Exploring the influence of stacking sequence on the mechanical properties of flax-ramie bi-quadratic layered hybrid epoxy composites

Over the past two decades, there has been a remarkable surge in the utilization of composites, with fiber-reinforced polymer composites gaining significant traction owing to their commendable structural performance. Concurrently, extensive research has been conducted to assess the behavior of composite materials, focusing on various parameters such as fiber type, lamination order, matrix type, and the inclusion of filler materials. In this study, we investigated the mechanical properties of flax (F)/ramie (R) epoxy hybrid composites, specifically exploring the effects of different lamination sequences. Four distinct lamination sequences were chosen: FFRR, FRFR, FRRF, and RFFR. The composites were fabricated using compression molding techniques, adhering to ASTM standards for the evaluation of mechanical properties including tensile strength, flexural strength, impact strength, and interlaminar shear strength. Among the laminate sequences studied, FRFR exhibited the lowest void content at 4.76%, while RFFR composites showed the highest void content at 16.67%. The most favorable results were observed with FRFR composites, boasting a tensile strength of 77.06 MPa, impact strength of 13.36 kJ m−2, and interlaminar shear strength of 8.13 MPa. Notably, the FFRR composite exhibited the highest flexural strength at 118.09 MPa. Additionally, scanning electron microscopy (SEM) analysis was conducted to investigate surface morphology and identify reasons for failure.


Introduction
The utilization of biowaste as a reinforcing material in materials science has accumulated significant attention from researchers over the past two decades [1].Owing to their appealing intrinsic properties, societal demands, and environmental responsibilities, bio-fiber composites are increasingly being considered in the automotive industry.Anticipated factors such as improved fuel efficiency, end-of-life cycle requirements, and governmental regulations are expected to further drive the adoption of bio-composites.Natural fibers such as jute, flax, sisal, and hemp find applications in manufacturing automotive components like package trays, headliners, door panels, dashboards, seat backs, and trunk liners.
Despite their advantages, natural fibers face several limitations that affect their widespread application in the automotive industry, including high moisture absorption, low temperature tolerance, susceptibility to microbial degradation, inadequate fire resistance, inferior mechanical properties and durability, quality variations, and price fluctuations due to seasonal crop production.Researchers have adopted various approaches such as hybridization of composites, fabrication methods, and the use of natural fillers to enhance binding action and strength [2].
A considerable number of studies have been conducted to quantify the mechanical properties of natural fiber-reinforced polymer composites.For instance, Umit Huner et al [3] demonstrated that NaOH surface treatment of flax fibers enhances their mechanical and adhesion properties.Javanshour et al [4] investigated the impact of graphene oxide-coated flax epoxy composites, reporting significant improvements in shear and transverse strength.Prabhakaran et al [5] explored the feasibility of producing composite laminates with improved acoustic and vibration damping capabilities using natural fiber-based materials, noting superior properties compared to glass fiber composites.Vinayagam Mohanavel et al [6] examined the reinforcement impact on hybrid composites using mechanical and thermogravimetric analysis, observing positive trends with continuous fiber composites.
Furthermore, research by Vijay Chaudary et al [7] highlighted the significant effect of moisture absorption on reducing tensile and flexural strength in jute/hemp/epoxy composites.Cavalcanti et al [8] investigated the mechanical properties of intra-laminar hybrid composites made of sisal, jute, and curaua fibers as reinforcements, noting significant improvements attributed to natural fibers and variations in fiber treatment methods.Mohanavel et al [9] explored the impact of fiber composition, layering patterns, and sequencing on the mechanical properties of hybrid composites, emphasizing the beneficial effects of adding jute fiber to glass fiber composites.
Moreover, Naveen Jesuarockiam et al [10] presented research on enhancing the thermal and dynamic mechanical properties of Kevlar/Cocosnucifera sheath (CS)/epoxy composites with graphene nanoplatelets (GNP), demonstrating the effective use of CS in structural applications.The investigation into the mechanical properties of flax/ramie hybrid epoxy composites with variations in stacking sequence suggests potential applications in automotive settings [11].Considerable outcomes have also been achieved in the field of natural fiber-reinforced composites using numerical methods [12][13][14].
The current understanding of hybridizing two distinct natural fibers, Flax and Ramie, and the consequent impact of their stacking sequence on mechanical properties remains limited.A comprehensive exploration of these factors is crucial for suggesting potential applications of natural fiber-reinforced materials in automotive settings.A detailed investigation into hybridization dynamics and stacking sequence effects will contribute to advancing knowledge in composite material science and optimizing the use of these materials in automotive applications.
Furthermore, Karimzadeh et al [15] investigated how the mechanical and moisture absorption characteristics deviate with changes in the stacking sequence of PALF/glass fiber epoxy composites, highlighting improvements in mechanical properties and water absorption characteristics with alternative stacking sequences.Similar investigations were conducted by Jafrey Daniel James D et al [16] and Venkatasudhahar et al [17], demonstrating the positive impact of stacking sequence variations on mechanical properties in bagasse/ sisal and carbon/jute/banana hybrid composites, respectively.Maruthi Prashanth BH et al [18] also investigated the effects of ply stacking sequence on the mechanical properties of hybrid jute banana fiber phenoplast composites, suggesting improvements for lightweight applications.
The objective of the present research is to analyze the effect of stacking sequence on the mechanical properties of hybrid (flax/ramie) epoxy composites, including tensile, flexural, impact, and interlaminar shear strength.SEM analysis will be employed to study microstructures, identifying void formation, fiber breakage, and delamination leading to composite failure.

Materials and methods
This section describes about the materials and fabrication methods that are used to prepare the composite laminate and the procedures of tests carried out to evaluate the mechanical properties of the laminates.The research article uses four laminates for a comprehensive exploration of stacking sequences, allowing for systematic testing of various configurations.This approach balances comprehensiveness and feasibility, as demonstrated in previous studies on kenaf/jute fibre hybrid composites [19].The use of a limited number of laminates provides valuable insights into the design space of hybrid composites.

Materials used
Hybrid composites were synthesized using flax and ramie fibers as reinforcing agents, in conjunction with an epoxy resin matrix.Procured from Fiber Region, Chennai, the raw materials, visually depicted in figures 1(a) and (b), exemplify the raw materials utilized in the composite manufacturing process.
The compositions, along with the physical and mechanical properties, of both flax and ramie fibers are thoroughly outlined in tables 1 and 2, respectively.

Method of fabrication
Four distinct types of hybrid composites were carefully fabricated through the variation of lamination order, employing the compression molding technique.Each composite assembly comprised two layers of flax fiber (F) intertwined with two layers of ramie fiber (R).The fabrication process involved the utilization of a 300 mm×300 mm MS mold.A matrix material blend of epoxy and hardener was prepared at a weight ratio of 10:1.Following mold completion, it was closed, subjecting it to a pressure of 10 Kg cm −2 at room temperature  for 24 h to facilitate curing.Refer to figure 2 for an illustrative representation of the compression molding process.
Prior to commencing the process, the fibers underwent treatment with a 5% wt.solution of NaOH and were subsequently dried for a duration of 24 h.In all considered composite materials, the fiber orientation remained consistent, aligned along the longitudinal direction.The lamination sequence employed during the manufacturing process is depicted in figure 3, while figure 4 showcases the resultant fabricated laminates.
It was widely acknowledged that the capability of flax fiber to absorb moisture is significantly augmented through chemical and physical treatments, thereby elevating the potential of the resulting composites.Furthermore, it was observed that the orientation of fibers concerning the thickness direction of samples profoundly influences the water absorption behavior of the composites due to the establishment of preferential water transport pathways along the fiber direction.Notably, fibers aligned at a 45-degree angle induce the highest water absorption in composites, whereas fibers arranged perpendicular to the thickness direction exhibit the least water absorption [20].
The details of the fabricated hybrid composites are furnished in table 3:

Density of composites
The theoretical density of composite materials is derived using rule of mixtures shown in the equation (1).
The experimental density was determined using a specific gravity tester at Kelvn Labs Pvt.Ltd, Hyderabad, in accordance with ASTM D 792 standards.Initially, the specimen was weighed in a dry condition, denoted as w d .Subsequently, the weight of the fully immersed sinker was recorded as w s .The specimen was then placed inside the immersed sinker, and the combined weight was noted as w sw .The specific gravity and density of the composite were calculated using the following equations, denoted as equations (2) and (3), respectively.with a loading rate of 10 mm min −1 .Tensile loads were applied in the longitudinal direction of the fibers.For each lamination sequence, five specimens were subjected to testing, and the resulting averages were recorded.

Flexural test
The flexural test was also conducted using the same Universal Testing Machine (UTM), following the test method and specimen preparation outlined in ASTM D 790, as depicted in figure 6. Specimens measuring 130 mm in length and 12.7 mm in width were prepared and subjected to three-point bending, adhering to the recommended span-to-depth ratio of 16:1.A load cell of 10 kN capacity was employed, and the test was executed at a speed of 2 mm min −1 .Five specimens underwent testing for each lamination sequence, and the average results were recorded.

Impact test
To evaluate the impact strength of the composite material, an Izod impact testing machine is utilized, coupled with a computerized cutting machine to fabricate a V-notch in accordance with the ASTM D-256 standard, as illustrated in figure 7. Specimens are subsequently prepared following this standard, and the impact test is conducted on these specimens to quantify their impact strength.

Inter laminar shear strength test
The interlaminar shear strength (ILSS) was assessed following the ASTM D2344-84 standard.A small beam, 45 mm in length with a square cross-section, was cut and subjected to three-point bending load at a rate of 1.3 mm min −1 .During testing, the specimen experienced both normal (bending) and transverse shear stresses as the loading cylinder applied downward force.Utilizing a short beam helps minimize the influence of bending loads on interlaminar shear failure, ensuring that cracking occurs predominantly along a horizontal plane between the laminates.The interlaminar shear strength was quantified using equation (5):

Results and discussions
In this section, the detailed results of the experiments were presented and discussed.

Density
The density of the composites with various laminate sequences is depicted in figure 8.It is evident that the density remains relatively consistent across different laminate sequences.Additionally, a slight decrease in composite density is observed compared to the densities of the fibers.The void content (%) of the laminates is calculated using equation (4) and summarized in table 4.
As indicated in table 4, the highest density (1.19 g cc −1 ) was observed in FRFR, attributable to a lower void content (4.79%), while the lowest density (1.04 g cc −1 ) was recorded in RFFR, attributed to a higher void content (16.67%).It is noteworthy that the lack of compatibility between ramie fiber and epoxy resin results in partial wetting of the ramie fiber by the matrix, leading to the formation of voids [21,22].These void contents may impact the mechanical properties of the composites.In comparison with the reinforced materials (flax and ramie), the FRFR composite is 22.72% lighter in weight.

Tensile properties
The positioning of fiber types during composite preparation profoundly impacts the characterization of mechanical properties.In this study, investigations were conducted to assess the effect of fiber type positioning in a four-layer composite.The tensile load, strength, modulus, and % elongation for various stacking sequences of flax-ramie laminated hybrid composites are depicted in figures 9(a)-(d), respectively.Among all the sequences analyzed (FFRR, FRFR, FRRF, RFFR composite types), FRFR demonstrated the highest response to the tensile load, as evident from figure 9(a).In figure 9(b), it was observed that the FRFR composite recorded the highest tensile strength of 77.06 MPa, while the lowest was recorded by the FRRF Composite at 63.86 MPa.Compared to the FRFR composite, FFRR, RFFR, and FRRF exhibited reductions of 7.16%, 14.12%, and 17.12% in tensile strength, respectively.The effective load transfer occurred when subjecting the composite to tensile loads, facilitated by arranging the fibers in FRFR, which efficiently distributed   the load between two materials of different natures.Additionally, the FFRR, FRFR, and RFFR composites displayed tensile moduli 1.82%, 3.36%, and 1.71% lower than that of FRRF, respectively, as observed in figure 9(c).Figures 9(d) and 10 illustrate the variation of % elongation and load versus displacement for the various composites.Despite utilizing the same type of material and composition, altering the position of the fiber resulted in varied responses to elongation.Among the considered composites (FFRR, FRFR, FRRF, RFFR), the FRRF composite exhibited the highest elongation.This can be attributed to the arrangement of the ramie fiber between the composite layers, collectively extending in response to the applied load.The remaining composites displayed 34.87%, 18.87%, and 30.89% less elongation than that of the FRRF composites.
SEM images capturing the fracture area are shown in figures 11(a)-(c).When subjected to tensile loading, the FRRF composite exhibits noticeable layer separation, leading to a diminished tensile strength (figure 11(a)).This is attributed to suboptimal bonding, resulting in a reduction in tensile strength.Conversely, the FRFR composite demonstrates robust load support, evident in fiber pull-out as illustrated in figure 11(b).The FFRR sequence achieves a moderate tensile strength, facilitated by a combination of fiber pull-out and fiber breakage (figure 11(c)).

Flexural properties
The flexural strength and modulus for different laminated sequences of flax-ramie epoxy hybrid composites are illustrated in figures 12(a) and (b), respectively.
A notable response is observed for the same composite considered in the study under bending.In this scenario, the FFRR sequence stands out by offering maximum benefits in terms of both flexural strength and flexural modulus.The high cellulose content and low lignin percentage of Ramie fiber contribute to excellent resistance against bending loads.Consequently, the FFRR sequence exhibits superior flexural modulus and flexural strength.

Impact strength
The impact strength for different lamination orders of flax-ramie epoxy hybrid composites is illustrated in figure 13.The maximum impact strength, 13.36 kJ m −2 , is recorded for FRFR composites.
When subjected to a sudden force, the sequence where ramie fiber was supported by flax fiber demonstrates superior performance compared to other sequences analyzed.Remarkably, even when ramie fiber is supported by the same fiber, it does not have a significant influence on impact strength compared to the combination of ramie and flax fibers.When compared with the FRFR composite laminate, FFRR, FRRF, and RFFR show 2.61%, 12.30%, and 11.47% less in impact strength, respectively.SEM images capturing the fracture area are presented in figures 14(a)-(c).
When subjected to impact loading, the FRRF composite exhibits noticeable fiber pullout, voids, and debris remaining on the surface, as shown in figure 14(a).This is attributed to suboptimal bonding, resulting in a

Inter laminar shear strength
The combination of similar or dissimilar fibers generates interlaminar shear stresses, and these stresses may surpass the shear strength of the same material, leading to failure.When recommending a composite material  for any industrial application, consideration of interlaminar stresses is crucial, in addition to tensile and flexural strength.
In this study, interlaminar shear strength is depicted in figure 15.The FRFR composite exhibits the highest shear strength.This superior performance is attributed to the effective support achieved between the flax and ramie fibers when arranged consecutively, surpassing other combinations considered in the study.The maximum strength of 8.13 MPa was obtained for the FRFR composite, as presented in figure 15.

Conclusions
The current study investigated how the stacking order affected the mechanical characteristics of hybrid composites made of flax, ramie, and epoxy.Based on the findings of the SEM analysis and mechanical analysis, the following conclusions can be drawn: • The results have indicated that FRFR composites, characterized by a lower void content (4.79%), are 22.72% lighter in weight compared to flax and ramie fibers.
• Experimental results have unveiled that tensile strength is influenced by the stacking sequence of the fibers.
The maximum tensile strength of 77.06 MPa is recorded with the FRFR laminate, while the composites FFRR, RFFR, and FRRF showed 7.16%, 14.12%, and 17.12% less tensile strength, respectively, compared to FRFR laminates.Additionally, the effect of stacking sequence on the tensile modulus is negligible.
• Experimental results have demonstrated that flexural strength is influenced by the stacking sequence of the fibers.The maximum flexural strength of 118.09MPa and flexural modulus of 17.59 GPa are recorded with the FFRR laminate, while the composites FRFR, FRRF, and RFFR exhibited 11.99%, 15.72%, and 41.82% less flexural strength, respectively, compared to FFRR laminates.
• The stacking sequence has demonstrated a significant effect on the impact strength.The ply sequence FRFR has recorded the highest impact strength of 13.36 (kJ/m 2 ), while the composites FFRR, FRRF, and RFFR exhibited 2.61%, 12.30%, and 11.47% less impact strength, respectively, compared to FRFR composite laminate.
• Interlaminar shear strength is also influenced by the ply sequence of the composite laminate.In the interlaminar shear strength test, the FRFR composite recorded a high value of 8.13 MPa, while the composites FFRR, FRRF, and RFFR exhibited 29.35%, 37.16%, and 44.77% less interlaminar shear strength, respectively, compared to the FRFR composite laminate.
• From the SEM micrographs, it was observed that the failure of the composites was mainly due to fiber pullout, fiber breakage, matrix cracks, and delamination between the fiber and matrix.
The best natural fiber hybrid composite for lightweight and high-strength applications was found to be the one with a flax-ramie-flax-ramie stacking sequence, according to the overall mechanical performance of the FRFR composite.The properties of the proposed composite laminate are presented in table 5.

Figure 2 .
Figure 2. Compression molding equipment setup at M/s Vaishnav composites, Hyderabad used for laminate fabrication.

Figure 3 .
Figure 3. Lamination sequence followed to fabricate the composite laminate.

ce 2 . 4 .
The void content within the composite laminate was determined in accordance with ASTM D 2734-70.The volume fraction of the void (V v ) is calculated using the equation (4).Tensile test The specimens for the tensile test were prepared in accordance with ASTM D 638 guidelines, as illustrated in figure5.Tensile testing was conducted using the Universal Testing Machine DXT at Kelvn Labs, Hyderabad,

Figure 9 .
Figure 9. (a) Tensile load of four laminates with different stacking sequence.(b) Tensile Strength of four laminates with different stacking sequence.(c) Tensile Modulus of four laminates with different stacking sequence.(d) % Elongation of four laminates with different stacking sequence.

Figure 10 .
Figure 10.Variation of % of elongation of the different composite considered for the study.

Table 3 .
The details of the fabricated hybrid composites.
S. no.

Table 4 .
% Void content of composite laminates.