Dynamic Mechanical Performance of Glass Microsphere-Loaded Carbon Fabric–Epoxy Composites Subjected to Accelerated UV Ageing

Abstract

This study investigates the effects of incorporating glass microspheres (GMSs) as fillers in carbon fabric–epoxy composites (CFECs) on their degradation behavior under environmental conditions such as moisture and ultraviolet rays. The GMS-filled composites were subjected to accelerated ageing and evaluated using dynamic mechanical analysis (DMA), the Charpy impact test, and inter-laminar shear strength (ILSS) tests. The results indicate that the addition of GMS fillers significantly improves the stiffness and viscoelastic behavior of the composites. However, the impact strength of the composites decreases with the addition of GMS fillers and accelerated ageing. The ILSS results demonstrate that the addition of GMS fillers improved the interfacial bonding between the carbon–epoxy matrix and fillers. This study provides insights into the mechanical properties of GMS-filled carbon–epoxy composites.

1. Introduction

Carbon fabric-reinforced epoxy composites (CFECs) are widely used in the aviation and automobile sector due to their high specific strength, superior fatigue resistance, and high stiffness properties. CFECs are not prone to corrosion and have a low coefficient of thermal expansion (CTE), preventing the weakening of the composite at high temperatures. Despite these magnificent properties, the longevity of carbon fabric–epoxy composites, especially in extreme conditions, remains a problem. The scientific community has carried out multiple investigations on the effect of UV (ultraviolet) rays [1], moisture, and temperature on CFECs [2]. Kumar et al. studied the effect of environmental ageing on composites; their study showed that aged carbon fiber composites showed a 29% decrease in the transverse properties of composites, while the longitudinal properties of the composites were unaffected [3]. Degradation of the composites accelerated when exposed to elevated temperatures [4]. Thermal cycling can induce cracks in laminates, and crack propagation may increase in oxidative environments [5]. Wang et al. studied the low-velocity impact damage behaviors of 3D angle-interlock woven carbon fiber/epoxy composites under accelerated aging at 90 °C and 180 °C temperature in an air environment [6]. Another group of researchers investigated the effect of long-term thermal aging on carbon fiber/epoxy composites. They found that the (inter-laminar shear strength) ILSS initially increased but then decreased with prolonged aging. Microstructural analysis revealed that thermal aging had little effect on the interface, contributing to better residual shear properties [7].

Siriruk et al. investigated the effect of the sea environment on the fatigue properties of composites; an 85% reduction in the fatigue strength of carbon fiber vinyl ester composites was reported [8]. Barbosa et al. carried out accelerated aging of carbon–epoxy composites under controlled conditions of temperature, humidity and UV radiations. Accelerated ageing of carbon fiber composites resulted in a decrease in glass transition temperature (T_g), while ILSS increased with ageing [9]. Sheng et al. examined the multi-scale damage in 3D braided CFECs caused by a γ-irradiation environment to the matrix, interfaces, and near interface regions at the atomic, microscopic, and macroscopic levels [10].

Boubakri et al. investigated the effect of accelerated ageing on thermoplastic polymers; they reported that short UV ageing of just 6 h significantly affected the mechanical properties of the polymers. The elastic modulus and stress at 200% strain decreased and the elastic modulus dropped by 14.7%, while stress reduced by 26.7% [11].

The degradation of carbon–epoxy composites occurs primarily due to moisture and UV rays, as moisture causes irreversible chemical changes in the polymer matrix, that lead to its poor mechanical properties. Capillary action along fibers accounts for a significant proportion of moisture uptake. Shrinkage of resin away from fibers during curing is a contributing factor in poor fiber–matrix interface leading to loss of efficiency in load transfer. In addition, ultraviolet rays cause photo-oxidation or weathering of polymeric composites. The UV ray spectrum comprises wavelengths between 290 nm and 400 nm, which corresponds to energy between 415 and 300 kJ/mol. These energies are in a range of different organic material bond energies. Chemical reactions are induced when a specific group absorbs ultra-violet radiation and free radicals are liberated in the process, triggering further reactions. The negative effect will be dependent on environmental factors, including temperature, humidity, and exposure time [11,12].

Several investigations have been carried out regarding the effect of environmental degradation on composites [13,14,15,16,17]. However, fewer studies have been carried out regarding the available methods to minimize the effect of ageing on the mechanical properties of composites. Yesu et al. investigated the effect of carbon nano tube (CNT) addition on the fracture toughness of CFECs after exposure to accelerated weathering. It was found that CNTs significantly improved the resistance of CFECs to weathering-induced degradation in fracture toughness. Even after prolonged exposure, CNT-modified CFECs maintained comparable fracture toughness [18].

The incorporation of inorganic fillers in carbon–epoxy composites could be a way of limiting the effect of ageing on carbon fabric composites. Khalili et al. reported that the addition of glass micro powders into epoxy resin raised the stiffness and bending strength of composites [19]. In another study, a glass/epoxy composite reinforced with silica particles showed lower water absorption and 2-wt% SiO₂ particles showed a 13.82% increase in the storage modulus of neat samples [20]. Alamri et al. reported in their study that addition of nanofiller reduced the water uptake and increased fracture toughness and impact strength compared to the wet neat epoxy [21]. Krishnarao studied the effect of hollow glass microspheres (HGMs) on the transverse properties of glass fiber-epoxy composites filled with HGMs. They reported that the addition of HGMs to the composite material resulted in a decrease in overall density, while the thermal stability and T_g were minimally affected [22]. Paramasivam also studied the effect of HGM on the tensile and flexural properties of glass fiber–epoxy composites subjected to various temperatures in the range of 40 °C to 120 °C. A decrease in the tensile and flexural properties has been reported with an increase in both HGM content and temperature [23].

In addition, numerous theoretical models explain the increase in mechanical properties of an epoxy matrix reinforced with ceramic fillers. These inorganic fillers cause crack deflection, twisting and tilting of cracks in the matrix around the filler material. Crack pining is another model in which the crack is pinned by filler due to the greater filler size than the crack. This model is mostly appropriate for micrometer-size fillers with a crack opening of ~1.7 µm [24].

Degradation of the polymer matrix (oxidation and chain scission, etc.) leads to the physical degradation of composites. The degradation of the matrix is a concern, especially in carbon composites used in high-end applications. Therefore, there is a need to study the effect of environmental factors on these composites and ways to minimize them.

This study focuses on the effect of short-time ageing (7 h) on carbon–epoxy composites reinforced with inorganic fillers. These composites were aged in an accelerated ageing chamber in order to observe the effect of accelerated ageing on the mechanical properties of composites, evaluated in terms of impact strength, inter-laminar shear strength, and dynamic mechanical analysis.

2. Materials and Methods

In this study, woven carbon fabric with a twill weave and an areal weight of 200 g/m² was used as reinforcement. Epikote 816 epoxy resin was used as the matrix material, while Epotec TH 7301 was used as a hardener, and both were mixed in a ratio of 5:3. Glass microspheres (GMSs) with a size range of 9–13 μm (obtained from Aldrich, Burlington, MA, USA) were used as a filler material. For composite fabrication the 8 plies of 300 × 300 mm carbon fabric were arranged manually in a [0/90]_4S orientation onto the mold. The composite materials were fabricated with varying filler concentrations (0, 2, 4 and 6%). The fillers were added to the resin and dispersed to obtain a homogeneous mixture using a sonicator. The epoxy–particle mixture was sonicated for 40 min using a Hawsin Powersonic 420 (Hawsin Technology, Phra Nakhon Si Ayutthaya, Thailand). Once the fabric plies were laid up, a vacuum bag was placed over the assembly. This bag was sealed around the mold and connected to a vacuum pump. Resin was then introduced into the bag using a resin infusion system. The vacuum was used to remove excess resin. The samples were left to cure for 24 h before being cut to appropriate dimensions for testing. Finally, the composites were subjected to accelerated ageing for 7 h. The factors and levels of the study are shown in

Table 1. Factors and levels of the study.

Factor	Units	Levels
Wt% of filler	Wt%	2	4		6
Ageing of composite	Hours	0		7
Fiber vol. fraction, Vf	-	0.60

. Figure 1 shows the fabrication process of composites.

2.1. Accelerated Ageing

Accelerated ageing of composites was carried out using Xenon Lab 325E (Mesdan Lab, Puegnago Del Garda, Italy). This type of ageing chamber stimulated environmental conditions like humidity, sunshine, and temperature. The standard test method ASTM G154 specifies different type of cycles (specifying irradiance and exposure time) for reference and also allows for other cycles to be adopted. The irradiance range was 0.49–1.55 W/m² for UV exposure times ranging from 4 h to 8 h. However, to have a significant effect on the composite materials, the irradiance value of 60 W/m² was decided, while keeping the exposure time to 7 h, as specified in the test method. The relative humidity of 50% and the temperature were regulated by the ageing chamber.

Table 2. Nomenclature adopted for the samples.

Filler Used	Conditioning	Wt% of Fillers
		0	2	4	6
Glass microspheres (GMSs)	Unaged	BaseUA	GMS2UA	GMS4UA	GMS6UA
	Aged	BaseA	GMS2A	GMS4A	GMS6A

lists the nomenclature used in this work.

2.2. Dynamic Mechanical Analysis

Dynamic mechanical analysis (DMA) of the composites was performed according to ASTM D4065 to characterize the temperature-dependent viscoelastic properties of the material described in

Table 3. Dynamic mechanical analysis results showing loss, storage modulus and tan delta.

Composite Samples	Storage Modulus	Loss Modulus	Tan δ	Tg (°C)
	E′ (GPa)	E″ (GPa)		By E″ Peak	By Tan Δ Peak
BaseUA	58.4	25.2	0.532	42.95	47.89
BaseA	233.7	68.5	0.527	48.23	52.05
GMS2UA	65.5	16.0	0.531	41.58	43.85
GMS2A	57.5	13.3	0.564	42.79	50.49
GMS4UA	273.9	69.3	0.579	40.07	44.99
GMS4A	269.2	58.5	0.517	46.35	51.57
GMS6UA	49.1	11.6	0.333	43.84	48.93
GMS6A	137.1	26.7	0.392	46.02	51.42

. Samples with dimensions of 60 mm × 13 mm × 2 mm were tested (Figure 2a). The tests were performed using DMA Q-800 from TA (New Castle, DE, USA) operating at a frequency of 1 Hz with a heating rate of 3 °C/min under a nitrogen atmosphere. A three-point bending mode was used for all the samples. Curves of storage modulus (E′), loss modulus (E″), and tan delta (tan δ) in the temperature range from 25 to 100 °C were recorded. These values are calculated using Equations (1), (2) and (3), respectively.

$$E^{\prime }={\displaystyle \frac{{\sigma }_0}{{\gamma }_0}}·\cos \delta$$

(1)

$$E^{″}={\displaystyle \frac{{\sigma }_0}{{\gamma }_0}}·\sin \delta$$

(2)

$$tan \delta ={\displaystyle \frac{E^{″}}{E^{\prime }}}$$

(3)

where

γ_ο = The strain amplitude;

σ_ο = The stress amplitude;

δ = The phase angle.

2.3. Short Beam Shear Test

The value of interlaminar shear strength was found by using the short-beam shear test method as per the ASTM D2344-22. A load was applied at the rate of 1.3 mm/min. The force applied at the time of failure was recorded and the stresses were determined using Equation (4). Five samples of each type were tested and the average of these was determined.

$$\tau ={\displaystyle \frac{3· P_R}{4·b·h}}$$

(4)

where

τ = Apparent interlaminar shear strength (MPa);

$P_R$= Maximum load at the moment of first failure (N);

$b$ = Width of the specimen (mm);

$h$ = Thickness of specimen (mm).

2.4. Charpy Impact Strength Test

Charpy impact testing of the composite was conducted as per the ASTM D256 D256–23^E1 standard test method. Three specimens of each sample of dimensions were 80 mm × 10 mm (Figure 2c). A Zwick/Roell HIT 5P machine was used for the Charpy impact test. The Charpy pendulum had a potential energy of 5 J and struck the specimen with an impact velocity of 2.9 m/s. An average of five samples was considered for all samples.

$$Impact strength={\displaystyle \frac{m·g·(H_i-H_f)}{b·h}}$$

(5)

where

$m$ = Mass of pendulum (g);

$g$ = Acceleration due to gravity (9.8 ms⁻²);

$H_i$ = Initial height of pendulum (mm);

$H_f$= Final height of pendulum (mm);

$b$ = Width of the specimen (mm);

$h$ = Thickness of specimen (mm).

3. Results and Discussion

3.1. Dynamic Mechanical Properties

In this study, the effects of particle addition on the dynamic mechanical properties of composites were studied to analyze how filler presence affects aged or unaged composites. Five samples of each type were tested to evaluate the effect of filler concentration and accelerated ageing on storage modulus E′, loss modulus E″, loss factor tan δ and glass transition temperature T_g. Table 3 summarizes the DMA values by averaging the results of five composite samples. An increase in the T_g was observed after the ageing of composite materials. Ageing causes a reduction in the free volume and changes in the molecular configuration of the matrix (increase in the cross-link density), thus leading to a small increase in T_g (about 5 °C) [25,26]. The presence of GMS also restricts the movement of polymer chains, contributing to an increase in T_g.

3.1.1. Storage Modulus

In the study, the storage modulus (E′) of unaged and accelerated aged carbon–epoxy composites reinforced with GMS was measured to evaluate their mechanical properties. Figure 3a shows that the addition of a lower concentration of GMS (GMS2UA) slightly increased the E′ compared to the base unaged sample (BaseUA) at a low temperature, indicating that GMS can enhance the composite’s ability to store energy when subjected to an external load. The GMS acts as a rigid filler, thus increasing the stiffness of the composite material. The presence of GMS also improves the load transfer between the matrix and fibers, enhancing the overall mechanical properties of the composite. However, no clear trend was observed with the addition GMS, as the E′ was higher at 4% loading and reduced at 6% filler loading. The significant increase in the modulus for 4% loading may be attributed to improved particle–matrix interactions in terms of interfacial adhesion and particle distribution [27]. The excessive amount of added particles in the GMS6UA sample negatively affected the fiber–matrix interface, resulting in a lower E′ value [28].

After accelerated ageing, Figure 3b shows that the GMS2A and GMS4A samples had decreased E′ values compared to their unaged counterparts, indicating a weakening of the composite’s mechanical properties due to ageing. This can be attributed to the hydrolysis reaction of the epoxy groups in the polymer backbone, which can cause chain scission, cross-linking, and/or a reduction in the degree of polymerization of the epoxy matrix in carbon–epoxy composites, leading to a decrease in the storage modulus [29]. As the content of inorganic fillers such as glass microspheres is increased, the epoxy resin substrate between GMS particles becomes thinner, affecting their bonding with the matrix and leading to a decrease in the overall mechanical properties, including E′ [30]. The BaseA sample, on the other hand, showed an increase in E′ after ageing due to cross-link formation, which enhances the material’s strength and stability. Marouani et al. [31] also showed an increase in the mechanical properties of composites in the first ageing stage of consolidation and this increase in properties is linked to post-curing reactions.

Interestingly, the GMS6A sample had a higher E′ value after ageing compared to the unaged sample, indicating that the ageing process had a positive effect on its mechanical properties. The possible explanations for this can be enhanced particle–matrix interaction, and stress redistribution. The accelerated aging might have improved the interfacial adhesion due to increased cross-linking or changes in the surface chemistry of the particles or the matrix. This improved interfacial adhesion could lead to a more effective stress distribution within the composite, reducing its strain under load, and contributing to a higher E′. However, it is important to note that the change in E′ was not significant for the GMS4UA and GMS2UA samples, showing no negative effects by ageing. The presence of GMS might have acted as a barrier, resulting in a slower degradation of the matrix, leading to a similar E’ value after aging [32].

3.1.2. Loss Modulus

The loss modulus E″ reflects the energy that is dissipated as heat during material deformation. Figure 4 displays loss modulus peaks for unaged and aged carbon–epoxy composites. The highest value of E″ is associated with the relaxation α-peak, which corresponds to the mobility of polymer chains as they transition from crystalline to amorphous molecular structures [33]. As shown in Figure 4a, the α-peaks shift to a higher temperature for composites with 6 wt% glass microspheres (GMS6UA), which indicates a decrease in epoxy chain flexibility. Loss modulus values decrease with increasing GMS content, suggesting that the composites’ damping behavior has improved.

Figure 4b depicts the loss modulus of aged composites, and the shift of curves for all filler-reinforced composites to lower temperatures indicates an increase in chain flexibility, which is a result of the composite’s ageing. As noted in Table 3, the addition of glass microspheres generally leads to a decrease in the loss modulus, particularly for aged samples. This could suggest that the particulate additives improved the material’s damping properties, reducing its vibrational energy losses. Additionally, the broadening of α-peaks for the composites may indicate fiber–matrix debonding, which is particularly pronounced in the GMS2A samples due to moisture, which reacts with the polymer matrix, causing irreversible chemical changes that result in its poor mechanical properties, while UV rays cause photo-oxidation or weathering of polymeric composites [34]. On the other hand, the shift of the BaseA α-peak to a higher temperature reveals a decrease in flexibility due to cross-linking during ageing treatment, as a high temperature and UV atmosphere create favorable conditions [35].

3.1.3. Tan Delta

The tan δ (loss factor) is the ratio of E″/E′, as mentioned in Equation 3, which represents the mechanical damping or molecular internal friction of a viscoelastic configuration. Figure 5 indicates the shift of peaks to a higher transition temperature with the addition of fillers indicating constrained polymer chains due to the interaction of polymers–particles [36,37]. The high weight fraction of inorganic fillers imposes restrictions against the molecular motion of polymer chains (due to the adsorption of polymer chains on the surface of the particles) resulting in a more elastic response of the material and fillers, which act as elastic to store energy rather than dissipate energy [38].

Figure 5a shows GMS-loaded composites on the tan delta graph. The unaged GMS2UA sample had a moderate damping behavior with a tan delta value of 0.531 and a relatively low flexibility in the polymer chains with a T_g value of 41.5 °C. In contrast, the unaged GMS6UA composite had a lower damping behavior and higher stiffness with a tan delta value of 0.333 and a higher T_g value of 43.84 °C, respectively. These results indicate that the addition of fillers can significantly impact the mechanical and viscoelastic properties of composite materials, even without ageing.

Upon accelerated ageing, the tan delta values of the composites increased, indicating an increase in the damping properties of the materials as shown in Figure 5b. The GMS2A composite exhibited a broadening in the tan delta peak, suggesting higher mobility of macromolecular chains due to degradation. Conversely, the GMS6UA composite displayed the lowest tan delta value among all samples, indicating lower damping behavior, which can be attributed to the high weight fraction of inorganic fillers restricting the molecular motion of polymer chains.

Regarding T_g, the base-aged composite shifted to a higher temperature, indicating lower flexibility in the polymer chains due to ageing conditions. In contrast, the filler-reinforced composites showed a shift of the tan delta peak to a lower temperature, indicating greater mobility of chains due to the ageing and degradation of the composites. The GMS6A sample demonstrated a significant shift in T_g and an increase in damping behavior, likely due to the high degree of degradation and greater mobility of polymer chains.

The changes in tan delta and T_g values upon accelerated ageing can be attributed to several factors, including degradation of polymer chains, breakdown of filler–polymer interactions, and increase in the weight fraction of inorganic fillers. These factors can lead to changes in the mobility of macromolecules and the overall viscoelastic behavior of the composites. In summary, the results suggest that the type and amount of filler used, as well as ageing conditions, play crucial roles in determining the mechanical and viscoelastic properties of composite materials.

3.2. Impact Strength

The impact strength of composites reinforced with GMS was evaluated to determine their ability to resist impact load. Figure 6 shows that the addition of GMS particles resulted in a decrease in the impact strength or impact absorption ability of these composites. The GMS2A composite showed an increase in the impact strength of the composite, while an increase in the weight percentage of GMS resulted in a decrease in impact strength due to the degradation of the composite caused by ageing. The reduction in impact absorption was attributed to the non-adherence of GMS particles in composites. The force vs. standard travel curves for unaged and aged composites indicated that ageing led to a decrease in the peak force and a reduction in the energy absorbed by the composite, which is consistent with the reduction in impact strength values observed after ageing. The fully fractured impact test samples indicated that the composites reached their ultimate failure point under the applied force, highlighting the poor impact strength of the GMS6A samples.

Figure 7 shows force vs. standard travel graphs that were generated for each sample. The graph plots the force required to break the sample against the standard travel distance. The area under the curve (AUC) of the graph represents the energy required to break the sample and thus can be used as an indicator of the impact strength. The relationship between impact strength and the AUC is directly proportional. The unaged sample graphs in Figure 7a show great AUC compared to the aged samples, as the AUC decreased significantly compared to their unaged counterparts, indicating a decrease in impact strength. The force vs. standard travel graphs of the aged samples in Figure 7b show a steeper increase in force and a sharper drop compared to the unaged samples, indicating a more brittle fracture behavior.

The difference in the AUC between the aged and unaged samples can be attributed to the degradation of the polymer matrix and filler–polymer interactions, leading to a decrease in the overall mechanical properties of the composites. Additionally, the increase in T_g values of the aged samples can contribute to the decrease in impact strength due to the decrease in chain mobility.

Figure 8 shows the post impact failure behavior of composite samples. The fully fractured impact test samples indicate that the composites reached their ultimate failure point under the applied force. This is a crucial observation in the analysis of the results, as it highlights the poor impact strength of GMS4A samples. These composites were unable to resist the applied force, resulting in complete failure. This observation is in line with the impact strength values of these composites, which were relatively low compared to the base-unaged sample. Figure 8b presents side-view optical micrographs of the test samples taken from the fracture sites. Both the images (top and bottom) reveal ply separations and broken fibers.

3.3. Inter-Laminar Shear Strength

The inter-laminar shear strength of composites was also evaluated. Figure 9 shows that increasing GMS content led to an increase in the ILSS of composites, which can be related to the effect of increasing the degree of adhesion at interfaces among the filler and matrix. The better adhesion at the interface results in efficient load transfer between the matrix and filler resulting in high ILSS. However, for GMS6A, the ILSS decreased due to the increasing void content or crack formation at the interface of composites. The ILSS graph showed an upward trend, indicating improved adhesion at the interface.

The accelerated ageing of composites caused an increase in the ILSS value for all the samples. One of the possible explanations for this observation is a decrease in the residual stresses left from the curing process of the laminates, which is probably caused by exposure to an elevated temperature and moisture. Moisture may have acted as a plasticizer of the polymer matrix and promoted a decrease in the residual stresses in the material [9]. However, in the case of GMS6A composites, the value was slightly reduced. A high concentration of GMS that interferes with stress relaxation may be the possible reason for this decrease. The trend for 2 and 6% is quite interesting for ILSS, as it does not follow the trend for previous results. Perhaps, in the case of ILSS, the load applied can be used to investigate the interlaminar performance of the composite and can be regarded as a static test (performed at a relatively slow rate), as compared to DMA and impact tests, which are dynamic tests.

Figure 10 shows the force vs. deformation graphs of both aged and unaged GMS filler composites. The plateau region in the curve indicates that shear stress dominates at the fiber–matrix interface. For the unaged composites, the graph showed that the ILSS increased with increasing filler content up to a certain limit. The GMS2UA composite had the highest ILSS value of 302 MPa, followed by GMS4A with 312 MPa. The GMS6UA composite had the lowest ILSS value of 246 MPa, likely due to the high concentration of inorganic fillers leading to agglomeration and poor adhesion at the interface between composite layers.

For the aged samples, the ILSS values increased for all the composites except for GMS6A, which had a lower ILSS value of 224 MPa. The AUC for the aged samples also increased for all composites except for GMS6A, which had a significantly lower AUC value than the other composites. The reduction in ILSS and AUC for the GMS6A composite can be attributed to the agglomeration and poor adhesion of inorganic fillers at the interface, which interfered with the stress transmission and resulted in poor mechanical properties.

The area under the curve for the aged and unaged samples represents the energy absorbed by the composites during the test and is compared in Figure 11. It can be observed that the AUC is the highest for the composites without glass fillers and decreased with the filler content. However, there is an increase observed in the AUC value for the aged composites for 2 and 4% concentrations. It is important to note that the overall effect of particle concentration and aging on the AUC is complex and may be attributed to new cross-linking, and particle–matrix interaction, and needs further investigations.

4. Conclusions

The incorporation of GMS into carbon fabric–epoxy composites demonstrated the complex relationship of the effects on the material’s properties. The increase in the storage modulus (E′) up to a certain concentration of GMS shows an improvement in the composite’s stiffness, which is desirable for many applications. However, excessive amounts of GMS can have a negative impact on the fiber–matrix interface, resulting in lower E′ values. Aging causes a decrease in E′ for some samples. While GMS significantly enhanced the composite stiffness and viscoelastic behavior, it simultaneously reduced impact strength, particularly under accelerated aging conditions. This trade-off highlights the need for careful optimization of GMS content. The 4% loading of GMS in composites can be regarded as the optimum filler loading in this particular study.

The shift in loss modulus peaks to a higher temperature for GMS6UA composites and the decrease in loss modulus, particularly for aged samples, suggest that the addition of GMS improved the material’s damping properties, reducing its vibrational energy losses. This is further supported by the impact test results, which show that the addition of GMS leads to improved impact resistance. The GMS improved the interfacial bonding between the carbon fiber–epoxy matrix and fillers, as evidenced by the enhanced interlaminar shear strength. The ILSS results also indicate an improvement in the interlaminar shear strength of composites with GMS, which is important for ensuring the structural integrity of the material. However, the degradation of impact performance under environmental stressors remains a critical area for further investigation to fully assess the long-term capability of GMS-reinforced carbon fiber-reinforced epoxy composites for practical applications.