Physical and Mechanical Behavior of Ramie and Glass Fiber Reinforced Epoxy Resin-Based Hybrid Composites

ABSTRACT In this study, the compression molding technique was used to fabricate hybrid composites using different wt.% of the glass and ramie fibers in different stacking sequences. The physical (density, water absorption, and wear resistance) and mechanical (tensile strength, hardness, and impact strength) and morphology studies were performed for the glass and ramie fibers and the effect of stacking sequencing was studied. A technique for order of preference by similarity to the ideal solution model was used to determine the optimal reinforcement composition in the hybrid composites. The stacking sequence played a crucial role in the physical and mechanical characteristics of the hybrid composites. The GRG (stacking sequence: Glass (4 wt.%)-Ramie (4 wt.%)-Glass(4 wt.%)) hybrid composite showed optimal characteristics; tensile strength: 114 MPa, hardness: 41 HV, impact energy: 3.5 J m−1, void content: 1.09% and wear: 63 μm. However, if glass fibers were sandwiched between the ramie fiber layer, the composite showed lower physical and mechanical characteristics; tensile strength: 72 MPa, hardness: 28 HV, impact energy: 3 J m−1. The glass fiber reinforced composites (GG and GGG) exhibited better water absorption characteristics. The RRR composite’s impact strength was comparable with the GGG composites due to the stacking of the composites. Moreover, the glass fiber composites stacking (RR to RGR) exhibited lower tensile strength than ramie fiber composites stacking (GG to GRG). The microstructural analysis of the fractured composite surface revealed voids, de-lamination, interfacial bonding of the fibers with the matrix, fiber pull-out, and matrix distribution.


Introduction
Environmental awareness has motivated researchers worldwide to study the natural fiber-reinforced polymer composite in recent years.Natural fiber composite can replace synthetic fiber reinforced composite owing to its advantages, such as being lightweight, bio-degradable (depending on polymer matrix) and environment friendly (Khalid et al. 2021;Thyavihalli Girijappa et al. 2019).However, major problems associated with natural fibers are poor moisture resistance, fiber wetting, and adhesion to the polymer matrix, which affect the physical and mechanical properties of composites.Therefore, to overcome such problems, a combination of natural and synthetic fibers can be used, which can create a balance of natural and synthetic fiber properties in the hybrid composites.
Several studies have shown better physical properties for glass and natural fiber hybrid composites than their corresponding pure composites (Aslan, Tufan, and Küçükömeroğlu 2018;Giridharan 2019;Thiyagu and NarendraKumar 2023).Natural and glass fiber-reinforced composites are cost-effective when used to make hybrid composites.Kumar and Anand (Kumar and Anand 2018) showed that incorporating ramie fiber into the epoxy matrix substantially enhanced the wear resistance of composites.The sliding wear characteristics of the glass fiber composite increased with fiber reinforcement (Billady and Mudradi 2020).Subhrajit et al (Ray, Rout, and KuSahoo 2017).studied the erosion behavior of the glass fiber composite incorporated with granite powder and showed that increasing the filler increased the composites' erosion wear resistance.Natural fiber composites exhibit high water content that may have a detrimental effect on the properties of the composites.However, it can be reduced with fiber loading.Vedanarayanan et al (Vedanarayanan et al. 2022).experimentally observed that the void content of hybrid composites of kevlar and Ramie fiber reduced when Ramie fiber loading was increased.The hybridization also improved the mechanical and physical properties of the composites.Romanini et al (Romanzini et al. 2012).studied the hybrid composite of ramie and glass fiber and observed that composites containing fibers with a length of 45 mm showed better mechanical characteristics than 25, 35, and 55 mm fiber lengths.Hybridization of Kevlar fiber with E-glass fiber significantly improved the impact characteristics of composites in applications such as aircraft, automobile bumpers and bulletproof jackets (Vasudevan et al. 2020).
Several studies have reported that the mechanical and physical properties of hybrid composites can be improved by different stacking sequences (Azlin et al. 2022;Hassan et al. 2022).Portella et al (Portella et al. 2016).found that the tensile strength of the fabricated composites depends upon the layers of the glass fiber.Mukesh et al (Kangokar Mukesh et al. 2022).studied the effect of stacking sequence and hybridization and reported that incorporating flax/kevlar in the epoxy matrix increased its mechanical characteristics.Bharath et al (Bharath et al. 2023).found that different stacking sequences of sheep wool and glass fiber composites improved the mechanical and chemical properties of the composite.Barouni et al (Barouni et al. 2022).investigated the fatigue behavior of a flax/glass hybrid composite and found that the fatigue strength increased with the inclusion of laminated flax fiber.The increased filler wt.% increases the vibrational characteristics of the natural fiber-reinforced composites (Prasad et al. 2022).Thiyagu and Narendrakumar (Thiyagu and Narendrakumar 2022) showed that the hybrid composite of glass and ramie fiber with two layers of ramie fiber exhibited good tensile properties.
Although hybrid composites of ramie and glass fibers have been studied, reports on the effect of stacking sequencing of the glass and ramie fibers on the physical and mechanical properties of hybrid composites are scarce in the literature.In the present study, hybrid composites were fabricated by reinforcing ramie and glass fibers (varying the wt.%) in different stacking sequences with epoxy polymer to study the physical and mechanical behavior of the composite.A total of seven different composites were fabricated to study the physical (density, water absorption, and wear resistance), and mechanical (tensile strength, hardness, and impact strength) characteristics.Scanning electron microscopy (SEM) micrographs of the fabricated hybrid composites were analyzed to study the morphological characteristics.

Materials
E-Glass fibers (300 gsm), poly(vinyl alcohol) (PVA), epoxy resin LY556, and epoxy hardener HY951 were procured from Amtech Esters Pvt. Ltd., New Delhi, India.Ramie fibers were procured from Go Green Products, Chennai, India.Table S1 (Supplementary Information) lists the Physical and mechanical properties of materials used in the present study.

Composite fabrication
The composites were fabricated with the help of a compression-molding machine (GRIMCO 30 Ton Hydraulic Press, Model 30-2-HT).The epoxy resin and hardener were mixed in a ratio of 10:1 at a curing temperature of 100 °C in a beaker such that no air was entrapped.Glass and ramie fibers were weighed in proportions mentioned in Table 1 and added to this mixture.The glass and woven ramie fibers were layered with 4 wt.% of the matrix as per the sequence shown in Table 1.Glass fibers were selected due to their chemical inertness, availability and low cost.Ramie fibers were selected for their higher mechanical properties and thermal stability and ramie fibers incorporation in the matrix increased the composite strength significantly.The mylar sheet of PVA was used in the mold (250 mm × 250 mm × 7 mm), and releasing agent was sprayed on the mylar sheet to remove the composite from the sheet effectively.The closed mold was loaded into the compression-molding machine (50 °C) and subjected to a compressive load.Proper care was taken to minimize the voids formations during the fabrication process of composites as the presence of voids has a detrimental effect on mechanical properties.The mold temperature slightly increased due to the exothermic polymerization reaction, which lowered the viscosity of the matrix system allowing the epoxy impregnation in the fibers.The compressive load was then reduced and maintained at the same load for 1 h to prevent resin flash and minimize fiber disturbance.The mold was cooled under a compressive load for 24 h to inhibit geometrical distortion of the composite plate.Finally, the fabricated composites of dimensions 250 mm × 250 mm × 7 mm were released from the mold.The weight of each fabricated composite was fixed (500 g).The test samples were prepared as per ASTM standards.Three similar samples were prepared for testing and the average of these reading were reported.

Density
The density affects the component's mechanical and physical properties.The theoretical density (ρ th ) of composites in terms of weight fraction was obtained using the relation (Prasad et al. 2020): where W f -weight fraction of fiber, W m -weight fraction of matrix, ρ f -density of fiber, ρ m -density of matrix.The composites' actual density (ρ exp ) was calculated with the simple water immersion technique of the composites.The volume fraction of voids (V v ) in the composites was determined using the ρ th and ρ exp density of composites using the following equation (Prasad et al. 2020): where ρ exp -experimental density of composite, respectively.

Water absorption test
The water absorption test in the present study of the composite sample (50.8 mm diameter and 7 mm thick) was performed as per ASTM standards.The sample weight was recorded using a digital weighing machine with an accuracy of ±1 mg.The test samples were immersed in water for 24 h.After 24 h, the water over the sample's surface was cleaned with tissue paper, and the weight was recorded.This was repeated until a steady state (no more water uptake by samples) was obtained for the test samples.The samples for the water absorption test are shown in Figure 1a.The percentage of water content (m %) was determined using the following equation (Keerthi Gowda et al. 2022): where: w t -the weight of the sample in time t, w d -the sample's initial or dried weight.Wear test A pin disc tribometer was used to assess the fabricated composites' wear behavior.In this test, the test samples were loaded against a rotating pin.The test samples (length: 32 mm, diameter: 6 mm with spherical tip) were prepared, and testing was done as per ASTM G99 standard.The rotating disc speed and load were kept at 500 rpm and 10 N. Figure 1b shows the wear test samples.

Hardness test
The Vickers hardness test was conducted per the ASTM standard.The hardness samples (Figure 1c) were polished before applying the load (test load: 5 kgf).The square base pyramid-shaped diamond indenter was used.The hardness of the fabricated composites was measured by a diamond indenter with an apical angle of 136°.

Tensile and impact test
The tensile test was performed as per ASTM D3039 standard on a HEICO universal testing machine (UTM) supplied by Hydraulic & Engineering Instruments, New Delhi, India.The test sample dimensions were taken as 115 × 6 × 7 mm.Figure 1e shows the tensile test samples.The ability to quantify the impact properties of composites leads to an advantage in product liability and safety.The Charpy test was used to conduct the impact test (Figure 1d) using a Charpy impact tester (Veekay Industries, India).The specimen (L × W × t = 55 mm × 10 mm × 2 mm, and with a V notch at the center) was freely rested, and the impactor (projectiles) was stuck at the center of the sample.

Morphological test
A scanning electron microscope (SEM) was used to study the morphological behavior of the tested composites.SEM analysis was performed using Carl Zeiss EVO 18 instrument.For SEM analysis, the fractured sample's surfaces were gold-sputtered using an ion-sputter device.The SEM images were used to evaluate the fractured surfaces, bonding between fibers and the epoxy, and provide information about voids.

Physical characteristics of composites
Table 2 shows the fabricated composites' theoretical density, experimental density, and voids fraction.
The experimental density values are lower than the theoretical density values due to the presence of voids and pores.A high void fraction (>5%) affects the mechanical properties of the composites (Prasad et al. 2019).The lowest void content (0.18%) was in the RR composite, while the maximum void content (1.64%) was in the GGG composite.The GGG composite was prepared using chopped glass fiber.The matrix material could not penetrate the chopped glass fiber mat properly, leading to voids in the GGG composite.The ramie layer consisted of a woven mat where warp and weft were at 90°.Hence, matrix material easily penetrated the ramie fiber mat, resulting in lesser voids.As the fiber loading (glass or ramie fiber) increased, the void content increased.Similar trends have been reported in previous studies (Wang et al. 2019;Zainudin et al. 2020).However, the increment rate in the ramie fiber composites (RR with 8 wt.% and RRR with 12 wt.%)was lesser than in glass fiber composites (GG with 8 wt.% and GGG with 12 wt.%).This can be attributed to the epoxy polymer matrix compatibilization of ramie fiber and the low packing ability of the glass fiber (Prasad et al. 2020).
The water absorption in the composites was less when glass fiber wt.% in the hybrid composites was increased since glass fiber is hydrophobic.The maximum water absorption was obtained for the RRR composite, and the minimum water absorption was obtained in the GG sample.Again, natural fibers (ramie) absorb more moisture than synthetic (glass).While comparing the RR and RRR composites, the water absorption in the former was lower, as it has lower ramie fiber content.This can be attributed to the ramie fiber containing many polar hydroxide groups, resulting in a high moisture absorption level (Bonniau and Bunsell 1981;Deo and Acharya 2010).Figure 2 shows the water absorption behavior of composites.All the composite samples achieved a steady state after 72 h of immersion.Other researchers have reported similar trends for natural fiber-reinforced composites (Negi et al. 2022;Prasad et al. 2020).The steady states for glass fiber reinforced composites (GG and GGG) were achieved much earlier (about 24 h).

Mechanical characteristics of composites
The fiber reinforcement, fiber loading and stacking affect the composite's hardness.The hardness was measured on both sides of the composites at random places and the average hardness values are reported.Figure 3 shows the hardness behavior of the fabricated composites.The maximum hardness was found for the GGG composite, followed by the GRG and GG composite.This was due to the higher wt.% of glass fiber, which is harder than the polymer matrix.The hardness of the composite RG improved by 26.92% when 4 wt.% of ramie fiber was replaced by the 4 wt.% of the glass fiber.The average hardness on ramie fiber and glass fiber sides of composite RG was 23 ± 0.7 and 42 ± 0.5 HV.The hardness increased by 42.30% (GG) when the glass fiber was used in place of ramie fiber (RR).This was due to the higher hardness of the glass fibers compared to ramie fibers.There was an increment in the hardness of the composite when the fiber loading wt.% increased (RR and RRR, 8 to 12 wt.%).The increase in hardness was also recorded when the glass fibers wt.% increased from 8 to 12 wt.%(GG and GGG).From the above results, it can be concluded that the hardness improved as the fiber content increased.Moreover, the addition of glass fibers improved the hardness compared to the addition of ramie fibers, due to hard glass fiber.Similar trends have been reported in other studies (Rudresh and Ravikumar 2017).
Figure 4 shows the tensile behavior of the fabricated composites.As the fiber content of glass fiber increased, the tensile strength of the composites increased.However, excessive glass fiber content led to voids forming in fabricated composite samples.Consequently, the GGG with 12 wt.%fiber loading slightly decreased tensile strength than GRG.The results showed a better tensile strength of composites with only glass fiber as a reinforcement than with only ramie fiber.This was attributed to the better tensile properties of the glass fibers (Giridharan 2019).With higher wt.% of fiber content, tensile strength increased.This can be attributed to the availability of more fibers to resist the same amount of deformation.From these results, it can be concluded that the hybridization showed better tensile strength compared to natural fiber and synthetic fiber alone.The fabricated composites showed better tensile strength compared to other studies.The maximum tensile strength was 114 MPa for GRG composite and 70 MPa for RRR.It was concluded that the hybridization of glass fiber enhanced more tensile strength than ramie fiber hybridization.Compared to existing literature, the composites without hybridization also showed good tensile behavior (Giridharan 2019).
The material's impact strength shows its toughness or the ability to resist high-speed stresses.Figure 5 shows the impact behavior of the fabricated composites.The impact strength of the composites increased with the fiber content irrespective of the fibers types (glass and ramie fibers).This was because more fibers resist high-speed loading (Prasad et al. 2020).The maximum impact energy was 4 J m −1 for the GGG composites, while the minimum was recorded for the RR composite (2 J m −1 ).At the same time, comparing the same amount of fiber by wt.% of glass and ramie fiber, as in the case of GGG and RRR and RR and GG, the glass fiber composites showed better impact strength.This can be attributed to the better bonding characteristics of glass fibers than ramie fibers.Consequently, better energy dissipation was obtained with the glass fiber composites.The same reason was responsible for the better impact behavior of the GRG composite compared to the RGR composite.It can be concluded from the above results that as the fibers content increased, the impact loading resistance of the composite increased.The glass fiber reinforced composites showed better impact resistance than ramie fiber reinforced composites.

Tribological behavior of the composites
The maximum wear was for the glass fiber reinforcement compared to ramie fiber reinforcement, as shown in Figure 6.The weight loss for each composite during the wear testing was reported in Table S2 (Supplementary Information).The composite's epoxy and fibers eroded during the test, and the average wear of two sides of the composites was reported.The maximum wear (80 μm) was for GGG, followed by the GG (73 μm) and GRG (63 μm).This can be attributed to the voids in the GGG composites.It can be concluded from these results that the limitation of glass fiber of poor abrasive resistance can be overcome by using hybrid ramie and glass fiber reinforced composites, as in the case of RGR (57 μm) composite, which showed lower wear.However, the minimum wear was recorded for RRR (37 μm), but the hardness was less.

Fractography of the hybrid composites
The SEM micrographs of the fractured hybrid composites are shown in Figure 7, depicting the damage caused to the fibers due to fracture.The extensive damage (fiber pull out and fiber breaking) was caused to the ramie fiber.The damages led to fiber breaking which was visible from the SEM images.Debonding of fiber and matrix was also visible.The mechanism for crack propagation, and consequently the composite mechanical strength, depends on the fiber/matrix adhesion (Djafar, Renreng, and Jannah 2020).The fibers were embedded in the matrix.However, some cavities formed in the composite.

Ranking of materials using the TOPSIS method
The technique of order preference similarity to the ideal solution (TOPSIS) method, was used for selecting the best possible options based on the physical, mechanical and tribological behavior.The comparisons were made using the TOPSIS method.This technique has been explained in our previous studies (Prasad et al. 2020(Prasad et al. , 2021)).Tables 3, S3-S5 (Supplementary Information), represent the decision matrix, normalization matrix, weight normalized matrix, separation measure, relative closeness, and ranking.The closeness factor shows the selectivity of the composites; the higher the closeness factor, the higher the selectivity.The composites with rank 1 (higher closeness factor), i.e.GRG, showed the optimal mechanical and physical characteristics.

Conclusions
Using different amounts of epoxy resin and hardener, efforts were made to improve the interlaminar adhesion of glass/ramie hybrid composites.Glass fiber being synthetic fiber showed a lower water absorption percentage than ramie fiber reinforced composites.The RRR composites showed 1.16% water absorption compared to only 0.24% for the GGG composite.The tensile strength of composites increased with glass fiber content and fiber loading.The tensile strength of the GGG composite (pure glass fiber reinforced) was 31.42% more than the RRR composite (pure ramie fiber reinforced).This was due to the high strength of the glass fiber than ramie fibers.Ramie fiber is a natural fiber, but with composites of ramie fiber, the impact strength is comparable to glass fiber's composites.Sample with pure ramie RRR showed an impact energy of 3 J m −1 , whereas the sample with pure glass GGG had an impact energy of 4 J m −1 .The results of composites prepared with one type of fiber were lower than the hybrid composites.Glass fibers  reinforced composites showed excellent mechanical properties.However, glass fiber's abrasive resistance is lower.The addition of natural fiber in glass fiber composite increased its wear resistance; simultaneously, there were lesser decrements in other properties of glass fiber composites.RRR composite showed higher wear resistance than GGG composite leading to such composites as bearing components.Glass fiber-reinforced composites absorbed very little moisture content, while the ramie fibers composites showed higher moisture content.At the same time, the hybrid composites showed better moisture characteristics.Therefore, a mixture of natural and glass fiber can be used to reduce glass fibers content leading to greener composites.The good bonding of ramie and glass fibers was visualized in SEM analysis of glass-ramie samples.From this study, it can be concluded that it is possible to assume that epoxy polymers could be substituted with ramie/epoxy composites for improved environmental and mechanical efficiency.Moreover, the findings show that the physical and mechanical characteristics of natural fibers composites are significantly affected by natural fiber hybridization and varying the fiber staking sequence.The fabricated composites are suitable for different structural and non-structural applications.

Highlights
• Hybrid composites using different wt.% of the glass and ramie fibers in different stacking sequences were fabricated.• The stacking sequence played a crucial role in the physical and mechanical characteristics of the composites.

Figure 1 .
Figure 1.Flowchart showing intermediate stages of this study containing ramie fibre and glass fibre and samples for (a) water absorption; (b) pin-on-disc wear; (c) hardness; (d) Impact; and (e) tensile test.

Figure 2 .
Figure 2. Water absorption behaviour of the composite samples.

Figure 3 .
Figure 3. Hardness behaviour of the composite samples.

Figure 4 .
Figure 4. Tensile behaviour of the composite samples.

Figure 5 .
Figure 5. Impact behaviour of the composite samples.

Figure 6 .
Figure 6.Wear characteristics of the composite samples.

Table 1 .
Designation, composition and stacking sequence of the fabricated composites.

Table 2 .
Density and void % of different composite samples.