. Pagar, P.L. Menezes, E. 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Ramakrishnan, A. Elkaseer, G. Venkatachalam, P.S. Velu, Low-Velocity Impact Behavior of 3D Printed Sandwich Composite with Polylactic Acid\u0026ndash;Micro-crystalline Cellulose Bio-inspired Xylotus Lattice Core: Energy Absorption and Crashworthiness, Journal of Materials Engineering and Performance (2025) 1-20.\u003c/li\u003e\n\u003cli\u003eS. Kalaimagal, M. Vasumathi, S. Rashia Begum, M. Saravana Kumar, V. Romanovski, B. Salah, Evaluation of impact resistance of sustainable sandwich structures with optimized FDM core geometries using low‐velocity drop impact tests, Polymer Composites (2025).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"3D printing, polylactic acid, natural fibers, sandwiched structure, biocomposites, NaOH treatment","lastPublishedDoi":"10.21203/rs.3.rs-7575389/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7575389/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe study presents an innovative method to enhance the structural performance of 3D-printed polylactic acid (PLA) composites by strategically incorporating continuous Agave sisalana fibers as a sandwiched core. This architecture aims to overcome existing limitations by enhancing fiber alignment, interfacial bonding, and mechanical performance, while maintaining the lightweight benefits of PLA. Alkaline treatment enhanced fiber/matrix interaction, confirmed via SEM analysis. Compared to neat 3D printed PLA, the treated fiber composite showed notable enhancement of 36.19% in tensile strength and 46.25% in interlaminar shear strength. The treated configuration showed tensile strength of 40.87 MPa and tensile modulus of 2420.88 MPa. While neat 3D printed PLA retained higher flexural strength and modulus, the treated fiber composite excelled in toughness (938.9 kJ/m\u0026sup3;) and energy resilience (307 kJ/m\u0026sup3;). Dynamic mechanical analysis revealed better thermal stability in the treated fiber composite (Tan δ\u0026thinsp;=\u0026thinsp;0.53 vs. 1.49 for neat 3D printed PLA). Impact testing showed that untreated fiber layers absorbed more energy, evidenced by the highest puncture force (349.78 N) and maximum deformation. Overall, the study confirms the benefits of a continuous fiber core in optimizing 3D-printed PLA composites and suggests future work on fiber arrangement and volume fraction for enhanced performance.\u003c/p\u003e","manuscriptTitle":"Performance of strategically sandwiched continuous sisal fiber core 3D printed PLA composites for engineering applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-01 07:51:28","doi":"10.21203/rs.3.rs-7575389/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revisions Needed","date":"2025-11-05T16:54:02+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-09-21T10:22:39+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-09-20T16:25:25+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-11T05:44:48+00:00","index":"","fulltext":""},{"type":"submitted","content":"The International Journal of Advanced Manufacturing Technology","date":"2025-09-09T11:25:20+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"the-international-journal-of-advanced-manufacturing-technology","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jamt","sideBox":"Learn more about [The International Journal of Advanced Manufacturing Technology](https://www.springer.com/journal/170)","snPcode":"170","submissionUrl":"https://submission.nature.com/new-submission/170/3","title":"The International Journal of Advanced Manufacturing Technology","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"84776378-be91-4c8a-933a-44143097da65","owner":[],"postedDate":"October 1st, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-02-23T16:03:45+00:00","versionOfRecord":{"articleIdentity":"rs-7575389","link":"https://doi.org/10.1007/s00170-026-17657-x","journal":{"identity":"the-international-journal-of-advanced-manufacturing-technology","isVorOnly":false,"title":"The International Journal of Advanced Manufacturing Technology"},"publishedOn":"2026-02-18 15:57:14","publishedOnDateReadable":"February 18th, 2026"},"versionCreatedAt":"2025-10-01 07:51:28","video":"","vorDoi":"10.1007/s00170-026-17657-x","vorDoiUrl":"https://doi.org/10.1007/s00170-026-17657-x","workflowStages":[]},"version":"v1","identity":"rs-7575389","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7575389","identity":"rs-7575389","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}