Experimental Investigation of Defect Geometry and Composite Type on the Pressure Resistance of Repaired Steel Pipelines

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Flaifel, Reza Mosalmani, Raheem Al-Sabur, Mohammad Shishesaz This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6038469/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 17 Oct, 2025 Read the published version in Discover Materials → Version 1 posted 6 You are reading this latest preprint version Abstract Metallic pipelines exposed to harsh environments, such as high humidity and temperatures, are susceptible to damage. Composite patching is a highly effective repair method, and the geometry of defects plays a crucial role, especially in irregularly shaped damage. This study investigates the influence of defect geometry on the effectiveness of composite repairs for steel pipelines under hydrostatic pressure. Three plain-woven composite materials—glass, carbon, and a hybrid—were evaluated. Results demonstrated that circular defects rehabilitated with glass/epoxy composites withstood 46.6% higher hydrostatic pressures compared to square defects of equal cross-sectional area. Furthermore, carbon fiber composites exhibited superior pressure resistance compared to glass fiber, and increasing the number of composite layers enhanced the overall failure pressure. Conversely, increasing the defect area from 1.4 cm² to 4.33 cm² significantly reduced failure pressure. A Taguchi analysis revealed the primary influence of reinforcing fabric type and layer number on failure pressure, with defect shape also playing a significant role. This study demonstrates that the reduction of stress concentration through a modification of defect shape can significantly increase failure pressure. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction Composite patches are made from fibers and resins and can be used to repair damaged areas of metallic pipes without the need for welding or other complex repair methods. Using a composite patch to repair damaged components is a standard procedure. In comparison with other procedures, this procedure is less damaging to the structure. When compared to mechanically fastened sleeves, the bonded patch exhibits better fatigue behavior, less corrosion, and is easier to conform to complex aerodynamic curves [1]. It has been demonstrated that composite materials can restore the initial load-carrying ability of corroded and damaged pipes. Reinforcing plastic pipes with FRP composites also improves the maximum allowable working pressure [2]. Composite patches have the greatest benefit of not creating new holes or weakening the structure. A patch minimizes the stress intensity factor (SIF), which slows down the process and prevents cracks from developing. A key advantage of patch repair technology is that it does not increase the weight of the structure significantly [3]. Hocine et al. [4] reviewed corrosion in steel pipelines and composite repair methods, highlighting their effectiveness in restoring structural integrity under ISO 24817 and ASME PCC-2 standards. Composite wraps offer safe, economical, and maintenance-free solutions for pipeline rehabilitation. Experimental studies demonstrated high burst pressures with composite repairs, showing reliability and signifacnt safety factors for restoring corroded pipelines, including those with localized defects, under diverse operating conditions. Budhe et al. [5] conducted a comprehensive review to present the most recent advancements and drawbacks associated with composite restoration systems for corroded metal pipelines. It was mentioned that current codes (ISO/TS 24817 and ASME PCC-2) need to be revised to account for the selection of materials and their mechanical properties. In addition, theoretical methods typically incorporate limiting assumptions that prevent them from being generalized. For a more accurate composite repair thickness calculation, the defect geometry should also be taken into account. It should be noted that the repair thickness for corroded pipes is calculated using ISO-24817 [6] and ASME PCC-2 [7] design codes, considering pipe diameter, remaining wall thickness, and composite properties. A numerical evaluation method has been presented by Sun et al. [8] for predicting the failure pressure of pipelines with irregular-shaped defects. In this study, the length and depth of defects are analyzed in relation to the failure pressure. It has been shown that the presented method is accurate based on experimental results. Savari [9] investigated the failure of composite repaired pipes under internal pressure numerically. The steel pipe has been examined using a bilinear isotropic hardening plastic flow theory. According to the results, the bond strength between the surface of the pipe and the polymer filler is more significant than the bond strength between the polymer filler and the composite patch. Using numerical and experimental methods, Mazurkiewicz et al. [10] evaluated the burst pressure of damaged and repaired steel pipes. As a result of using a fiberglass sleeve with a 6 mm thickness to repair a steel pipe with a wall thickness of 6 to 2.4 mm, the pipe's pressure resistance increased even more than that of the undamaged pipe. Silva and Zhou [11] analyzed the impact of defect width on the burst capacity of composite-repaired pipelines. Their finite element analysis revealed that localized defects significantly reduce burst capacity compared to full-circumferential defects. An empirical defect width correction factor was proposed to enhance the predictive accuracy of the ASME PCC-2 burst capacity model. Heggab et al. [12] conducted numerical investigations on CFRP-rehabilitated corroded steel pipes, analyzing various corrosion parameters through 144 finite element models. Their findings demonstrated that CFRP rehabilitation increased burst capacity by 20-80% depending on defect depth, with optimal performance in intermediate defects. The study developed predictive equations for hoop stress and provided design recommendations that showed better agreement with FEM results than existing standards. Arifin et al. [13] investigated the impact of defect geometries on composite-repaired pipes and putty performance using finite element analysis and Box-Behnken design methodology. Their findings revealed that defect depth had the most significant negative impact on putty strength and pipe burst pressure, while defect length and width showed complex interactions. The study emphasized the importance of matching putty properties to specific defect geometries for optimal repair effectiveness. Shabibi et al. [14] evaluated composite repair suitability for high-pressure pipelines using finite element analysis. The study demonstrated that repair effectiveness depends on parameters such as thickness, length, and fiber orientation, with repair thickness and fiber orientation having the most significant impact on failure pressure. In order to repair the corroded circumferential welds in the steel pipes, Junior et al. [15] used polymeric composites. As a result of the composite repair system, it was demonstrated that the pipe's lifetime could be extended as well as downtime reduced. Mattos et al. [16] investigated the repair of corroded thin-walled metallic pipes with polymeric composites. A methodology was presented to predict the failure pressure for reinforced pipelines with arbitrary geometries of corrosion and composite repair systems. Additionally, hydrostatic tests were conducted in order to validate the methodology presented. However, it should be noted that the corroded region was considered a system of rectangular defects. Using composite materials, Mahdi and Eltai [17] developed a more cost-effective system for repairing oil and gas pipelines. The pipeline was repaired with woven fabric in the region of decreased thickness. The failure pressure is significantly affected by the fabric orientation. Further, repaired damaged pipes show a higher failure pressure than those that are undamaged. Shamsuddoha et al. [18] studied the failure of corroded steel pipes that had metal loss of 20 to 80%. The polymeric composite sleeve was reinforced by carbon or glass fibers, and the grout was an epoxy-based infill. As a result of the numerical results, it was found that the tensile strength of the infill grout influenced the performance of this repair system. Budhe et al. [19] presented a methodology to predict the failure pressure of metallic pipes with wall losses and repair with composite materials. Hydrostatic tests were used to validate this methodology. In comparison with ISO/TS 24817 (32.3 MPa), the predicted failure pressure (28.66 MPa) is more conservative, whereas the experimental failure pressure is 36.28 MPa. The work of Kong et al. [20] investigated the failure mechanisms of wrapped carbon fiber-reinforced polymer composites and the use of putty in the repair of steel pipes. Further, the effects of mechanical properties of the applied putty and defect dimensions on the burst stress of the repaired pipe were examined. In the design of pipeline repair codes and standards, the focus is often placed on the depth of the defect rather than other defect geometries like length and width. However, it is important to take into account the geometry of defects when analyzing and designing pipe repair techniques. To assess the effect of defect width on the burst capacity of composite repaired pipelines, Leong et al. [21] examined the burst pressure of composite repaired pipes with varying defect widths. A finite element analysis was performed on the composite repaired pipe with a rectangular flaw. Three different defect lengths and depths were chosen. There is a more complex relationship between irregular-shaped defects and their failure pressure than with regular-shaped defects. Sun et al. [8] present a case study on the failure pressure of pipelines with irregular-shaped defects, analyzing the effects of length, depth, and longitudinal spacing on the failure pressure. In addition, a new evaluation method is proposed for predicting failure pressure in steel pipelines with irregular-shaped defects. The method's accuracy is verified by comparing it with experimental results for different steel grades. It has been demonstrated by Li et al. [22] that square patches are more efficient in terms of repair than other patches. According to Li et al. [23], the square patch performed the best among the five patches tested (circle, hexagon, trapezoid, square, and lozenge). It was found that the trapezoidal patch was more appropriate in view of the adhesive residual strength. Using three-dimensional finite elements, Rachid et al. [24] studied the effect of patch shape on the efficacy and durability of bonded composite repairs in aircraft structures. It was shown that the rectangular shape of the patch could be enhanced through the use of an "H" shape. The semi-circular patch performed better in decreasing the SIF and lowering adhesive stress than the circular and elliptical composite patches, according to Benyahia et al. [25]. Using ABAQUS finite element software, Saffar et al. [26] developed a model that predicts critical pressure for various defect types under static pressure loading conditions. A ductile damage criterion examines the effect of various defect patterns in API X65 steel pipes. It was found [27] that there is a significant reduction in stress magnitude around the crack tip after implementing composite patch reinforcement. Composite patches show high efficacy in various applications. Muda et al. [28] developed a computational model using artificial neural networks (ANN) to predict the burst pressure strength of CFRP-repaired pipelines, taking into account the defect geometry defined by length, width, and depth. This technique, which has been validated against revised finite element method solutions, provides a quick process for the CFRP repair of such pipes. Using numerical analysis and experimental data, Khaisem et al. [29] examined failure pressure in metallic pipelines with wall loss defects. They optimized composite thickness for repair, taking into account three cases: non-defective, defective wall loss, and composite pipe repair. The ISO/TS 24817 standard for wall loss defect pipe provides a theoretical failure pressure estimation that is significantly conservative when compared to the numerical failure pressure obtained for the specified composite repair thickness. Saeed et al. [30] used analytical equations and the finite element method to model different design scenarios. They found that repair thickness is independent of live pressure, which suggests that the current design equation needs to be changed. Köpple et al. [31] examined the impact of through-wall defects in pipes because of internal corrosion on installed overwrap. To prevent composite damage and its detachment from the steel substrate, an analytical assessment approach using linear elastic fracture mechanics and finite element analysis was developed. Then, this approach was compared to ISO/TS 24817. Li et al. [29] investigated finite element models of corroded pipelines with a colony of defects, concentrating on 13 different types. On the basis of failure pressure ratios to the failure pressure of the corresponding base case, the interaction between defects is determined, and a new rule is proposed to determine whether interaction occurs. During his study, Netto [32] examined the narrow and long defects caused by corrosion in pipelines caused by water, sediment, or chemical contaminants. Small-scale experiments and nonlinear numerical analyses demonstrate that corrosion defects influence offshore pipeline collapse pressure. Utilizing parametric models and 2-D and 3-D numerical models, his research determines the collapse pressure of pipelines with narrow defects involving various defect geometries and their interaction with pipe ovalization. Shafaee Fallah et al. [33] investigated the use of glass fiber-reinforced composite patches for repairing cracked steel pipes, analyzing fiber orientation, layer number, and curing effects. Experimental results, validated by numerical simulations, reveal that pressure-bearing capacity increases with additional layers and full curing, with up to a threefold improvement in strength for glass-polyester composites. It is possible to categorize corrosion defects as irregularly shaped or regularly shaped (with a smooth depth profile) [29]. The analysis by Khaise et al. [29] focused on the results obtained from pressure tests conducted on four tubular specimens that contained defects of irregular or complex shapes. The laboratory tests involved the measurement of burst pressures, which were then compared to the predicted values obtained from six different assessment methodologies. The study conducted by Benjamin et al. [34] focused on the examination of pipeline behavior in the presence of long, nonuniform-depth corrosion defects. The project includes doing burst tests on two tubular specimens, each containing a single exterior nonuniform depth defect. Additionally, two finite element models—a shell model and a solid model—are used in the investigation. It was established that the solid model is more accurate, although both models are capable of simulating the failure mode of defects with a long and shallow corrosion patch with deep defects. As mentioned before, the geometry of defects can have a significant effect on the design of rehabilitation process for damaged pipes using composite patches. According to ISO/TS 24817 [6] and ASME PCC-2 [7], the repair area needs to be free of sharp changes in geometry, and sharp geometry has to be faired-in. Consequently, the present study utilized hydrostatic testing as a means to investigate the effects of defect shape. To clarify, the objective of this study is to assess the resistance of multiple steel pipes, which possess holes of different geometries (square and circular shapes), to internal hydrostatic pressure subsequent to the application of composite patches. The experimental investigation and subsequent discussion will focus on the examination of key characteristics that contribute to effectiveness, including hole defect shape and its size, reinforcing fabric type, and the number of composite layers. In addition, to determine the importance of the above factors, a statistical analysis is carried out using the Taguchi method. 2. Materials The following materials were used to examine the shape of defects in steel pipe rehabilitation using composite materials: 6-inch carbon steel pipes that were 1.25 meter long and 7.11 millimetres thick were utilized. It is noteworthy to notice that two plates, made from the same material and possessing a uniform thickness of 8 mm, have been welded to the opposing ends of the pipe. The left plate was equipped with a nozzle designed for the connection of the pump tube used in the hydrostatic test. On the right side, a nozzle was included for attaching the pressure gauge. There were two main types of plain-woven fiber-reinforced polymers (FRP) used: epoxy-reinforced with T300 carbon fiber (200 g/m 2 with a tensile strength of 3500 MPa) and E-glass fiber (200 g/m 2 with a tensile strength in weft and warp directions of 44 N/mm and 48 N/mm). The utilized epoxy resin, known as Araldite LY 5052, possesses a tensile strength of 60 MPa and was mixed with its corresponding hardener, Aradur 5052, in a weight ratio of 100:38. The epoxy resin and plain-woven fibers were mixed in a way to keep the fiber weight fraction at about 50%. Additionally, to carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP), a combination of carbon and glass (hybrid) fiber reinforced polymer (HFRP) was used in this study. 3. Problem Statement and Experiment Uniform corrosion, pitting corrosion, cavitation and erosion corrosion, stray current corrosion, and microbiologically influenced corrosion are among the common types of corrosion that occur on oil and gas pipelines. Uniform corrosion causes material loss along the pipe's surface, resulting in continuous wall erosion, which can potentially lead to leakage or rupture. At high pressure, a combination of cavitation, erosion, and corrosion can result in extremely severe pitting corrosion [35]. Fig. 1 displays multiple pipeline failures attributed to corrosion. As shown in this figure, the damaged area may have an irregular shape and sharp changes. The repair location must be free of sharp changes in geometry and sharp geometry should be faired-in, according ISO/TS 24817 [6] and ASME PCC-2 [7]. Nevertheless, the influence of defect shape has not been taken into consideration in these codes. The present investigation aims to examine the influence of defect shape on steel pipe repair using composite materials. To achieve this objective, a number of steel pipes containing circular or square holes in the middle have been prepared (see Fig. 2). In order to make the defects (holes) in the pipes, a CNC machine was used for square shapes and a drilling machine was utilized for circular shapes. The surfaces of the pipes have been sanded and cleaned to ensure that they are smooth and free of any contamination. In this operation, no special putty was used; therefore, the pipe surface was coated with a thin layer of the same epoxy resin and hardener. The resin thickness was quite thin, and it was applied to increase the bondage between the composite layer and the pipe surface. After that, layers of wet woven fiber with a 30 cm width were wrapped around the pipe. Then, the pipe that had the composite applied to it was allowed to stay at room temperature for a period of 72 hours to ensure that the epoxy resin completely cured. The process of attaching composite layers to each and every pipe was carried out in the same manner as before. During the experiment, a 60-bar compressor was utilized to gradually raise the water pressure in the pipes by filling them with water. The damage (water leaking out or rupture of composite layers) that occurs in the composite when subjected to varying pressures has been traced, and the pressure level that results in damage has been recorded here. Fig. 3 depicts the test configuration carried out. 4. Results and discussion In this section, a comprehensive examination of the factors affecting failure pressure is provided through a series of experimental investigations. This examination revolves around four separate cases. In case 1, the study examines the relationship between the shape of defects and the type of reinforced fibers in relation to the failure pressure. case 2 investigates the combined influence of the number of layers and the type of reinforcing woven fabric. case 3 focuses on understanding the effects caused by variations in defect cross-sectional areas. Finally, Case 4 utilizes the Taguchi method to provide a statistical analysis of the parameters mentioned above. Case 1: Effects of defect shape and reinforcing woven fabric type on failure pressure The first case involved repairing square and circular defect shapes with the same area of 1.44 cm 2 using 8 layers of glass or carbon fibers. As shown in Table 1, the circular defect bearing failure pressure was greater than one square, and there was no rupture (NF) or water leakage occurring in carbon FRP with 10 layers despite exceeding the maximum applied pressure of 60 bar. It should be highlighted that the possible failure that was reported is a water leakage; there was no rupture or leakage in the composite. In relation to glass (FRP), in comparison to the square defect, the circular defect resulted in improved load-bearing capacity, as demonstrated by a significant increase of around 46.6% in the failure pressure. Under identical conditions, it has been found that carbon fiber-reinforced polymer (FRP) exhibits the ability to withstand failure pressures that are twice as high as those experienced by glass FRP. In addition, the defect shape could considerably affect the resistance of composite repair. It could be explained that defects with higher stress concentrations can impose greater stress on the intermediate (bonding) phase between composite layers and the pipe surface. In this situation, the bonding phase is susceptible to failure. In addition, it is evident from the findings that the magnitude of this applied stress is contingent upon the mechanical characteristics of composite materials. A schematic failure of the composite patch due to water leakage is illustrated in Fig. 4. Table 1. Effects of defect shape on the failure pressure at the constant cross-sectional area of 1.44 cm 2 and applying 8 composite layers. Fiber type Carbon FRP Glass FRP Defect type Square Circular Square Circular Number of layers 8 8 8 8 Failure pressure (bar) 59 NR 30 44 Case 2: Effects of number of composite layers and reinforcing woven fabric type on failure pressure The primary objective of this case study was to examine the effectiveness of carbon FRP, glass FRP, and hybrid FRP as repair composite materials for a square hole with an area of 1.44 cm 2 . This case study utilized 4, 8, and 10 layers of both carbon FRP and glass FRP materials. The hybrid FRP configuration incorporates three layers of glass and three layers of carbon fibers. In hybrid FRP, each carbon fiber layer was followed by a glass fiber layer. This study indicates that for the square defect shape, glass FRP failed at a lower pressure than carbon, approximately 50% less than carbon FRP when 8 layers were utilized. The failure pressure of glass fiber-reinforced polymer (FRP) rises to 57 bar when the number of layers is increased to 10. In contrast, the carbon fiber-reinforced polymer (FRP) exhibited no signs of rupture (NR) or leakage when subjected to a pressure of 60 bar, with 10 layers being utilized. The results are displayed in Table 2. As anticipated, carbon FRP demonstrated greater resistance to failure under identical conditions, whereas hybrid FRP is more influenced by its number of carbon layers. The findings indicate that internal pressure did not create a pathway for leakage within the composite layers. However, there was a failure in the bonding between the composite layers and the surface of the pipe. It appears that the stress concentration around a defect depends on the mechanical properties and thickness of the composite material. Table 2. Number of layers Effects on the failure pressure for square defect with area of 1.44 cm 2 Fiber type Carbon FRP Glass FRP Hybrid FRP Defect type Square Square Square Number of layers 4 8 10 8 10 8 Failure pressure (bar) 42 59 NR 30 57 54 Case 3: Effects of defect cross-sectional area on failure pressure In the previous cases, it was indicated that in the same condition, circular defects can withstand higher internal pressure than square defects. Therefore, in this case, just a circular defect using four layers to repair was investigated. However, the cross-sectional area of the circular defect was increased from 1.44 cm 2 to 4.3 cm 2 to investigate the effects of defect cross-sectional area. As indicated in Table 3, the failure pressure decreased significantly with increasing defect area. The decrease in failure pressure was about 38% for glass fibers and 44% for carbon fibers. It was shown that the stress concentration around a circular hole in a cylinder under internal pressure is a direct function of the radius of the hole. In other words, under the same conditions, the stress concentration around a circular hole in a cylinder under internal pressure increases significantly by increasing the radius of the circular hole [39]. Therefore, it is reasonable that at a bigger hole, the failure stress decreases significantly. Table 3. Effects of cross-sectional area on the failure pressure for the circular defect and applying 8 composite layers. Fiber type Carbon FRP Glass FRP Defect area (cm 2 ) 1.44 4.33 1.44 4.33 Number of layers 4 4 4 4 Failure Pressure (bar) 52 29 29 18 Case 4: Statistical analysis using Taguchi method The Taguchi method is a powerful tool in the field of experimental design, selecting appropriate levels for each factor, and analyzing the results using statistical techniques. This method is widely used to analyze the effect of multiple factors on a given response. In this study, the effects of three factors (reinforcing woven fabric type (fiber type), number of composite layers, and defect type) each at three levels have been studied on the failure pressure using Taguchi method. The Taguchi method was employed, utilizing Minitab software. The levels of selected factors (fiber type, number of composite layers, and defect type) were given in Table 4. By varying these factors at various levels, it becomes possible to determine the optimal combination that yields the maximum failure pressure. The reinforcing woven fabric effect has been investigated using glass, carbon, and hybrid fibers. Half of the fiber leayrs in hybrid composites are carbon, and the other half are glass. Following each carbon fiber layer was a glass fiber layer. 4, 6, and 8 layers have been selected as the number of composite layers. In order to examine the impact of defect shape and its cross-sectional area, an examination was conducted on three different shapes of defect holes. These shapes included a square hole with a cross-sectional area of 1.4 cm 2 , as well as circular defect holes with cross-sectional areas of 1.4 cm 2 and 4.3 cm 2 . Table 5 represents the configurations of Taguchi's experimental design utilizing the L9 orthogonal array, along with the corresponding results of failure pressure and S/N ratio. Table 4. Levels of the parameters (factors) used in the experiment. Factors Level 1 Level 2 Level 3 Fiber Type CFRP GFRP HFRP Defect type Square (1.4cm 2 ) Circular (1.4cm 2 ) Circular (4.3cm 2 ) Number of layers 4 6 8 Table 5. Taguchi experimental design using L9 orthogonal array with failure pressure and S/N ratio results. Exp. No. Fiber type Defect type Number of layers Failure pressure (bar) S/N ratio 1 CFRP Square (1.4cm 2 ) 4 42 32.4650 2 CFRP Circular (1.4cm 2 ) 6 55 34.8073 3 CFRP Circular (4.3cm 2 ) 8 46 33.2552 4 GFRP Square (1.4cm 2 ) 6 27 28.6273 5 GFRP Circular (1.4cm 2 ) 8 44 32.8691 6 GFRP Circular (4.3cm 2 ) 4 18 25.1055 7 HFRP Square (1.4cm 2 ) 8 54 34.6479 8 HFRP Circular (1.4cm 2 ) 4 30 29.5424 9 HFRP Circular (4.3cm 2 ) 6 32 30.1030 Fig. 5 depicts the mean signal-to-noise ratio (S/N ratio) of failure pressure. On the basis of the S/N ratio diagrams, it can be deduced that the number of composite layers and the type of reinforcing fibers in the composite have significant effects on failure pressure. In particular, carbon fiber-reinforced polymer (FRP) performs better than hybrid and glass FRPs. Furthermore, there is an obvious trend showing that pressure failure rises as the number of composite layers increases. However, it is important to note that the replacement of glass fibers with carbon fibers, along with an increase in the quantity of composite layers, leads to a rise in composite repair financial costs. In relation to the shape of the defect, it can be observed that a circular defect hole is more effective than a square defect hole when considering the same cross-sectional area. Even though the defect type has a lower mean S/N ratio than the other two classifications, it is possible to significantly increase the failure pressure by considering the stress concentration caused by the defect geometry and its cross-sectional area. Based on the mean Signal-to-Noise (S/N) results, the best repair configuration entails the use of carbon Fiber Reinforced Polymer (CFRP) including 8 composite layers for the restoration of a defect characterized by a circular hole having a cross-sectional area of 1.4 cm 2 . The Taguchi method predicts that this combination will withstand pressures higher than 60 bar, specifically 61.33 bar with a S/N ratio of 36.97. The experimental evidence presented in Table 1 supports this prediction, demonstrating that there is no rupture or leakage of water observed for this specific combination up to an internal pressure of 60 bar. 5. Conclusion According to ISO/TS 24817 and ASME PCC-2, repair areas should be devoid of sharp geometric variations, with sharp transitions faired-in. However, these codes have not considered the influence of defect shape. This study employed hydrostatic testing to examine the effect of defect shape and its cross-sectional area on steel pipe rehabilitation using composite patches. Therefore, the resistance of steel pipes with different shapes of defect holes to internal hydrostatic pressure was assessed. Three types of plain-woven composites, reinforced either with glass, carbon, or a hybrid of the two, were evaluated. Alongside a parametric investigation, a Taguchi method-based statistical analysis was conducted to determine the significance of the aforementioned parameters. The key findings were: With constant cross-sectional area and layer number of glass/epoxy composite, pipes with circular defects resisted hydrostatic pressure 46.6% more effectively than those with square defects. Carbon/epoxy-reinforced pipes with circular defects showed no water leakage up to 60 bar, whereas those with square defects failed at 59 bar. Carbon fibers exhibited greater hydrostatic pressure resistance than glass fibers under the same conditions. By increasing the composite layers, the failure pressure was raised. Elevating carbon/epoxy layers from 4 to 8 led to a 40.47% increase in failure pressure, whereas this increase was 90% for glass/epoxy from 8 to 10 layers. However, increasing the defect's cross-sectional area from 1.4 cm 2 to 4.33 cm 2 notably reduced failure pressure. The Taguchi method was used to examine statistically the impact of three factors (reinforcing woven fabric type, number of composite layers, and defect type) on failure pressure. While reinforcing woven fabric type and composite layer number proved predominant factors, defect geometry also held considerable influence. Reducing stress concentration because of defect shape has a significant impact on failure pressure, while alterations in reinforcing fabric type and composite layer number may escalate repair costs. Declarations Funding: This research received no grant from any funding agency in the public, commercial, or the others. Ethics declaration: Not applicable. Consent to Participate declaration: Not applicable. Consent to Publish declaration: Not applicable. Competing interests: The authors declare no competing interests. Clinical Trial: Not applicable. References S. Mohammadi, M. Yousefi, and M. Khazaei, “A review on composite patch repairs and the most important parameters affecting its efficiency and durability,” J. Reinf. Plast. Compos. , vol. 40, no. 1–2, pp. 3–15, Jan. 2021, doi: 10.1177/0731684420941602. F. G. Alabtah, E. Mahdi, and F. F. Eliyan, “The use of fiber reinforced polymeric composites in pipelines: A review,” Compos. Struct. , vol. 276, p. 114595, Nov. 2021, doi: 10.1016/j.compstruct.2021.114595. C. Ramakrishna and J. K. Balu, “Finite Element Analysis of the Composite Patch Repairs of the Plates,” Int. J. Eng. Res. 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Anal. , vol. 59, pp. 223–236, Jan. 2016, doi: 10.1016/j.engfailanal.2015.10.007. E. Mahdi and E. Eltai, “Development of cost-effective composite repair system for oil/gas pipelines,” Compos. Struct. , vol. 202, pp. 802–806, Oct. 2018, doi: 10.1016/j.compstruct.2018.04.025. M. Shamsuddoha, A. Manalo, T. Aravinthan, M. Mainul Islam, and L. Djukic, “Failure analysis and design of grouted fibre-composite repair system for corroded steel pipes,” Eng. Fail. Anal. , vol. 119, p. 104979, Jan. 2021, doi: 10.1016/j.engfailanal.2020.104979. S. Budhe, M. D. Banea, N. R. F. Rohem, E. M. Sampaio, and S. de Barros, “Failure pressure analysis of composite repair system for wall loss defect of metallic pipelines,” Compos. Struct. , vol. 176, pp. 1013–1019, Sep. 2017, doi: 10.1016/j.compstruct.2017.06.044. D. Kong, X. Huang, M. Xin, and G. Xian, “Effects of defect dimensions and putty properties on the burst performances of steel pipes wrapped with CFRP composites,” Int. J. Press. Vessel. Pip. , vol. 186, p. 104139, Sep. 2020, doi: 10.1016/j.ijpvp.2020.104139. K. E. Leong, K. S. Lim, A. S. Sulaiman, S. C. Chin, and N. Yahaya, “Effect of Defect Width upon Burst Capacity of Composite Repaired Pipe,” IOP Conf. Ser. Mater. Sci. Eng. , vol. 712, p. 012018, Jan. 2020, doi: 10.1088/1757-899X/712/1/012018. C. Li, Q. Zhao, J. Yuan, Y. Hou, and Y. Tie, “Simulation and experiment on the effect of patch shape on adhesive repair of composite structures,” J. Compos. Mater. , vol. 53, no. 28–30, pp. 4125–4135, Dec. 2019, doi: 10.1177/0021998319853033. K. Li, C. Li, Y. Tie, and Y. Yu, “Influence of patch parameters on damage and residual strength of adhesively bonded composite repair under fatigue loading,” Mater. Phys. Mech. , vol. 24, pp. 391–402, 2015, [Online]. Available: https://www.ipme.ru/e-journals/MPM/no_42415/MPM424_11_li.pdf. M. Rachid, B. Serier, B. Bachir Bouiadjra, and M. Belhouari, “Numerical analysis of the patch shape effects on the performances of bonded composite repair in aircraft structures,” Compos. Part B Eng. , vol. 43, no. 2, pp. 391–397, Mar. 2012, doi: 10.1016/j.compositesb.2011.08.047. F. Benyahia, A. Albedah, and B. A. B. Bouiadjra, “Elliptical and circular bonded composite repair under mechanical and thermal loading in aircraft structures,” Mater. Res. , vol. 17, no. 5, pp. 1219–1225, Oct. 2014, doi: 10.1590/1516-1439.259613. A. Saffar, A. Darvizeh, R. Ansari, A. Kazemi, and M. Alitavoli, “Prediction of Failure Pressure in Pipelines with Localized Defects Repaired by Composite Patches,” J. Fail. Anal. Prev. , vol. 19, no. 6, pp. 1801–1814, Dec. 2019, doi: 10.1007/s11668-019-00781-0. P. S. Shinde, V. K. Tripathi, P. Kumar, P. K. Sarkar, and K. K. Singh, “Review Paper on Analysis of Composite Patches as a Crack Arrestor,” J. Eng. Appl. Sci. , vol. 6, no. 3, pp. 222–226, Mar. 2011, doi: 10.3923/jeasci.2011.222.226. M. F. Muda et al. , “Burst pressure strength of corroded subsea pipelines repaired with composite fiber-reinforced polymer patches,” Eng. Fail. Anal. , vol. 136, p. 106204, Jun. 2022, doi: 10.1016/j.engfailanal.2022.106204. M. Khaise, S. de Barros, N. Rohem, M. Banea, and S. Budhe, “Numerical analysis of repaired wall loss defect pipelines for optimum composite wrap thickness,” Frat. ed Integrità Strutt. , vol. 17, no. 63, pp. 153–168, Dec. 2022, doi: 10.3221/IGF-ESIS.63.14. N. Saeed, H. Ronagh, and A. Virk, “Composite repair of pipelines, considering the effect of live pressure-analytical and numerical models with respect to ISO/TS 24817 and ASME PCC-2,” Compos. Part B Eng. , vol. 58, pp. 605–610, Mar. 2014, doi: 10.1016/j.compositesb.2013.10.035. M. F. Köpple, S. Lauterbach, and W. Wagner, “Composite repair of through-wall defects in pipework – Analytical and numerical models with respect to ISO/TS 24817,” Compos. Struct. , vol. 95, pp. 173–178, Jan. 2013, doi: 10.1016/j.compstruct.2012.06.023. T. A. Netto, “On the Effect of Narrow and Long Corrosion Defects on the Collapse Pressure of Pipelines,” in Volume 3: Pipeline and Riser Technology; CFD and VIV , Jan. 2007, pp. 429–441, doi: 10.1115/OMAE2007-29457. A. Shafaee Fallah, M. Sadeghian, and M. E. Golmakani, “Experimental and Numerical Study on the Strength of Repaired Steel Pipes with Composite Patches under Internal Pressure,” Mech. Adv. Compos. Struct. , vol. 10, no. 2, pp. 437–448, 2023, doi: 10.22075/macs.2023.29108.1459. A. C. Benjamin and E. Q. de Andrade, “Predicting the Failure Pressure of Pipelines Containing Nonuniform Depth Corrosion Defects Using the Finite Element Method,” in Volume 2: Safety and Reliability; Pipeline Technology , Jan. 2003, pp. 557–564, doi: 10.1115/OMAE2003-37072. H. R. Vanaei, A. Eslami, and A. Egbewande, “A review on pipeline corrosion, in-line inspection (ILI), and corrosion growth rate models,” Int. J. Press. Vessel. Pip. , vol. 149, pp. 43–54, Jan. 2017, doi: 10.1016/j.ijpvp.2016.11.007. R-TECH MATERIALS, “CASE STUDY: Failure Analysis of a Crude Oil Pipeline,” 2023. https://www.r-techmaterials.com/news-and-blog/case-study-failure-analysis-crude-oil-pipeline/. Área Académica de Metalurgia, “Tipos de corrosión que ocurren en sistemas de tuberías industriales,” 2023. https://www.areametalurgia.com/post/tipos-de-corrosión-que-ocurren-en-sistemas-de-tuberías-industriales. Horizon Distributors, “CORROSION AND CATHODIC PROTECTION,” 2020 . https://www.horizononline.com/corrosion-and-cathodic-protection/. N. J. I. Adams, “Stress Concentration in a Cylindrical Shell Containing a Circular Hole,” J. Eng. Ind. , vol. 93, no. 4, pp. 953–961, Nov. 1971, doi: 10.1115/1.3428089. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 17 Oct, 2025 Read the published version in Discover Materials → Version 1 posted Reviewers agreed at journal 01 Apr, 2025 Reviewers agreed at journal 25 Mar, 2025 Reviewers invited by journal 24 Mar, 2025 Editor assigned by journal 18 Mar, 2025 Submission checks completed at journal 18 Mar, 2025 First submitted to journal 15 Feb, 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. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6038469","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":436775154,"identity":"2e7c4b6a-cd44-49ab-a364-d990fb8bab62","order_by":0,"name":"Thaer R. Flaifel","email":"","orcid":"","institution":"Shahid Chamran University of Ahvaz","correspondingAuthor":false,"prefix":"","firstName":"Thaer","middleName":"R.","lastName":"Flaifel","suffix":""},{"id":436775155,"identity":"1238a9d0-8a8b-4f11-b440-2cd47e26d565","order_by":1,"name":"Reza Mosalmani","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAzklEQVRIiWNgGAWjYLCCB0DMxt4A5TEToyUBpIXnAKlaGCQSiHSTbnv7ww+JOXZ5fJLPH37mYbCTZ2DnfYBXi9mZM8YSiduSi9mkc4yleRiSDRuY2Q3wa7mRwwDUwpzYJp3DxszDwJzAwMyG32FmN9If/0jcVp/YJnn8GVBLPTFaEsyAthxObJNgMANqOUyEljNnzCwStx1PbOPJMZacY3DcsI2gluPtj2983FadOL/9+MMPbyqq5fn5j+HXggaAYUXAjlEwCkbBKBgFxAAAVCE6WD6TWBkAAAAASUVORK5CYII=","orcid":"","institution":"Shahid Chamran University of Ahvaz","correspondingAuthor":true,"prefix":"","firstName":"Reza","middleName":"","lastName":"Mosalmani","suffix":""},{"id":436775156,"identity":"f8396a51-4dc0-4cba-b458-6f9564167dcc","order_by":2,"name":"Raheem Al-Sabur","email":"","orcid":"","institution":"University of Basrah","correspondingAuthor":false,"prefix":"","firstName":"Raheem","middleName":"","lastName":"Al-Sabur","suffix":""},{"id":436775157,"identity":"dc8c3e6f-82f4-4ec1-b1be-4708e3979d1b","order_by":3,"name":"Mohammad Shishesaz","email":"","orcid":"","institution":"Shahid Chamran University of Ahvaz","correspondingAuthor":false,"prefix":"","firstName":"Mohammad","middleName":"","lastName":"Shishesaz","suffix":""}],"badges":[],"createdAt":"2025-02-15 21:23:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6038469/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6038469/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s43939-025-00398-1","type":"published","date":"2025-10-17T15:57:31+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":79744142,"identity":"1a4aecc8-5dab-4a41-ba6a-687c6afc696b","added_by":"auto","created_at":"2025-04-02 08:29:30","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":245210,"visible":true,"origin":"","legend":"\u003cp\u003eFailed pipelines because of corrosion: (a) [36] , (b) [37] and (c) [38].\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6038469/v1/95b89e651ef1121d2c8c1bb5.png"},{"id":79744143,"identity":"32b67a06-0f8a-4a28-bff6-ee6e64a84943","added_by":"auto","created_at":"2025-04-02 08:29:30","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":243235,"visible":true,"origin":"","legend":"\u003cp\u003eSteel pipe with (a) square and (b) circular hole.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6038469/v1/557e113854a63b723d04186f.png"},{"id":79744147,"identity":"dc5db495-a7d1-47ed-ae8d-cf3acc4ceca0","added_by":"auto","created_at":"2025-04-02 08:29:30","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":607445,"visible":true,"origin":"","legend":"\u003cp\u003e(a) test configuration, (b) pressure gauge, (c) applying carbon fiber plain woven fabric, (d) applying glass fiber plain woven fabric.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6038469/v1/0248bb3e1b1f68bfe51c1b8d.png"},{"id":79744145,"identity":"e39f87aa-459a-4400-b6c1-9b09908b843f","added_by":"auto","created_at":"2025-04-02 08:29:30","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":151436,"visible":true,"origin":"","legend":"\u003cp\u003eThe water leakage in the rehabilitated pipe with (a) glass/epoxy and (b) carbon/epoxy plain woven composites.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6038469/v1/b1262d1b1399a09ed19d2426.png"},{"id":79744152,"identity":"3de2a25e-b277-4211-b0b5-0cdd4a0f5990","added_by":"auto","created_at":"2025-04-02 08:29:30","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":32055,"visible":true,"origin":"","legend":"\u003cp\u003eThe mean Signal-to-Noise ratio (S/N ratio) of failure pressure.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6038469/v1/dcc14b4dac3cc319e756fe3f.png"},{"id":93956742,"identity":"8f121b21-dabd-4fdb-9e0a-514100722c3a","added_by":"auto","created_at":"2025-10-20 16:12:04","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1926463,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6038469/v1/a030ed14-6e17-4f1d-84c4-8cae507a98e1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Experimental Investigation of Defect Geometry and Composite Type on the Pressure Resistance of Repaired Steel Pipelines","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eComposite patches are made from fibers and resins and can be used to repair damaged areas of metallic pipes without the need for welding or other complex repair methods. Using a composite patch to repair damaged components is a standard procedure. In comparison with other procedures, this procedure is less damaging to the structure. When compared to mechanically fastened sleeves, the bonded patch exhibits better fatigue behavior, less corrosion, and is easier to conform to complex aerodynamic curves [1]. It has been demonstrated that composite materials can restore the initial load-carrying ability of corroded and damaged pipes. Reinforcing plastic pipes with FRP composites also improves the maximum allowable working pressure [2]. Composite patches have the greatest benefit of not creating new holes or weakening the structure. A patch minimizes the stress intensity factor (SIF), which slows down the process and prevents cracks from developing. A key advantage of patch repair technology is that it does not increase the weight of the structure significantly [3].\u003c/p\u003e\n\u003cp\u003eHocine et al. [4] reviewed corrosion in steel pipelines and composite repair methods, highlighting their effectiveness in restoring structural integrity under ISO 24817 and ASME PCC-2 standards. Composite wraps offer safe, economical, and maintenance-free solutions for pipeline rehabilitation. Experimental studies demonstrated high burst pressures with composite repairs, showing reliability and signifacnt safety factors for restoring corroded pipelines, including those with localized defects, under diverse operating conditions.\u0026nbsp;Budhe et al.\u0026nbsp;[5]\u0026nbsp;conducted a comprehensive review to present the most recent advancements and drawbacks associated with composite restoration systems for corroded metal pipelines. It was mentioned that current codes (ISO/TS 24817 and ASME PCC-2) need to be revised to account for the selection of materials and their mechanical properties. In addition, theoretical methods typically incorporate limiting assumptions that prevent them from being generalized. For a more accurate composite repair thickness calculation, the defect geometry should also be taken into account.\u0026nbsp;It should be noted that the repair thickness for corroded pipes is calculated using ISO-24817\u0026nbsp;[6]\u0026nbsp; and ASME PCC-2\u0026nbsp;[7]\u0026nbsp;design codes, considering pipe diameter, remaining wall thickness, and composite properties.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eA numerical evaluation method has been presented by Sun et al. [8]\u0026nbsp;for predicting the failure pressure of pipelines with irregular-shaped defects. In this study, the length and depth of defects are analyzed in relation to the failure pressure. It has been shown that the presented method is accurate based on experimental results. Savari [9] investigated the failure of composite repaired pipes under internal pressure numerically. The steel pipe has been examined using a bilinear isotropic hardening plastic flow theory. According to the results, the bond strength between the surface of the pipe and the polymer filler is more significant than the bond strength between the polymer filler and the composite patch. Using numerical and experimental methods, Mazurkiewicz et al. [10] evaluated the burst pressure of damaged and repaired steel pipes. As a result of using a fiberglass sleeve with a 6 mm thickness to repair a steel pipe with a wall thickness of 6 to 2.4 mm, the pipe's pressure resistance increased even more than that of the undamaged pipe. Silva and Zhou [11] analyzed the impact of defect width on the burst capacity of composite-repaired pipelines. Their finite element analysis revealed that localized defects significantly reduce burst capacity compared to full-circumferential defects. An empirical defect width correction factor was proposed to enhance the predictive accuracy of the ASME PCC-2 burst capacity model.\u003c/p\u003e\n\u003cp\u003eHeggab et al. [12] conducted numerical investigations on CFRP-rehabilitated corroded steel pipes, analyzing various corrosion parameters through 144 finite element models. Their findings demonstrated that CFRP rehabilitation increased burst capacity by 20-80% depending on defect depth, with optimal performance in intermediate defects. The study developed predictive equations for hoop stress and provided design recommendations that showed better agreement with FEM results than existing standards. Arifin et al. [13] investigated the impact of defect geometries on composite-repaired pipes and putty performance using finite element analysis and Box-Behnken design methodology. Their findings revealed that defect depth had the most significant negative impact on putty strength and pipe burst pressure, while defect length and width showed complex interactions. The study emphasized the importance of matching putty properties to specific defect geometries for optimal repair effectiveness. Shabibi et al. [14] evaluated composite repair suitability for high-pressure pipelines using finite element analysis. The study demonstrated that repair effectiveness depends on parameters such as thickness, length, and fiber orientation, with repair thickness and fiber orientation having the most significant impact on failure pressure.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn order to repair the corroded circumferential welds in the steel pipes, Junior et al. [15] used polymeric composites. As a result of the composite repair system, it was demonstrated that the pipe's lifetime could be extended as well as downtime reduced. Mattos et al. [16] investigated the repair of corroded thin-walled metallic pipes with polymeric composites. A methodology was presented to predict the failure pressure for reinforced pipelines with arbitrary geometries of corrosion and composite repair systems. Additionally, hydrostatic tests were conducted in order to validate the methodology presented. However, it should be noted that the corroded region was considered a system of rectangular defects. Using composite materials, Mahdi and Eltai [17] developed a more cost-effective system for repairing oil and gas pipelines. The pipeline was repaired with woven fabric in the region of decreased thickness. The failure pressure is significantly affected by the fabric orientation. Further, repaired damaged pipes show a higher failure pressure than those that are undamaged. Shamsuddoha et al. [18] studied the failure of corroded steel pipes that had metal loss of 20 to 80%. The polymeric composite sleeve was reinforced by carbon or glass fibers, and the grout was an epoxy-based infill. As a result of the numerical results, it was found that the tensile strength of the infill grout influenced the performance of this repair system.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBudhe et al. [19] presented a methodology to predict the failure pressure of metallic pipes with wall losses and repair with composite materials. Hydrostatic tests were used to validate this methodology. In comparison with ISO/TS 24817 (32.3 MPa), the predicted failure pressure (28.66 MPa) is more conservative, whereas the experimental failure pressure is 36.28 MPa. The work of Kong et al. [20] investigated the failure mechanisms of wrapped carbon fiber-reinforced polymer composites and the use of putty in the repair of steel pipes. Further, the effects of mechanical properties of the applied putty and defect dimensions on the burst stress of the repaired pipe were examined.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn the design of pipeline repair codes and standards, the focus is often placed on the depth of the defect rather than other defect geometries like length and width. However, it is important to take into account the geometry of defects when analyzing and designing pipe repair techniques. To assess the effect of defect width on the burst capacity of composite repaired pipelines, Leong et al. [21] examined the burst pressure of composite repaired pipes with varying defect widths. A finite element analysis was performed on the composite repaired pipe with a rectangular flaw. Three different defect lengths and depths were chosen. There is a more complex relationship between irregular-shaped defects and their failure pressure than with regular-shaped defects. Sun et al. [8] present a case study on the failure pressure of pipelines with irregular-shaped defects, analyzing the effects of length, depth, and longitudinal spacing on the failure pressure. In addition, a new evaluation method is proposed for predicting failure pressure in steel pipelines with irregular-shaped defects. The method's accuracy is verified by comparing it with experimental results for different steel grades.\u003c/p\u003e\n\u003cp\u003eIt has been demonstrated by Li et al. [22] that square patches are more efficient in terms of repair than other patches. According to Li et al. [23], the square patch performed the best among the five patches tested (circle, hexagon, trapezoid, square, and lozenge). It was found that the trapezoidal patch was more appropriate in view of the adhesive residual strength. Using three-dimensional finite elements, Rachid et al. [24] studied the effect of patch shape on the efficacy and durability of bonded composite repairs in aircraft structures. It was shown that the rectangular shape of the patch could be enhanced through the use of an \"H\" shape.\u003c/p\u003e\n\u003cp\u003eThe semi-circular patch performed better in decreasing the SIF and lowering adhesive stress than the circular and elliptical composite patches, according to Benyahia et al. [25]. Using ABAQUS finite element software, Saffar et al. [26] developed a model that predicts critical pressure for various defect types under static pressure loading conditions. A ductile damage criterion examines the effect of various defect patterns in API X65 steel pipes. \u0026nbsp;It was found [27] that there is a significant reduction in stress magnitude around the crack tip after implementing composite patch reinforcement. Composite patches show high efficacy in various applications.\u003c/p\u003e\n\u003cp\u003eMuda et al. [28] developed a computational model using artificial neural networks (ANN) to predict the burst pressure strength of CFRP-repaired pipelines, taking into account the defect geometry defined by length, width, and depth. This technique, which has been validated against revised finite element method solutions, provides a quick process for the CFRP repair of such pipes. Using numerical analysis and experimental data, Khaisem et al. [29] examined failure pressure in metallic pipelines with wall loss defects. They optimized composite thickness for repair, taking into account three cases: non-defective, defective wall loss, and composite pipe repair. The ISO/TS 24817 standard for wall loss defect pipe provides a theoretical failure pressure estimation that is significantly conservative when compared to the numerical failure pressure obtained for the specified composite repair thickness.\u003c/p\u003e\n\u003cp\u003eSaeed et al. [30] used analytical equations and the finite element method to model different design scenarios. They found that repair thickness is independent of live pressure, which suggests that the current design equation needs to be changed. Köpple et al. [31] examined the impact of through-wall defects in pipes because of internal corrosion on installed overwrap. To prevent composite damage and its detachment from the steel substrate, an analytical assessment approach using linear elastic fracture mechanics and finite element analysis was developed. Then, this approach was compared to ISO/TS 24817.\u003c/p\u003e\n\u003cp\u003eLi et al. [29] investigated finite element models of corroded pipelines with a colony of defects, concentrating on 13 different types. On the basis of failure pressure ratios to the failure pressure of the corresponding base case, the interaction between defects is determined, and a new rule is proposed to determine whether interaction occurs. During his study, Netto [32] examined the narrow and long defects caused by corrosion in pipelines caused by water, sediment, or chemical contaminants. Small-scale experiments and nonlinear numerical analyses demonstrate that corrosion defects influence offshore pipeline collapse pressure. Utilizing parametric models and 2-D and 3-D numerical models, his research determines the collapse pressure of pipelines with narrow defects involving various defect geometries and their interaction with pipe ovalization. Shafaee Fallah et al. [33] investigated the use of glass fiber-reinforced composite patches for repairing cracked steel pipes, analyzing fiber orientation, layer number, and curing effects. Experimental results, validated by numerical simulations, reveal that pressure-bearing capacity increases with additional layers and full curing, with up to a threefold improvement in strength for glass-polyester composites.\u003c/p\u003e\n\u003cp\u003eIt is possible to categorize corrosion defects as irregularly shaped or regularly shaped (with a smooth depth profile) [29]. The analysis by Khaise et al. [29] focused on the results obtained from pressure tests conducted on four tubular specimens that contained defects of irregular or complex shapes. The laboratory tests involved the measurement of burst pressures, which were then compared to the predicted values obtained from six different assessment methodologies. The study conducted by Benjamin et al. [34] focused on the examination of pipeline behavior in the presence of long, nonuniform-depth corrosion defects. The project includes doing burst tests on two tubular specimens, each containing a single exterior nonuniform depth defect. Additionally, two finite element models—a shell model and a solid model—are used in the investigation. It was established that the solid model is more accurate, although both models are capable of simulating the failure mode of defects with a long and shallow corrosion patch with deep defects.\u003c/p\u003e\n\u003cp\u003eAs mentioned before, the geometry of defects can have a significant effect on the design of rehabilitation process for damaged pipes using composite patches. According to ISO/TS 24817 [6] and ASME PCC-2 [7], the repair area needs to be free of sharp changes in geometry, and sharp geometry has to be faired-in. Consequently, the present study utilized hydrostatic testing as a means to investigate the effects of defect shape. To clarify, the objective of this study is to assess the resistance of multiple steel pipes, which possess holes of different geometries (square and circular shapes), to internal hydrostatic pressure subsequent to the application of composite patches. The experimental investigation and subsequent discussion will focus on the examination of key characteristics that contribute to effectiveness, including hole defect shape and its size, reinforcing fabric type, and the number of composite layers. In addition, to determine the importance of the above factors, a statistical analysis is carried out using the Taguchi method.\u003c/p\u003e"},{"header":"2.\tMaterials ","content":"\u003cp\u003eThe following materials were used to examine the shape of defects in steel pipe rehabilitation using composite materials:\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003e6-inch carbon steel pipes that were 1.25 meter long and 7.11 millimetres thick were utilized. It is noteworthy to notice that two plates, made from the same material and possessing a uniform thickness of 8 mm, have been welded to the opposing ends of the pipe. The left plate was equipped with a nozzle designed for the connection of the pump tube used in the hydrostatic test. On the right side, a nozzle was included for attaching the pressure gauge.\u003c/li\u003e\n \u003cli\u003eThere were two main types of plain-woven fiber-reinforced polymers (FRP) used: epoxy-reinforced with T300 carbon fiber (200 g/m\u003csup\u003e2\u003c/sup\u003e with a tensile strength of 3500 MPa) and E-glass fiber (200 g/m\u003csup\u003e2\u003c/sup\u003e with a tensile strength in weft and warp directions of 44 N/mm and 48 N/mm). The utilized epoxy resin, known as Araldite LY 5052, possesses a tensile strength of 60 MPa and was mixed with its corresponding hardener, Aradur 5052, in a weight ratio of 100:38. The epoxy resin and plain-woven fibers were mixed in a way to keep the fiber weight fraction at about 50%. Additionally, to carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP), a combination of carbon and glass (hybrid) fiber reinforced polymer (HFRP) was used in this study.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":" 3. Problem Statement and Experiment","content":"\u003cp\u003eUniform corrosion, pitting corrosion, cavitation and erosion corrosion, stray current corrosion, and microbiologically influenced corrosion are among the common types of corrosion that occur on oil and gas pipelines. Uniform corrosion causes material loss along the pipe\u0026apos;s surface, resulting in continuous wall erosion, which can potentially lead to leakage or rupture. At high pressure, a combination of cavitation, erosion, and corrosion can result in extremely severe pitting corrosion [35]. Fig. 1 displays multiple pipeline failures attributed to corrosion. As shown in this figure, the damaged area may have an irregular shape and sharp changes. The repair location must be free of sharp changes in geometry and sharp geometry should be faired-in, according ISO/TS 24817 [6] and ASME PCC-2 [7]. Nevertheless, the influence of defect shape has not been taken into consideration in these codes.\u003c/p\u003e\n\u003cp\u003eThe present investigation aims to examine the influence of defect shape on steel pipe repair using composite materials. To achieve this objective, a number of steel pipes containing circular or square holes in the middle have been prepared (see Fig. 2). In order to make the defects (holes) in the pipes, a CNC machine was used for square shapes and a drilling machine was utilized for circular shapes. The surfaces of the pipes have been sanded and cleaned to ensure that they are smooth and free of any contamination. In this operation, no special putty was used; therefore, the pipe surface was coated with a thin layer of the same epoxy resin and hardener. The resin thickness was quite thin, and it was applied to increase the bondage between the composite layer and the pipe surface.\u0026nbsp;After that, layers of wet woven fiber with a 30 cm width were wrapped around the pipe. Then, the pipe that had the composite applied to it was allowed to stay at room temperature for a period of 72 hours to ensure that the epoxy resin completely cured.\u0026nbsp;The process of attaching composite layers to each and every pipe was carried out in the same manner as before. During the experiment, a 60-bar compressor was utilized to gradually raise the water pressure in the pipes by filling them with water. The damage (water leaking out or rupture of composite layers) that occurs in the composite when subjected to varying pressures has been traced, and the pressure level that results in damage has been recorded here. Fig. 3 depicts the test configuration carried out.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e"},{"header":"4.\tResults and discussion ","content":"\u003cp\u003eIn this section, a comprehensive examination of the factors affecting failure pressure is provided through a series of experimental investigations. This examination revolves around four separate cases. In case 1, the study examines the relationship between the shape of defects and the type of reinforced fibers in relation to the failure pressure. case 2 investigates the combined influence of the number of layers and the type of reinforcing woven fabric. case 3 focuses on understanding the effects caused by variations in defect cross-sectional areas. Finally, Case 4 utilizes the Taguchi method to provide a statistical analysis of the parameters mentioned above.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCase 1: Effects of defect shape and reinforcing woven fabric type on failure pressure\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe first case involved repairing square and circular defect shapes with the same area of 1.44 cm\u003csup\u003e2\u003c/sup\u003e using 8 layers of glass or carbon fibers. As shown in Table 1, the circular defect bearing failure pressure was greater than one square, and there was no rupture (NF) or water leakage occurring in carbon FRP with 10 layers despite exceeding the maximum applied pressure of 60 bar. It should be highlighted that the possible failure that was reported is a water leakage; there was no rupture or leakage in the composite. In relation to glass (FRP), in comparison to the square defect, the circular defect resulted in improved load-bearing capacity, as demonstrated by a significant increase of around 46.6% in the failure pressure. Under identical conditions, it has been found that carbon fiber-reinforced polymer (FRP) exhibits the ability to withstand failure pressures that are twice as high as those experienced by glass FRP. In addition, the defect shape could considerably affect the resistance of composite repair. It could be explained that defects with higher stress concentrations can impose greater stress on the intermediate (bonding) phase between composite layers and the pipe surface. In this situation, the bonding phase is susceptible to failure. In addition, it is evident from the findings that the magnitude of this applied stress is contingent upon the mechanical characteristics of composite materials. A schematic failure of the composite patch due to water leakage is illustrated in Fig. 4.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1.\u003c/strong\u003e Effects of defect shape on the failure pressure at the constant cross-sectional area of 1.44 cm\u003csup\u003e2\u003c/sup\u003e and applying 8 composite layers.\u0026nbsp;\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eFiber type\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003eCarbon FRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003eGlass FRP\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eDefect type\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSquare\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSquare\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eNumber of layers\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eFailure pressure (bar)\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e59\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eNR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e44\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCase 2: Effects of number of composite layers and reinforcing woven fabric type on failure pressure\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe primary objective of this case study was to examine the effectiveness of carbon FRP, glass FRP, and hybrid FRP as repair composite materials for a square hole with an area of 1.44 cm\u003csup\u003e2\u003c/sup\u003e. This case study utilized 4, 8, and 10 layers of both carbon FRP and glass FRP materials. The hybrid FRP configuration incorporates three layers of glass and three layers of carbon fibers. In hybrid FRP, each carbon fiber layer was followed by a glass fiber layer. This study indicates that for the square defect shape, glass FRP failed at a lower pressure than carbon, approximately 50% less than carbon FRP when 8 layers were utilized. The failure pressure of glass fiber-reinforced polymer (FRP) rises to 57 bar when the number of layers is increased to 10. In contrast, the carbon fiber-reinforced polymer (FRP) exhibited no signs of rupture (NR) or leakage when subjected to a pressure of 60 bar, with 10 layers being utilized. The results are displayed in Table 2. As anticipated, carbon FRP demonstrated greater resistance to failure under identical conditions, whereas hybrid FRP is more influenced by its number of carbon layers. The findings indicate that internal pressure did not create a pathway for leakage within the composite layers. However, there was a failure in the bonding between the composite layers and the surface of the pipe. It appears that the stress concentration around a defect depends on the mechanical properties and thickness of the composite material.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTable 2.\u0026nbsp;Number of layers Effects on the failure pressure for square defect with area of 1.44 cm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\n\u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eFiber type\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\"\u003e\n \u003cp\u003eCarbon FRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\"\u003e\n \u003cp\u003eGlass FRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e\u003cstrong\u003eHybrid FRP\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eDefect type\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"3\" valign=\"top\"\u003e\n \u003cp\u003eSquare\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\" valign=\"top\"\u003e\n \u003cp\u003eSquare\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eSquare\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eNumber of layers\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eFailure pressure (bar)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e59\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eNR\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e54\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003e\u003cstrong\u003eCase 3: Effects of defect cross-sectional area on failure pressure\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn the previous cases, it was indicated that in the same condition, circular defects can withstand higher internal pressure than square defects. Therefore, in this case, just a circular defect using four layers to repair was investigated. However, the cross-sectional area of the circular defect was increased from 1.44 cm\u003csup\u003e2\u003c/sup\u003e to 4.3 cm\u003csup\u003e2\u003c/sup\u003e to investigate the effects of defect cross-sectional area. As indicated in Table 3, the failure pressure decreased significantly with increasing defect area. The decrease in failure pressure was about 38% for glass fibers and 44% for carbon fibers. It was shown that the stress concentration around a circular hole in a cylinder under internal pressure is a direct function of the radius of the hole. In other words, under the same conditions, the stress concentration around a circular hole in a cylinder under internal pressure increases significantly by increasing the radius of the circular hole [39]. Therefore, it is reasonable that at a bigger hole, the failure stress decreases significantly.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 3.\u003c/strong\u003e Effects of cross-sectional area on the failure pressure for the circular defect and applying 8 composite layers.\u0026nbsp;\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eFiber type\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003eCarbon FRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd colspan=\"2\"\u003e\n \u003cp\u003eGlass FRP\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eDefect area (cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e1.44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4.33\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e1.44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4.33\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eNumber of layers\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003eFailure Pressure (bar)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e29\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e29\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e18\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cstrong\u003eCase 4: Statistical analysis using Taguchi method\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Taguchi method is a powerful tool in the field of experimental design, selecting appropriate levels for each factor, and analyzing the results using statistical techniques. This method is widely used to analyze the effect of multiple factors on a given response. In this study, the effects of three factors (reinforcing woven fabric type (fiber type), number of composite layers, and defect type) each at three levels have been studied on the failure pressure using Taguchi method. \u0026nbsp;The Taguchi method was employed, utilizing Minitab software. The levels of selected factors (fiber type, number of composite layers, and defect type) were given in Table 4. By varying these factors at various levels, it becomes possible to determine the optimal combination that yields the maximum failure pressure. The reinforcing woven fabric effect has been investigated using glass, carbon, and hybrid fibers. Half of the fiber leayrs in hybrid composites are carbon, and the other half are glass. Following each carbon fiber layer was a glass fiber layer. 4, 6, and 8 layers have been selected as the number of composite layers. In order to examine the impact of defect shape and its cross-sectional area, an examination was conducted on three different shapes of defect holes. These shapes included a square hole with a cross-sectional area of 1.4 cm\u003csup\u003e2\u003c/sup\u003e, as well as circular defect holes with cross-sectional areas of 1.4 cm\u003csup\u003e2\u003c/sup\u003e and 4.3 cm\u003csup\u003e2\u003c/sup\u003e. Table 5 represents the configurations of Taguchi\u0026apos;s experimental design utilizing the L9 orthogonal array, along with the corresponding results of failure pressure and S/N ratio.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 4.\u0026nbsp;\u003c/strong\u003eLevels of the parameters (factors) used in the experiment.\u003c/p\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eFactors\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eLevel 1\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eLevel 2\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eLevel 3\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eFiber Type\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eGFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eHFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eDefect type\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSquare (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (4.3cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eNumber of layers\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cstrong\u003eTable 5.\u0026nbsp;\u003c/strong\u003eTaguchi experimental design using L9 orthogonal array with failure pressure and S/N ratio results.\u003c/p\u003e\n \u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eExp. No.\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eFiber type\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eDefect type\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eNumber of layers\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e\u003cstrong\u003eFailure pressure (bar)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e\u003cstrong\u003eS/N ratio\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eCFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSquare (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e42\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e32.4650\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eCFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e55\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e34.8073\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eCFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (4.3cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e46\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e33.2552\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eGFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSquare (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e27\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e28.6273\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eGFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e32.8691\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eGFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (4.3cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e18\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e25.1055\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eHFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eSquare (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e54\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e34.6479\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eHFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (1.4cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e29.5424\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003eHFRP\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003eCircular (4.3cm\u003csup\u003e2\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e32\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\"\u003e\n \u003cp\u003e30.1030\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eFig. 5 depicts the mean signal-to-noise ratio (S/N ratio) of failure pressure. On the basis of the S/N ratio diagrams, it can be deduced that the number of composite layers and the type of reinforcing fibers in the composite have significant effects on failure pressure. In particular, carbon fiber-reinforced polymer (FRP) performs better than hybrid and glass FRPs. Furthermore, there is an obvious trend showing that pressure failure rises as the number of composite layers increases. However, it is important to note that the replacement of glass fibers with carbon fibers, along with an increase in the quantity of composite layers, leads to a rise in composite repair financial costs. In relation to the shape of the defect, it can be observed that a circular defect hole is more effective than a square defect hole when considering the same cross-sectional area. Even though the defect type has a lower mean S/N ratio than the other two classifications, it is possible to significantly increase the failure pressure by considering the stress concentration caused by the defect geometry and its cross-sectional area. Based on the mean Signal-to-Noise (S/N) results, the best repair configuration entails the use of carbon Fiber Reinforced Polymer (CFRP) including 8 composite layers for the restoration of a defect characterized by a circular hole having a cross-sectional area of 1.4 cm\u003csup\u003e2\u003c/sup\u003e. The Taguchi method predicts that this combination will withstand pressures higher than 60 bar, specifically 61.33 bar with a S/N ratio of 36.97. The experimental evidence presented in Table 1 supports this prediction, demonstrating that there is no rupture or leakage of water observed for this specific combination up to an internal pressure of 60 bar.\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eAccording to ISO/TS 24817 and ASME PCC-2, repair areas should be devoid of sharp geometric variations, with sharp transitions faired-in. However, these codes have not considered the influence of defect shape. This study employed hydrostatic testing to examine the effect of defect shape and its cross-sectional area on steel pipe rehabilitation using composite patches. Therefore, the resistance of steel pipes with different shapes of defect holes to internal hydrostatic pressure was assessed. Three types of plain-woven composites, reinforced either with glass, carbon, or a hybrid of the two, were evaluated. Alongside a parametric investigation, a Taguchi method-based statistical analysis was conducted to determine the significance of the aforementioned parameters. The key findings were:\u003c/p\u003e\n\u003cul\u003e\n \u003cli\u003eWith constant cross-sectional area and layer number of glass/epoxy composite, pipes with circular defects resisted hydrostatic pressure 46.6% more effectively than those with square defects. Carbon/epoxy-reinforced pipes with circular defects showed no water leakage up to 60 bar, whereas those with square defects failed at 59 bar.\u003c/li\u003e\n \u003cli\u003eCarbon fibers exhibited greater hydrostatic pressure resistance than glass fibers under the same conditions. By increasing the composite layers, the failure pressure was raised. Elevating carbon/epoxy layers from 4 to 8 led to a 40.47% increase in failure pressure, whereas this increase was 90% for glass/epoxy from 8 to 10 layers. However, increasing the defect's cross-sectional area from 1.4 cm\u003csup\u003e2\u003c/sup\u003e to 4.33 cm\u003csup\u003e2\u003c/sup\u003e notably reduced failure pressure.\u003c/li\u003e\n \u003cli\u003eThe Taguchi method was used to examine statistically the impact of three factors (reinforcing woven fabric type, number of composite layers, and defect type) on failure pressure. While reinforcing woven fabric type and composite layer number proved predominant factors, defect geometry also held considerable influence. Reducing stress concentration because of defect shape has a significant impact on failure pressure, while alterations in reinforcing fabric type and composite layer number may escalate repair costs.\u003c/li\u003e\n\u003c/ul\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e\u0026nbsp; This research received no grant from any funding agency in the public, commercial, or the others.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics declaration:\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate declaration:\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish declaration:\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u0026nbsp;\u003c/strong\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical Trial:\u0026nbsp;\u003c/strong\u003eNot applicable.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eS. 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Şahin, \u0026ldquo;Progressive fatigue failure behavior of glass/epoxy (\u0026plusmn;75)2 filament-wound pipes under pure internal pressure,\u0026rdquo; \u003cem\u003eMater. Des.\u003c/em\u003e, vol. 30, no. 10, pp. 4293\u0026ndash;4298, Dec. 2009, doi: 10.1016/j.matdes.2009.04.025.\u003c/li\u003e\n\u003cli\u003eS. Budhe, M. D. Banea, and S. de Barros, \u0026ldquo;Composite repair system for corroded metallic pipelines: an overview of recent developments and modelling,\u0026rdquo; \u003cem\u003eJ. Mar. Sci. 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Adams, \u0026ldquo;Stress Concentration in a Cylindrical Shell Containing a Circular Hole,\u0026rdquo; \u003cem\u003eJ. Eng. Ind.\u003c/em\u003e, vol. 93, no. 4, pp. 953\u0026ndash;961, Nov. 1971, doi: 10.1115/1.3428089.\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":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"dime","sideBox":"Learn more about [Discover Materials](https://www.springer.com/journal/43939)","snPcode":"","submissionUrl":"","title":"Discover Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6038469/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6038469/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Metallic pipelines exposed to harsh environments, such as high humidity and temperatures, are susceptible to damage. Composite patching is a highly effective repair method, and the geometry of defects plays a crucial role, especially in irregularly shaped damage. This study investigates the influence of defect geometry on the effectiveness of composite repairs for steel pipelines under hydrostatic pressure. Three plain-woven composite materials—glass, carbon, and a hybrid—were evaluated. Results demonstrated that circular defects rehabilitated with glass/epoxy composites withstood 46.6% higher hydrostatic pressures compared to square defects of equal cross-sectional area. Furthermore, carbon fiber composites exhibited superior pressure resistance compared to glass fiber, and increasing the number of composite layers enhanced the overall failure pressure. Conversely, increasing the defect area from 1.4 cm² to 4.33 cm² significantly reduced failure pressure. A Taguchi analysis revealed the primary influence of reinforcing fabric type and layer number on failure pressure, with defect shape also playing a significant role. This study demonstrates that the reduction of stress concentration through a modification of defect shape can significantly increase failure pressure.","manuscriptTitle":"Experimental Investigation of Defect Geometry and Composite Type on the Pressure Resistance of Repaired Steel Pipelines","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-02 08:29:26","doi":"10.21203/rs.3.rs-6038469/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"136046534263419460536843288929676000601","date":"2025-04-01T09:03:37+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"22732352021318267697287497989124224763","date":"2025-03-25T05:19:34+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-03-24T09:48:46+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-03-18T13:32:44+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-03-18T13:31:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Materials","date":"2025-02-15T21:15:05+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"dime","sideBox":"Learn more about [Discover Materials](https://www.springer.com/journal/43939)","snPcode":"","submissionUrl":"","title":"Discover Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e3f7e473-112f-4e56-9b14-5e39c0c69da1","owner":[],"postedDate":"April 2nd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-10-20T16:09:06+00:00","versionOfRecord":{"articleIdentity":"rs-6038469","link":"https://doi.org/10.1007/s43939-025-00398-1","journal":{"identity":"discover-materials","isVorOnly":false,"title":"Discover Materials"},"publishedOn":"2025-10-17 15:57:31","publishedOnDateReadable":"October 17th, 2025"},"versionCreatedAt":"2025-04-02 08:29:26","video":"","vorDoi":"10.1007/s43939-025-00398-1","vorDoiUrl":"https://doi.org/10.1007/s43939-025-00398-1","workflowStages":[]},"version":"v1","identity":"rs-6038469","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6038469","identity":"rs-6038469","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}