Characterization of Physical and Mechanical Properties of Rice Straw Particles and Furcraea foetida Fiber Reinforced Hybrid Composite

ABSTRACT

The biodegradable characteristics and abundant availability of the fiber sources have gained the attention of various industries to produce natural fiber-based composites. As a sustainable alternative to the non-biodegradable fiber-based products, the natural composites provide a viable solution to reduce the environmental pollution caused by synthetic materials. This study developed rice straw particle (RS_p) and Furcraea foetida (FF) fiber reinforced hybrid composite and investigated its physical and mechanical properties. The addition of 15 wt.% of RS_p reduced the density of the test samples by 41.87% and its water absorption (WA) increased with the increase in fiber concentration. The composite with 5 wt.% and 15 wt.% of RS_p showed maximum tensile strength (σ_t: 29.45 MPa) and modulus (σ_tm: 3.67 GPa), respectively. At 15 wt.% of RS_p, the maximum flexural strength (σ_f: 43.12 MPa) and modulus (σ_fm: 2.09 GPa) was achieved and at 10 wt.% of RS_p showed the highest impact strength (σ_i: 101.01 J/m). The σ_t (40.21%) and σ_f (7.76%) of the RS_p reinforced composite were improved by the hybridization of FF (20 wt.%) fiber reinforcement.

摘要

纤维源的生物可降解特性和丰富的可用性引起了各行业生产天然纤维基复合材料的关注. 作为不可生物降解纤维基产品的可持续替代品，天然复合材料为减少合成材料对环境的污染提供了可行的解决方案. 本研究开发了稻草颗粒（RSp）和黄曲霉（FF）纤维增强混杂复合材料，并对其物理力学性能进行了研究. 添加15 wt.%的RSp使测试样品的密度降低了41.87%，并且其吸水率（WA）随着纤维浓度的增加而增加. RSp含量为5 wt%和15 wt%的复合材料分别表现出最大拉伸强度（σt:29.45MPa）和模量（σtm:3.67GPa）. 在15 wt.%的RSp下，获得了最大的弯曲强度（σf:43.12MPa）和模量（σfm:2.09GPa），在10 wt.%时，RSp显示出最高的冲击强度（σi:101.01J/m）. FF（20 wt.%）纤维增强复合材料的σt（40.21%）和σf（7.76%）得到了改善.

KEYWORDS:

• Rice straw
• Furcraea foetida
• hybrid composite
• mechanical properties
• SEM analysis

关键词:

• 稻草
• 鹅掌楸
• 杂交复合材料
• 机械性能
• SEM分析

Introduction

The continuous consumption of polymer materials for various industrial applications is drastically increasing the global demand (380 million tons in FY 2018) for synthetic materials (Alabi et al. 2019). The polymers are non-biodegradable in nature and cause severe environmental pollutions. They also release harmful gasses/particles due to the continuous exposure to cyclic weathering conditions and trigger health problems in living beings (Li et al. 2023; Thompson et al. 2009). Even with the continuous efforts toward recycling and regulating these materials, their consumption is rapidly rising and severely impacting the environment. Most of these synthetic wastes pollute aquatic and land resources, and an estimated plastic waste of 1.15-to-2.41-million-ton flow into the oceans through rivers annually (Lebreton et al. 2017). Hence it is essential to regulate and replace synthetic materials with sustainable and eco-friendly alternatives. Natural composites are highly promising materials that can potentially replace synthetic materials in civil, automobile, aerospace and packaging and other industrial applications (Khalid et al. 2021; Yahas et al. 2019). Natural composites are reinforced with materials extracted from natural resources such as plants, animals and minerals. Plant fibers such as jute, hemp, sisal, abaca, ramie, bamboo are majorly employed in different applications as they are biodegradable, low cost and easily available (Arpitha and Yogesha 2017; Rozali et al. 2017). Recently the use of two or more natural fibers to fabricate hybrid composites are popularly practiced for improving the functionalities of the composites. Also, various studies have shown that the addition of multiple fibers in the composite improve the mechanical properties of the composites. Studies on hybrid hazelnut/walnut shell-filled acrylonitrile butadiene styrene composite showed better tensile (σ_t: 34 MPa) and flexural (σ_f: 59 MPa) strength compared to composites made with individual fibers (Kuram 2022). The hybrid Aloevera/pistachio shell particle reinforced epoxy composite showed higher σ_t (130 MPa), σ_f (178 MPa) and impact strength (6.7 J) compared to other composite test samples during the study (Suganya et al. 2022). The addition of Sirisha bark filler in coir fiber-reinforced hybrid polypropylene composite showed better flexural modulus (3.6 GPa), impact strength (3.25 J/cm), and it also improved flame resistance in the composite (Khuntia and Biswas 2022). Adding sawdust fillers in banyan fiber increased the tensile modulus and impact strength of the hybrid epoxy composite (Raja et al. 2022). The addition of wood particles and nano clay fillers in hemp/polypropylene composite exhibited better σ_t (35.18 MPa) and σ_f (40.93 MPa). Also, the moisture resistance in the test samples was improved (Alshgari et al. 2022). Ricciardi et al. (2019) prepared hybrid basalt/flax/epoxy composite which exhibited higher impact properties compared to individual single fiber reinforced composite. The σ_t (24 MPa) and σ_f (49.5 MPa) of the bambara nutshell/clay hybrid polyester composite improved by the addition of clay in the composite (Okonkwo et al. 2020). Bekele, Lemu, and Jiru (2022) improved the tensile (38.8%) and flexural (12.6%) strength of polyester resin by reinforcing enset and sisal fibers (Vijaykumar et al. 2020). prepared cost-effective and eco-friendly materials as an alternative to gypsum boards by utilizing groundnut shell and rice straw reinforced hybrid polypropylene composite for civil structural application. Jani et al. (2021) observed significant improvement in the mechanical properties by the addition of coconut and palm shell filler in hemp/Kevlar composite. Similarly, the tribological properties of Bauhinia-vahlii/sisal fiber reinforced epoxy composite improved by the hybridization of rice husk fillers (Kumar et al. 2019). The rice straw/PLA composite prepared by Agirgan, Agirgan, and Taskin (2022) showed good thermal insulation (0.01618 W/m K) and sound absorption coefficient (0.99) and suggested for its use in civil applications. In this study, agricultural waste rice straw is considered for the preparation of composites. Further, to improve the mechanical properties of the resulting composite, they are hybridized with an unexplored novel Furcraea foetida fiber which showed attractive reinforcing properties in the preliminary studies. This work attempts to explore the effect of hybridization of rice straw particles with novel FF fiber for creating cladding panels particularly for thermal insulation of the living spaces.

Materials and methods

The Lapox L12 Epoxy and K6 hardener combination was used for the preparation of test specimens and was procured from Atul Pvt. Ltd, Gujrat, India. The rice straw and FF plant leaves were collected locally in the Udupi district. The cleaned and segregated rice straws are chopped into a smaller particle of length <2 mm and are then ground to produce rice straw particles (Madival et al. 2022). The FF fiber is extracted from its plant leaf through water retting process similar to the process followed by Abhishek et al. (2022) and Madival et al. (2023). The extracted FF fiber was segregated, and a unidirectional fiber mat is prepared for the fabrication of the composite. The detailed steps followed for the extraction of FF fiber are shown in Figure 1.

Figure 1. Extraction process of Furcraea foetida fiber. (a) FF plant leaves, (b) soaking of FF leaves in water, (c) separation of FF fibers, (d) drying of FF fiber under sunlight and (e) FF fiber mat.

Preparation of hybrid RS_p/FF composite

The composite was fabricated by hand layup process (Poonia et al. 2022). Initially, RS_p and epoxy mixture is prepared and a layer of this mixture is coated on the mold. Then, the FF fiber mat is placed on this layer and gently pressed by hand and another layer of RS_p and epoxy mixture is added on top of the FF mat. This step is repeated until the required thickness of the laminate is achieved. Finally, the top of the mold is covered with a metal plate and is allowed to cure under atmospheric conditions for 24 h. Finally, the cured hybrid RS_p/FF composite was removed from the mold. Figure 2 shows the detailed steps of the fabrication process. Table 1 shows the details of test samples blending and the corresponding sample codes for describing the test samples.

Figure 2. Process of hybrid RS_p/FF polymer composite production. (a) Epoxy and hardener combination, (b) prepared RS particles, (c) RS_p and epoxy mixture, (d) FF fiber Mat, (e) preparation of metal mold, (f) layup process of laminate, (g) curing and (h) cured R10F20 composite.

Table 1. Material composition for hybrid composites.

Sl. no	Sample code	RSp (WRS)	FF (WFF)	Epoxy (Wm)
1	Neat	0	0	1
2	R05F20	0.05	0.20	0.75
3	R10F20	0.10	0.20	0.70
4	R15F20	0.15	0.20	0.65

Study of physical properties

The effect of fiber reinforcement on the physical and mechanical properties of the composite is evaluated by conducting different experimental trials. The theoretical density (ρ_t) of the composite samples is evaluated by rule of mixture as given by Eq. (1), where ρ_m,f = density (g/cm³) and V_m,f = volume fractions of matrix and fiber, respectively. The experimental density (ρ_e) is calculated by determining the volume of water displaced by the test samples using Eq. (2), where W_s = weight of the sample, V_d = volume of water displaced. The voids developed in the test sample are estimated by considering ρ_t and ρ_e of the composite samples using Eq. (3) (Sydow et al. 2021), whereas the WA in the fabricated hybrid RS_p/FF composite samples is evaluated by immersing the test samples in water for 720 h as per ASTM D570–98. The WA in the composite samples is calculated using Eq. (4), where W₁ (g) = initial weight, W₂ (g) = final weight of specimens (Wang et al. 2020).

(1) ${\rho }_{\mathrm{t}}(\mathrm{g}/\mathrm{c}{\mathrm{m}}^3)={\rho }_{\mathrm{m}}{\mathrm{V}}_{\mathrm{m}}+{\rho }_{\mathrm{f}}{\mathrm{V}}_{\mathrm{f}}$(1)

(2) ${\rho }_e(\mathrm{g}/\mathrm{c}{\mathrm{m}}^3)={\mathrm{W}}_{\mathrm{s}}/{\mathrm{V}}_{\mathrm{d}}$(2)

(3) ${\mathrm{V}}_c(\mathrm{\%})=\frac{{\rho }_{\mathrm{t}}-{\rho }_{\mathrm{e}}\hspace{0.17em}}{{\rho }_{\mathrm{t}}}\times 100$(3)

(4) $\mathrm{W}\mathrm{A}(\mathrm{\%})=\frac{{\mathrm{W}}_2-{\mathrm{W}}_1}{{\mathrm{W}}_2}\times 100$(4)

Study of mechanical properties

The mechanical properties of the composite are studied to evaluate the strength and load bearing capabilities of the fabricated composite materials. The microhardness of the hybrid RS_p/FF composite is evaluated as per ASTM E382–17 using Vickers hardness (V_h) instrument (MMT-X, Matsuzawa Co., Ltd., Toshima, Akita, Japan). The σ_t of the composite specimens was measured using universal testing machine (Biss, Itw, Universal Testing Machine) according to ASTM D 3039 testing standards. The σ_f is evaluated as per ASTM D 790 standards using universal testing machine (UNITEK 9940, Fuel Instruments and Engineers Pvt. Ltd., Kolhapur, Maharashtra, India), whereas the σ_i of the test samples is evaluated by Izod impact tester (HIT50P, Zwick/Roell, Ulm, Germany) as per ASTM D4812 testing standard. The equations used for calculating the mechanical properties of the test samples are listed in Table 2, where F_a = the applied force (N), A_i = the area of indentation (mm²), F_m = maximum force (N), A_c = cross-sectional area (mm²), L = length (mm), w = width (mm) and d = thickness (mm), D = displacement, m = slope of the tangent to the initial straight-line portion of the load-deflection curve and ε_t, ε_f = strain under tensile and flexural load, respectively.

Table 2. List of equations used to calculate the mechanical properties.

Mechanical property	Equation
Vickers Hardness (HV)	${\mathrm{V}}_h=1.854\frac{{\mathrm{F}}_{\mathrm{a}}}{{\mathrm{A}}_{\mathrm{i}}^2}$	(5)
Tensile strength (MPa)	${\sigma }_{\mathrm{t}}=\frac{{\mathrm{F}}_{\mathrm{m}}}{{\mathrm{A}}_{\mathrm{c}}}$	(6)
Tensile strain	${\epsilon }_{\mathrm{t}}=\frac{D}{\mathrm{L}}$	(7)
Tensile modulus (GPa)	${\sigma }_{\mathrm{t}\mathrm{m}}(\mathrm{G}\mathrm{P}\mathrm{a})=\frac{\mathrm{\Delta }\sigma }{\mathrm{\Delta }\epsilon }$	(8)
Flexural strength (MPa)	${\sigma }_{\mathrm{f}}=\frac{3{\mathrm{F}}_{\mathrm{m}}\mathrm{L}}{2\mathrm{w}{\mathrm{d}}^2}$	(9)
Flexural strain	${\epsilon }_f=\frac{6\mathrm{D}\mathrm{d}}{{\mathrm{L}}^2}$	(10)
Flexural modulus (GPa)	${\sigma }_{\mathrm{f}\mathrm{m}}=\frac{{\mathrm{L}}^{3}m}{4\mathrm{w}{\mathrm{d}}^3}$	(11)

Results and discussion

Density and void content

Figure 3 shows the ρ_t and ρ_e of hybrid RS_p/FF composite. It is observed that the ρ_t and ρ_e of the test specimens decreased with the increase in the RS_p concentration. The lower density of RS_p compared to the density of epoxy resulted in a decrease in the density of the hybrid composites. The ρ_e of R15F20 test sample decreased by 41.52% in comparison with the neat laminate. It is also observed that the ρ_e of test samples is lower than the ρ_t. This is due to the formation of porous structure by the entrapment of air/gaseous volatiles released during the fabrication process (Chowdari, Prasad, and Devireddy 2020; Shivamurthy et al. 2015). The study also revealed that the addition of RS_p reduced the void content in the composite samples. The RS_p spread across the FF fiber mat filled the gaps between intermediate FF fiber and epoxy regions which reduced the porosity in test specimens (Rashid et al. 2017). The void content (%) in R15F20 composite is reduced by 33.44% compared to R05F20. The neat, R05F10, R10F20 and R15F20 composites showed void content of 1.6%, 6.25%, 4.87% and 4.16%, respectively.

Figure 3. Density of the fabricated composites.

Water absorption (WA)

Figure 4 shows the percentage of WA in different test samples examined at an interval of 24 h for 30 days. The neat composite is water resistant; therefore, it did not show considerate amount of water absorption. The addition of fibers in epoxy increased the WA due to the hydrophilic nature of the natural fibers. The hydroxyl groups present in the natural fibers store water molecules through hydrogen bonding in the fiber cell walls (Norizan et al. 2017). The increase in the natural fiber concentration in the laminate simultaneously increased the quantity of hydroxyl groups and accelerated the moisture uptake. Initially, there was a rapid rise in the WA (4–7%) in the hybrid test samples up to 24 h of immersion and thereafter the WA rate gradually increased during the study period. Under continuous water immersion, the test samples absorbed maximum possible moisture content and reached a saturation point (>648 h) above which the WA in the test samples was negligible. The composite with 15 wt.% of RS_p showed highest WA (14.54%) during the immersion period of 720 h, and the neat composite showed lowest WA (0.08%). The study conducted by Ogabi et al. (2016) (bagasse/sisal/coir), Girimurugan et al. (2021) (banana/camellia sinensis) and Bekele, Lemu, and Jiru (2022) (enset/sisal) observed similar trends in WA compared to neat and RS_p/FF composites.

Figure 4. Water absorption in hybrid RS_p/FF composite.

Micro hardness

Figure 5 shows the hardness of neat, R05F20, R10F20 and R15F20 test specimens. The neat composite exhibited the highest hardness of 21.14 HV compared to other test samples. The increase in the RS_p content decreased the hardness of the hybrid composites. The top surface layer of the hybrid composite is covered by a thin layer of epoxy. During indentation, the applied load on the surface of the hybrid composite is transferred to the RS_p which is relatively softer and possesses lower rigidity compared to the cross-linked epoxy material (Shivamurthy et al. 2019). Therefore, the overall hardness of the epoxy material is decreased by the RS_p reinforcement. In addition, the smaller filler size and the micro voids on the surface of the composite are also considered to be the reason for the decrease in the hardness of hybrid composites (Pujar and Mani 2022). The addition of 15 wt.% of RS_p in epoxy decreased the hardness of the composite by 48.15%.

Figure 5. Hardness of RS_p/FF hybrid composites.

Tensile properties of hybrid RS_p/FF composites

Figure 6 shows the σ_t and σ_tm of composites with different concentrations of rice straw and FF fiber. The neat sample showed highest σ_t of 31.95 MPa, whereas the R05F20 showed maximum σ_t of 29.45 MPa. The standard deviation of composites during tensile test slightly increased with the increase in the fiber content due to the higher inhomogeneity developed by the natural reinforcements. The addition of RS_p decreased the σ_t of the epoxy material. The σ_t of R15F20 composite is reduced by 20.06% compared to the neat sample. The interphase between the RS_p and epoxy acted as stress concentration zones in the laminate and the cracks propagated through these interfaces under the tensile load. These stress concentration zones were increased with the increase in the RS_p content and thus reduced the σ_t of the composite. Similar trends in σ_t were observed by Zhang and Hu (2014); Fu et al. (2008) and Srivastava (1990) where the addition of fibers decreased the σ_t of the composites. The increased RS_p content also resulted in poor wetting of FF fiber and developed weak interfacial adhesion between the intermediate fiber phases in the composite affecting the σ_t of composites (Joshi, Pramendra, and Samrat 2022; Kaatubi et al. 2022). Due to the above cited reasons, overall σ_t of the hybrid RS_p/FF composite decreased compared to neat composite.

Figure 6. Tensile properties of the composites.

It is also observed from Figure 6 that the σ_tm of the test samples increased with the increase in RS_p content. The tensile modulus was marginally improved with the addition of RS_p from 5 to 15 wt.%. However, compared to neat epoxy, the tensile modulus of the hybrid composite with maximum combination of RS_p and FF was improved by 69.75%. The R15F20 combination showed highest σ_tm of 3.67 GPa. Figure 7 shows the stress-strain plots of composites subjected to tensile load. The neat sample showed highest strain (0.024) compared to hybrid composites. This is due to the lower intermolecular interactions in the polymers which exhibited lower tensile modulus and correspondingly higher strain under the applied tensile load (Li et al. 2018). However, the maximum strain resulted in the composite decreased with the addition of 15 wt.% RS_p and 20 Wt. % of FF fiber. This is due to the rigid nature of natural fibers which developed higher stiffness in the composite resulting in increased σ_tm in the hybrid composite (Kilinc et al. 2016). Similar results were also observed by Saba et al. (2019) (Date palm/Epoxy), Azlina et al. (2019) (oil palm empty fruit bunch/bagasse/Phenol Formaldehyde) and Mahjoub et al. (2014) (Kenaf/Epoxy) in their study of hybrid composites.

Figure 7. Stress-strain plot of the composites subjected to tensile load.

Flexural properties of RS_p/FF composites

Figure 8 shows the σ_f and σ_fm of test specimens. It is observed that the σ_f and σ_fm of epoxy composite reduced by the addition of RS_p and FF fiber reinforcements and the neat test sample showed highest σ_f (107.63 MPa) and σ_fm (3.39 GPa) compared to hybrid composites. This is due to the lower load bearing capacity of the fibers in the direction perpendicular to the fiber axis (Bachmann, Wiedemann, and Wierach 2018; Jagadeesh et al. 2021). Also, the voids and weak interfacial adhesion between the fiber and matrix material further reduced the flexural strength of the composite. Interestingly, the σ_f and σ_fm of hybrid composites marginally improved with the increase in the RS_p content in the composite. This improvement is attributed to the higher stiffness attained by the laminate by the addition of RS_p which delayed the crack initiation under the applied flexural load (Yu et al. 2016; Yussuf, Massoumi, and Hassan 2010). The increase in the RS_p content from 5 wt.% to 15 wt.% improved the σ_f and σ_fm of hybrid composite by 22.79% and 13.39%, respectively.

Figure 8. Flexural properties of the hybrid composites.

Figure 9 shows the stress-strain values of test specimens subjected to flexural load. The maximum strain developed in the neat composite is found to be decreased by the addition of fiber reinforcement. However, the strain induced in the hybrid RS_p/FF composite gradually increased upto 10 wt.% of RS_p in the laminate and it also showed highest strain (0.03) compared to other composites. This improvement in the strain is due to the delay in the crack initiation by the incorporation of optimum concentration of RS_p in the laminate. Further increase in the RS_p (>10 wt.% of RS_p) resulted in insufficient fiber wetting which induced higher rigidity and reduced the maximum strain induced in the laminate. Similar observations were made by Mourad, Abu-Jdayil, and Hassan (2020) (Emirati red shale/unsaturated polyester), Ravichandran, Rathnakar, and Santhosh (2021) (HNT and nano alumina particles/glass-epoxy).

Figure 9. Stress-strain plot of the composites subjected to flexural load.

Impact strength of RS_p/FF composites

Figure 10 shows the σ_i of hybrid RS_p/FF composite. It is observed that the addition of RS_p from 5 wt.% to 10 wt.% with 20 wt.% FF fiber content in the matrix resulted in improving the σ_i of the hybrid composite. At 10 wt.% RS_p concentration, the σ_i of 101.01 J/m is produced in the hybrid composite. Further addition of rice straw particles beyond 10 wt.% up to 15 wt.% resulted in reducing the σ_i by 24.87% with reference to peak σ_i obtained by R10F20 composite. Increasing the rice straw concentration beyond the optimum level (10 wt.%) resulted in improper fiber wetting and thus created the weaker interfacial bonding between the fiber and the matrix material (Ramesh et al. 2022; Salman et al. 2015). This leads to a reduction in the ability of the resulting composite to absorb and transfer the impact load effectively. Hence, the σ_i gets reduced at higher concentration of RS_p.

Figure 10. Impact strength of RS_p/FF hybrid composites.

Table 3 shows the comparative details of physical and mechanical properties of hybrid RS_p/FF (R15F20) and single fiber reinforced RS_p and FF composites from the earlier works by the authors. It is observed that the hybrid composite showed lower mechanical properties compared to FF composite. However, the σ_t (40.21%) and σ_f (7.76%) of the RS_p composite is improved by the hybridization of FF fibers. The hardness (34.76%), σ_t (20.53%) and σ_f (45.03%) of FF composite are reduced compared to hybrid RS_p/FF composite. It is seen that the incorporation of long FF fiber provides better structural stability and enhances the mechanical properties of the composite. Further, the mechanical properties of the fabricated hybrid RS_p/FF (R15F20) composite in the present study are compared with other similar hybrid composites. The details of the composites and the respective mechanical properties are listed in Table 4.

Table 3. Comparison of hybrid RS/FF composite with single fiber reinforced composite.

Fiber	Matrix	ρe (g/cm^3)	Vh (HV)	σt (MPa)	σf (MPa)	References
RSp/FF (15/20 Wt.%)	Epoxy	0.69	10.96	25.54	43.12	Present Study
RSp (25 Wt.%)	Epoxy	0.65	17.80	15.27	39.77	(Madival et al. 2022)
FF (30 Wt.%)	Epoxy	1.03	16.80	32.14	78.45	(Abhishek et al. 2022)

Table 4. Comparison of tensile and flexural strength of various hybrid composite materials.

Matrix	Fiber	Fiber	σt (MPa)	σf (MPa)	σi	Reference
Epoxy	Rice straw	Furcraea foetida	25.54	43.12	101.01 J/m	Present study
Epoxy	Pineapple leaf	Pineapple	89.43	79.55	88.63 J/m	Saha et al.
Epoxy	PP shell	Kenaf/Sisal	86.94	287.84	61.68 J/m	(Saha, Kumar, and Kumar 2021)
PLA	Hemp/sisal	Sisal	46.25	94.83	11.08 kJ/m^2	(Jagadeesh et al. 2021)
Polyester	Coconut shell	Sugarcane	12	41	0.465 J/mm^2	(Pappu, Pickering, and Thakur 2019)
Polyester	Coconut shell	Roselle	32.17	74	17.97 J	(Gokul, Prabhu, and Rajasekaran 2017)
Epoxy	Banana	Sisal	19.93	60.86	20.22 kJ/m^2	(Sadyan and Athijayamani 2016)
Epoxy	Flax	Banana/glass	30	13.54	16 J	(Narayanan et al. 2011)
Polyester	Sisal	Roselle	32.4	51.3	1.41 kJ/m^2	(Srinivasan et al. 2014)
PF	Banana	Glass fibers	45	54.5	70 kJ/m^2	(Athijayamani et al. 2009)
B-CF	Jute	Rice husk	10	14	-	(Joseph et al. 2008)
Epoxy	Banyan	Saw dust	31.15	34.37	18 J	(Arbelaiz et al. 2005)

B-CF= Bisphenol-C formaldehyde, PF = Phenol formaldehyde, PLA = Polylactic acid, Pongamia Pinnata =PP.

Morphological study of fractured surfaces

The tensile-tested specimens are shown in Figure 11. It is observed that the test specimens failed almost perpendicular to the axis of the fibers in the matrix and the cut surface did not show any cracks and delaminations. Figure 12 shows the microscopic images (EVO MA18, Carl Zeiss Ltd., Cambridge, United Kingdom) of the tensile failed specimens. The surface from the failure region shows hemi-spherical cavities/voids of size varying from 46.79 µm to 214 µm (Figure 12(a–e)). It is observed from Figure 12(a, b) that the FF fibers were split along the line of failure, and they were found to be firmly intact in the epoxy indicating the good interphase between the FF fiber and the matrix. Due to the higher ductility of hybrid laminates, the crack is propagated rapidly and resulted in catastrophic brittle failure of test samples (Azlina et al. 2019; Singh et al. 2021) which is evident from Figure 12(b).

Figure 11. Tensile load failed hybrid RS_p/FF test samples under tensile load.

Figure 12. Microscopic images from the failure region (a), (b) R05F20 (c), (d) R10F20 and (e), (f) R15F20.

The RS_p showed debonding in the failure zones, and this is due to shorter lengths of the fibers which made the fiber detached from the epoxy during the failure (Srivastava and Prakash 2005). It is observed that the RS_p which are present in the direction of matrix failure are split along the axis of the fiber as seen in Figure 12(f). As seen from the figures, the FF fiber is split in the fracture region and exposed cross-sectional area of the FF fiber. The microfibrils observed in Figure 12(c, d) are indicative of the successful transfer of load from the matrix phase to FF fibers (de Lima et al. 2022).

Figure 13(b–d) shows the microscopic images (Olympus, BX53MRF-S) of cracked samples subjected to flexural load. Under the flexural load, the test samples did not break completely and the FF fiber was intact with the matrix material. However, due to the applied load, it appears that the entire layers in the laminate were deformed at the point of loading, due to which the upper fiber layer of the laminate appears to get shifted causing the delamination on the top layer (Fiore, Valenza, and Di Bella 2012). The impact test failed test samples are shown in Figure 14. It is observed fromFigure 14 that there is more fiber pull-out for the samples with 5 and 10 wt.% of RS_p and qualitatively less fiber pull-out for the test sample with 15 wt.% RS_p (Singleton et al. 2003). The shear angle at the cut surface of test specimen with 15 wt.% of RS_p was around 14.51°.

Figure 13. Flexural load failed hybrid RS_p/FF composite. (a) Test samples post flexural test and (b) R05F20 (c) R10F20 (d) R15F20 hybrid composites.

Figure 14. Impact test failed test samples.

Conclusions

The hybrid RS_p/FF composite is fabricated by varying the wt.% of RS_p in the laminate, and its effects on the physical and mechanical properties are studied. It is observed that the density of the RS_p and FF fiber reinforced hybrid composite is decreased by 41.87% compared to neat epoxy composite. The maximum water absorption in the hybrid composites is 6.39% for the first 24 hours, and the rate of water absorption gradually increased and reached a saturation point (>648 hours) with the increase in the immersion period. The addition of RS_p and FF fiber reinforcements increased the stiffness and ductility in the composite. The 5, 10 and 15 wt.% of RS_p along with 20 wt.% of FF fiber is observed to be the optimum concentration for achieving maximum σ_t (29.45 MPa), σ_i (101.01 J/m) and σ_f (43.12 MPa), respectively, compared to other combinations of fiber concentrations. The addition of discontinuous RS_p reinforcement in the chopped form created intermediate stress concentrating zones and hence reduced the mechanical properties of hybrid composite. Compared to hybrid RS_p/FF composite, the unidirectional FF fiber composite showed improved σ_t (20.53%) and σ_f (45.03%). The microscopic images of RS_p/FF composite showed brittle fracture with no fiber pull-out on the fractured surfaces of the hybrid composites.

Acknowledgements

The authors would like to acknowledge Manipal Academy of Higher Education, Manipal, for providing the instrumental and infrastructural facilities to conduct the experimentations.

Disclosure statement

No potential conflict of interest was reported by the author(s).