Dynamic mechanical analysis of alkali-treated flax/hemp yarn on SS304 wire mesh reinforced epoxy hybrid composites

Natural fiber reinforced hybrid composites are emerging because of the burgeoning need for lightweight, sustainable and high-performance materials for use in automotive and engineering applications. But traditional natural fibre composites have some drawbacks like insufficient stiffness, weak adhesion between the fibers and poor thermal stability in dynamic conditions. The present study aimed at the development of hybrid epoxy composites reinforced with flax fibers and SS304 wire mesh to improve dynamic mechanical properties of the developed composite. Two laminate constructions were made by hand lay-up technique, namely, F1 [Flax/SS304/Flax] and F2 [Flax/(Flax–Hemp Yarn on SS304 Mesh)/Flax]. The two hand lay-up laminate constructions were made as F1 (Flax/SS304/Flax) and F2 (Flax/(Flax–Hemp Yarn on SS304 Mesh)/Flax). Before fabrication, flax fibers and SS304 wire mesh were treated with 5% NaOH to enhance the fiber–matrix interfacial bonding. Dynamic Mechanical Analysis (DMA) was carried out in dual-cantilever configuration, ranging from 25 °C to 150 °C at frequencies of 1, 2.5 and 5 Hz. Viscoelastic properties of the developed laminates have been investigated through the storage modulus, loss modulus, damping factor (tan δ), complex modulus and glass transition temperature. It was observed from the results that the woven hybrid laminate (F2) had a higher value of storage modulus, loss modulus, and complex modulus than the non-woven laminate (F1) in all cases, which means that it is stiffer, more energy dissipative, and more thermo-mechanically stable. Moreover, F2 showed a lower tan δ and slightly higher glass transition temperatures, suggesting better interfacial adhesion and reduced molecular mobility in the epoxy matrix. The combined effect of woven composite flax-hemp yarns and SS304 wire mesh enhanced the dynamic mechanical properties of the composite, which has a great potential for lightweight application in automotive industry, vibration damping and semi-structural construction.

The global automotive industry is currently undergoing a significant paradigm shift driven by stringent environmental regulations and the urgent need for fuel efficiency (Ganesan et al., 2026). To meet these challenges, manufacturers are increasingly turning toward lightweight, sustainable materials that can replace conventional synthetic counterparts without compromising structural integrity or passenger safety (Arul Murugan and Selva Kumar, 2021; Krishnasamy et al., 2022). Natural fiber reinforced polymers (NFRPs) have emerged as a frontrunner in this transition due to their low density, high specific strength, and favorable environmental footprint compared to traditional glass or carbon fiber composites (Sergi et al., 2022; Gnanakumar et al., 2025). Among the diverse array of available natural reinforcements, flax fiber (Linum usitatissimum) is particularly notable for its exceptional mechanical performance, boasting a tensile strength ranging from 345 to 1500 MPa and a Young’s modulus between 20 and 70 GPa (Singh and Rajamurugan, 2021; Rajamurugan et al., 2024). Furthermore, with a density of approximately 1.54 g/cm 3, flax fibers offer a high specific modulus (26–46 GPa), making them ideal for weight-sensitive applications in vehicle interiors and semi-structural components (Elumalai et al., 2022; Krishnasamy et al., 2022).

Despite their advantages, the widespread implementation of flax-based composites is often hindered by inherent limitations such as high hydrophilicity, susceptibility to moisture absorption, and relatively poor interfacial adhesion with hydrophobic polymer matrices like epoxy (Banu et al., 2021; Kanagaraj et al., 2021). These factors can lead to premature failure and inconsistent mechanical behavior under varying service conditions. To mitigate these issues, surface modification of fibers has become a standard practice in composite manufacturing. Chemical treatments, specifically alkalization (NaOH treatment), are widely employed to remove hemicellulose, lignin, and other impurities from the fiber surface, thereby increasing surface roughness and improving mechanical interlocking with the matrix (Ferede, 2020; Radzi et al., 2023; Elayaraja and Rajamurugan, 2024). In this study, a 5 wt% sodium hydroxide (NaOH) treatment was applied to the flax and hemp fibres to modify their surface characteristics and promote improved interaction with the epoxy matrix.

A promising strategy to overcome the mechanical deficiencies of NFRPs is hybridization the combination of two or more distinct reinforcements within a single matrix. Hybridizing natural fibers with metallic reinforcements, such as stainless steel (SS) wire mesh, can significantly enhance the impact resistance, stiffness, and durability of the resulting laminate (Bakhbergen et al., 2024). Stainless steel grade 304 (SS 304) is an exemplary candidate for such hybridization due to its high ultimate tensile strength (515 MPa) and exceptional corrosion resistance, attributed to its carefully balanced alloy composition (18%–20% Chromium and 8%–10.5% Nickel) (Specification for Chromium, 2022). The integration of a ductile metallic phase like SS 304 wire mesh (0.43 mm thickness, 2.11 mm grid opening) into a flax/epoxy system not only improves load distribution but also acts as a barrier to crack propagation and moisture ingress, thereby addressing the primary weaknesses of purely organic composites (Moften et al., 2022; Elayaraja and Rajamurugan, 2024; Elayaraja and Rajamurugan, 2026).

Dynamic Mechanical Analysis (DMA) serves as a critical diagnostic tool in characterizing the performance of such hybrid systems. Unlike static tests, DMA evaluates the viscoelastic properties of materials storage modulus (E′), loss modulus (E″), and damping factor (tan δ) as a function of temperature and frequency (Menard, 2008). These parameters provide fundamental insights into the material’s stiffness, energy dissipation capacity, and glass transition temperature (Tg), which are vital for predicting the long-term durability and vibrational behavior of automotive components subjected to cyclic loading and varying thermal environments (Botelho et al., 2005; Vinu Kumar et al., 2020). While the literature extensively covers the static mechanical properties of hybrid fiber-metal laminates, research focused specifically on the dynamic mechanical behavior of flax/SS 304 hybrid systems remains limited. Furthermore, the configuration of the reinforcement whether in a woven or non-woven architecture and the specific stacking sequence are known to play a decisive role in the final performance of the composite (Fiore and Calabrese, 2019; Jeyaguru et al., 2024). The interaction between the bidirectional flax layers and the metallic mesh layer can vary significantly depending on their spatial arrangement within the matrix, influencing both the mechanical load-bearing capacity and the viscoelastic response.

With recent advances in computational engineering, experimental measurements are increasingly being coupled with data-driven prediction, reliability assessment and intelligent decision-support methods. To model complex engineering systems, predict degradation behaviour and improve the reliability of systems under variable conditions, evolutionary and machine-learning approaches have been used (Ngnassi Djami and Nzié, 2025; Ngnassi Djami and Abdelhakim, 2026; Ngnassi Djami et al., 2026b). In the field of computational mechanics and statistical engineering, comparable advances enable the incorporation of the material responses measured in experiments and the use of predictive models for the assessment of the structural performance (Ngnassi Djami et al., 2024a; Ngnassi Djami and Nzié, 2026). But, experimental characterization is still needed to develop reliable material-level dataset for the development of such predictive framework. The aim of the present DMA investigation is to obtain experimentally determined viscoelastic parameters of flax/SS304 hybrid laminates as a foundation for further thermo-mechanical modelling, uncertainty quantification and reliability-based investigations.

The objective of this research is to investigate the dynamic mechanical properties of a flax/SS 304 hybrid composite system. Using a combination of LY 556 epoxy and HY 951 curing agent as the matrix, laminates were fabricated with variations in weaving and non-weaving stacking sequences. This study aims to fill the existing research gap by providing a comprehensive analysis of the storage modulus, loss modulus, and damping behavior of these hybrid laminates, evaluated alongside the efficacy of the 5% NaOH chemical treatment. The findings of this study are expected to provide valuable guidelines for the design of high-performance, sustainable hybrid composites tailored for impact-critical automotive applications.

Materials

Flax fibre fabric with an areal density of 292 GSM was used as the primary reinforcement for the preparation of composite laminates. The flax fiber mat was obtained from Go Green Products, Chennai, India. In addition to the flax and hemp yarns were incorporated as reinforcing materials to improve the structural stability and load transfer characteristics of the composites. These flax and hemp yarns were purchased from the local textile market in Salem, Tamil Nadu, India, where natural fibre yarns are widely available for textile and engineering applications. Flax fibre typically exhibits a density of about 1.4–1.5 g/cm 3, tensile strength ranging between 500 and 900 MPa, and a Young’s modulus of approximately 50–70 GPa. Similarly, hemp fibre possesses a density of nearly 1.48 g/cm 3, tensile strength in the range of 550–900 MPa, and a Young’s modulus between 30 and 60 GPa, making these fibres suitable for reinforcement in polymer composites.

To further enhance the mechanical strength and wear resistance of the laminates, stainless steel 304 (SS304) wire mesh was incorporated as a hybrid reinforcement layer. The SS304 wire mesh was procured from a local market in Katpadi, Vellore, Tamil Nadu. The mesh had an approximate wire diameter of 0.43 mm with uniform square openings, which helps distribute stresses more effectively within the composite structure. Stainless steel 304 is commonly used in engineering materials due to its excellent corrosion resistance and good mechanical properties, with a density of approximately 7.9–8.0 g/cm 3, tensile strength around 515–620 MPa, and a Young’s modulus close to 193 GPa.

The matrix system used for the fabrication of the composites consisted of epoxy resin (LY556) and hardener (HY951). The epoxy resin has a density of about 1.15–1.20 g/cm 3, while the hardener has a density of 0.97–0.99 g/cm 3. Epoxy resins are widely used in fibre-reinforced composites because they provide strong adhesion with reinforcement materials, good chemical stability, and resistance to moisture, which improves the durability and performance of the final composite.

The alkaline treatment condition was selected as a relatively mild surface-modification treatment to improve fibre-matrix compatibility while avoiding excessive degradation of the lignocellulosic reinforcement. A 5 wt% NaOH solution was selected based on previous investigations showing that moderate alkaline concentrations can effectively modify the fibre surface, whereas excessively concentrated solutions may adversely affect fibre structure and mechanical integrity. The flax and hemp fibres were immersed in the NaOH solution for 4 h at room temperature. After treatment, the fibres were repeatedly washed with distilled water to remove residual alkali and then dried at room temperature before laminate fabrication. This procedure was adopted to provide a cleaner and moderately roughened fibre surface for improved mechanical interlocking with the epoxy matrix (Mahjoub et al., 2014; Aslain Brisco Ngnassi DjamiFossi, 2024).

Composite fabrication and specimen preparation

The hybrid composite laminates were prepared using the hand lay-up fabrication method, which is commonly adopted in laboratory-scale production of fibre-reinforced polymer composites because of its simplicity and low processing cost. Before starting the fabrication process, the mould surface was thoroughly cleaned to remove dust and impurities. A mould release agent was then applied to the surface to ensure that the composite laminate could be removed easily after the curing process. The matrix system used in the present work consisted of epoxy resin (LY556) and hardener (HY951). These two components were blended in a 10:1 weight ratio to obtain a uniform epoxy mixture suitable for composite preparation. After mixing, a thin layer of the resin mixture was first applied on the mould surface to act as the initial base layer for laminate formation. In this study, two types of hybrid composite laminates were produced in order to evaluate the effect of reinforcement configuration. A square mould measuring 300 mm × 300 mm was used to fabricate each laminate. In the first configuration, flax fibre fabric (282 GSM) was used as the outer layers at the top and bottom, while stainless steel 304 (0.43 mm wire diameter) wire mesh was positioned as the middle reinforcement layer.

The incorporation of the metallic mesh in the central layer helps improve load distribution and enhances the structural strength of the laminate. In the second configuration, flax fibre fabric was again placed as the top and bottom reinforcement layers, whereas the central layer consisted of flax and hemp yarns woven over the SS304 wire mesh. This arrangement was introduced to improve fibre interlocking and to strengthen the bonding between the natural fibres, SS 304 wire mesh, and the epoxy matrix shown in Figure 1. Table 1 shown the material composition of the F1 and F2 hybrid laminates. During the fabrication process, each reinforcement layer was carefully arranged on the mould and impregnated with the prepared epoxy mixture to ensure proper wetting of the fibres. A roller was used to spread the resin evenly and to remove trapped air pockets, thereby improving the consolidation of the laminate. After completing the stacking sequence, the laminate was covered with a release sheet and pressed using a flat plate to maintain uniform thickness. The fabricated laminates were produced with an approximate thickness of 3 mm ± 0.5 mm. The laminates were allowed to cure under room temperature conditions for approximately 24 h, allowing the epoxy matrix to solidify completely. After curing, the composite plates were carefully removed from the mould and trimmed to obtain smooth edges. The laminates were subsequently cut into test specimens of required dimensions using a precision cutting machine, following the relevant ASTM standards. For the assessment of viscoelastic properties through Dynamic Mechanical Analysis (DMA), the specimens were prepared according to ASTM D5023 (Vinu Kumar et al., 2020).

The temperature and frequency-dependent DMA response is attributed to the competition between the deformation of the epoxy matrix and load transfer through the fibre–metal reinforcement network. Below T g, the epoxy matrix is relatively stiff and makes a significant contribution to the measured storage modulus. When the temperature approaches the transition region, the polymer chains move in the form of segments with increasing activity, giving rise to a marked decrease in stiffness. The arrangement of woven flax–hemp yarns create supplementary mechanical interlocking and load transferring paths around the SS304 mesh, potentially modifying the local deformation behaviour and the relaxations in the matrix. With a higher oscillation frequency, the time for each oscillation is shorter, meaning that molecular rearrangement occurs for a shorter time, which leads to a frequency dependent response.

Dynamic mechanical analysis (DMA) at 1 Hz frequency

Dynamic Mechanical Analysis (DMA) was done at 1 Hz to study the viscoelastic property of flax/SS304 hybrid composites. To investigate the effect of reinforcement architecture on the thermo-mechanical behaviour of the laminates, the storage modulus (E′), loss modulus (E″), damping factor (tan δ) and complex modulus (E*) were investigated as a function of temperature. The DMA curves of F1 (Flax/SS304/Flax) and F2 (Flax/(Flax-Hemp Yarn on SS304 Mesh)/Flax) are shown in Figures 2–5. The results show that the stiffness, energy dissipation ability and thermal stability of the composite laminate were greatly enhanced by the use of woven flax-hemp yarns on the wire mesh SS304 (Djami et al., 2024b).

Storage modulus (E′)

The storage modulus as a function of temperature for a frequency of 1 Hz is shown in Figure 2. The storage modulus is the elastic part of the material response and is a measure of the capacity of the composite to store energy of deformation under cyclic loading. Both composites had high stiffnesses at lower temperatures because of the glassy state of the epoxy matrix. F2 exhibited higher storage modulus values over most of the investigated temperature range, although F1 showed a slightly higher storage modulus than F2 at room temperature.

The storage moduli of F1 and F2 at room temperature were 755 MPa and 705 MPa respectively. An increase in modulus was observed with increasing temperature for both F2 and F1 with the higher F2 value of 1736 MPa being obtained around 82 °C and the lower F1 value of 1381 MPa being obtained around 88 °C. The enhanced is due to the architecture of flax-hemp woven yarns, which increased load transfer efficiency and limited molecular movement in the epoxy matrix. The fibers from the natural fibers and the SS304 mesh formed a more rigid reinforcement network, which increases the stiffness of the laminate. The same trend has been noted in the case of natural fiber composites reinforced with wire mesh, where the reinforcement architecture is found to be the most important factor to enhance dynamic stiffness (Rajamurugan et al., 2024).

Outside the glass transition region both composites showed a rapid drop in storage modulus, which was caused by the change in matrix from glassy to rubbery state. However, F2 showed higher residual modulus than F1, which suggested better adhesion between the fibers and the matrices, and higher thermal resistance.

The dynamic mechanical response of the laminates is influenced by the interaction between the natural-fibre reinforcement and the epoxy matrix. Modification of the fibre surface can improve its compatibility with the polymer matrix and facilitate more effective transfer of applied stresses across the interface. Earlier experimental investigations of natural-fibre/epoxy systems have shown that alkaline surface treatment can modify fibre characteristics and influence fibre matrix interfacial strength (Aslain Brisco Ngnassi DjamiFossi, 2024). In the present study, the woven arrangement of flax hemp yarns together with the SS304 mesh provides additional physical interlocking within the laminate, which may contribute to more efficient stress transfer. This combined reinforcement arrangement can therefore be associated with the improved retention of thermo-mechanical properties observed for F2, particularly at elevated temperatures and near the glass-transition region.

Loss modulus (E″)

The loss modulus is plotted as a function of temperature in Figure 3. Loss modulus is related to the viscous response of the material, and is a measure of the energy dissipated as heat during cyclic deformation. The loss modulus values of the F2 laminate were significantly higher than those of the F1 laminate in the whole testing range.

The loss modulus of F2 at 24 °C was about 12,500 MPa, while that of F1 was about 9,600 MPa. The higher loss modulus of F2 suggests that the internal friction is greater and the energy dissipating capacity is also better. This could be due to the large number of interfaces formed by woven flax-hemp yarns and the SS304 wire mesh which encouraged micro-frictional interactions during dynamic loading.

The loss modulus for both laminates decreased slowly with temperature, then dropped in a sharp manner from 75 °C–85 °C, which is the onset of matrix softening and glass transition. Since F2 has better energy absorption properties, it is considered to be suitable for vibration-sensitive applications in engineering, for automotive parts with repeated dynamic loads. Similar enhancements of the energy dissipation have been reported for hybrid fiber-metal laminates with metallic mesh structures (Botelho et al., 2005; Bakhbergen et al., 2024).

Damping factor (tan δ)

The damping factor (tan δ) curves in Figure 4 were obtained for both laminates. The damping factor is the ratio of the loss modulus to the storage modulus and is a measure of how well the material is able to dissipate mechanical energy. It also provides valuable information regarding fiber-matrix interfacial bonding and molecular mobility. Both composites showed a high temperature region of tan δ peak around 85 °C–90 °C which is related to the glass transition temperature (Tg) of the epoxy matrix. The peak value of F1 was 0.47 and that of F2 was 0.42, which was comparatively low.

The decrease of tan δ peak for F2 shows that the adhesion force between the reinforcement and matrix phases is enhanced. The woven flax-hemp yarns combined with the SS304 mesh, reduced the mobility of polymer chains and the internal friction, which led to lower damping behavior. Generally, reduction in tan δ value of a hybrid composite indicates the improvement of stress transfer efficiency and improvement in interfacial bonding (Vinu Kumar et al., 2020; Kanagaraj et al., 2021).

The tan δ peak was used to determine the value of Tg which was 86 °C for F1 and ∼88 °C for F2. The improvement of the thermal stability is reflected by the slight increase of Tg for F2, which is attributed to the improvement of reinforcement matrix behaviors interaction and effectiveness of alkaline surface treatment.

Complex modulus (E*)

The change of the complex modulus with temperature is shown in Figure 5. Complex modulus is a measure which includes both elastic and viscous response of viscoelasticity to dynamic deformation.

The complex modulus values of the F2 laminate were found to be much higher throughout the temperature range. The complex modulus of F2 was 12,567 MPa, and F1 was 9,622 MPa at room temperature. The superior load-bearing capacity and dynamic stiffness of woven hybrid structure is confirmed by the high complex modulus. The outstanding performance of F2 is due to the synergy created by flax fibres, hemp yarns and SS304 wire mesh. The woven reinforcement structure provided good stress distribution and limited deformation under cyclic loading. F2 had higher modulus even in the rubbery state as compared to both composites around the glass transition region though both composites showed a drop in complex modulus around the glass transition region, showing the better thermo-mechanical stability.

The overall assessment of the performance of the DMA

The DMA results obtained at 1 Hz clearly show that the reinforcement architecture significantly affects the viscoelastic properties of the flax/SS304 hybrid composite. Storage modulus, Loss modulus, Complex modulus and Glass transition temperature were the properties for which the woven configuration (F2) was superior to the non-woven configuration (F1). Additionally, the damping performance of F2 was found to be lower, indicating better transfer of stresses between the two layers and higher interfacial bond strength of the composite structure.

The superior thermo-mechanical property of F2 is attributed to the following: alkaline treatment, fiber interlocking and the incorporation of SS304 wire mesh with flax-hemp yarn. These properties improved stiffness, energy dissipation property and thermal stability, presenting F2 laminate as an attractive material for lightweight automotive panels, vibration-damping structures and other engineering components that demand high dynamic mechanical properties.

Dynamic mechanical analysis at 2.5 Hz frequency

In order to investigate the viscoelastic response of the flax/SS304 hybrid composites under moderate cyclic loading condition, Dynamic Mechanical Analysis (DMA) was conducted at a testing frequency of 2.5 Hz. The variation of storage modulus (E′), loss modulus (E″), damping factor (tan δ) and complex modulus (E) with temperature was compared and examined to determine the effect of the reinforcement architecture on the thermo-mechanical properties of the laminates. The DMA results of these materials (F1, Flax/SS304/Flax and F2, Flax/(Flax–Hemp Yarn on SS304 Mesh)/Flax) are presented in Figures 6–9. The use of woven flax-hemp yarns on the SS304 wire mesh further improved the viscoelastic properties of the composite system, which leads to better stiffness, energy dissipation properties, interfacial bonding and thermal stability. The observed enhancements indicate that the woven reinforcement architecture is an important factor in improving load transfer mechanisms and reducing molecular mobility within the epoxy matrix to improve the overall dynamic mechanical properties of the hybrid laminate. Similar observation have been recorded for woven natural fibres and metal-mesh reinforcement composite systems where the thermo-mechanical behaviours and structural stability are greatly affected by the types of reinforcements arrangement.

Storage modulus (E′)

The storage modulus values as a function of temperature at 2.5 Hz are plotted in Figure 6. The storage modulus is the elastic component of the material and indicates the resistance to deformation when the material is loaded dynamically. Both composites had high stiffness in the glassy region with a gradual decrease towards the glass transition region with increasing temperature.

The storage modulus values of the F2 laminate were significantly higher for the entire test. The modulus of the laminates, both were 730–760 MPa at room temperature. The increase in temperature, however, caused F2 to reach a peak storage modulus of 1754 MPa around 82.67 °C, while the F1 only achieved around 1418 MPa. The superior stiffness of F2 was thought to be the result of the flax-hemp yarns architecture woven with the SS304 mesh, that promoted a better stress transfer and limited the mobility of the molecular chains in the epoxy matrix. The same enhancements in dynamic stiffness have been observed with hybrid natural fiber composites containing metallic mesh structures (Rajamurugan et al., 2024). After the glass transition region, both composites showed a significant drop in storage modulus, which is attributed to the softening of the matrix. However, the residual stiffness of F2 was higher than that of F1 in the rubbery region, representing the good compatibility of the rubber and better thermal stability at the interfaces.

Loss modulus (E″)

The curves of the loss modulus shown in Figure 7 are the curves of the heat that is lost during cyclic deformation. The higher the loss modulus the more of the energy will be absorbed and the more internal friction.

The loss modulus values of the F2 laminate were significantly higher than F1 for the entire temperature range. For lower temperatures, F2 showed a loss modulus of 12,700 MPa, whereas the loss modulus of F1 was around 9,600 MPa. The F2 exhibited a higher loss modulus, which implied that it had more energy dissipation, due to higher interfacial interactions between flax fibers, hemp yarns, SS304 mesh and the epoxy matrix.

For both laminates the loss modulus gradually decreased with increasing temperature and then dropped sharply between 80 °C and 90 °C due to the softening of the matrix and the glass transition temperature. The better energy dissipation property of F2 implies better vibration damping and dynamic loading absorption properties. This characteristic is consistent with the results of other studies on hybrid fiber-metal laminates, where it has been demonstrated that metallic reinforcement improves the frictional energy dissipation mechanisms.

Damping factor (tan δ)

The damping factor (tan δ) curves shown in Figure 8 give clues to the molecular mobility and bonding of the fiber-matrix interface. The glass transition temperature (Tg) of polymer composite is usually estimated from the maximum value of tan δ.

A very high peak tan δ value was observed for both laminates between 89 °C–for F1 it was 0.48, whereas it was 0.43 for F2. The lower damping peak in F2 suggests that the molecular mobility is decreased and that the adhesion between the reinforcement constituents and the epoxy matrix is increased.

The flax-hemp woven yarns around the SS304 wire mesh may have facilitated mechanical interlocking and limited the ability of the segments to move relative to one another, leading to less energy loss due to chain relaxation. From the tan δ values, the load transfer efficiency and bond strength between the fiber and the matrix are generally higher for the lower value of tan δ, consistent with previous DMA investigations on woven hybrid composites. The glass transition temperature calculated by the peak of tan δ was 89 °C for F1 and 89.6 °C for F2, indicating the woven structure produced better thermal resistance.

Complex modulus (E*)

The variation of complex modulus with temperature is shown in Figure 9. The complex modulus is the total resistance of the material to dynamic deformation and describes both the elastic and viscous properties of the material.

All the F2 laminate samples had much higher complex modulus values than F1 during the test. The F2 had a greater complex modulus of 12,763 MPa compared to 9,637 MPa for F1 at room temperature. The difference indicates the effectiveness of the woven flax-hemp yarn and the SS304 mesh reinforcement in achieving the overall structural rigidity.

With the increase in temperature up to the glass transition region, the complex modulus of both composites decreased rapidly. F2 however consistently exhibited higher values, suggesting enhanced thermo-mechanical stability and resistance to dynamic deformation. The high performance of F2 is explained by the improved stress distribution and higher interface bonding of the woven natural fibers and metallic mesh, and the synergistic reinforcement.

Overall DMA performance

It is clearly shown in the DMA results at 2.5 Hz that the viscoelastic properties of hybrid composites is strongly affected by the reinforcement architecture. The storage modulus, loss modulus, and complex modulus of the woven configuration (F2) were higher than that of the non-woven laminate (F1), which showed higher stiffness, energy absorption capacity, and dynamic mechanical stability. Additionally, the lower damping factor and slightly higher glass transition temperature measured for the F2 material was indicative of better fiber/matrix interfacial adhesion and thermal resistance. The synergistic effects of alkaline treated flax fibers, hemp yarn weaving and the use of SS304 wire mesh are responsible for these improvements as they improve the stress transfer efficiency and limit the mobility of molecules within the epoxy matrix. As a result, it shows enhanced application potential for automotive, vibration damping or lightweight structural applications with excellent dynamic mechanical properties.

Dynamic mechanical analysis at 5 Hz frequency

The viscoelastic properties of flax/SS304 hybrid composites were examined by Dynamic Mechanical Analysis (DMA) at 5 Hz frequency level for higher cyclic loading conditions. The temperature-dependent responses of storage modulus (E′), loss modulus (E″), damping factor (tan δ), and complex modulus (E) were analyzed and the effect of reinforcement configuration on the dynamic and thermo-mechanical performance of the laminates, evaluated. The DMA profiles for the F1 (Flax/SS304/Flax) and F2 [Flax/(Flax–Hemp Yarn on SS304 Mesh)/Flax] are shown in the following figures (Figures 10–13). The results have shown that the higher loading frequency had the highest resistance to dynamic deformation for both composites, as it minimized molecular relaxation in the epoxy matrix. The dynamic mechanical properties of the investigated laminates showed that F2 had the best properties; this could be explained by the synergy between the woven flax-hemp yarns and the SS304 wire mesh. The new reinforcement structure led to more efficient transfer of stresses, higher stiffness, higher energy absorption and thermal stability. The results of this research are in accord with earlier studies which concluded that natural fiber woven structures and metallic mesh reinforcement had a strong effect on the dynamic mechanical property and structural integrity of polymer composite systems subjected to cyclic loadings.

Storage modulus (E′)

The variation of the storage modulus with temperature at 5 Hz is presented in Figure 10. The storage modulus is the energy stored while the composite material is being cyclically deformed and is also considered to determine the stiffness of the composite material. Both composites had high storage modulus at lower temperatures as the epoxy matrix was rigid glassy material. F2 was, however, found to be consistently stiffer than F1 in the temperature range.

The maximum storage modulus recorded for the F2 laminate was 1858 MPa at about 82.67 °C while it was 1520 MPa for the F1 laminate. The enhanced storage modulus of F2 is thought to be the result of the woven flax-hemp yarn and SS304 wire mesh structure and its role in increasing the efficiency of stress transfer and curbing the freedom of motion of the molecular chains of epoxy. The synergistic interaction of the natural fibers and the metallic mesh made possible a rigid reinforcement network that was able to carry higher dynamic loads. Woven natural fibre composites have also demonstrated similar increases in storage modulus (Elayaraja and Rajamurugan, 2024; Elayaraja and Rajamurugan, 2026; Jeyaguru et al., 2024; Rajamurugan et al., 2024) as well as hybrid metal mesh composites.

Both laminates showed significant drop in storage modulus as the temperature moved beyond the glass transition region, due to matrix softening. However, the residual stiffness of F2 in the rubbery region was higher than F1, suggesting that the fiber-matrix bonding was stronger and the thermal stability was better. The higher modulus retention suggests that the woven structure was effective at enhancing the load-bearing capacity at high temperatures (Vinu Kumar et al., 2020; Kanagaraj et al., 2021).

Loss modulus (E″)

The curves for the loss modulus (shown in Figure 11) are indicative of the viscous component of the composite response and show the amount of mechanical energy dissipated as heat in the cyclic deformation. The loss modulus values of the F2 laminate were significantly higher than those of the F1 laminate in the whole temperature range.

The loss modulus of F2 is 12,803 MPa at room temperature while the F1 is about 9,617 MPa at room temperature. The increased loss modulus of F2 is attributed to the increased interfacial interactions between the flax fibers, the hemp yarns, SS304 mesh and epoxy matrix. The woven reinforcement structure created new interfaces to absorb energy through frictional processes and localized deformation.

With increasing temperature both composites exhibited a gradual decrease in the loss modulus, followed by a steep drop in the loss modulus between 80 °C–90 °C, which is related to the transition of the epoxy matrix from glassy to rubbery state. F2 has a superior loss modulus, which means that it has a better ability to dissipate energy and better properties of vibration damping. This type of behavior has been reported in fiber-metal laminates where the presence of metal reinforcements resulted in higher frictional energy absorption and dynamic load resistance (Botelho et al., 2005; Bakhbergen et al., 2024).

The elevated values of loss modulus with the higher testing frequency were attributed to the higher amount of molecular friction and low relaxation time in the polymer matrix at 5 Hz compared to 1 Hz (Menard, 2008).

Damping factor (tan δ)

The damping factor (tan δ) curves shown in Figure 12 give insight into energy dissipation and molecular mobility, and into the bonding of the interface between the two constituents of the composite structure. Typically, the glass transition temperature (Tg) is defined from the peak in the tan δ curve.

The two laminates both had a clear damping peak between 88 °C and 95 °C with maximum values of tan δ of 0.49 and 0.43 for F1 and F2, respectively. The appearance of the lower damping peak for F2 suggests that there is a higher interfacial adhesion between the reinforcement constituents and the epoxy matrix. The energy dissipated as a result of internal friction is inhibited because of good interfacial bonding, which restricts molecular motion.

The woven flax-hemp yarns, which were woven with the SS304 mesh, increased mechanical interlocking and increased efficiency of stress transfer, which decreased polymer chain mobility. Thus, F2 had less damping characteristics and increased structural integrity under cyclic loading applications. Lower tan δ values were related to better interfacial bonding and higher composite stiffness in woven flax composites, basalt-flax hybrid composites and fiber-metal laminates (Botelho et al., 2005; Ferede, 2020; Vinu Kumar et al., 2020).

F1 and F2 had a glass transition temperature measured by the tan δ peak of 90 °C and 91.35 °C, respectively. It is observed that the increase in the Tg is due to the effect of the woven architecture and the alkaline treatment of the composite system (Ferede, 2020; Radzi et al., 2023).

Complex modulus (E*)

For the complex modulus, the change of modulus with temperature is shown in Figure 13. Complex modulus takes into account both elastic and viscous components and is a measure of the overall resistance of the composite material to dynamic deformation.

The complex modulus values for F2 were much higher than the values for F1 in the whole temperature range. F2 had a higher complex modulus than F1 at room temperature (12,830 MPa and 9,646 MPa, respectively). The superior complex modulus test results indicate that the woven flax-hemp yarn and SS304 mesh reinforcement has significantly improved dynamic stiffness and structural stability.

Both laminates showed a large drop in complex modulus as the temperature moved toward the glass transition region because of matrix softening. F2 however showed high values even at the rubbery regime, higher thermo-mechanical stability and stronger interactions between reinforcement and matrix. The improvement in complex modulus is due to the synergic effect of natural fibers and metallic mesh that improved load distribution and reduced the viscoelastic deformations during cyclic loading.

The results of the high frequency complex modulus (5 Hz) compared to the low frequency complex modulus further support the frequency dependence of polymer composites, as this will limit molecular relaxation at the higher loading frequencies and hence increase the apparent stiffness (Menard, 2008; Vinu Kumar et al., 2020).

Overall DMA performance

The viscoelastic characteristics of the woven flax-hemp yarn reinforced laminate (F2) and non-woven laminate (F1) are clearly shown in the DMA results obtained at 5 Hz. The F2 exhibited higher storage modulus, loss modulus, and complex modulus values which suggests that the storage F2 had greater stiffness, energy absorption and resistance to dynamic deformation. In addition, the lower damping factor value and higher glass transition temperature obtained in F2 shows that strong interfacial bond and high thermal stability are obtained.

The better performance of F2 was believed to be due to the synergistic effect of the alkaline treated flax fibers, hemp yarn weaving, SS304 wire mesh, and epoxy matrix, which enhanced fiber interlocking, transfer of stress, and limited molecular mobility. The results obtained are consistent with the earlier studies conducted on woven natural fiber composites, fiber-metal laminates and wire mesh reinforcement hybrid composites, that showed reinforcement architecture to be an important factor in improving the dynamic mechanical properties (Elayaraja and Rajamurugan, 2024; Elayaraja and Rajamurugan, 2026; Jeyaguru et al., 2024; Rajamurugan et al., 2024). Therefore, the F2 laminate is a potential material for automotive applications, where the primary requirement is low weight, vibration sensitive components, and lightweight engineering applications with high thermo-mechanical performance.

With a lower maximum damping factor, one can get a quantitative sense of the difference in energy dissipation behavior between the two architectures. Compared to F1, maximum tan ⁡ δ of F2 was found to decrease by about 10.6%, 10.4% and 12.2% at frequencies 1Hz, 2.5Hz and 5Hz, respectively. This decrease is in line with limited molecular mobility and optimal transfer of stress in woven reinforcement configuration. Note, however, that while tan ⁡ δ is an indirect measure of the interfacial behavior, it is not necessarily a measure of the strength of the interfacial bond.

The experimental results of the viscoelastic behavior of the material when operated at various frequencies and temperatures provide useful material-level input for future computational modeling and reliability analyses. For instance, the measured characteristics of the storage modulus, damping and glass-transitions can be the input parameters in predictive frameworks to assess the structural behavior under different service conditions. More recently, data-driven stochastic and physics-informed methods have been shown to be effective in engineering-system prediction and degradation assessment, as well as in decision-making for reliability (Tatsinkou Fogang et al., 2024; Ngnassi Djami and Nzié, 2025; Djami et al., 2026; Ngnassi Djami and Abdelhakim, 2026; Ngnassi Djami et al., 2026b). These methodologies may also be applied to the present hybrid laminate system once adequate experimental datasets and reliable estimates of the associated uncertainties have been obtained.

Comparative effect of testing frequency

The results of the DMA response at the three test frequencies 1, 2.5 and 5 Hz have been compared and show that the hybrid laminates have definite frequency dependence. This means that with higher frequency more time for rearrangement of epoxy chains is not available in each loading cycle. As a result, the material is more resistant to dynamic deformation. The movement of the damping peak towards higher temperature as the frequency increases is typical of limited molecular relaxation. In general, F2 showed an advantage of its woven flax–hemp/SS304 reinforcement architecture, in all three frequencies, especially in regard to storage modulus, transition behaviour and modulus retention. From these observations, the dynamic response of the laminate is related to both the reinforcement architecture and loading frequency (Djami et al., 2024a).

Frequency dependence and apparent activation energy

The study of the frequency dependence of the glass-transition temperature was performed with an Arrhenius-type relationship. The relationship between frequency and T g was written as ln ⁡ f = ln ⁡ f 0 − E a R T g and E a is the apparent activation energy of the molecular relaxation process with the universal gas constant R and the T g in Kelvin. The slope of the linear relationship of ln ⁡ f with 1 / T g was used to determine the apparent activation energy. The activation energies calculated were around 412 kJ/mol for F1 and 525 kJ/mol for F2 using Equation 4. The increased apparent activation energy, obtained for F2, suggests that the relaxation process for F2 must be thermally less favourably assisted than that of F1. It may be linked to the woven architecture of flax-hemp yarns and SS304 mesh. It could be related to the restricted molecular mobility resulting from the architecture of woven flax–hemp yarn and SS304 mesh. The enhanced reinforcement interaction can affect the motion of segments of the epoxy matrix and therefore affect the glass transition behavior with frequency.

In this study, the effects of the reinforcement architecture on the dynamic mechanical properties of flax fiber-reinforced epoxy composites with SS304 (stainless steel 304) wire mesh and woven flax-hemp yarns were examined. The storage modulus, loss modulus, damping, complex modulus and glass transition temperature of the hybrid developed laminates were analysed by Dynamic Mechanical Analysis (DMA) at frequencies of 1, 2.5 and 5 Hz.

The superior dynamic mechanical performance of the woven hybrid configuration F2 in many respects over the non-woven configuration F1 was more accentuated in the high temperature and glass transition regions. The room-temperature storage modulus was not greater than that of F1 at 1 Hz, but F2 had higher values of the storage modulus for significant portions of the temperature range and retained a higher value of the storage modulus near and above the transition region. From the overall response, reinforcement architecture plays a significant role on the thermo-mechanical response of the hybrid laminate.

The damping factor analysis showed that the tan δ peak values were lower for F2, indicating that there was a strong fiber–matrix interfacial adhesion and stress transfer between the fibers and the matrix was more efficient in the composite structure. Moreover, higher glass transition temperature of F2 than F1 was observed at all frequencies, indicating higher thermal stability and lower molecular mobility of the epoxy matrix. The complex modulus results also confirmed the improved dynamic stiffness and thermo-mechanical strength of the woven hybrid laminate.

The storage and loss modulus values as well as the complex modulus values of both composites increased with frequency from 1 Hz to 5 Hz, showing the viscoelastic behavior of the hybrid system containing epoxy. The shift of glass transition region towards higher temperature at higher frequency also demonstrated the limited relaxation under high frequency cyclic loading.

In summary, stiffness, energy dissipation, interfacial bonding and thermal resistance of hybrid composite were significantly enhanced by embedding woven flax-hemp yarns into SS304 wire mesh. The superior dynamic mechanical performance of the F2 laminate demonstrates its suitability for lightweight structural parts, automotive interior and semi-structural applications, vibration-damping systems, and for other lightweight and vibration-damping applications in the field of engineering under dynamic service conditions demanding high thermo-mechanical stability.

Future research should be continued on the experimental characterisation in the present work, but extended to include uncertainty-aware thermo-mechanical modelling and reliability assessment of the hybrid laminates in the presence of prolonged cyclic and environmental loading. The experimental data on the storage and loss moduli, the damping factor and the glass-transition behaviour could serve as input parameters for stochastic degradation models, predictive algorithms and physics-informed computational models. In recent years, the fusion of experimental material characterisation with computational decision-support has been shown to have potential for combining experimental and computational methods for optimising reliability, for stochastic degradation modelling, for machine-learning based prognosis, and for physics-informed prediction (Djami et al., 2024a; Ngnassi Djami et al., 2024c, Ngnassi Djami et al., 2024b; Ngnassi Djami et al., 2026c, Ngnassi Djami et al., 2026a; Ngnassi Djami, 2025). This integrated approach may allow for the prediction of property degradation, uncertainty propagation and long-term structural dependability of natural fibre/metal hybrid composites.