Thermal Influence on the Mechanical Performance and Deformation Characteristics of Symmetric and Asymmetric GFRP Laminates

Abstract

The present study investigated the tensile behavior, failure mechanisms and deformation characteristics of glass fiber-reinforced polymer (GFRP) composites with symmetric [0°/90°/90°/0°] and asymmetric [0°/90°/0°/90°] stacking sequences across a temperature range of 30–150 °C. Tensile testing revealed superior mechanical performance in the symmetric lay-up, with higher tensile strength and failure strain sustained across elevated temperatures. Failure mode analysis revealed a transition from ductile failure to brittle failure with increasing temperature, which was more pronounced in the asymmetric lay-up, along with increased delamination and reduced fiber pull-out. Failure surface examination supported these findings, revealing better interfacial bonding and matrix integrity in the symmetric lay-up. Deformation analysis further confirmed a more homogeneous distribution of strain and longer failure time in symmetric laminates. Across all the metrics, including toughness, energy absorption, and strain uniformity, the symmetric configuration outperformed the asymmetric counterpart, underscoring the critical role of balanced stacking in enhancing the thermal durability. The observed temperature-induced degradation and its impact on mechanical and failure behavior emphasize the need for temperature-sensitive design strategies in GFRP-based structures.

1. Introduction

The aerospace and automotive industries are increasingly adopting lightweight structures. Fiber-reinforced polymers (FRPs) represent an advanced option, with standard metal alternatives being heavier in terms of their attributes. A high strength-to-weight ratio makes FRPs perfect for applications with weight-saving needs and structural reliability [1,2]. They also deliver excellent vibration and noise dampening, improving passenger comfort while lowering operational noise levels [3]. Their high-impact energy absorption enhances safety by reducing crash damage [4,5]. FRPs also have excellent fatigue resistance, which ensures long-term durability under cyclic stress [6,7]. Furthermore, their corrosion resistance enhances the lifespan of components in severe environments, which may include extremely high and low temperatures and humidities, especially in the aerospace industry [7]. For these reasons, the characteristics of FRPs make them very suitable materials for manufacturing very lightweight, fuel-efficient, and high-performance vehicles and aircraft [8].

Among the many fiber-reinforced polymers, glass fiber-reinforced polymers (GFRPs) have emerged as a cornerstone in the field of applied sciences, mainly owing to their superior strength-weight ratio, corrosion resistance, and design flexibility. GFRPs are made of glass fibers embedded in polymer substrates and find applications in aviation, automotive, maritime, and civil engineering [9]. The mechanical properties of GFRPs, primarily tensile performance, are fundamental to their structural performance. Therefore, mechanical properties are sensitive to environmental variables such as temperature, which can drastically change a material’s response when loaded [10]. Therefore, understanding how temperature affects the performance of GFRPs is crucial, as this understanding will increase the possibility of design and increase the reliability of GFRPs when used in real utilitarian applications.

Many researchers have extensively investigated the mechanical behavior of GFRPs under various environmental conditions. For example, Torabizadeh et al. [11] investigated the tensile, compressive, and in-plane shear strengths of unidirectional (UD) GFRP composites by varying the temperature (25 °C, −20 °C, and −60 °C). Experimental testing has indicated that the failure mode of these composites is significantly affected by a decrease in temperature. The strength and modulus increased with decreasing test temperature for all loading modes; however, the strain to failure decreased with decreasing temperature. Piyush et al. [12] examined the tension and compression creep response of GFRP composites under constant loading at temperatures of 25 °C, 50 °C, and 80 °C. At 25 °C, the composite continued to strain under creep loading indefinitely. In contrast, failure occurred within an hour at higher temperatures after 80% of the ultimate stress was reached. Hawileh et al. [13] studied the tensile strength and elastic modulus of carbon, glass, and carbon–glass laminates at temperatures up to 300 °C. After the temperature was maintained for 45 min, followed by 24 h of cooling, the carbon, glass, and carbon–glass sheets experienced elastic modulus reductions of 28%, 26%, and 9%, respectively, and tensile strength reductions of 42%, 31%, and 35%, respectively. The failure modes revealed that brittle rupture was dominant in the 100–150 °C range, whereas in the 200–250 °C range, the loss of epoxy and fiber splitting dominated. Hamad et al. [10] investigated the effects of high temperatures (up to 450 °C) on the mechanical performance of basalt fiber-reinforced polymers (BFRPs), carbon fiber-reinforced polymers (CFRPs), and GFRP bars, as well as their bonding characteristics with concrete. GFRP and BFRP completely melted at 450 °C, losing all the tensile strength. The tensile strength and elastic modulus of the CFRP bars decreased by approximately 55% and approximately 30%, respectively, at 325 °C. The interfacial bonding strength between the concrete and FRP bars was reduced by approximately 81.5% at 325 °C. An empirical model was suggested for predicting the heating bond stress-slip curve, with a perfect correlation with the test results. Ashrafi et al. [14] investigated the tensile properties of GFRP and CFRP bars with 4, 6, 8, and 10 mm diameters at a high temperature of up to 450 °C. In this study, FRP bars at 450 °C lost approximately 50–70% their tensile strength. In addition, Ashrafi et al. reported that FRP bars with larger diameters maintained greater tensile strength than smaller-diameter bars when exposed to elevated temperatures. Hamzeh et al. [15] evaluated the tensile strength of GFRP bars subjected to different temperatures ranging from 25 °C to 500 °C while loading the composite between 25% and 70% of tensile strength. According to their findings, GFRP bars can retain the design service stress level of 25% of their original tensile strength up to at least 400 °C; hence, this temperature can be regarded as the critical temperature in design. Milad et al. [16] studied the long-term mechanical behavior of GFRP pultruded structural sections exposed to harsh environmental conditions such as seawater at different temperatures, wetting and drying cycles, and alkaline and acidic solutions. These findings indicate that immersing the specimens in an alkaline solution caused a significant loss in mechanical strength, whereas higher seawater temperatures led to a more substantial loss in strength. Allan Manalo et al. [17] studied the temperature-dependent mechanical properties of GFRP composites in the longitudinal and transverse directions via three-point static bending tests at a maximum temperature of 200 °C. The results indicated that the shear modulus of the specimens in the transverse direction decreased with increasing temperature.

The orientations of the fibers and the stacking sequence of the GFRP laminates also impact their mechanical performance. Landesmann et al. [18] conducted a mechanical analysis of detailed tension, compression, flexural bending, pin-bearing, and interlaminar shear deformation tests on a GFRP structural element. The obtained results showed that GFRP can be used as a structural element. Yunfu Ou et al. [19] used a servo-hydraulic high-rate testing machine to examine the effects of strain rate (25, 50, 100, and 200 s⁻¹) and temperature (−25, 0, 25, 50, 75, and 100 °C) on the mechanical properties and failure modes of unidirectional GFRPs. Tests were conducted on glass yarn at strain rates of 40, 80, 120, and 160 s⁻¹ and at temperatures of 25, 50, 75, and 100 °C, respectively; the Weibull statistics model was used to assess the stress-strain behavior and determine the tensile strength variability for engineering applications. Shaohua et al. [20] investigated the effects of fiber orientation on the tensile strength and elastic modulus of pultruded GFRP-based specimens with fiber angles ranging from 0° to 90° under uniaxial tensile loading. A generalized Hankinson’s formula for predicting the off-axis properties was validated with literature results, rendering it available in structural design guides for pultruded GFRPs. H.W. Wang et al. [21] studied the Young’s modulus of unidirectional GFRP composites for wind energy application through analytical, numerical, and experimental approaches. Fiber orientation angles between 0° and 90° were considered, and it was reported that the Young’s modulus varied with fiber inclination and the volume fraction of the glass. Finite element simulations in ABAQUS were performed, and the contribution of the shear modulus to the stiffness further suggested building design guidelines to optimize the GFRP microstructure. Rizal et al. [22] studied the long-term performance and durability of pultruded glass fiber-reinforced polymer (p-GFRP) composites for cross-arms in transmission towers, incorporating the effects of a fiber layer stacking sequence. Five different stacking sequences of fibers were examined in quasistatic and creep tests in four-point bending mode. The best configuration, with nine layers in the sequence [0°/45°/0°/−45°/0°/−45°/0°/45°/0°], presented the highest ultimate flexural strength, low-creep deflection, and high elastic and apparent creep moduli and was thus suitable for structural applications over time. Viren Modi et al. [23] reviewed the interlaminar shear stress (ILSS) performance of multidirectional GFRP laminates prepared with various stacking sequences from bidirectional plain-weave glass fabric. Five types of laminates, with stacking sequences of [0°/(0°)₂/0°]_S, [0°/(15°)₂/0°]_S, [0°/(30°)₂/0°]_S, [0°/(60°)₂/0°]_S, and [0°/(75°)₂/0°]_S, were tested in terms of the ILSS following the ASTM D2344-16 standard, with a strain rate of 1 mm/s. The results indicated that the ILSS varied with respect to ply orientation, with the 0°-ply laminates showing the minimum ILSS, whereas the maximum ILSS was observed for the 60°-ply laminates.

On the basis of in-depth reviews of previous works, GFRP has shown superior mechanical properties. However, few studies have been conducted on the failure modes and variations in tensile properties concerning different stacking sequences and temperatures. Despite these advancements, a sizable void remains in the research regarding how specific stacking sequences, specifically those with symmetric and asymmetric configurations, affect the tensile properties of GFRPs across varying temperature ranges. Although independent studies on the effects of temperature and stacking sequence had previously taken place, there were very few instances where the same perspective on their interaction was considered with respect to the tensile behavior of GFRPs. The temperature-dependent behavior of GFRPs with different stacking sequences has not been explored in detail. Understanding these gaps is crucial in developing more performance-oriented GFRP products for temperature-sensitive applications. This research intends to investigate the influence of varying stacking sequences and temperature conditions on the mechanical properties of GFRP. This research seeks to bridge these gaps by conducting tensile tests on symmetric and asymmetric GFRP composites at room temperature (RT) and elevated temperatures (50 °C, 100 °C, and 150 °C). The maximum testing temperature was limited to 150 °C, as a previous study showed that the glass transition temperature of Epoxy A45 was 105 °C [24]. Digital Image Correlation (DIC) was used to measure the full-field strain and displacement data, allowing for a detailed examination of deformation patterns and failure modes. The microstructural features of the failed specimens were examined in greater detail by using scanning electron microscopy (SEM), providing some understanding of how the failure modes correlate with the temperature and stacking sequence. By combining these methods, the present study aims to investigate the temperature-dependent tensile properties of GFRPs, providing valuable insight into the tailoring of composite materials for high-strength temperature-sensitive applications. This research contributes to the development of thermally durable composite materials and supports the United Nations Sustainable Development Goals, particularly by advancing resilient material systems, encouraging longer service life and resource efficiency, and promoting sustainable material choices that address climate challenges.

2. Materials and Methods

The present study focuses on understanding the effects of temperature on the tensile properties of symmetric and asymmetric GFRP laminates. The laminates were prepared by stacking glass fiber prepregs. The GFRP prepreg consisted of a unidirectional E-glass fiber with a density of 2.52 g/cm³ and an Epoxy A45 resin system with a density of 1.15 g/cm³, which was procured from Bhor Chemicals and Plastics Pvt. Ltd., Nasik, India. The E-glass fiber and A45 resin moduli were 80 GPa and 3.1 GPa, respectively. The prepregs were prepared with fiber and matrix volume fractions of 65% and 35%, respectively.

The laminates were manufactured via a hand lay-up process followed by autoclave (Unique Chemoplant Equipment, Mumbai, India) curing, which involved stacking the prepreg sheets in a metallic mold. In the present study, the laminates consisting of 4 layers were made of two stacking sequences: symmetric [0°/90°/90°/0°] and asymmetric [0°/90°/0°/90°]. To prepare the laminate, the release agent was sprayed onto the mold surface, and then, the prepreg laminas of 250 × 250 mm were placed according to the desired stacking sequences, followed by the peel ply and breather. After the completion of lay-up, the laminates were subjected to a debulking process via a vacuum-bagging technique and kept in the autoclave for curing, as shown in Figure 1. The curing process of the laminates is carried out in two stages on the basis of data provided by the supplier, and the curing cycle adopted in the process is shown in Figure 2. In the first stage and second stage of the heating cycle, the temperature was increased at constant rates of 2 °C/min and 1 °C/min, respectively. After the final dwell period of the second stage, the laminates were cooled to 80 °C at a rate of 1 °C/min. The specimen is allowed to cool to room temperature inside the autoclave.

After laminate preparation, dumbbell-shaped tensile specimens were prepared according to ASTM D638 via water jet machining at the Stonemax waterjet cutting center in Kochin, India. A schematic and photograph of the test specimen are shown in Figure 3. A tensile specimen has a gauge length of 80 mm, width of 12.29 ± 0.13 mm and thickness of 1.21 ± 0.22 mm.

The physical properties, specimen dimensions and test conditions of the symmetric and asymmetric laminates are reported in

Table 1. Tensile test conditions and specimen details.

Stacking Sequence	Average Thickness (mm)	Density (g/cm3)	Temperature (°C)	Gauge Length (mm)	Strain Rate (mm/min)
[0°/90°/90°/0°]	1.21	1.37 ± 0.09	RT	80	2
			50
			100
			150
[0°/90°/0°/90°]	1.21	1.39 ± 0.13	RT	80	2
			50
			100
			150

.

A hydraulic 50 kN universal testing machine (BiSS Bengaluru, Karnataka, India) was used to conduct the tensile tests. The test was conducted in displacement mode at a 2 mm/min loading rate, and the data were captured via a data acquisition system at 30 Hz. To obtain the average failure stress and failure strain, five specimens were used in each test cases. For specimens to be tested at different temperatures, they were first heated in the chamber at a rate of 2 °C/min. After the temperature stabilized, a tensile test was carried out.

The DIC technique has become a popular and reliable approach for dynamic testing of composite materials. By comparing the digital images before and after deformation, DIC allows direct estimation of displacement and strains with pixel accuracy [25,26,27,28]. To implement DIC technology to capture strain development in the specimen, speckle patterns were created on the specimen. A uniform layer of white paint was first sprayed over the surface, followed by a light spray of black paint to generate random black dots of uniform size, creating speckle patterns. A Basler acA4096-40um USB 3.0 camera with a Sony IMX255 CMOS sensor procured from AlphaTechSys Automation, Pune, India, was used to measure the strain and Poisson’s ratio. During the tensile tests, images were captured at 30 fps, and strain map images of the specimen were generated via the open-source MATLAB-based digital image correlation software Ncorr 1.2.2. The transverse and longitudinal strain data were subsequently extracted with Ncorr Post software by placing virtual extensometers at three distinct positions along and across the loading direction. The average strain values obtained in each direction were then used to calculate the Poisson’s ratio of the composite.

The tensile failure surface morphology of the symmetric and asymmetric laminates tested at room temperature was studied via a Zeiss EVO MA18 SEM apparatus at 2500Xx magnification (Zeiss, Oberkochen, Germany). The images obtained from the SEM analysis were compared with the tensile failure behaviors of the specimens.

3. Results and Discussion

Tensile characterization of symmetric and asymmetric GFRP laminates under the influence of temperature was carried out in the present study. The effects of temperature on the Young’s modulus, failure strain, failure stress, and failure behavior were studied. The subsequent sections discuss the stress strain behavior, Young’s modulus variation, failure modes of the laminates, strain mapping via DIC and failure surface morphology of the symmetric and asymmetric GFRP laminates.

3.1. Stress-Strain Behavior

The stress-strain response of the different stacking sequences was examined to determine the deformation behavior under tensile loading. Figure 4a,b show the stress-strain behavior of both laminates. Compared with the asymmetric stacking sequence, the symmetric stacking sequence resulted in greater failure strain and failure stress at room temperature, as shown in Figure 4a. With increasing temperature, the stress-strain behavior of the stacking sequences decreased in strength and stiffness. The symmetric stacking sequence at any temperature had better stress-strain behavior than did the asymmetric stacking sequence, reflecting improved thermal stability. The failure stress, the stress at which a material fails by any mode of failure, decreases with increasing temperature for both lay-up orientations, indicating a loss of load-carrying capability.

Table 2. Mechanical properties of symmetric and asymmetric GFRPs at different temperatures.

Testing Temperature	Stacking Sequence	Young’s Modulus (GPa)	Failure Strain (mm/mm)	Failure Stress (MPa)
RT	Symmetric	12.45 ± 0.93	0.0434	286.29 ± 39.64
	Asymmetric	12.44 ± 0.67	0.0345	284.72 ± 28.38
50 °C	Symmetric	11.38 ± 0.64	0.0356	257.02 ± 28.13
	Asymmetric	10.52 ± 0.53	0.0373	249.92 ± 21.08
100 °C	Symmetric	10.69 ± 0.58	0.0463	243.58 ± 21.82
	Asymmetric	10.16 ± 0.84	0.0422	249.81 ± 32.73
150 °C	Symmetric	9.67 ± 0.29	0.0427	220.31 ± 19.63
	Asymmetric	9.76 ± 0.97	0.0354	217.97 ± 27.34

shows the variation in the mechanical properties of the symmetric and asymmetric laminates with temperature. From Table 2, the failure stresses at RT are appreciably equal for the symmetric and asymmetric orientations at 286.29 ± 39.64 MPa and 284.72 ± 28.38 MPa, respectively. However, at 150 °C, the failure stress of the symmetric configuration decreases to 220.31 ± 19.63 MPa, which is marginally greater than that of the asymmetric configuration, 217.97 ± 27.34 MPa. At all temperatures, the failure stress of the symmetric lay-up configuration was consistently lower than that of the asymmetric configuration, indicating that the symmetric lay-up exhibited more uniform and reliable mechanical performance. The stress-strain behaviors (Figure 4) also depict these trends, with the symmetric configuration showing a consistently better response, higher failure strain and marginally greater strength throughout the range of temperatures. The postpeak stress drop increases at elevated temperatures, more so at 150 °C, indicating a change toward brittle failure with increasing temperature.

The failure strain, which indicates the ductility of a material, varies with temperature and lay-up configuration. For the symmetric lay-up configuration at RT, there was a failure strain of 0.0434 mm/mm, which then increased to 0.0463 mm/mm at 100 °C (Table 2), indicating improved ductility at elevated temperatures. The asymmetric stacking sequence had a failure strain of 0.0345 mm/mm at RT and 0.0422 mm/mm at 100 °C. At 150 °C, the failure strain of the symmetric configuration was 0.0427 mm/mm, which was greater than that of the asymmetric configuration at 0.0354 mm/mm. This reveals that the symmetric lay-up configuration is more resistant to deformation under high-temperature loading. The superior ductility at higher temperatures shows a much better performance of the symmetric configuration in applications where deformation resistance is critical.

The knee point, or the onset of matrix failure (e.g., cracking), was determined from the stress-strain curves (Figure 4a,b) through a bilinear fit method in Origin Lab. The knee point stress and strain for the symmetric and asymmetric laminates are shown in

Table 3. Knee Point Strain and Stress for Symmetric and Asymmetric GFRP Laminates.

Testing Temperature	Stacking Sequence	Knee Point Strain	Knee Point Stress (MPa)
RT	Symmetric	0.0260	171.77
	Asymmetric	0.0207	170.83
50 °C	Symmetric	0.0214	154.21
	Asymmetric	0.0224	149.95
100 °C	Symmetric	0.0278	146.15
	Asymmetric	0.0253	149.88
150 °C	Symmetric	0.0256	132.19
	Asymmetric	0.0212	130.78

. The symmetric laminate tested at RT exhibited knee points at 0.0260 strain and 171.77 MPa stress. In contrast, the asymmetric laminate exhibited a knee point at 0.0207 and 170.83 MPa, which is indicative of premature matrix failure for the asymmetric laminate because of its unbalanced lay-up. With increasing temperature to 150 °C, the knee point values decreased to 0.0256 and 132.19 MPa (symmetric) and 0.0212 and 130.78 MPa (asymmetric), indicating matrix softening [29,30,31]. Symmetric laminates in all the cases presented higher knee point strains, indicating greater resistance to initial matrix failure, especially at high temperatures [32].

3.2. Young’s Modulus

Figure 5 shows the variation in the Young’s modulus with temperature for both stacking sequences and highlights the stable stiffness of the symmetric configuration at elevated temperatures. The Young’s modulus, a stiffness measure of materials, decreases with increasing temperature for both lay-up sequences [16,30,31], reflecting the thermal softening of the matrix. Tests at RT revealed relatively similar values of Young’s modulus in the symmetric and asymmetric lay-up sequences, with values of 12.45 ± 0.93 GPa and 12.44 ± 0.67 GPa, respectively. As the temperature increased to 150 °C, the Young’s modulus of the specimen in the symmetric lay-up sequence decreased to 9.67 ± 0.29 GPa, and that of the samples in the asymmetric lay-up sequence decreased to 9.76 ± 0.97 GPa. The results also revealed that the symmetric lay-up sequence was slightly more stable with increasing temperature, as indicated by the lower standard deviation over all temperatures, particularly at 150 °C (0.29 GPa compared with 0.97 GPa for the asymmetric configuration).

3.3. Failure Modes of the Laminates

Tensile tests on GFRP composites with symmetric and asymmetric stacking sequences revealed unique failure patterns across RT, 50 °C, 100 °C, and 150 °C, as reflected in the fractured specimens shown in Figure 6a–h. For the symmetric laminate, the specimen at RT (Figure 6a) showed extensive fiber pull-out with long, projecting fibers that signify extensive energy absorption via progressive fiber–matrix debonding. This type of behavior reflects the ability of the balanced structure to carry tensile stress equally between the 0° and 90° plies, retarding catastrophic failure. At 50 °C, the specimen (Figure 6b) transitioned to a combination of splitting and fiber pull-out with a decreasing pull-out length when the matrix softened, indicating the onset of thermal effects on interfacial adhesion. At 100 °C and 150 °C, specimens (Figure 6c,d) exhibited little fiber pull-out, with cleaner, more brittle fracture due to matrix softening close to the glass transition temperature (T_g) (100–150 °C). The Tg plays a crucial role in determining the mechanical performance of thermoset polymer composites. In this study, the Tg was not directly measured for the fabricated laminates; however, according to the differential scanning calorimetry (DSC) analysis reported by Shah et al. [24] For the same epoxy system (Epoxy A45), the matrix reported a Tg of approximately 105 °C. As the test temperature increases above the T_g, the mobility of the epoxy molecules increases in the matrix material and becomes softer, and the load transfer decreases. Additionally, exposure to higher temperatures reduces the interfacial strength of the fiber/matrix interphase. This initiates and accelerates the formation of microcracks at the interface. This degradation of the fiber-matrix interface led to a transition from ductile failure to brittle failure, with delamination becoming more pronounced at 150 °C, indicating a temperature-dependent reduction in the load-carrying ability of the matrix [32,33,34,35,36].

For the asymmetric lay-up, the specimen (Figure 6e) at RT experienced moderate fiber pull-out, which was less severe than its symmetric counterpart because the nonmirrored stacking sequence resulted in an uneven stress distribution and more sudden initiation of failure. At 50 °C, the specimen (Figure 6f) exhibited fiber splitting with minimal pull-out, reflecting premature matrix softening and decreased interfacial bonding in the unbalanced structure. The specimens deformed in a brittle fashion at 100 °C and 150 °C, with minimal fiber pull-out since the softened matrix lost its ability to engage fibers properly, resulting in a high level of delamination as well as tidy fracture surfaces (Figure 6g,h). The asymmetry of lay-up is susceptible to localized damage and increases further at higher temperatures. In this case, the lack of symmetry accelerated stress buildup and matrix failure. Symmetric and balanced glass/epoxy laminates have better mechanical performance because their design eliminates bending–extension coupling, allowing stresses to be distributed more evenly across the thickness and delaying layer-by-layer failure. This symmetry also helps reduce the interlaminar stresses at the edges, lowering the chances of delamination and related strength loss. In addition, the mirrored ply arrangement ensures that loads are shared more uniformly between layers, leading to consistent stiffness, whereas the balanced configuration supports gradual and predictable damage progression, enhancing the laminate’s overall toughness and reliability [34,35,36]. These failure modes tend to highlight the combined influence of the stacking sequence and temperature on the behavior of GFRP composites. Owing to its balanced stress distribution, the symmetric lay-up consistently exhibited greater fiber pull-out and energy absorption at lower temperatures. In contrast, the asymmetric lay-up exhibited more abrupt and localized failure due to its structural imbalance. The increased temperatures reduced the fiber pull-out of both lay-up configurations, which shifted toward brittle failure with increasing matrix degradation at higher temperatures, particularly above 100 °C, when the glass transition effect began to dominate.

3.4. Strain Mapping Using DIC

DIC analysis was conducted on symmetric and asymmetric stacking laminates at RT to study the strain distribution during tensile loading. Figure 7a shows the symmetric lay-up specimen before testing. Figure 7b shows the strain map image just after loading was initiated, revealing the scattered strain distribution throughout the specimen in the early loading stage. The DIC strain map of the specimen just before failure is shown in Figure 7c, which reveals a failure-prone zone near the gripping end, where the fracture initiates before failure. The DIC analysis indicated a relatively consistent strain distribution over the gauge length, with a minimum strain of 0.0086 and a maximum strain of 0.0280. The strain maps indicate zones of localized high strains close to the fracture location, which are correlated with the widespread fiber pull-out and matrix cracking detected via SEM analysis. Lay-up symmetry enables a smooth strain gradient, indicating effective load sharing and energy dissipation, which is consistent with the ductile failure mode at RT. Localized strain concentrations at the ply interfaces and minor delamination, can be expected as previously reported.

For the asymmetric lay-up, Figure 8a,b show the specimen before testing and a strain map image just after the application of the load begins. Figure 8c shows the strain map image just before failure. The strain maps in specimen with an asymmetric lay-up are relatively inhomogeneous compared to the specimen with a symmetric lay-up. DIC showed a more inhomogeneous strain distribution, with a minimum value of 0.0027 and a maximum value of 0.0302. The strain maps show distinct high-strain locations close to the fracture, agreeing with the moderate fiber pull-out and localized matrix cracking observed via SEM, indicative of sudden failure initiation due to stress concentrations. The nonuniform strain pattern indicates decreased load-sharing effectiveness, increasing the propensity of the asymmetric lay-up for brittle behavior at room temperature, whereas the strain concentration at the ply interfaces aggravates damage development.

The DIC results confirm that the symmetric lay-up is better than the asymmetric lay-up in terms of strain homogeneity. The balanced configuration of the symmetric lay-up makes it possible to achieve a more homogenized deformation that leads to toughness. In contrast, the structural asymmetry of the asymmetric lay-up leads to localized strain peaks and is prone to premature failure. The failure time, measured on the stress-strain curve at a gauge length of 250 mm and a crosshead velocity of 2 mm/min, is exactly 314.25 s (5 min 14.25 s) for the symmetric lay-up and 261.75 s (4 min 21.75 s) for the asymmetric lay-up, the latter failing first owing to its prior strain localization and reduced load-carrying capability. Figure 9 and Figure 10 show the graphs of the transverse versus longitudinal strain values of the symmetric and asymmetric lay-up specimens, respectively. Poisson’s ratio is the slope obtained from the linear fitting of the experimental data, which is 0.16 and 0.14 for the symmetric and asymmetric lay-up specimens, respectively.

3.5. Failure Surface Morphology

The failure surface morphology of the tensile fractured specimens at RT for both the symmetric and asymmetric specimens studied via SEM is shown in Figure 11a and b, respectively. For the symmetric lay-up, SEM micrographs (Figure 11a) indicate widespread pull-out of the fibers, with long, extending fibers observed in a matrix with gradual debonding. The uniform distributions of the 0° and 90° planes allow for balanced stress transfer, resulting in a rough fracture surface with visible matrix cracking and fiber-matrix interfacial separation. The energy-absorbing pull-out mechanism, characterized by the exposed fiber length, demonstrates firm interfacial bonding at room temperature.

The failure surface of the asymmetric lay-up revealed (Figure 11b) significant fiber pull-out with reduced fiber lengths and a rougher fracture surface than those of the symmetric equivalent. The asymmetric stacking sequence results in a nonuniform stress distribution with matrix cracking and sharp interfacial failure localization, as evidenced in Figure 6e. The shorter pull-out length indicates poorer fiber–matrix adhesion, which is caused by stress concentrations at the ply interfaces that are enhanced by the absence of symmetry. Matrix debris and fiber splitting occur occasionally, which indicates that the asymmetric lay-up undergoes more sudden energy release during failure, as seen from its tendency toward localized damage.

Compared with the SEM images, the symmetric lay-up results in a more ductile failure mode with longer fiber pull-out and interfacial debonding owing to its balanced stress state, which facilitates greater energy absorption at room temperature. The asymmetric lay-up, however, results in more brittle failure with shorter pull-out lengths and localized damage owing to its structural imbalance, which exacerbates stress concentrations. These microstructural variations support the macroscopic trends of tensile testing, in which the symmetric specimens (Figure 6a) performed better than the asymmetric specimens (Figure 6e) in terms of energy dissipation. The results indicate that for the RT usage symmetric lay-ups are optimal with increased toughness. These micrograph observations suggest that asymmetric configurations may require reinforcement or redesign to prevent premature failure, providing valuable directions for improving and optimizing GFRP composite designs.

4. Conclusions

This paper presents a comprehensive study of the tensile behavior, failure modes, microstructural properties, and deformation characteristics of GFRP composites with symmetric [0°/90°/90°/0°] and asymmetric [0°/90°/0°/90°] stacking sequences, tested according to ASTM standards at temperatures ranging from 30 °C to 150 °C, as described by the tensile properties, failure mode, microstructural observations, and deformation analysis. The following conclusions are drawn from the studies of the test results:

• The tensile tests revealed a greater ultimate tensile strength (286.29 ± 39.64 MPa at 30 °C, reducing to 220.31 ± 19.63 MPa at 150 °C) and higher failure strain (0.0434 at 30 °C, having an optimum value of 0.0463 at 100 °C) in the symmetric lay-up than in the asymmetric lay-up (284.72 ± 28.38 MPa at 30 °C, reducing to 217.97 ± 27.34 MPa at 150 °C, with a failure strain of 0.0345 at 30 °C), indicating better mechanical behavior and heat resistance at higher temperatures.
• The failure behavior exhibited a shift from ductile failure with fiber pull-out over a large area in the symmetric lay-up at 30 °C to brittle failure with very little pull-out and extensive delamination at 150 °C, with the asymmetric lay-up exhibiting some pull-out of the fibers at 30 °C, moving toward brittle failure with little pull-out and high delamination at 150 °C, indicating the influence of the stacking sequence on failure behavior.
• Microstructural findings were confirmed at 30 °C, with the symmetric lay-up exhibiting a considerable length of fiber pull-out and matrix cracking due to excellent interfacial bonding. In contrast, the asymmetric lay-up exhibited a smaller pull-out length and localized matrix cracking, reflecting lower adhesion and a tendency toward brittleness under ambient conditions.
• The deformation analysis verified a homogeneous strain distribution for the symmetric lay-up with an upper strain of 0.0419 and a failure time of 314.25 s versus a nonhomogeneous strain distribution for the asymmetric lay-up with an upper strain of 0.0349 and a failure time of 261.75 s, verifying the greater deformation capacity of the symmetric lay-up and the early failure of the asymmetric lay-up at 30 °C.
• Compared with the symmetric lay-up samples, all the asymmetric lay-up specimens showed lower toughness, energy absorption, and strain uniformity. Overall, symmetric stacking improved the tensile properties, failure resistance, microstructural performance, and deformation behavior of the samples, increasing their durability. In contrast, the structural imbalance in the asymmetric lay-up led to premature failure, necessitating potential reinforcement for high-temperature applications.
• Between 30 °C and 150 °C, the matrix gradually weakened, leading to a shift from ductile failure to brittle failure. This transition, confirmed by microstructural deterioration and strain localization during deformation, highlights the need for temperature-dependent design strategies in GFRP applications exposed to high-temperature conditions. Together, these results highlight the dominant position of the stacking sequence in optimizing GFRP composite performance under changing thermal conditions, offering a basis for designing resilient temperature-resistant structures.