Comparative analysis of thermophysical properties of functional composites reinforced with epoxy-glass and epoxy-carbon fibers in the context of heat transfer anisotropy

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PANAS, Zbigniew LECIEJEWSKI, Judyta SIENKIEWICZ, Mirosław NOWAKOWSKI This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6629055/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The paper concerns comprehensive, complementary studies of thermophysical properties of functional composite structures. The term functional in this case means the study of the structure while maintaining its post-production imperfections, as opposed to the study of material samples prepared solely for this purpose. The paper presents the results of experimental studies, followed by an analysis of thermophysical properties characterizing heat accumulation and anisotropic heat transfer of two types of utility composites. A composite with an epoxy matrix and two types of fillers: glass mat and carbon fabric were studied. The research program included micro- and macrostructural analysis and comprehensive thermogravimetric, microcalorimetric and thermal diffusivity measurements. In the studies of heat transfer phenomena, the directional dependence of properties was taken into account. Attention was focused on maintaining high temperature resolution of measurements, and the effect of repeated temperature exposure was also determined also. The results of the research are the determined quantitative and qualitative characteristics, including the temperature dependence of a set of thermophysical properties of the tested materials. Analysis of the results provides insight into possible design and operational problems of real structures in relation to model data. Physical sciences/Engineering Physical sciences/Materials science Physical sciences/Physics epoxy−glass mat composite epoxy –graphite fabric composite thermophysical properties comparative studies Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 Figure 19 Figure 20 Figure 21 Figure 22 Figure 23 Figure 24 INTRODUCTION As is commonly known, composites are material structures composed of two or more distinct phases, each with unique properties, combined in a manner that results in a material with enhanced or superior properties compared to those of the individual components when used independently or simply added together [ 1 ]. Structural composites represent a broad and diverse class of materials distinguished by the type of components used, the shape and size of the reinforcing phase, and the specific manufacturing technologies employed. Through the careful selection of components tailored to meet the stringent demands of modern applications, these composites exhibit exceptional strength properties. Additionally, they are often engineered to meet specific performance requirements, such as maintaining stability under high temperatures or cyclic thermal changes. Composites, especially carbon composites, have excellent thermal properties, i.e. low CTE expansion coefficient and high mechanical strength at elevated temperatures. Owing to these exceptional properties, composites find applications in the production of aircraft engine nozzles and thermal assemblies for combustion chambers, as well as aircraft brake discs. These applications span across the aerospace, military, and civilian sectors [ 2 ], [ 3 ], [ 4 ]. In the above-mentioned applications, the thermophysical properties of composites, which are strongly influenced by their microstructure, including density and porosity, are crucial, however, reports on them are scarce. Epoxy-glass and epoxy-graphite composites are extensively employed in aircraft construction due to their favourable strength-to-weight ratios and excellent mechanical properties. Very good structural properties of composite structures - in given applications - allow for the extension of the range of permissible variability of permissible production deviations of parameters. However, this does not eliminate the source problem. And the problem concerns both carbon composite structures and those with glass filling, in particular it is related to the problem of anisotropy of properties. Epoxy-glass composites comprise an epoxy resin matrix reinforced with glass fibres. The microstructure of these composites is intricate, featuring several distinct phases. The amorphous epoxy resin matrix consists of cross-linked polymer chains, forming a continuous phase. Embedded within this matrix are glass fibres, providing the composite with its primary strength and stiffness. The interfacial region between the fibre and matrix is crucial for load transfer and overall composite performance. This region, typically a few micrometres thick, contains a mixture of epoxy resin and fibre surface treatment. Under microscopic observation, glass fibres appear as elongated strands embedded within the epoxy matrix. Fiber orientation significantly influences the composite's mechanical properties. Unidirectional arrangements, where fibres align in a single direction, offer high strength and stiffness along the fibre axis but exhibit lower properties in transverse directions. Bidirectional arrangements, with fibres aligned in two perpendicular directions, provide more balanced properties but may not achieve the same axial strength as unidirectional composites. Epoxy-graphite composites, in turn, consist of a graphite fibre reinforcement embedded within an epoxy resin matrix. Similar to epoxy-glass composites, these materials commonly utilize unidirectional or bidirectional fibre orientations. Microscopic examination reveals graphite fibres as elongated structures dispersed within a continuous epoxy matrix. Fibers typically exhibit micrometer-scale diameters and millimeter-scale lengths. The microstructure of epoxy-graphite composites is influenced by factors such as manufacturing processes and fibre surface treatments. These treatments enhance fibre-matrix adhesion, impacting fibre distribution and orientation within the composite. The resulting microstructure is engineered to produce a high-strength, lightweight material with excellent wear and corrosion resistance, making it ideal for demanding applications in aerospace and sporting goods. The epoxy matrix serves as a load-bearing medium, binding and transferring loads between the graphite fibres. The volume fraction of the matrix and the quality of the fibre-matrix interface significantly influence the overall mechanical performance of the composite. The common use of composites in combination with the specificity of production technology, as well as the availability of their production technology, however, raises the problem of the lack of full repeatability of the structure properties. This applies in particular to composites with an epoxy resin matrix. This research investigates properties of industrial, or in other words utility composite structures. This means that samples of the production material were tested with the preservation of natural structural imperfections. The subject of the tests were thermophysical properties. Thermophysical properties extend beyond classical thermodynamic parameters such as specific heat, thermal expansion, and thermal conductivity [ 5 ], [ 6 ], [ 7 ], [ 8 ]. This includes thermomechanical properties, notably rheological characteristics, including viscoelasticity [ 8 ]. The study of thermophysical properties necessitates not only quantifying the temperature dependence of individual physical parameters but also comprehensively characterizing phase transformations. Furthermore, it is crucial to obtain data that accurately predict material behaviour under various thermal loading conditions. At present case the attention was focused on heat accumulation and heat transfer parameters taking into account their dependence on temperature, as well as anisotropy in the case of heat conduction. The mutual purpose of the work is to provide insight into possible design and operational problems of real structures in relation to model data of “perfect” structure. This also applies to the interpretation of test results for properties - results may depend on the measurement procedure. In this dimension, the work falls into metrological categories. MATERIALS AND METHODS This research utilized three types of composite materials: two epoxy-glass fibre mat composites (3A Composities Mobility S.A., Mielec, Poland) with different thicknesses: - with a thickness of 6.2-6.4 mm - designated "szGR"; - with a thickness of 4.5-5.2 mm - designated "szCI"; and an epoxy-graphite composite (Rega Yacht sp. z o.o., Ropczyce, Poland) reinforced with carbon fabric layers (designated "graf"). Initially, tested composites were subjected to microstructural analysis. Imaging techniques included a KEYENCE VHX-6000 digital microscope and a Phenom ProX scanning electron microscope. Microscopic images were analyzed to determine individual layer thicknesses, porosity levels, and the volume fractions of constituent phases. A Netzsch TG 209 F3 Tarsus thermobalance was used to conduct thermogravimetric tests. The measuring range of the thermobalance covers the temperature range from room temperature (TP) to 1000° C, the resolution is 0.1mg, the maximum weight range is 2000 mg, the rates of temperature change from 0.001° C/min to 100° C/min, and the capacity of a standard alumina sample pan is 85ml. DSC (Dynamic Scanning Calorimetry) microcalorimetric studies were performed using a Pyris 1 power-compensated scanning microcalorimeter from Perkin-Elmer with a temperature range of ‒30° C to 600° C or from room the temperature (RT) to 710° C and a claimed accuracy of enthalpy and specific heat determination of ± 2%. Attention in the study was focused on the determination of specific heat. The three-curve method ([8], [9], [10], [11]) was used to determine the specific heat. The study used a dedicated temperature change program described in publications [12], [13] that enables which allows for obtaining reliable results in both heating and cooling microcalorimeter operation modes. Each sample was tested first according to the thermal program with the maximum exposition temperature limited to 130 °C, and then according to the program specified as high-temperature (extended range). The low temperature program corresponds to the typical operating temperature range of aircraft composites. Complementing the study of thermophysical properties were measurements of thermal diffusivity, i.e. the ratio of thermal conductivity to volumetric heat capacity. A modified temperature oscillation method was used to determine this parameter [14], [15], [16]. A description of the test stand, together with a presentation of the procedures for testing directional properties, are discussed in publications [16], [17], [18], [19]. The method used is characterized by the ability to perform measurements with high-temperature resolution [13]. Processing of the measurement signals results in two independently calculated values of thermal diffusivity: one determined by comparing the amplitude of the periodic temperature change excitation signal and the periodic response, the so-called amplitude value a y , and the other, determined by the phase shift, the phase value a j . In the model case of one-dimensional heat flow, without convective losses from lateral surfaces, both values should be equal. That allows for a preliminary check of the correctness of the measurement. The thermal diffusivity of the tested composite structures was determined taking into account the property directional differences. For measurements in the direction perpendicular to the surface(transversal/out-of-plane component), square-shaped specimens with a side length of 40 mm were used, while specimens with a length of about 60 mm and a width of about 10 mm were used to measure the thermal diffusivity in the longitudinal direction (longitudinal/in-plane component). In the case of measuring transverse diffusivity, an outside bilateral symmetric forcing was used with the measurement of the response signal at the contact surface of two samples put together with square surfaces- the symmetry surface of the system. In this case, the characteristic dimension for calculating the thermal diffusivity was equal to the sample thickness, i.e. approximately 6.2 and 5.3 mm for epoxy-mat composites and approximately 3.5 mm for epoxy-fabric composites. The measurement system makes it possible to perform thermal diffusivity measurements in the range from about -10° C to about 100° C. The period of excitation was selected each time according to the properties of the tested material and ranged from 30 s for graphite composites to 60 s or 120 s for glass composites. Bilateral excitation was also used to study the longitudinal component along the shorter side of the prismatic specimen. Due to the dimensions of the specimen lengths, which were too large concerning the dimensions of the system measuring head, it was not possible to eliminate the lateral losses to a sufficient extent. Therefore, the tests were repeated with single-sided excitation and thermal imaging of the temperature (comp. [18] and [16]). It should be mentioned that the specimens were tested without removing the paint coating, and the registration of temperature changes was made from the opposite side. To carry out the measurements, the surfaces of the samples were coated with a layer of GRAPHITE 33 flake graphite with a thickness of no more than 20mm. Changes in the temperature distribution were recorded using a Flir SC5600 thermal imaging camera with a recording frequency of 1 Hz for a 120 s excitation period and 0.25 Hz for a 240 s excitation period. For data processing, temperature change signals were collected as the spatial mean temperature of the control lines. The control lines were declared along the expected isotherms. The lines were spaced at approximately 2.5 mm intervals. RESULTS AND DISCUSSION MACROSTUCTURE AND MICROSTRUCTURE Illustrative photographs of the tested composites are depicted in Fig. 1 . Figure 1a) shows the surfaces of the upper and lower surfaces. All tested specimens were coated with factory paint on one side (grey surfaces visible in the left column of Fig. 1a ). Figure 1b represents a macroscopic cross-section view of the original specimen from which the samples were cut. Figure 2 presents cross-sectional views of the "szGR" and "szCI" epoxy-glass laminates, revealing their macrostructures. Red arrows in the images indicate pores and the paint layer applied to the laminates' undersides. The macrostructure exhibits pores of varying shapes and sizes, primarily concentrated in the interlayer spaces. Larger pores tend to form clusters, while finer, more spherical pores are distributed more evenly throughout the laminate volume. Two distinct porosity types were identified: cylindrical voids between individual fibres and spherical voids located between fibre bundles. In the "szGR" composite, significant pores or holes are visible beneath the surface. Notably, the areas surrounding these largest pores lack glass reinforcement. Furthermore, Fig. 2 also showcases the surface of the "szCI" epoxy-glass composite. The image reveals fibres arranged at varying angles, indicating non-uniform fibre distribution within the composite. Additionally, particles of epoxy resin are visible within the image. Higher magnification images of the microstructure of the epoxy-glass composite "szGR" and "szGR" are shown in Fig. 3 and Fig. 4 , respectively. Figure 5 a) presents a low-magnification view of the epoxy-graphite composite's microstructure. Characteristic of epoxy-graphite composites reinforced with woven carbon fabric, the image reveals fibres oriented both parallel and perpendicular to the viewing plane, indicative of the traditional interlacing pattern. Figure 5 b) provides a closer look at the composite structure, highlighting the paint layer and the individual graphite-fiber fabrics comprising the laminate. Furthermore, Fig. 5 c) showcases the microstructure of a sample sectioned at a 45° angle. These micrographs collectively confirm the layered structure of the carbon fabric with its inherent interlacing pattern. In addition to the fibres' varied orientations, the images also reveal areas with increased resin density (appearing darker grey), corresponding to regions with a lower concentration of graphite fibres. Figure 6 presents scanning electron microscope (SEM) images of the epoxy-graphite composite's microstructure. The images reveal a generally uniform fibre distribution, although some resin-rich areas are evident. The microstructure further illustrates the arrangement of fibres within individual fabric bundles. Within these bundles, fibres are randomly distributed with minimal inter-fibrevoiding. A small number of pores, broken fibres, and fibre detachments were observed within the resin matrix. Microstructural analysis indicated low porosity in the tested composite (low void volume fraction, V v < 0.2%) and a high fibre volume fraction, V f ~57%. The diameter of the graphite fibres was determined to be in the range of 5–6 µm. Microstructural analysis revealed that the epoxy-glass "szGR" composite exhibited the largest fibre diameter, while the epoxy-graphite composite displayed the smallest (Fig. 7 ). Density and porosity measurements for all analyzed composites are summarized in Table 1 . Table 1 Porosity and density of the structures studied Type of structure Filling Indication Porosity, % Density (effective), kg/m 3 epoxy-glass mat, layers szGR 33.0 1407 epoxy-glass mat, layers szCI 3.5 1677 epoxy-graphite fabric, layers graf 0.2 1484 Additionally, taking into account density and determined volume fraction of the components of tested composites (Table 2 ) the mass fraction of reinforcement was calculated by using Eq. 1 : $$\:{\omega\:}_{\text{r}}=\frac{{V}_{\text{r}}{\rho\:}_{\text{r}}}{{V}_{\text{r}}{\rho\:}_{\text{r}}+\left(1-{V}_{\text{r}}\right){\rho\:}_{\text{m}}}$$ 1 , where: \(\:{\omega\:}_{\text{r}}\) is the mass fraction of reinforcement, V r is the volume fraction of reinforcement, \(\:{\rho\:}_{\text{r}}\) is a density of reinforcement, \(\:{\rho\:}_{\text{m}}\) is a density of matrix. Table 2 Calculated mass fraction of reinforcement based on image analysis Code Volume fraction of reinforcement, % Density of reinforcement, kg/m 3 Density of matrix, kg/m 3 Mass fraction of reinforcement, % szGR 56 ± 7 2.43 1.4 69 szCI 43 ± 6 2.43 1.4 57 graf 57 ± 4 1.94 1.2 68 THERMOPHYSICAL PROPERTIES The analyses carried out in the “Macrostructure and Microstructure” section identified structural anisotropy and the associated expected variation in directional properties in two main characteristic directions: in the direction perpendicular to the plate surface i.e. traversal or out-of-plane in other words and in the longitudinal i.e. in-plane direction. In the case of glass composites, due to the stochastic variation in the longitudinal directions of the arrangement of individual mat fibres, all longitudinal characteristic directions appear to be equal. When studying graphite composite, the direction of the cut of the specimens for longitudinal characteristic testing may be of importance. This is determined by the regular orthogonal structure of the carbon fabric. The thermal behaviour of composites is highly dependent on fibre arrangement, a fact underscored by [ 20 ]. Composites with aligned fibres show a strong directional dependence of thermal properties, while chopped fibre composites, though more uniform, exhibit localized fluctuations in thermal diffusivity. While the anisotropy of the tested materials, as a typical feature of most composite structures, should not pose major research problems, two geometric-structural features that prevent the direct application of standard materials testing procedures were recognized at the outset of the visual evaluation. The first of these features is the variation of the structure and surface condition of the face of the plates concerning the inner surface (Fig. 1a). This is related not only to the presence of the paint layer but also to the variation in topography: the inner surface of each structure shows greater irregularity. In this case, it is necessary to take this fact into account when interpreting the test results, and, what is particularly important, to distinguish the result of the measurements from the actual material property [ 18 ]. The second problem relates to local differences in the thickness of the plates supplied for testing. In the case of the GFRP (Glass Fibred Reinforced Polymer)composite structure, this is compounded by high porosity with an irregular distribution of gaseous inclusions near the inner surface (Fig. 1b). The variation in slab thickness and the presence of unevenly distributed pores are accompanied by local variations in composition. In the homogenization interpretation, the studied structure is composed of locally differentiated materials. Given this, the determination of the actual properties becomes problematic. Only the possibility of determining representative properties remains, and only for selected test cases. However, the results obtained should be fully useful both for characterizing the structure and for carrying out structural calculations - also taking into account operational recommendations. The reliability of the data in this regard depends on the accurate demonstration of metrological limitations. The two primary objectives of the study were: 1. Providing material data in terms of the thermodynamic (thermophysical) parameters studied for engineering calculations; 2. Determination of thermal stability and determination of parameters of characteristic transformation of the matrix material of the studied composite structures. The thermal behaviour of composite materials is a critical factor in their performance, particularly in demanding applications like aerospace. Consequently, thorough thermophysical testing is indispensable. While methodologies employing techniques such as Differential Scanning Calorimetry (DSC), Laser Flash Analysis (LFA), and Dynamic Mechanical Thermal Analysis (DMTA) have been proposed [ 21 ], comprehensive studies in this area remain relatively limited. TG/DTG THERMOGRAVIMETRIC STUDIES The weight data of the samples are included in Table 3 . The table also includes the results of measuring the total change - in the case of loss - of the weight of the samples tested. Table 3 Weighting data of samples subjected to TG test: initial weight, total weight change in test, final weight Structure Sample indication m post , mg Δ m , Mg m kon , mg Residue, % by weight. szGR pr1 7.61 -5.03 2.58 33.90 pr2 12.45 -7.04 5.41 43.45 szCI pr1 21.52 -10.74 10.78 50.09 pr2 10.09 -5.03 5.06 50.15 graf pr1 32.05 -10.86 21.19 66.12 pr2 50.36 -18.43 31.93 63.40 Due to the lack of precise information on the composition of the material and the inhomogeneity of the structure, it was decided to set the upper temperature of the furnace heater ("STC off" mode) at 600° C and the rate of temperature change at 10 K/min. Such settings mean a slightly lower value for the maximum temperature and slightly lower values for the rate of temperature change during heating. A single measurement consisted of subjecting the test sample to heating and then cooling according to the temperature variation program shown in Fig. 8 . The control system used the furnace heater control mode, and the flow rates of nitrogen gas in the chamber and of the guard zone gas were set at 20 ml/min. Assuming that the epoxy resin material is completely decomposed into released gaseous products, the result of the final mass measurement allows the determination of approximate values of the mass shares of the filling material. The approximate value of the mass share of graphite in the “graf” structure is about 65 wt%, and of glass mat fibres in the “szCI” structure is about 50 wt%. Determining the mass share of glass in the “szGR” composite to be about 40 wt. % may be considered problematic, but the sheer fact of the lower fill content in this structure is highly probable. To develop the results, TG and DTG (differential: derivatives of mass changes after time) thermograms of the key first heating stage plotted as a function of temperature were prepared (Fig. 9). The results of developing the TG and DTG signals are shown in Fig. 9a )and Fig. 9b ), respectively. The TG waveforms visualized in Fig. 9 show a qualitative similarity in the mass changes - matrix phase distribution - for the tested samples of the epoxy-glass structure “szGR” and “szCI”, along with similar values of the temperature of the conventional onset of the ONSET transformation for the TG curves of approximately 294 °C, and small but clear differences in the mass changes of the graphite “graf” composite compared to the glass composites and a significantly different ONSET TG temperature of approximately 355 °C. The results of in-depth analyses carried out for DTG signals bring to light the fact of the two-stage transformation of matrix material decomposition. In the case of glass composites, the process starts at a lower temperature value (ONEST DTG about 295°C) and initially proceeds smoothly, and then, above about 350° C it accelerates until complete decomposition in the vicinity of 440 °C. (Fig. 9, szGR_pr2 and szCI_pr2 curves). The two-stage transformation is a characteristic of graphite composite structure matrix i.e. epoxy resin decomposition. The temperature at the beginning of the transformation is about 340 °C, and the end is about 460 °C. Given the differences in the ONSET TG and ONSET DTG temperature values, the lower values of the DTG parameters: 230° C for the epoxy-glass structure and 340° C for the epoxy-graphite structure should be taken as the basis for determining the limits for other measurements and setting possible operating limits. MICROCALORIMETRIC STUDIES The basic data for the programs are provided in Table 4 , and the temperature changes over time for both programs in actual tests are illustrated in Fig. 10 . In light of the results of the structure evaluation, because of the high porosity and heterogeneity of the “szGR” structure, microcalorimetric measurements were limited to the study of “szCI” and “graf” structures. Due to the small size of DSC samples, obtaining reliable qualitative results for heterogeneous materials requires laborious repetitive testing of many separate samples. The parameters of the “szCI” structure as a representative glass should not generally differ from the properties of the “szGR” structure. This is indicated by the convergent results of TG measurements of both structures. Table 4 DSC temperature program data Program Temperature range [ t min ; t max ], °C Rate of temperature change, K/min. Number of subintervals oftherange [ t min ; t max ] Number of repetitions of cycles baseline 042 [‒20; 120] ± 5 4 2 extended 059 [‒20; 280] ± 10 6 2.5 Each sample was first tested according to program indicated as 042 and then according to 059. The direct results of developing the thermograms into calculated specific heat values using the three-curve method for the low-temperature measurements are illustrated in Fig. 11 . Figure 12compares the results of the high-temperature tests. A summary of the direct results of all the tests performed is illustrated in Fig. 13 . Analyzing the results of the low-temperature measurement shown in Fig. 11 , the following can be concluded: i. As expected, the higher specific heat values of the glass-filled structure sample compared to the graphite structure. This is determined by the higher specific heat of glass than the specific heat of carbon [ 7 ] and the higher values of the mass share of graphite filling shown by thermogravimetric studies. ii. Stability and compatibility of thermal properties of the tested structures in the range of low-temperature excitation, iii. Possible minor effects of moisture release (slightly higher specific heat values in the first heating cycles in the temperature range above 60 °C for glass composite and 90 °C for graphite composite. In addition to moisture, these phenomena may be due to the release of other, unidentified volatile components. From the point of view of operational requirements, the most important thing is the repeatability of the results of testing the properties of a material subjected to cyclic thermal loading. The results of the high-temperature tests are unfortunately burdened by mass loss effects. Nevertheless, they provide an opportunity to determine the correspondence of the determined values of specific heat in the initial phase of the high-temperature cycle with the results of an earlier low-temperature measurement. In Fig. 13 , it is not difficult to see fragments of concordant curves: marked in blue and amaranth for the “szCI” sample and marked in red and green for the “graf” sample. Direct results, especially in the form of raw thermograms, are a good illustration of metrological problems of testing real composite structures. However, even in such a situation it is possible to estimate some representative and reliable data. In the case of the TG test, this was information on the composition of the tested samples. If DSC data are taken into account, it is even recommended to present representative c p data for calculating the tested samples based on the results of the thermal diffusivity measurement. In the discussed case, such data were obtained by B-spline approximation [ 12 ] of the specific heat values obtained for 2 cycles - i.e. the state of thermally stabilized samples in the range of thermal load. The results are shown in Fig. 14 . THERMAL DIFFUSIVITY Before discussing the obtained thermal diffusivity measurement results, it should be noted that due to the non-homogeneous structure of the tested materials, the presented results should be treated in terms of effective diffusivity. The results of the transverse diffusivity measurement of the “szGR” structure are shown in Fig. 15 , Fig. 16 , Fig. 17 and Fig. 18 . In light of the phenomena identified and characterized in the previous sections of the differences between the first and subsequent thermal loading cycles, it is important to pay attention to the results obtained in the first measurements. The temperature program of the first measurement of the structure in question is shown in Fig. 15 . The results of this measurement, illustrated in Fig. 16 , do not show clear signs of phase transitions, residual polymerization effects or moisture release: the heating and cooling characteristics of the sample (in fact, the two samples) coincide. This provides a basis for using geometric mean of the amplitude and phase thermal diffusivity data to determine a representative relationship (comp. [ 14 ] and [ 16 ]). Repeating the measurements for a changed orientation of the sample set highlights possible differences in the measurement results (see Fig. 17 ). Treating the obtained results as a measure of uncertainty, a decision was made to select results that were quantitatively consistent for determining a representative approximation characteristic for the longitudinal diffusivity of the “szGR” composite. In light of the presented considerations, the results of the geometric mean-square approximation of the geometric mean data of the calculated amplitude and phase values can be considered as an upper limit for the analysis. The baseline (ampl. and phase), input data and calculation results are illustrated in Fig. 18 . A summary of the results of the “szCI” structure tests is illustrated in Fig. 19 and Fig. 20 , while the results of the “graf” structure tests are shown in Fig. 21 . The study of the longitudinal component of thermal diffusivity was carried out in two stages: separately for the “szGR” sample, and separately for the “szCI” and “graf” samples. The reason was that the thickness of the “szGR” sample differed too much the others. An example of the arrangement of samples in the measuring head is shown in Fig. 22 . In turn, the image of the temperature distribution recorded during the measurements and the result of reading the temperature changes is illustrated in Fig. 23 . The temperature change signals shown in this figure include, among other things, the values of the average line temperature read from the control lines visible on the thermogram. In determining the position of each line, the uniformity of the spatial distribution was checked, which is an indication of one-dimensional heat flow -a condition for methodological correctness. The measure in mm of distances between the lines were determined photogrammetrically. Thermal diffusivity values were calculated for pairs of signals, one of which was the excitation and the other was the thermal response. The distance from the edge of the end (top) of the sample was also included in the calculations. The determined average values of the longitudinal component of thermal diffusivity are shown in Fig. 24 . CONCLUSIONS This study investigated the thermophysical properties of two epoxy-glass composites (reinforced with glass mat) and an epoxy-graphite composite (filled with graphite fabric) to provide data for engineering calculations, operational limitations, and structural analyses. The research focused on the effects of cyclic thermal loading and high-temperature behaviour. Key findings are summarized below: Thermogravimetric analysis - thermogravimetric (TG) analysis revealed qualitative similarities in mass change profiles for the two epoxy-glass composites ("szGR" and "szCI"), with similar conventional onset temperatures (ONSET) of approximately 294°C. However, distinct differences in weight change and a significantly higher ONSET temperature (approximately 355°C) for the epoxy-graphite composite compared to the glass composites. Given the differences in ONSET TG and ONSET DTG temperatures, the lower DTG values of 230°C for the epoxy-glass composites and 340°C for the epoxy-graphite composite should be used as a basis for establishing limits for subsequent measurements and defining operational limits. Microcalorimetric analysis – (i) a glass transition in the epoxy-graphite composite, shifting from 95-110°C (as-delivered) to 110-130°C after heating to 280°C; (ii) a strong likelihood of a similar glass transition in the glass-filled composite, with an onset temperature above 120°C in the post-annealed state; (iii) a slight decrease in specific heat for the epoxy-graphite composite after exposure to high temperatures. Diffusivity studies - the epoxy-graphite composite exhibited the highest transverse and longitudinal thermal diffusivity. While the longitudinal diffusivity was approximately twice the transverse diffusivity for the glass composites, this ratio was closer to 10:1 for the graphite composite, consistent with previous findings [16]. Moreover, for carbon fibre/epoxy resin, a similar relationship—exhibiting higher longitudinal thermal diffusivity than transverse diffusivity—had also been reported previously e.g. in [22]. The study investigates the thermal anisotropy of reinforced carbon fibre/epoxy composites, revealing that the obtained anisotropy ratio was even higher than in our research, primarily due to the enhanced in-plane thermal conductivity achieved through the introduction of high thermal conductive coaxial PAN/PBO carbon fibres. Declarations FUNDING DECLARATION Research and Development Project No. O ROB 0046 03 001 on “35 mm KDA automatic cannon with a fire control system using the ZGS-158 Integrated Tracking Warhead – made in naval version” subsidized in 2012-2023 by the Polish National Centre for Research and Development. COMPETING INTERESTS The authors declare no competing interests. DATA AVAILABILITY The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. References S.T. Peters, Handbook of Composites , Second edition. Springer New York, NY, 1998. doi: 10.1007/978-1-4615-6389-1. R. 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Warszawa: WNT Publishing House, 2012 (in Polish). K.D. Maglić, A. Cezairliyan, andV.E. Peletsky, Eds., Compendium of Thermophysical Property Measurement Methods . New York: Plenum Press, 1984. doi: 10.1007/978-1-4615-3286-6. J.L. McNaughton and C.T. Mortimer, Differential Scanning Calorimetry , IRS; Physical Chemistry Series 2, Volume 10. (London: Butterworths; Norwalk: reprinted by Perkin-Elmer Corp.), 1975. A. Cezairliyan, Specific Heat of Solids . Hemisphere Publishing Corporation, 1988. W. Wendlandt, Ed., Thermal Analysis , Third edition. New York: John Willey & Sons, 1986. A.J. Panas and D. Panas, “DSC investigation of binary iron-nickel alloys”. High Temperatures-High Pressures , vol. 38, no. 1, pp. 63–78, 2009. A.J. Panas, Comparative-Complementary Investigations of Thermophysical Properties – High Thermal Resolution Procedures In Practice. In Thermophysics 2010, Brno University of Technology, Faculty of Chemistry, 2010, ISBN 978-80-214-4166-8, pp 218-235. A.J. Ångström, “NeueMethode, das Warmeleitungsvermogen der Korperzu Bestimmen”. Annalen der Physic und Chemie , vol. 114, 1861. J.M. Belling and J. Unsworth, “Modified Angström’s method for measurement of thermal diffusivity of materials with low conductivity”. Review of Scientific Instruments , vol. 58, no. 6, pp. 997–1002, 1987, doi: 10.1063/1.1139589. A.J. Panas, “IR Support of Thermophysical Property Investigation - Study of Medical and Advanced Technology Materials”. In Infrared Thermography , InTech, 2012. doi: 10.5772/30397. R. Szczepaniak, A.J. Panas, M. Nowakowski, and J. Panas, “Investigation of the Thermal Diffusivity of Water Temperature Oscillation (in Polish),” Problemymechatroniki. Uzbrojenie , lotnictwo, inżynieria bezpieczeństwa / Probl. Mechatronics. ArmamentAviat. Saf. Eng. , vol. 3, no. 3, pp. 85–98, 2012. A.J. Panas and M. Talkowski, “Investigation on directional thermal diffusivity for graphite composite,” Journal of KONES Powertrain and Transport , vol. 23, no. 4, 2016, doi: 10.5604/01.3001.0010.2390. A.J. Panas. et al. , Investigation of termophysical and thermomechanical properties of aviation epoxy-glass composite (in Polish). In Mechanika w lotnictwie ML-XVIII 2018 , pp. 163–178, 2018. G. Bresson et al. , “Thermographic and tomographic methods for tridimensional characterization of thermal transfer in silica/phenolic composites”. Compos B Eng , vol. 104, pp. 71–79, Nov. 2016, doi: 10.1016/j.compositesb.2016.08.022. B. Weidenfeller and S. Kirchberg, “Thermal and mechanical properties of polypropylene-iron-diamond composites”. Compos B Eng , vol. 92, pp. 133–141, May 2016, doi: 10.1016/j.compositesb.2016.02.011. M. Hao et al. , “Enhanced both in-plane and through-thickness thermal conductivity of carbon fiber/epoxy composites by fabricating high thermal conductive coaxial PAN/PBO carbon fibers”. Compos B Eng , vol. 229, Jan. 2022, doi: 10.1016/j.compositesb.2021.109468. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted 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-6629055","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":468249940,"identity":"904c2b8c-3e47-41e5-9a70-7c44593e3962","order_by":0,"name":"Andrzej J. 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SIENKIEWICZ","email":"data:image/png;base64,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","orcid":"","institution":"Military University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Judyta","middleName":"","lastName":"SIENKIEWICZ","suffix":""},{"id":468249944,"identity":"45c235a7-bb80-4fa8-95d1-7824643e3066","order_by":3,"name":"Mirosław NOWAKOWSKI","email":"","orcid":"","institution":"Air Force Institute of Technology","correspondingAuthor":false,"prefix":"","firstName":"Mirosław","middleName":"","lastName":"NOWAKOWSKI","suffix":""}],"badges":[],"createdAt":"2025-05-09 13:23:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6629055/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6629055/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":84305433,"identity":"915acdc1-9149-410a-a0d0-d44fcbc5cb01","added_by":"auto","created_at":"2025-06-10 11:24:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":500729,"visible":true,"origin":"","legend":"\u003cp\u003eView of samples taken from individual plates of composite structures (a) and side view of samples (b)\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/14a2f3f0206f5058a417833d.png"},{"id":84305997,"identity":"e4e901d2-8065-4ad5-ba4f-1ccfd8dec193","added_by":"auto","created_at":"2025-06-10 11:24:27","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":926713,"visible":true,"origin":"","legend":"\u003cp\u003eCross-sectional view of epoxy-glass composite \"szGR\" and \"szCI\" macrostructure together with the view of the surface of the epoxy-glass composite\" szCI\"\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/31fa75a473b5bef216cd6413.png"},{"id":84306043,"identity":"7421b412-8842-4da6-b09f-f85d80f607c6","added_by":"auto","created_at":"2025-06-10 11:24:29","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":401796,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of epoxy-glass composite \"szGR\" microstructure\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/43bb0d4da41b55399ae5ab52.png"},{"id":84306047,"identity":"d03770f7-af04-4002-bdf1-7dc2937bc26a","added_by":"auto","created_at":"2025-06-10 11:24:29","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":322592,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of epoxy-glass composite \"szCI\" microstructure\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/64d4b56e16562f9d35e0be5a.png"},{"id":84305407,"identity":"93878224-8341-47f1-a382-9ea07ecfeeb1","added_by":"auto","created_at":"2025-06-10 11:24:12","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":881674,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eMacrostructure of graphite composite - a) lower magnification, b) c) image taken on a specimen cut at an angle of 45 °\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/8c98ed38832ecc8d484771a0.png"},{"id":84306089,"identity":"a9f37af6-ba5d-4e70-a5c4-7ee4a736d331","added_by":"auto","created_at":"2025-06-10 11:24:33","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":856415,"visible":true,"origin":"","legend":"\u003cp\u003eSEM images of the microstructure of the epoxy-graphite composite\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/75630527b3adab8b1d27c63b.png"},{"id":84305483,"identity":"939c9a30-d779-4ebb-960e-478047a715cd","added_by":"auto","created_at":"2025-06-10 11:24:15","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":31449,"visible":true,"origin":"","legend":"\u003cp\u003eA graph showing the diameter of reinforcing fibres for each composite\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/b90c34dab640dee9b1720a88.png"},{"id":84305431,"identity":"2e252063-6fe2-4691-b975-aed63ee1ac4e","added_by":"auto","created_at":"2025-06-10 11:24:13","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":118326,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature changes of the tested samples during the TG test (dashed lines, right axis of ordinate) and a summary representation of the test thermograms of all samples with a list of global mass changes (Table 3)\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/4e315c2c784b153b319b0918.png"},{"id":84305424,"identity":"13b3ac69-31ce-4587-8396-422806098f35","added_by":"auto","created_at":"2025-06-10 11:24:13","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":192592,"visible":true,"origin":"","legend":"\u003cp\u003eThermograms of thermogravimetric measurements and the results of their post-processing in the form of determined DTG differential waveforms (dashed curves) and ONSET characteristic temperature values of the transformations for TG (a) and DTG (b) signals\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/e9845a4bc8918a2737879858.png"},{"id":84305417,"identity":"dd3c9dee-769e-49ad-97d8-3798ead65ffc","added_by":"auto","created_at":"2025-06-10 11:24:13","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":46160,"visible":true,"origin":"","legend":"\u003cp\u003eTemperature variation programs of tests 042 and 059 (Table 4)\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/c45f35eb453f8749775c66c8.png"},{"id":84305484,"identity":"186e0574-19d7-4913-9b2a-e705f118ada5","added_by":"auto","created_at":"2025-06-10 11:24:15","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":54086,"visible":true,"origin":"","legend":"\u003cp\u003eDirect results of the DSC Heat Flow thermogram processing of the low-temperature test into specific heat (with artifacts of irregular heat transfer effects at the beginning of each individual heating/cooling ramp)\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/f7d987c2d47dd2c29a7c6bae.png"},{"id":84306915,"identity":"4ca72c2c-c176-480e-a6d4-1ac0465b47bc","added_by":"auto","created_at":"2025-06-10 11:32:28","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":67161,"visible":true,"origin":"","legend":"\u003cp\u003eDirect results of the DSC Heat Flow thermogram processing of the high temperature extended range test into specific heat\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/eb98d4dd42c23688cece5390.png"},{"id":84306086,"identity":"50de3246-100f-4816-afac-5b899d801ea0","added_by":"auto","created_at":"2025-06-10 11:24:32","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":80162,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of calculated specific heat characteristics of the tested samples obtained in all tests\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/ebf9ae3c36e0a9154511e04c.png"},{"id":84306039,"identity":"37cafc1a-4f62-4fc7-820c-7a7f81acbe22","added_by":"auto","created_at":"2025-06-10 11:24:28","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":27531,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eResults of a B-spline approximation of the calculated specific heat capacity values recommended for characterization of the investigated composite structures\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/fdd578cac5f6832818ccf2ba.png"},{"id":84305488,"identity":"6fb9b865-b26d-4f9b-b70e-41af62fbf555","added_by":"auto","created_at":"2025-06-10 11:24:15","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":19089,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTemperature program of the first measurement of the GFRP structure tests\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/0e6b005450eae05e02f196ec.png"},{"id":84305413,"identity":"28b8efd0-bb62-4bb3-a19a-c9567451a480","added_by":"auto","created_at":"2025-06-10 11:24:12","extension":"png","order_by":16,"title":"Figure 16","display":"","copyAsset":false,"role":"figure","size":28709,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eResults of the first measurement performed for the “szGR” structure - illustration of the magnitude of differences between the amplitude and phase results, the effects of disordered heat transfer (peaks for the extremes of the temperature interval) and the geometric mean data used to determine representative characteristics\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"16.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/6365ed0ac3d82a13f0259643.png"},{"id":84305831,"identity":"509f43fd-553a-4d84-addd-807d21794163","added_by":"auto","created_at":"2025-06-10 11:24:18","extension":"png","order_by":17,"title":"Figure 17","display":"","copyAsset":false,"role":"figure","size":31046,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eResults of all six “szGR” structure tests - illustration of the differences in results due to changing the configuration of the samples\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"17.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/ad4e7dcb846580b9a6f0c0fa.png"},{"id":84305882,"identity":"26079fff-1f4c-453e-89eb-dfbeef198558","added_by":"auto","created_at":"2025-06-10 11:24:22","extension":"png","order_by":18,"title":"Figure 18","display":"","copyAsset":false,"role":"figure","size":45196,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eResults of measurements used to determine a representative transversal thermal diffusivity -temperature relationship for the “szGR” structure, input data for calculations including the geometric mean and resulting approximation characteristic\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"18.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/9a86113724c435bbc8d6731d.png"},{"id":84306904,"identity":"056fa4e7-a76e-4672-a987-dd353caec8c9","added_by":"auto","created_at":"2025-06-10 11:32:15","extension":"png","order_by":19,"title":"Figure 19","display":"","copyAsset":false,"role":"figure","size":60489,"visible":true,"origin":"","legend":"\u003cp\u003eSummary of the results of thermal diffusivity tests of the transverse component - for samplesof the “szCI”structure\u003c/p\u003e","description":"","filename":"19.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/9e56ed0bf55dc0c95801b20c.png"},{"id":84305839,"identity":"41c59e30-0648-4faf-ab73-6a1949cfa7b8","added_by":"auto","created_at":"2025-06-10 11:24:19","extension":"png","order_by":20,"title":"Figure 20","display":"","copyAsset":false,"role":"figure","size":45189,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eData for determination and resulting approximation characteristics of thermal diffusivity of transverse structure of “szCI”\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"20.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/5e4abd690efdd423c3bb371f.png"},{"id":84305426,"identity":"6cf46a01-f756-41a0-a210-5dfa015c949e","added_by":"auto","created_at":"2025-06-10 11:24:13","extension":"png","order_by":21,"title":"Figure 21","display":"","copyAsset":false,"role":"figure","size":64164,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSummary of measurement results (ampl., phas.), data for determination (aprox.) and resulting approximation characteristics of thermal diffusivity of transverse “graf”structure\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"21.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/96877b93712940aae042c259.png"},{"id":84305487,"identity":"74e5bf55-1ab5-4341-b8ef-6d5e176e7874","added_by":"auto","created_at":"2025-06-10 11:24:15","extension":"png","order_by":22,"title":"Figure 22","display":"","copyAsset":false,"role":"figure","size":176933,"visible":true,"origin":"","legend":"\u003cp\u003eView of samples of composite structures mounted in a holder for recording temperature changes with a thermal imaging camera in measurements of the longitudinal component of thermal diffusivity: from the left two samples of “szCI”DMA, on the right two samples of “graf”DMA\u003c/p\u003e","description":"","filename":"22.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/aeac507efcdefd4285bba8fd.png"},{"id":84305971,"identity":"9859ab0f-8325-4187-bc94-6d77eebeb16b","added_by":"auto","created_at":"2025-06-10 11:24:26","extension":"png","order_by":23,"title":"Figure 23","display":"","copyAsset":false,"role":"figure","size":255117,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eImage from the thermal imaging camera with plotted readout lines of temperature change signals (left) and depiction of recorded temperature changes (right)\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"23.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/ef949b77ecdf2efa64f8d633.png"},{"id":84305498,"identity":"2aa9db7c-e11b-4315-ba4c-404c8322c152","added_by":"auto","created_at":"2025-06-10 11:24:16","extension":"png","order_by":24,"title":"Figure 24","display":"","copyAsset":false,"role":"figure","size":23499,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of longitudinal thermal diffusivity values of the tested composite structures\u003c/p\u003e","description":"","filename":"24.png","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/756ffdb2d2fbb0e9b7482d67.png"},{"id":86887419,"identity":"4a30bf3a-7d1a-407d-83f5-ea7100f30673","added_by":"auto","created_at":"2025-07-16 18:16:27","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":6573803,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6629055/v1/10199b63-ace8-4027-83cd-25e2ff410634.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Comparative analysis of thermophysical properties of functional composites reinforced with epoxy-glass and epoxy-carbon fibers in the context of heat transfer anisotropy","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eAs is commonly known, composites are material structures composed of two or more distinct phases, each with unique properties, combined in a manner that results in a material with enhanced or superior properties compared to those of the individual components when used independently or simply added together [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eStructural composites represent a broad and diverse class of materials distinguished by the type of components used, the shape and size of the reinforcing phase, and the specific manufacturing technologies employed. Through the careful selection of components tailored to meet the stringent demands of modern applications, these composites exhibit exceptional strength properties. Additionally, they are often engineered to meet specific performance requirements, such as maintaining stability under high temperatures or cyclic thermal changes.\u003c/p\u003e \u003cp\u003eComposites, especially carbon composites, have excellent thermal properties, i.e. low CTE expansion coefficient and high mechanical strength at elevated temperatures. Owing to these exceptional properties, composites find applications in the production of aircraft engine nozzles and thermal assemblies for combustion chambers, as well as aircraft brake discs. These applications span across the aerospace, military, and civilian sectors [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. In the above-mentioned applications, the thermophysical properties of composites, which are strongly influenced by their microstructure, including density and porosity, are crucial, however, reports on them are scarce. Epoxy-glass and epoxy-graphite composites are extensively employed in aircraft construction due to their favourable strength-to-weight ratios and excellent mechanical properties.\u003c/p\u003e \u003cp\u003eVery good structural properties of composite structures - in given applications - allow for the extension of the range of permissible variability of permissible production deviations of parameters. However, this does not eliminate the source problem. And the problem concerns both carbon composite structures and those with glass filling, in particular it is related to the problem of anisotropy of properties.\u003c/p\u003e \u003cp\u003eEpoxy-glass composites comprise an epoxy resin matrix reinforced with glass fibres. The microstructure of these composites is intricate, featuring several distinct phases. The amorphous epoxy resin matrix consists of cross-linked polymer chains, forming a continuous phase. Embedded within this matrix are glass fibres, providing the composite with its primary strength and stiffness. The interfacial region between the fibre and matrix is crucial for load transfer and overall composite performance. This region, typically a few micrometres thick, contains a mixture of epoxy resin and fibre surface treatment. Under microscopic observation, glass fibres appear as elongated strands embedded within the epoxy matrix. Fiber orientation significantly influences the composite's mechanical properties. Unidirectional arrangements, where fibres align in a single direction, offer high strength and stiffness along the fibre axis but exhibit lower properties in transverse directions. Bidirectional arrangements, with fibres aligned in two perpendicular directions, provide more balanced properties but may not achieve the same axial strength as unidirectional composites.\u003c/p\u003e \u003cp\u003eEpoxy-graphite composites, in turn, consist of a graphite fibre reinforcement embedded within an epoxy resin matrix. Similar to epoxy-glass composites, these materials commonly utilize unidirectional or bidirectional fibre orientations. Microscopic examination reveals graphite fibres as elongated structures dispersed within a continuous epoxy matrix.\u003c/p\u003e \u003cp\u003eFibers typically exhibit micrometer-scale diameters and millimeter-scale lengths. The microstructure of epoxy-graphite composites is influenced by factors such as manufacturing processes and fibre surface treatments. These treatments enhance fibre-matrix adhesion, impacting fibre distribution and orientation within the composite. The resulting microstructure is engineered to produce a high-strength, lightweight material with excellent wear and corrosion resistance, making it ideal for demanding applications in aerospace and sporting goods.\u003c/p\u003e \u003cp\u003eThe epoxy matrix serves as a load-bearing medium, binding and transferring loads between the graphite fibres. The volume fraction of the matrix and the quality of the fibre-matrix interface significantly influence the overall mechanical performance of the composite.\u003c/p\u003e \u003cp\u003eThe common use of composites in combination with the specificity of production technology, as well as the availability of their production technology, however, raises the problem of the lack of full repeatability of the structure properties. This applies in particular to composites with an epoxy resin matrix.\u003c/p\u003e \u003cp\u003eThis research investigates properties of industrial, or in other words utility composite structures. This means that samples of the production material were tested with the preservation of natural structural imperfections. The subject of the tests were thermophysical properties. Thermophysical properties extend beyond classical thermodynamic parameters such as specific heat, thermal expansion, and thermal conductivity [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e], [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e], [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. This includes thermomechanical properties, notably rheological characteristics, including viscoelasticity [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The study of thermophysical properties necessitates not only quantifying the temperature dependence of individual physical parameters but also comprehensively characterizing phase transformations. Furthermore, it is crucial to obtain data that accurately predict material behaviour under various thermal loading conditions. At present case the attention was focused on heat accumulation and heat transfer parameters taking into account their dependence on temperature, as well as anisotropy in the case of heat conduction. The mutual purpose of the work is to provide insight into possible design and operational problems of real structures in relation to model data of \u0026ldquo;perfect\u0026rdquo; structure. This also applies to the interpretation of test results for properties - results may depend on the measurement procedure. In this dimension, the work falls into metrological categories.\u003c/p\u003e"},{"header":"MATERIALS AND METHODS","content":"\u003cp\u003eThis research utilized three types of composite materials: two epoxy-glass fibre mat composites (3A Composities Mobility S.A., Mielec, Poland) with different thicknesses:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e- with a thickness \u0026nbsp;of 6.2-6.4 mm - designated \"szGR\";\u003c/p\u003e\n\u003cp\u003e- with a thickness of 4.5-5.2 mm - designated \"szCI\";\u003c/p\u003e\n\u003cp\u003eand\u0026nbsp;an epoxy-graphite composite (Rega Yacht sp. z o.o., Ropczyce, Poland) reinforced with carbon fabric layers\u0026nbsp;(designated \"graf\").\u003c/p\u003e\n\u003cp\u003eInitially, tested composites were subjected to microstructural analysis. Imaging techniques included a KEYENCE VHX-6000 digital microscope and a Phenom ProX scanning electron microscope. Microscopic images were analyzed to determine individual layer thicknesses, porosity levels, and the volume fractions of constituent phases.\u003c/p\u003e\n\u003cp\u003eA Netzsch TG 209 F3 Tarsus thermobalance was used to conduct thermogravimetric tests. The measuring range of the thermobalance covers the temperature range from room temperature (TP) to 1000°\u0026nbsp;C, the resolution is 0.1mg, the maximum weight range is 2000 mg, the rates of temperature change from 0.001°\u0026nbsp;C/min to 100°\u0026nbsp;C/min, and the capacity of a standard alumina sample pan is 85ml.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDSC (Dynamic Scanning Calorimetry) microcalorimetric studies were performed using\u0026nbsp;\u003cbr\u003e\u0026nbsp;a Pyris 1 power-compensated scanning microcalorimeter from Perkin-Elmer with a temperature range of\u0026nbsp;‒30°\u0026nbsp;C to 600°\u0026nbsp;C or from room the temperature (RT) to 710°\u0026nbsp;C and a claimed accuracy of enthalpy and specific heat determination of\u0026nbsp;±\u0026nbsp;2%. Attention in the study was focused on the determination of specific heat. The three-curve method ([8], [9], [10], [11]) was used to determine the specific heat. The study used a dedicated temperature change program described in publications [12], [13] that enables which allows for obtaining reliable results in both heating and cooling microcalorimeter operation modes. Each sample was tested first according to the thermal program with the maximum exposition temperature limited to 130\u0026nbsp;°C, and then according to the program specified as high-temperature (extended range). The low temperature program corresponds to the typical operating temperature range of aircraft composites.\u003c/p\u003e\n\u003cp\u003eComplementing the study of thermophysical properties were measurements of thermal diffusivity, i.e. the ratio of thermal conductivity to volumetric heat capacity. A modified temperature oscillation method was used to determine this parameter\u0026nbsp;[14], [15], [16]. A description of the test stand, together with a presentation of the procedures for testing directional properties, are discussed in publications\u0026nbsp;[16], [17], [18], [19]. The method used is characterized by the ability to perform measurements with high-temperature resolution\u0026nbsp;[13]. Processing of the measurement signals results in two independently calculated values of thermal diffusivity: one determined by comparing the amplitude of the periodic temperature change excitation signal and the periodic response, the so-called amplitude value \u003cem\u003ea\u003c/em\u003e\u003csub\u003ey\u003c/sub\u003e, and the other, determined by the phase shift, the phase value \u003cem\u003ea\u003c/em\u003e\u003csub\u003ej\u003c/sub\u003e. In the model case of one-dimensional heat flow, without convective losses from lateral surfaces, both values should be equal. That allows for a preliminary check of the correctness of the measurement.\u003c/p\u003e\n\u003cp\u003eThe thermal diffusivity of the tested composite structures was determined taking into account the property directional differences. For measurements in the direction perpendicular to the surface(transversal/out-of-plane component), square-shaped specimens with a side length of 40 mm were used, while specimens with a length of about 60 mm and a width of about 10 mm were used to measure the thermal diffusivity in the longitudinal direction (longitudinal/in-plane component). In the case of measuring transverse diffusivity, an outside bilateral symmetric forcing was used with the measurement of the response signal at the contact surface of two samples put together with square surfaces- the symmetry surface of the system. In this case, the characteristic dimension for calculating the thermal diffusivity was equal to the sample thickness, i.e. approximately 6.2 and 5.3 mm for epoxy-mat composites and approximately 3.5 mm for epoxy-fabric composites. The measurement system makes it possible to perform thermal diffusivity measurements in the range from about -10°\u0026nbsp;C to about 100°\u0026nbsp;C. The period of excitation was selected each time according to the properties of the tested material and ranged from 30 s for graphite composites to 60 s or 120 s for glass composites.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eBilateral excitation was also used to study the longitudinal component along the shorter side of the prismatic specimen. Due to the dimensions of the specimen lengths, which were too large concerning the dimensions of the system measuring head, it was not possible to eliminate the lateral losses to a sufficient extent. Therefore, the tests were repeated with single-sided excitation and thermal imaging of the temperature (comp.\u0026nbsp;[18]\u0026nbsp;and\u0026nbsp;[16]). It should be mentioned that the specimens were tested without removing the paint coating, and the registration of temperature changes was made from the opposite side. To carry out the measurements, the surfaces of the samples were coated with a layer of GRAPHITE 33 flake graphite with a\u0026nbsp;thickness of no more than 20mm. Changes in the temperature distribution were recorded using a Flir SC5600 thermal imaging camera with a recording frequency of 1 Hz for a 120 s excitation period and 0.25 Hz for a 240 s excitation period. For data processing, temperature change signals were collected as the spatial mean temperature of the control lines. The control lines were declared along the expected isotherms. The lines were spaced at approximately 2.5 mm intervals.\u003c/p\u003e"},{"header":"RESULTS AND DISCUSSION","content":"\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003eMACROSTUCTURE AND MICROSTRUCTURE\u003c/h2\u003e\n \u003cp\u003eIllustrative photographs of the tested composites are depicted in \u003cstrong\u003eFig.\u0026nbsp;1\u003c/strong\u003e. Figure\u0026nbsp;1a) shows the surfaces of the upper and lower surfaces.\u003c/p\u003e\n \u003cp\u003eAll tested specimens were coated with factory paint on one side (grey surfaces visible in the left column of \u003cstrong\u003eFig.\u0026nbsp;1a\u003c/strong\u003e). Figure 1b represents a macroscopic cross-section view of the original specimen from which the samples were cut.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e presents cross-sectional views of the \u0026quot;szGR\u0026quot; and \u0026quot;szCI\u0026quot; epoxy-glass laminates, revealing their macrostructures. Red arrows in the images indicate pores and the paint layer applied to the laminates\u0026apos; undersides. The macrostructure exhibits pores of varying shapes and sizes, primarily concentrated in the interlayer spaces. Larger pores tend to form clusters, while finer, more spherical pores are distributed more evenly throughout the laminate volume. Two distinct porosity types were identified: cylindrical voids between individual fibres and spherical voids located between fibre bundles. In the \u0026quot;szGR\u0026quot; composite, significant pores or holes are visible beneath the surface. Notably, the areas surrounding these largest pores lack glass reinforcement. Furthermore, Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e also showcases the surface of the \u0026quot;szCI\u0026quot; epoxy-glass composite. The image reveals fibres arranged at varying angles, indicating non-uniform fibre distribution within the composite. Additionally, particles of epoxy resin are visible within the image.\u003c/p\u003e\n \u003cp\u003eHigher magnification images of the microstructure of the epoxy-glass composite \u0026quot;szGR\u0026quot; and \u0026quot;szGR\u0026quot; are shown in Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e and Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, respectively.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ea) presents a low-magnification view of the epoxy-graphite composite\u0026apos;s microstructure. Characteristic of epoxy-graphite composites reinforced with woven carbon fabric, the image reveals fibres oriented both parallel and perpendicular to the viewing plane, indicative of the traditional interlacing pattern. Figure \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003eb) provides a closer look at the composite structure, highlighting the paint layer and the individual graphite-fiber fabrics comprising the laminate. Furthermore, Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003ec) showcases the microstructure of a sample sectioned at a 45\u0026deg; angle. These micrographs collectively confirm the layered structure of the carbon fabric with its inherent interlacing pattern. In addition to the fibres\u0026apos; varied orientations, the images also reveal areas with increased resin density (appearing darker grey), corresponding to regions with a lower concentration of graphite fibres.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e presents scanning electron microscope (SEM) images of the epoxy-graphite composite\u0026apos;s microstructure. The images reveal a generally uniform fibre distribution, although some resin-rich areas are evident. The microstructure further illustrates the arrangement of fibres within individual fabric bundles. Within these bundles, fibres are randomly distributed with minimal inter-fibrevoiding. A small number of pores, broken fibres, and fibre detachments were observed within the resin matrix. Microstructural analysis indicated low porosity in the tested composite (low void volume fraction, \u003cem\u003eV\u003c/em\u003e\u003csub\u003ev\u003c/sub\u003e\u0026lt; 0.2%) and a high fibre volume fraction, \u003cem\u003eV\u003c/em\u003e \u003csub\u003ef\u003c/sub\u003e ~57%. The diameter of the graphite fibres was determined to be in the range of 5\u0026ndash;6 \u0026micro;m.\u003c/p\u003e\n \u003cp\u003eMicrostructural analysis revealed that the epoxy-glass \u0026quot;szGR\u0026quot; composite exhibited the largest fibre diameter, while the epoxy-graphite composite displayed the smallest (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e). Density and porosity measurements for all analyzed composites are summarized in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003ePorosity and density of the structures studied\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eType of structure\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eFilling\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eIndication\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003ePorosity,\u003c/p\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDensity (effective),\u003c/p\u003e\n \u003cp\u003ekg/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eepoxy-glass\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emat, layers\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eszGR\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e33.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1407\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eepoxy-glass\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003emat, layers\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eszCI\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e3.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1677\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eepoxy-graphite\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003efabric, layers\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003egraf\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e0.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1484\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\u003eAdditionally, taking into account density and determined volume fraction of the components of tested composites (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) the mass fraction of reinforcement was calculated by using Eq. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e:\u003c/p\u003e\n \u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\n \u003cdiv class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e$$\\:{\\omega\\:}_{\\text{r}}=\\frac{{V}_{\\text{r}}{\\rho\\:}_{\\text{r}}}{{V}_{\\text{r}}{\\rho\\:}_{\\text{r}}+\\left(1-{V}_{\\text{r}}\\right){\\rho\\:}_{\\text{m}}}$$\u003c/div\u003e\n \u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\n \u003c/div\u003e,\u003cp\u003ewhere: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\omega\\:}_{\\text{r}}\\)\u003c/span\u003e\u003c/span\u003e is the mass fraction of reinforcement, \u003cem\u003eV\u003c/em\u003e\u003csub\u003er\u003c/sub\u003e is the volume fraction of reinforcement, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{\\text{r}}\\)\u003c/span\u003e\u003c/span\u003eis a density of reinforcement,\u0026nbsp;\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\rho\\:}_{\\text{m}}\\)\u003c/span\u003e\u003c/span\u003e is a density of matrix.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eCalculated mass fraction of reinforcement based on image analysis\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCode\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVolume fraction of reinforcement,\u003c/p\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDensity of reinforcement,\u003c/p\u003e\n \u003cp\u003ekg/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eDensity of matrix, kg/m\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eMass fraction of reinforcement,\u003c/p\u003e\n \u003cp\u003e%\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eszGR\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e56\u0026thinsp;\u0026plusmn;\u0026thinsp;7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e69\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003eszCI\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e43\u0026thinsp;\u0026plusmn;\u0026thinsp;6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e57\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003egraf\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e57\u0026thinsp;\u0026plusmn;\u0026thinsp;4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.94\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e1.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e68\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n\u003c/div\u003e\n\u003ch3\u003eTHERMOPHYSICAL PROPERTIES\u003c/h3\u003e\n\u003cp\u003eThe analyses carried out in the \u0026ldquo;Macrostructure and Microstructure\u0026rdquo; section identified structural anisotropy and the associated expected variation in directional properties in two main characteristic directions: in the direction perpendicular to the plate surface i.e. traversal or out-of-plane in other words and in the longitudinal i.e. in-plane direction. In the case of glass composites, due to the stochastic variation in the longitudinal directions of the arrangement of individual mat fibres, all longitudinal characteristic directions appear to be equal. When studying graphite composite, the direction of the cut of the specimens for longitudinal characteristic testing may be of importance. This is determined by the regular orthogonal structure of the carbon fabric. The thermal behaviour of composites is highly dependent on fibre arrangement, a fact underscored by [\u003cspan class=\"CitationRef\"\u003e20\u003c/span\u003e]. Composites with aligned fibres show a strong directional dependence of thermal properties, while chopped fibre composites, though more uniform, exhibit localized fluctuations in thermal diffusivity.\u003c/p\u003e\n\u003cp\u003eWhile the anisotropy of the tested materials, as a typical feature of most composite structures, should not pose major research problems, two geometric-structural features that prevent the direct application of standard materials testing procedures were recognized at the outset of the visual evaluation. The first of these features is the variation of the structure and surface condition of the face of the plates concerning the inner surface (Fig.\u0026nbsp;1a). This is related not only to the presence of the paint layer but also to the variation in topography: the inner surface of each structure shows greater irregularity. In this case, it is necessary to take this fact into account when interpreting the test results, and, what is particularly important, to distinguish the result of the measurements from the actual material property [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e]. The second problem relates to local differences in the thickness of the plates supplied for testing. In the case of the GFRP (Glass Fibred Reinforced Polymer)composite structure, this is compounded by high porosity with an irregular distribution of gaseous inclusions near the inner surface (Fig. 1b). The variation in slab thickness and the presence of unevenly distributed pores are accompanied by local variations in composition. In the homogenization interpretation, the studied structure is composed of locally differentiated materials. Given this, the determination of the actual properties becomes problematic. Only the possibility of determining representative properties remains, and only for selected test cases. However, the results obtained should be fully useful both for characterizing the structure and for carrying out structural calculations - also taking into account operational recommendations. The reliability of the data in this regard depends on the accurate demonstration of metrological limitations. The two primary objectives of the study were:\u003c/p\u003e\n\u003cp\u003e\u003cspan\u003e\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003e1. Providing material data in terms of the thermodynamic (thermophysical) parameters studied for engineering calculations;\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003e2. Determination of thermal stability and determination of parameters of characteristic transformation of the matrix material of the studied composite structures.\u003c/p\u003e\n\u003c/span\u003e\n\u003cp\u003eThe thermal behaviour of composite materials is a critical factor in their performance, particularly in demanding applications like aerospace. Consequently, thorough thermophysical testing is indispensable. While methodologies employing techniques such as Differential Scanning Calorimetry (DSC), Laser Flash Analysis (LFA), and Dynamic Mechanical Thermal Analysis (DMTA) have been proposed [\u003cspan class=\"CitationRef\"\u003e21\u003c/span\u003e], comprehensive studies in this area remain relatively limited.\u003c/p\u003e\n\u003ch3\u003eTG/DTG THERMOGRAVIMETRIC STUDIES\u003c/h3\u003e\n\u003cp\u003eThe weight data of the samples are included in Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e. The table also includes the results of measuring the total change - in the case of loss - of the weight of the samples tested.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eWeighting data of samples subjected to TG test: initial weight, total weight change in test, final weight\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eStructure\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eSample indication\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003em\u003c/em\u003e\u003csub\u003epost\u003c/sub\u003e,\u003c/p\u003e\n \u003cp\u003emg\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u0026Delta;\u003cem\u003em\u003c/em\u003e,\u003c/p\u003e\n \u003cp\u003eMg\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003e\u003cem\u003em\u003c/em\u003e\u003csub\u003ekon\u003c/sub\u003e,\u003c/p\u003e\n \u003cp\u003emg\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eResidue, % by weight.\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003e\u003cem\u003eszGR\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003epr1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e7.61\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-5.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e2.58\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e33.90\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003epr2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e12.45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-7.04\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.41\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e43.45\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003e\u003cem\u003eszCI\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003epr1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e21.52\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-10.74\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10.78\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e50.09\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003epr2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e10.09\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-5.03\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e5.06\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e50.15\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" rowspan=\"2\"\u003e\n \u003cp\u003e\u003cem\u003egraf\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003epr1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e32.05\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-10.86\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e21.19\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e66.12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003epr2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e50.36\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e-18.43\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e31.93\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e63.40\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\u003eDue to the lack of precise information on the composition of the material and the inhomogeneity of the structure, it was decided to set the upper temperature of the furnace heater (\u0026quot;STC off\u0026quot; mode) at 600\u0026deg; C and the rate of temperature change at 10 K/min. Such settings mean a slightly lower value for the maximum temperature and slightly lower values for the rate of temperature change during heating. A single measurement consisted of subjecting the test sample to heating and then cooling according to the temperature variation program shown in Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e. The control system used the furnace heater control mode, and the flow rates of nitrogen gas in the chamber and of the guard zone gas were set at 20 ml/min.\u003c/p\u003e\n\u003cp\u003eAssuming that the epoxy resin material is completely decomposed into released gaseous products, the result of the final mass measurement allows the determination of approximate values of the mass shares of the filling material. The approximate value of the mass share of graphite in the \u0026ldquo;graf\u0026rdquo; structure is about 65 wt%, and of glass mat fibres in the \u0026ldquo;szCI\u0026rdquo; structure is about 50 wt%. Determining the mass share of glass in the \u0026ldquo;szGR\u0026rdquo; composite to be about 40 wt. % may be considered problematic, but the sheer fact of the lower fill content in this structure is highly probable.\u003c/p\u003e\n\u003cp\u003eTo develop the results, TG and DTG (differential: derivatives of mass changes after time) thermograms of the key first heating stage plotted as a function of temperature were prepared (Fig. 9). The results of developing the TG and DTG signals are shown in \u003cstrong\u003eFig.\u0026nbsp;9a\u003c/strong\u003e)and \u003cstrong\u003eFig.\u0026nbsp;9b\u003c/strong\u003e), respectively. The TG waveforms visualized in \u003cstrong\u003eFig.\u0026nbsp;9\u003c/strong\u003eshow a qualitative similarity in the mass changes - matrix phase distribution - for the tested samples of the epoxy-glass structure \u0026ldquo;szGR\u0026rdquo; and \u0026ldquo;szCI\u0026rdquo;, along with similar values of the temperature of the conventional onset of the ONSET transformation for the TG curves of approximately 294 \u0026deg;C, and small but clear differences in the mass changes of the graphite \u0026ldquo;graf\u0026rdquo; composite compared to the glass composites and a significantly different ONSET TG temperature of approximately 355 \u0026deg;C.\u003c/p\u003e\n\u003cp\u003eThe results of in-depth analyses carried out for DTG signals bring to light the fact of the two-stage transformation of matrix material decomposition. In the case of glass composites, the process starts at a lower temperature value (ONEST DTG about 295\u0026deg;C) and initially proceeds smoothly, and then, above about 350\u0026deg; C it accelerates until complete decomposition in the vicinity of 440 \u0026deg;C. (Fig.\u0026nbsp;9, szGR_pr2 and szCI_pr2 curves). The two-stage transformation is a characteristic of graphite composite structure matrix i.e. epoxy resin decomposition. The temperature at the beginning of the transformation is about 340 \u0026deg;C, and the end is about 460 \u0026deg;C.\u003c/p\u003e\n\u003cp\u003eGiven the differences in the ONSET TG and ONSET DTG temperature values, the lower values of the DTG parameters: 230\u0026deg; C for the epoxy-glass structure and 340\u0026deg; C for the epoxy-graphite structure should be taken as the basis for determining the limits for other measurements and setting possible operating limits.\u003c/p\u003e\n\u003ch3\u003eMICROCALORIMETRIC STUDIES\u003c/h3\u003e\n\u003cp\u003eThe basic data for the programs are provided in Table \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, and the temperature changes over time for both programs in actual tests are illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eIn light of the results of the structure evaluation, because of the high porosity and heterogeneity of the \u0026ldquo;szGR\u0026rdquo; structure, microcalorimetric measurements were limited to the study of \u0026ldquo;szCI\u0026rdquo; and \u0026ldquo;graf\u0026rdquo; structures. Due to the small size of DSC samples, obtaining reliable qualitative results for heterogeneous materials requires laborious repetitive testing of many separate samples. The parameters of the \u0026ldquo;szCI\u0026rdquo; structure as a representative glass should not generally differ from the properties of the \u0026ldquo;szGR\u0026rdquo; structure. This is indicated by the convergent results of TG measurements of both structures.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable id=\"Tab4\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eDSC temperature program data\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eProgram\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eTemperature range\u003c/p\u003e\n \u003cp\u003e[\u003cem\u003et\u003c/em\u003e\u003csub\u003emin\u003c/sub\u003e; \u003cem\u003et\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e],\u003c/p\u003e\n \u003cp\u003e\u0026deg;C\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eRate of temperature change,\u003c/p\u003e\n \u003cp\u003eK/min.\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNumber of subintervals oftherange\u003c/p\u003e\n \u003cp\u003e[\u003cem\u003et\u003c/em\u003e\u003csub\u003emin\u003c/sub\u003e; \u003cem\u003et\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eNumber of repetitions of cycles\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003ebaseline 042\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[‒20; 120]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026plusmn; 5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eextended 059\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e[‒20; 280]\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e\u0026plusmn; 10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2.5\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\u003eEach sample was first tested according to program indicated as 042 and then according to 059. The direct results of developing the thermograms into calculated specific heat values using the three-curve method for the low-temperature measurements are illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e. Figure 12compares the results of the high-temperature tests. A summary of the direct results of all the tests performed is illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e.\u003c/p\u003e\n\u003cp\u003eAnalyzing the results of the low-temperature measurement shown in Fig. \u003cspan class=\"InternalRef\"\u003e11\u003c/span\u003e, the following can be concluded:\u003c/p\u003e\n\u003cp\u003e\u003cspan\u003e\u003c/span\u003e\u003c/p\u003e\n\u003cp\u003ei. As expected, the higher specific heat values of the glass-filled structure sample compared to the graphite structure. This is determined by the higher specific heat of glass than the specific heat of carbon [\u003cspan class=\"CitationRef\"\u003e7\u003c/span\u003e] and the higher values of the mass share of graphite filling shown by thermogravimetric studies.\u003c/p\u003e\u003cspan\u003e\n \u003cp\u003eii. Stability and compatibility of thermal properties of the tested structures in the range of low-temperature excitation,\u003c/p\u003e\n\u003c/span\u003e\u003cspan\u003e\n \u003cp\u003eiii. Possible minor effects of moisture release (slightly higher specific heat values in the first heating cycles in the temperature range above 60 \u0026deg;C for glass composite and 90 \u0026deg;C for graphite composite. In addition to moisture, these phenomena may be due to the release of other, unidentified volatile components.\u003c/p\u003e\n\u003c/span\u003e\n\u003cp\u003eFrom the point of view of operational requirements, the most important thing is the repeatability of the results of testing the properties of a material subjected to cyclic thermal loading.\u003c/p\u003e\n\u003cp\u003eThe results of the high-temperature tests are unfortunately burdened by mass loss effects. Nevertheless, they provide an opportunity to determine the correspondence of the determined values of specific heat in the initial phase of the high-temperature cycle with the results of an earlier low-temperature measurement. In Fig. \u003cspan class=\"InternalRef\"\u003e13\u003c/span\u003e, it is not difficult to see fragments of concordant curves: marked in blue and amaranth for the \u0026ldquo;szCI\u0026rdquo; sample and marked in red and green for the \u0026ldquo;graf\u0026rdquo; sample.\u003c/p\u003e\n\u003cp\u003eDirect results, especially in the form of raw thermograms, are a good illustration of metrological problems of testing real composite structures. However, even in such a situation it is possible to estimate some representative and reliable data. In the case of the TG test, this was information on the composition of the tested samples. If DSC data are taken into account, it is even recommended to present representative \u003cem\u003ec\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e data for calculating the tested samples based on the results of the thermal diffusivity measurement. In the discussed case, such data were obtained by B-spline approximation [\u003cspan class=\"CitationRef\"\u003e12\u003c/span\u003e] of the specific heat values obtained for 2 cycles - i.e. the state of thermally stabilized samples in the range of thermal load. The results are shown in Fig. \u003cspan class=\"InternalRef\"\u003e14\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003eTHERMAL DIFFUSIVITY\u003c/h2\u003e\n \u003cp\u003eBefore discussing the obtained thermal diffusivity measurement results, it should be noted that due to the non-homogeneous structure of the tested materials, the presented results should be treated in terms of effective diffusivity. The results of the transverse diffusivity measurement of the \u0026ldquo;szGR\u0026rdquo; structure are shown in Fig. \u003cspan class=\"InternalRef\"\u003e15\u003c/span\u003e, Fig. \u003cspan class=\"InternalRef\"\u003e16\u003c/span\u003e, \u003cstrong\u003eFig.\u0026nbsp;17\u003c/strong\u003e and \u003cstrong\u003eFig.\u0026nbsp;18\u003c/strong\u003e. In light of the phenomena identified and characterized in the previous sections of the differences between the first and subsequent thermal loading cycles, it is important to pay attention to the results obtained in the first measurements. The temperature program of the first measurement of the structure in question is shown in Fig. \u003cspan class=\"InternalRef\"\u003e15\u003c/span\u003e. The results of this measurement, illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e16\u003c/span\u003e, do not show clear signs of phase transitions, residual polymerization effects or moisture release: the heating and cooling characteristics of the sample (in fact, the two samples) coincide.\u003c/p\u003e\n \u003cp\u003eThis provides a basis for using geometric mean of the amplitude and phase thermal diffusivity data to determine a representative relationship (comp. [\u003cspan class=\"CitationRef\"\u003e14\u003c/span\u003e] and [\u003cspan class=\"CitationRef\"\u003e16\u003c/span\u003e]).\u003c/p\u003e\n \u003cp\u003eRepeating the measurements for a changed orientation of the sample set highlights possible differences in the measurement results (see \u003cstrong\u003eFig.\u0026nbsp;17\u003c/strong\u003e). Treating the obtained results as a measure of uncertainty, a decision was made to select results that were quantitatively consistent for determining a representative approximation characteristic for the longitudinal diffusivity of the \u0026ldquo;szGR\u0026rdquo; composite.\u003c/p\u003e\n \u003cp\u003eIn light of the presented considerations, the results of the geometric mean-square approximation of the geometric mean data of the calculated amplitude and phase values can be considered as an upper limit for the analysis. The baseline (ampl. and phase), input data and calculation results are illustrated in \u003cstrong\u003eFig.\u0026nbsp;18\u003c/strong\u003e.\u003c/p\u003e\n \u003cp\u003eA summary of the results of the \u0026ldquo;szCI\u0026rdquo; structure tests is illustrated in \u003cstrong\u003eFig.\u0026nbsp;19\u003c/strong\u003eand Fig. \u003cspan class=\"InternalRef\"\u003e20\u003c/span\u003e, while the results of the \u0026ldquo;graf\u0026rdquo; structure tests are shown in Fig. \u003cspan class=\"InternalRef\"\u003e21\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eThe study of the longitudinal component of thermal diffusivity was carried out in two stages: separately for the \u0026ldquo;szGR\u0026rdquo; sample, and separately for the \u0026ldquo;szCI\u0026rdquo; and \u0026ldquo;graf\u0026rdquo; samples. The reason was that the thickness of the \u0026ldquo;szGR\u0026rdquo; sample differed too much the others. An example of the arrangement of samples in the measuring head is shown in Fig. \u003cspan class=\"InternalRef\"\u003e22\u003c/span\u003e. In turn, the image of the temperature distribution recorded during the measurements and the result of reading the temperature changes is illustrated in Fig. \u003cspan class=\"InternalRef\"\u003e23\u003c/span\u003e. The temperature change signals shown in this figure include, among other things, the values of the average line temperature read from the control lines visible on the thermogram. In determining the position of each line, the uniformity of the spatial distribution was checked, which is an indication of one-dimensional heat flow -a condition for methodological correctness.\u003c/p\u003e\n \u003cp\u003eThe measure in mm of distances between the lines were determined photogrammetrically. Thermal diffusivity values were calculated for pairs of signals, one of which was the excitation and the other was the thermal response. The distance from the edge of the end (top) of the sample was also included in the calculations.\u003c/p\u003e\n \u003cp\u003eThe determined average values of the longitudinal component of thermal diffusivity are shown in Fig. \u003cspan class=\"InternalRef\"\u003e24\u003c/span\u003e.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"CONCLUSIONS","content":"\u003cp\u003eThis study investigated the thermophysical properties of two epoxy-glass composites (reinforced with glass mat) and an epoxy-graphite composite (filled with graphite fabric) to provide data for engineering calculations, operational limitations, and structural analyses. The research focused on the effects of cyclic thermal loading and high-temperature behaviour. Key findings are summarized below:\u003c/p\u003e\n\u003col\u003e\n \u003cli\u003eThermogravimetric analysis - thermogravimetric (TG) analysis revealed qualitative similarities in mass change profiles for the two epoxy-glass composites (\"szGR\" and \"szCI\"), with similar conventional onset temperatures (ONSET) of approximately 294°C. However, distinct differences in weight change and a significantly higher ONSET temperature (approximately 355°C) for the epoxy-graphite composite compared to the glass composites. Given the differences in ONSET TG and ONSET DTG temperatures, the lower DTG values of 230°C for the epoxy-glass composites and 340°C for the epoxy-graphite composite should be used as a basis for establishing limits for subsequent measurements and defining operational limits.\u003c/li\u003e\n \u003cli\u003eMicrocalorimetric analysis – (i) a glass transition in the epoxy-graphite composite, shifting from 95-110°C (as-delivered) to 110-130°C after heating to 280°C; (ii)\u0026nbsp;\u003cbr\u003ea strong likelihood of a similar glass transition in the glass-filled composite, with an onset temperature above 120°C in the post-annealed state; (iii) a slight decrease in specific heat for the epoxy-graphite composite after exposure to high temperatures.\u003c/li\u003e\n \u003cli\u003eDiffusivity studies - the epoxy-graphite composite exhibited the highest transverse and longitudinal thermal diffusivity. While the longitudinal diffusivity was approximately twice the transverse diffusivity for the glass composites, this ratio was closer to 10:1 for the graphite composite, consistent with previous findings [16]. Moreover, for carbon fibre/epoxy resin, a similar relationship—exhibiting higher longitudinal thermal diffusivity than transverse diffusivity—had also been reported previously e.g. in [22]. The study investigates the thermal anisotropy of reinforced carbon fibre/epoxy composites, revealing that the obtained anisotropy ratio was even higher than in our research, primarily due to the enhanced in-plane thermal conductivity achieved through the introduction of high thermal conductive coaxial PAN/PBO carbon fibres.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFUNDING DECLARATION\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eResearch and Development Project No. O ROB 0046 03 001 on “35 mm KDA automatic cannon with a fire control system using the ZGS-158 Integrated Tracking Warhead – made in naval version” subsidized in 2012-2023 by the Polish National Centre for Research and Development.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCOMPETING INTERESTS\u003c/strong\u003e\u003cbr\u003e\u0026nbsp;The authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDATA AVAILABILITY\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eS.T. Peters, \u003cem\u003eHandbook of Composites\u003c/em\u003e, Second edition. Springer New York, NY, 1998. doi: 10.1007/978-1-4615-6389-1.\u003c/li\u003e\n \u003cli\u003eR. Luo, \u0026ldquo;Friction performance of C/C composites prepared using rapid directional diffused chemical vapor infiltration process\u0026rdquo;.\u003cem\u003eCarbon\u003c/em\u003e, vol. 40, no. 8, pp.1279-1285, 2002. doi: 10.1016/S0008-6223(01)00283-4.\u003c/li\u003e\n \u003cli\u003eR. Luo, T. Liu, J. Li, H. Zhang, Z. Chen, and G. Tian, \u0026ldquo;Thermophysical properties of carbon/carbon composites and physical mechanism of thermal expansion and thermal conductivity\u0026rdquo;. \u003cem\u003eCarbon\u003c/em\u003e, vol. 42, no. 14, pp. 2887\u0026ndash;2895, 2004, doi: 10.1016/j.carbon.2004.06.024.\u003c/li\u003e\n \u003cli\u003eO. Siron, G. Chollon, H. Tsuda, H. Yamauchi, K. Maeda, and K. Kosaka, \u0026ldquo;Microstructural and mechanical properties of filler-added coal-tar pitch-based C/C composites: the damage and fracture process in correlation with AE waveform parameters\u0026rdquo;. \u003cem\u003eCarbon\u003c/em\u003e, vol. 38, no. 9, pp. 1369-1389. 2000.\u003c/li\u003e\n \u003cli\u003eG. Grimval, \u003cem\u003eThermophysical Properties of Materials\u003c/em\u003e. Amsterdam: Elsevier, 1986.\u003c/li\u003e\n \u003cli\u003eS. Wiśniewski and T. S. Wiśniewski, \u003cem\u003eHeat Transfer\u003c/em\u003e. Warszawa: WNT Publishing House, 2000 (in Polish).\u003c/li\u003e\n \u003cli\u003eS. Wiśniewski, \u003cem\u003eTechnical Thermodynamics\u003c/em\u003e. Warszawa: WNT Publishing House, 2012 (in Polish).\u003c/li\u003e\n \u003cli\u003eK.D. Maglić, A. Cezairliyan, andV.E. Peletsky, Eds., \u003cem\u003eCompendium of Thermophysical Property Measurement Methods\u003c/em\u003e. New York: Plenum Press, 1984. doi: 10.1007/978-1-4615-3286-6.\u003c/li\u003e\n \u003cli\u003eJ.L. McNaughton and C.T. Mortimer, \u003cem\u003eDifferential Scanning Calorimetry\u003c/em\u003e, IRS; Physical Chemistry Series 2, Volume 10. (London: Butterworths; Norwalk: reprinted by Perkin-Elmer Corp.), 1975.\u003c/li\u003e\n \u003cli\u003eA. Cezairliyan, \u003cem\u003eSpecific Heat of Solids\u003c/em\u003e. Hemisphere Publishing Corporation, 1988.\u003c/li\u003e\n \u003cli\u003eW. Wendlandt, Ed., \u003cem\u003eThermal Analysis\u003c/em\u003e, Third edition. New York: John Willey \u0026amp; Sons, 1986.\u003c/li\u003e\n \u003cli\u003eA.J. Panas and D. Panas, \u0026ldquo;DSC investigation of binary iron-nickel alloys\u0026rdquo;.\u003cem\u003eHigh Temperatures-High Pressures\u003c/em\u003e, vol. 38, no. 1, pp. 63\u0026ndash;78, 2009.\u003c/li\u003e\n \u003cli\u003eA.J. Panas, Comparative-Complementary Investigations of Thermophysical Properties \u0026ndash; High Thermal Resolution Procedures In Practice. In \u003cem\u003eThermophysics\u003c/em\u003e 2010, Brno University of Technology, Faculty of Chemistry, 2010, ISBN 978-80-214-4166-8, pp 218-235.\u003c/li\u003e\n \u003cli\u003eA.J. \u0026Aring;ngstr\u0026ouml;m, \u0026ldquo;NeueMethode, das Warmeleitungsvermogen der Korperzu Bestimmen\u0026rdquo;. \u003cem\u003eAnnalen der Physic und Chemie\u003c/em\u003e, vol. 114, 1861.\u003c/li\u003e\n \u003cli\u003eJ.M. Belling and J. Unsworth, \u0026ldquo;Modified Angstr\u0026ouml;m\u0026rsquo;s method for measurement of thermal diffusivity of materials with low conductivity\u0026rdquo;.\u003cem\u003eReview of Scientific Instruments\u003c/em\u003e, vol. 58, no. 6, pp. 997\u0026ndash;1002, 1987, doi: 10.1063/1.1139589.\u003c/li\u003e\n \u003cli\u003eA.J. Panas, \u0026ldquo;IR Support of Thermophysical Property Investigation - Study of Medical and Advanced Technology Materials\u0026rdquo;. In \u003cem\u003eInfrared Thermography\u003c/em\u003e, InTech, 2012. doi: 10.5772/30397.\u003c/li\u003e\n \u003cli\u003eR. Szczepaniak, A.J. Panas, M. Nowakowski, and J. Panas, \u0026ldquo;Investigation of the Thermal Diffusivity of Water Temperature Oscillation (in Polish),\u0026rdquo; \u003cem\u003eProblemymechatroniki.\u0026nbsp;\u003c/em\u003e\u003cem\u003eUzbrojenie , lotnictwo, inżynieria bezpieczeństwa / Probl. Mechatronics. ArmamentAviat. Saf. Eng.\u003c/em\u003e, vol. 3, no. 3, pp. 85\u0026ndash;98, 2012.\u003c/li\u003e\n \u003cli\u003eA.J. Panas and M. Talkowski, \u0026ldquo;Investigation on directional thermal diffusivity for graphite composite,\u0026rdquo; \u003cem\u003eJournal of KONES Powertrain and Transport\u003c/em\u003e, vol. 23, no. 4, 2016, doi: 10.5604/01.3001.0010.2390.\u003c/li\u003e\n \u003cli\u003eA.J. Panas. \u003cem\u003eet al.\u003c/em\u003e, Investigation of termophysical and thermomechanical properties of aviation epoxy-glass composite (in Polish). In \u003cem\u003eMechanika w lotnictwie ML-XVIII 2018\u003c/em\u003e, pp. 163\u0026ndash;178, 2018.\u003c/li\u003e\n \u003cli\u003eG. Bresson \u003cem\u003eet al.\u003c/em\u003e, \u0026ldquo;Thermographic and tomographic methods for tridimensional characterization of thermal transfer in silica/phenolic composites\u0026rdquo;. \u003cem\u003eCompos B Eng\u003c/em\u003e, vol. 104, pp. 71\u0026ndash;79, Nov. 2016, doi: 10.1016/j.compositesb.2016.08.022.\u003c/li\u003e\n \u003cli\u003eB. Weidenfeller and S. Kirchberg, \u0026ldquo;Thermal and mechanical properties of polypropylene-iron-diamond composites\u0026rdquo;.\u003cem\u003eCompos B Eng\u003c/em\u003e, vol. 92, pp. 133\u0026ndash;141, May 2016, doi: 10.1016/j.compositesb.2016.02.011.\u003c/li\u003e\n \u003cli\u003eM. Hao \u003cem\u003eet al.\u003c/em\u003e, \u0026ldquo;Enhanced both in-plane and through-thickness thermal conductivity of carbon fiber/epoxy composites by fabricating high thermal conductive coaxial PAN/PBO carbon fibers\u0026rdquo;. \u003cem\u003eCompos B Eng\u003c/em\u003e, vol. 229, Jan. 2022, doi: 10.1016/j.compositesb.2021.109468.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"epoxy−glass mat composite, epoxy –graphite fabric composite, thermophysical properties, comparative studies","lastPublishedDoi":"10.21203/rs.3.rs-6629055/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6629055/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe paper concerns comprehensive, complementary studies of thermophysical properties of functional composite structures. The term functional in this case means the study of the structure while maintaining its post-production imperfections, as opposed to the study of material samples prepared solely for this purpose. The paper presents the results of experimental studies, followed by an analysis of thermophysical properties characterizing heat accumulation and anisotropic heat transfer of two types of utility composites. A composite with an epoxy matrix and two types of fillers: glass mat and carbon fabric were studied. The research program included micro- and macrostructural analysis and comprehensive thermogravimetric, microcalorimetric and thermal diffusivity measurements. In the studies of heat transfer phenomena, the directional dependence of properties was taken into account. Attention was focused on maintaining high temperature resolution of measurements, and the effect of repeated temperature exposure was also determined also. The results of the research are the determined quantitative and qualitative characteristics, including the temperature dependence of a set of thermophysical properties of the tested materials. Analysis of the results provides insight into possible design and operational problems of real structures in relation to model data.\u003c/p\u003e","manuscriptTitle":"Comparative analysis of thermophysical properties of functional composites reinforced with epoxy-glass and epoxy-carbon fibers in the context of heat transfer anisotropy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-10 11:23:39","doi":"10.21203/rs.3.rs-6629055/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a39b6583-bfab-429c-9767-70a08ed2bbcf","owner":[],"postedDate":"June 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":49710808,"name":"Physical sciences/Engineering"},{"id":49710809,"name":"Physical sciences/Materials science"},{"id":49710810,"name":"Physical sciences/Physics"}],"tags":[],"updatedAt":"2025-07-16T18:08:13+00:00","versionOfRecord":[],"versionCreatedAt":"2025-06-10 11:23:39","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6629055","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6629055","identity":"rs-6629055","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}