Evaluation of Mechanical Properties of Sabai Grass (Eulaliopsis binata) Fibers and Epoxy Resin Composite Laminates Using Fly Ash as Filler Material

Abstract

The integration of sabai grass fibers and fly ash in epoxy resin combines the strengths of both materials for developing a tailor-made composite laminate that balances performance, sustainability, and cost-efficiency. This innovative blend of natural fibers and industrial waste promotes environmental conservation. The laminates produced could also be used in diverse industrial and structural applications. This study investigated the mechanical properties of composite laminates reinforced with sabai grass fibers, fly ash filler, and epoxy resin as the matrix. In this work, the hand lay-up method was used to fabricate composites with two stacking configurations ((0°/0°/0°/0°) and (0°/90°/90°/0°)) and filler contents of 1.5 wt.%, 3 wt.%, and 5 wt.%. Various weight fractions of fly ash filler and sabai grass fiber were integrated into the epoxy resin to evaluate their impact on tensile strength, flexural strength, and hardness. The experimental results indicate that adding fly ash significantly improves the composite’s hardness to 27 HV in the composites containing 5 wt.% filler, while sabai grass fibers contribute to enhanced tensile strength and flexural strength. The composites with (0°/0°/0°/0°) fibers and 5 wt.% filler showed a higher tensile strength of 63.5 MPa and flexural strength of 118.5 MPa. The fractured sample was analyzed with the help of FESEM images. The XRD analysis confirmed the presence of fly ash components suitable for forming a bond with epoxy. EDX was conducted to determine the elemental composition of the fly ash. FTIR analysis verified the removal of impurities such as dust, dirt, and lignin from the fiber surface following NaOH treatment.

1. Introduction

Composite laminates with reinforcing fibers embedded in a matrix material provide superior mechanical properties and enable extensive applications in various industries. The modification of the mechanical behaviors of composite laminates to meet specific requirements has prompted substantial research and development efforts. One important area of exploration involves examining the influence of fillers on a composite’s mechanical properties. Filler materials can be natural or synthetic, and each filler material has a unique effect on the reinforcement and matrix materials to which it is added [1,2]. With the addition of 5 wt.% commercially available graphite to epoxy, the material’s compressive strength increases its thermal conductivity. Thereby, graphite’s varied effect presents opportunities to create epoxy composites with desired qualities for various purposes [3]. The study of waste and bio-compatible materials as fillers in composites has gained interest. This research focuses on specific fillers and fly ash and their impact on the mechanical behaviors of sabai grass/epoxy composite laminates.

Replacing synthetic fibers with natural alternatives reduces dependence on petroleum-based products, contributing to resource conservation. Naturally occurring sabai grass fibers have a lower carbon footprint, as they are more compatible with the environment than synthetic fibers, which are manufactured using energy. The sabai grass composite laminates can be used in a variety of applications, including automotive panels, furniture, construction materials, and consumer goods. They can be customized by adjusting the fiber type, orientation, and matrix, and composites can be tailored for specific applications. Sabai grass fibers have a unique texture and appearance, providing a natural and appealing aesthetic for interior and exterior designs. The advantage of manufacturing sabai grass fiber epoxy composites lies in their ability to contribute to a sustainable future while maintaining functionality and cost-effectiveness in diverse applications. Moreover, developing a new composite will add to the materials repository, providing new, additional choices to designers for material selection.

Fly ash, derived from coal combustion in thermal power plants, and marble, a naturally occurring stone crushed into a fine powder, are being investigated as potential fillers to improve the performance and functionality of composite laminates. Including fly ash and marble fillers offers several advantages, including cost-effectiveness, waste utilization, and improved mechanical properties [4,5,6]. Fly ash is a filler that improves mechanical properties such as tensile and flexural strength. It also improves hardness, as fly ash contains ceramic particles, which enhance wear resistance. Using fly ash in composites promotes industrial waste recycling, contributing to environmental sustainability.

Filler addition in composite laminates has gained attention due to its abundant availability as an industrial waste product. When properly dispersed within the composite matrix, fly ash particles can contribute to increased strength, stiffness, and thermal stability [7,8]. They also act as a reinforcing phase, providing load-bearing capabilities and reducing the overall weight of the composite laminate. Moreover, fly ash can modify the curing behavior of the matrix. Micro-sized cenosphere particle reinforcement affects the tribological characteristics of vinyl ester composites under dry sliding conditions, potentially influencing the composite’s mechanical properties [9].

Nanocomposites improve the fracture mechanism at the interface and the toughness of the composites [10]. Meanwhile, the composition of rice husks (30%) and PLA (70%) can increase the stiffness of the composites [11]. According to the findings of the research, at a specified binder-to-aggregate ratio that was determined to be ideal, the addition of fly ash (FA) or silica fume (SF) (10%) and fibers effectively compensated for their respective detrimental effects, improving durability and mechanical/microstructural qualities [12]. Low-cost bamboo composite (LCBC) columns are made from bamboo columns combined with bio-based resins. According to a previous study, bio-resin-based LCBC columns may eventually replace their more carbon-intensive counterparts [13].

This research encompasses various studies aimed at examining the impacts of fly ash on the mechanical characteristics of composite laminates. The filler material concentration (1.5 wt.%, 3 wt.%, and 5 wt.%), average particle size (80 µm), and surface treatment of the fiber (with 5% NaOH) were considered to study the interfacial adhesion and understand their influence on the overall mechanical behavior. The research outcomes provide a valuable understanding of the potential benefits and limitations of using fly ash and marble fillers in composite laminate.

Fly ash filler offers cost-effective and sustainable composite manufacturing opportunities while enhancing strength, stiffness, thermal stability, hardness, and wear resistance. The following sections will examine and explore the influence of fly ash fillers in composite laminates. Considering the advantages of fly ash filler and fibers as potential reinforcements, following the alkali treatment methods, this research work aimed to fabricate a layered sabai fiber/fillers/epoxy composite using the hand lay-up method to develop a class of composite materials and to characterize its properties. The specific objectives are listed as follows.

• Develop composite laminates using four-layered sabai fiber and epoxy matrix, integrating fly ash fillers at concentrations of 1.5 wt.%, 3 wt.%, and 5 wt.%.
• Analyze the impact of varying fly ash filler concentrations on the density and porosity of the composite laminates fabricated in stacking sequences of [0°/0°/0°/0°] and [0°/90°/90°/0°].
• Assess the tensile and flexural properties of the composite laminates (both [0°/0°/0°/0°] and [0°/90°/90°/0°]) as influenced by different concentrations of fly ash fillers.

2. Materials and Methods

2.1. Reinforcement Material

Sabai grass (Eulaliopsis binate) is a perennial grass found in Asian countries and belongs to the Poaceae family (Figure 1a). It is mostly used for rope making. The height and quality of this grass depend on the soil quality and the cultivation process. In general, the length of the grass is 3–8 feet, and the thickness of the grass varies across the length. The chemical composition of sabai grass is 52.34% cellulose, 16.07% lignin, 27.2% hemicellulose and 4.16% ash. Sabai grass is characterized by high cellulose content, low levels of lignin, and ash content higher than that of bamboo. The elevated degree of fermentation (polymerization) and molecular weight observed in sabai grass suggest its potential as a resource for producing high-strength fiber [14,15]. The physical properties of sabai grass can be summarized as strength of 493 MPa, modulus of 20.9 GPa, elongation of 3.0%, and moisture of approximately 8.8%, and the tensile strength of sabai strands is at 76 MPa [16]. Therefore, it is a potential candidate for reinforcement because the properties of these fibers are like those of other natural fibers.

2.2. Fly Ash Filler

A source of waste fly ash is steel power plants. The chemical composition of fly ash is presented in

Table 1. Chemical contents in the filler materials with EDS.

Elements	O	Mg	Si	Ca	Fe	Al
Marble powder (%)	58.62	11.21	10.41	19.76	-	-
Fly ash (%)	50.94	-	23.96	-	2.55	22.54

. Sieve analysis is used to obtain desired particle sizes in the 53–106 µm range, and the average particle size was 80 µm for this experimental work. An FESEM image of fly ash powder is shown in Figure 1b and Table 1. It shows the chemical constituents of fly ash, and it conforms with the EDS, as shown in Figure 2.

2.3. Surface Modification and Characterization of Fiber

Chemical treatment, particularly with NaOH, effectively removes hemicellulose, lignin, and pectin, enhancing the fiber’s surface texture and promoting better adhesion with the matrix, which otherwise adversely affects the properties of the fabricated composite with epoxy resins. NaOH also improves the wettability of fibers, activates functional groups, and increases surface roughness, all of which contribute to improved mechanical interlocking in composites [17,18,19]. A sabai fiber surface modified with 5% NaOH is used as a reinforcement. The detailed process of fiber surface modification and tensile properties of chemically modified fibers are explained in previous work [20]. The fibers were immersed in different concentrations of NaOH solution for 5 h. Here, the treatment with 5% NaOH is reported to have optimum results. The tensile strength of fibers was 258.5 MPa.

2.4. Matrix Material

The resin components are Diglycidyl Ether of Bisphenol A (DGEBA, epoxy) and Triethylene Tetra-Amine (TETA), a room-temperature curing hardener. Adolac Chemicals, Kolkata, India, is the supplier of these materials. Because of its superior mechanical and electrical properties, good adhesion to various fibers, and efficient performance at high temperatures, epoxy was employed as a matrix. It also exhibits little shrinkage after curing and good chemical resistance. It works well with both vacuum bag resin transfer molding [1] and manual lay-up [21]. The characteristics of the hardener are as follows: (K6) a refractive index of 25 °C, −1.4940–1.5000, and lapox epoxy (L12); viscosity of 9000–12,000 mPa·s; density of 1.1 g/cc; and curing time-at 25 °C of 14–24 h [20].

2.5. Composite Fabrication

The hand lay-up process was used to fabricate composite laminates. Specific composite fabrication steps followed in this work are shown in Figure 3.

This means that the NaOH-treated sabai fiber and fillers (fly ash powder) were employed with different concentrations (1.5%, 3%, 5%). A mechanical starrier was used for 5 min to homogenously mix fillers in the resin material. This experimental work used dry and desiccated fibers and filler as a reinforcement. The combination of the sabai fiber mat alignments and filler concentrations is presented in

Table 2. Designations of composite laminates.

Group	Designation	Reinforcement Orientation in 4 Layers of Sabai Fibers	Fiber Treatment	Filler	wt.% of Filler
	CWF	0°/0°/0°/0°	5% NaOH	Fly ash	0
Group 1	D				1.5
	E				3
	F				5
	CWF	0°/90°/90°/0°			0
Group 2	J		5% NaOH	Fly ash	1.5
	K				3
	L				5

.

3. Composite Characterization

Density and Porosity of Composite Laminates

The density and porosity of the fabricated composite laminates (D, E, F, J, K, and L) are shown in

Table 3. Density and porosity of developed composite laminates.

Composite Laminate	Reinforcement (wt.%)	Density of Matrix (ρm.)	Density of Fibers (ρf)	Density of Fillers (ρfillers)	Expt. Density (ρexp.)	Theo. Density (ρth.)	Porosity (%)
CWF	20.72	1.17	1.67	-	1.17	1.2	2.564
D	21.21	1.17	1.67	2.690	1.184	1.254	5.551
E	23.84	1.17	1.67	2.690	1.182	1.257	5.909
F	25.81	1.17	1.67	2.690	1.185	1.267	6.413
CWF	20.11	117	1.67	2.690	1.15	1.19	3.361
J	21.39	1.17	1.67	2.690	1.188	1.274	6.831
K	23.50	1.17	1.67	2.690	1.184	1.275	7.123
L	25.26	1.17	1.67	2.690	1.178	1.276	7.697

CWF—composite without filler.

. The density of composite laminates was calculated using Archimedes’ principle. The experimental density (ρ_e), theoretical density (ρ_t), and porosity (%) with ASTM D2734-09 were calculated using Equations (1)–(3).

$$\mathrm{Experimental} \mathrm{density} ({\mathsf{\rho }}_e)={\displaystyle \frac{\mathrm{Weight} \mathrm{in} \mathrm{Air}}{\mathrm{Weight} \mathrm{in} \mathrm{air}-\mathrm{Weight} \mathrm{in} \mathrm{liquid} }}\times \mathrm{density} \mathrm{of} \mathrm{liquid}$$

(1)

Theoretical Density

$$({\mathsf{\rho }}_{t })={\displaystyle \frac{100}{\frac{w_f}{{\rho }_f}+\frac{w_m}{{\rho }_m}}}$$

(2)

$$\mathrm{Porosity} \left(\%\right)={\displaystyle \frac{\mathrm{Theoretical} \mathrm{density} -\mathrm{Experimental} \mathrm{density}}{\mathrm{Theoretical} \mathrm{density} }}\times 100$$

(3)

where $w_f, w_m$ are the weight of fiber and matrix, respectively, and ${\rho }_f,{\rho }_m$ are the density of fiber and matrix, respectively.

As evident from Table 3, the experimentally determined density is lower than the theoretical density. While preparing the composite using the hand lay-up method, removing the entrapped air in the bubble form is quite difficult. Air entrapment may occur during mechanical stirring and pouring of the resin; thereby, the porosity in the fabricated composite laminates remains. It can be noticed from Table 3 that the porosities increased as the filler (fly ash) contents increased in each group of composite laminates. The orientation of fibers in the 0°/90°/90°/0° stacking sequence is considerably more prone to entrapping bubbles in fly ash-reinforced composites. The percentage of porosity varies in the range of approximately 5.5–7.6%. The presence of porosity may affect the mechanical properties of a developed composite. According to the literature, the inclusion of fly ash filler in composites results in an increased volume fraction corresponding to the filler content. This tendency is more likely to elevate the void percentage in the developed composite. This phenomenon is attributed to the entrapment of air during the hand lay-up fabrication process [9].

4. Mechanical Characterization

4.1. Tensile and Flexural Test

Tensile and flexural composite specimens were prepared following the ASTM 3039M-17 and ASTM D790-03 standards, respectively. These specimens were tested under tensile and flexural conditions using a Tinius universal testing machine (UTM) (Tinius Olsen India Pvt. Ltd, Noida, Uttar Pradesh, India) with a 25 kN load capacity and a 1 mm/min crosshead speed. The tensile and flexural strength, strain, and modulus of elasticity were measured and recorded during the tests.

4.2. Field Emission Scanning Electron Microscopy (FESEM)

FESEM (Model-JEOL JSM-7610F Plus) (JEOL Ltd.,Tokyo, Japan) micrographs of samples after tensile failure were taken to study their failure mechanisms, the cause in tensile and flexural mode, and the effect of alkali treatment.

4.3. Fourier-Transform Infrared Spectroscopy (FTIR)

FTIR analysis of treated sabai fiber (with 5% NaOH), fly ash, and the composite containing both fly ash and sabai fiber was conducted at room temperature using a Nicolet 6700 spectrometer (HYPERION2000, Bruker Co., Ltd., Bremen, Germany) to identify the presence of lignin, cellulose, and other functional groups.

5. Results and Discussion

5.1. FTIR Analysis

Fourier-transform infrared spectroscopy (FTIR) spectra were obtained to characterize the materials and understand their structural behavior. The comparative results of FTIR spectra for treated sabai fibers, fly ash, and composite (FA1.5%) are shown in Figure 4. They show that removal of the peaks of hemicellulose and lignin is evident at 1735 cm⁻¹ after 5% NaOH treatment. Figure 4 shows the fly ash FTIR spectrum as it was received. The fly ash has two distinct peaks at 1047 cm⁻¹ and 777 cm⁻¹. These peaks, which have an intensity band at 778 cm⁻¹ and another significant band at 1047 cm⁻¹, correspond to Si-O-Si and Si-O-Al asymmetric stretching vibrations [22]. The composite with 1.5 wt.% fly ash demonstrates the same peaks as fly ash. However, the hydroxyl groups (OH) of cellulose molecules that exhibit O-H stretching vibration in treated sabai fibers are represented by the broad band at 3580–3034 cm⁻¹. The sabai grass was treated with NaOH to eliminate impurities and expose the OH groups so that the epoxy could bond. Alkali groups with C-H stretching vibration and aliphatic bonds found in lignin, hemicellulose, and cellulose make up the peak bands between 2907 and 2710 cm⁻¹. The acetyl group of hemicellulose peaks at about 1735 cm⁻¹, attributed to C=O stretching vibration [23]. The hemicellulose component and the presence of aryl alkyl–ether compounds in lignin with the C-O stretching vibration molecules are linked to the peak at 1253 cm⁻¹, according to research [24].

5.2. X-Ray Diffraction Analysis

The X-ray diffraction (XRD) pattern of fly ash powder is presented in Figure 5. The predominant crystalline phases identified in fly ash powder, alongside minor phases like hematite (Fe₂O₃), include alpha quartz (SiO₂) and mullite (3Al₂O₃, 2SiO₂). The visibility of strong peaks in the XRD of fly ash powder is attributed to the presence of alpha quartz. Additionally, 2θ values falling between 18 and 35 are associated with the amorphous aluminosilicate phase. This observation aligns well with the findings reported in other studies in the literature [22,25]. The fly ash samples have three main crystalline phases: mullite, hematite, and quartz. Mullite (aluminosilicate) peaks at 16.32, 25.98, 30.99, and 40.72 in the [110], [120], [001], and [121] planes. Hematite (Fe₂O₃) exhibits peaks at 33.06, 36.40, and 68.10 in the [100] and [110] planes, and quartz (SiO₂) shows peaks at 20.46, 26.32, and 35.94 in the [101] and [100] planes. The intensity of peaks is largest in fly ash, followed by the composite with 5 wt.% fly ash content. Neat epoxy represents the amorphous phase, and others represent the crystalline phase with sharp peaks. Since there are no apparent peaks in the neat epoxy X-ray diffraction curve, this confirms that the material is amorphous.

5.3. Effect of Fly Ash Filler on Mechanical Properties

5.3.1. Tensile Properties of Composites

Table 4. Mechanical properties of composite laminates.

Composite	Tensile Strength (MPa)	Modulus (GPa)	Break Strain (%)	Flexural Strength (MPa)	Modulus (GPa)	Break Strain (%)
CWF	43.15 ± 2.5	1.63 ± 0.32	1.6 ± 0.2	61.9 ± 0.3	6.10 ± 0.33	1.98 ± 0.43
D	45.15 ± 7.09	2.90 ± 0.60	1.215 ± 0.29	62.6 ± 3.5	5.5 ± 0.26	1.58 ± 0.03
E	48.25 ± 6.1	4.06 ± 0.71	1.211 ± 0.21	95.2 ± 4.2	6.8 ± 0.27	1.40 ± 0.71
F	63.35 ± 7.35	4.91 ± 0.43	1.120 ± 0.21	118.5 ± 2.62	8.2 ± 0.40	1.28 ± 0.20
CWF	30.15 ± 1.5	1.43 ± 0.22	1.5 ± 0.3	55.6 ± 0.3	4.1 ± 0.32	1.27 ± 0.41
J	29.25 ± 2.33	1.87 ± 0.44	1.35 ± 0.13	58.1 ± 1.6	4.8 ± 0.3	1.405 ± 0.04
K	40.3 ± 1.13	2.93 ± 0.29	1.24 ± 0.2	80.45 ± 7.99	5.6 ± 0.21	1.32 ± 0.03
L	42.45 ± 2.16	3.39 ± 0.35	1.51 ± 0.1	100.3 ± 2.14	7.53 ± 0.94	1.23 ± 0.01

shows fly ash filler’s effect on composites’ mechanical properties. It signifies the tensile strength, Young’s modulus, and break strain of sabai fiber and fly-ash-reinforced (with 1.5%, 3%, and 5%) composite laminates. As the concentration of filler is increased, the tensile strength and modulus also increases. This is evident in Figure 6a,b. For the 0°/0°/0°/0° fiber orientation, the tensile strength of the composite with 5 wt.% fly ash is 63.35 MPa, approximately 46.83% greater than the composite without filler (Figure 6a). In the case of the 0°/90°/90°/0° fiber orientation, an increasing trend of tensile strength and modulus is observed, but the tensile strength values are lower than those of the (0°/0°/0°/0°) orientation of fibers (Figure 6a). Here, the composite with 5 wt.% added fly ash fillers has a tensile strength of 42.45 MPa, approximately 13.42% greater than the composite without filler (Figure 6a).

Fibers are the primary load-bearing components in laminates. In a laminate with a stacking sequence of [0°/0°/0°/0°], where the fibers in each layer are aligned with the loading direction, all fibers effectively share the total load. In contrast, in a laminate with a stacking sequence of [0°/90°/90°/0°], only the fibers aligned with the loading direction contribute significantly to carrying the applied load. As a result, the tensile strength of laminates varies depending on the stacking sequence. In the case of the [0°/90°/90°/0°] laminate, effective inter-layer load transfer is limited due to early inter-laminar matrix cracking/brittle fracture between the two adjacent fibers aligned at 90° to the loading direction. Therefore, the two aligned layers primarily support the laminate’s load-carrying capacity. But in this alignment [0°/90°/90°/0°]), it has better strength in the orthogonal direction than laminates with a [0°/0°/0°/0°] stacking sequence.

As shown in Figure 6b, the Young’s modulus shows an increasing pattern with an increase in the fly ash filler material proportion, which is similar to the observed trend in tensile strength. This is due to the enhanced stiffness of the rigid fly ash particles as per the rule of mixtures. The addition of fly ash increases void development (Table 3) and causes dislocation clouds to form (as can be increasingly seen in Figure 6a,b), and the dislocation density restricts deformation against load and hence increases the stiffness. Additionally, the strong interfacial bonding between fly ash and the polymer matrix restricts deformation, resulting in a higher Young’s modulus. The composite containing 5 wt.% fly ash filler and a fiber orientation of [0°/0°/0°/0°] has a tensile modulus of 4.91 GPa, while the composite with 5 wt.% marble dust and a fiber orientation of [0°/90°/90°/0°] has 3.39 GPa in its respective group. The composite with a [0°/90°/90°/0°] alignment has a smaller break strain of 1.59%, representing brittle fracture. The addition of fly ash filler makes the composites stronger and brittle. The cracks found in the sample after testing represent the composite’s brittle behavior, as seen in the FESEM image (Figure 7a). In Figure 7b, there is no tearing of fibers, but small pull-out and maximum cross-sectional fracture of fibers can be observed. This shows that the fiber–matrix adhesion is good at the interface and transfers the maximum load to the matrix. Thereby, it demonstrates higher strength than the remaining composites.

In fly ash, SiO₂ is the major constituent (63.71%), and Al₂O₃ comprises 25.43%. SiO₂ particles have a high surface area and establish a strong physical and mechanical connection with the epoxy matrix. During curing, Si-O-Si bonds are formed between the SiO₂ surface and epoxy functional groups, enhancing mechanical properties and thermal stability. It should be noted that no thermal stability study was carried out in the present work. The hydroxyl groups on the surface of SiO₂ form hydrogen bonds with epoxy functional groups and enrich the interfacial bonding. Al₂O₃ particles also interact with epoxy through physical entanglement and hydrogen bonding, which improves mechanical strength [22,25]. XRD analysis is evidence of this chemical bonding, as explained in the above section. It revealed the brittle nature of the composites after the addition of 5 wt.% fly ash. The evidence of increments in the filler contents can be seen in Figure 6b. It also exhibits the sharp fracture of fiber and good adhesion at the fiber–matrix interface. These improve the strength of the composite.

The void formation is considerably greater when fly ash is used as filler material. This is also reported in Table 3. The spherical shape of fly ash allows more space to create voids. Because of the good bonding, the fiber pull-out is reduced, as seen in the FESEM image in Figure 7a,b. After severe breakdown, residual fly ash remains in the matrix due to good adhesion with the filler material.

5.3.2. Flexural Properties of Composites

The composites were subjected to a three-point flexural test with a sabai fiber stacking sequence of (0°/0°/0°/0°)/(0°/90°/90°/0°) and fillers (fly ash at 0 wt.%, 1.5 wt.%, 3 wt.%, and 5 wt.%), and the results are presented in Table 4. As illustrated in Figure 8a,b, the composite samples’ flexural strength and modulus exhibited a similar increasing trend.

The composite with 5 wt.% fly ash and the (0°/0°/0°/0°) fiber alignment has the highest flexural strength (118.5 MPa), and that of the composite with 5 wt.% fly ash and the (0°/90°/90°/0°) fiber alignment is 100.3 MPa. In all (0°/0°/0°/0°) lay-up tests, each laminate was in the same direction so that each layer could contribute to the flexural strength. There was a large shearing area between the layers. However, in lay-up composites (0°/90°/90°/0°), only two laminates (longitudinal alignment) contributed to flexural strength. The other two laminates (transversely aligned) contributed little to the flexural strength (only interface bonding contributed to the flexural strength). After the failure of the fourth layer, the crack propagated up to the second layer, as shown in Figure 9. However, the flexural strength increased as the fly ash contents were increased from 0 wt.% to 5 wt.% (Figure 8a). It was also found that 5 wt.% fly-ash-reinforced composite had the highest flexural strength in both groups (1) and (2). The fly ash fillers have shear behaviors in the composite that help to improve flexural strength. The particles experienced lateral distortion during loading, which caused the sabai fiber to receive shear stresses. This caused more significant lateral deformation, which improved loads and postponed fracture timing. A similar trend of flexural strength with marble and basalt fiber has also been observed [26]. The laminate with (0°/90°/90°/0°) fiber lay-up with fillers provides crack propagation resistance. Combining fibers with various orientations and improving filler toughness can increase a material’s overall strength by preventing cracks from propagating from one layer to another. A comparison of the mechanical behaviors of the alkali-treated natural fiber-reinforced composites is shown in

Table 5. Effect of alkali treatment on mechanical behaviors of natural-fiber-reinforced composites.

Fibers	Tensile Strength (MPa)	Flexural Strength (MPa)	Reference
Sabai grass+fly ash/epoxy	63.5	118.5	Current study
Henequen fiber/PA6	50	35	[17]
Hemp fiber	55	-	[18]
Sisal fiber	118	60	[19]
Bamboo fiber	135.2–210.3	117–330	[27]

. It shows that the sabai-grass-reinforced composite with fly ash as filler material has good mechanical properties compared to other natural fibers and justifies the applications.

5.3.3. Effect of Filler on Surface Hardness

A Vickers hardness test was performed on composites with fly ash (1.5 wt.%, 3 wt.%, and 5 wt.%) and sabai fiber with (0°/0°/0°/0°) stacking sequences as per ASTM E92 and with a load of 10 N. The selected surface was polished and cleaned. The hardness measurements were taken at three different, randomly selected points across the surface of the composite laminate. The average bulk Vickers hardness value from the individual measurements was calculated using Formula (4).

$$H_{v=1.8544 \times {\displaystyle \frac{P}{d^2}} }$$

(4)

The results are presented in Figure 10. The results demonstrate an increase in the fly ash concentrations from 1.5 wt.% to 5 wt.%. The surface hardness of the material increases and shows a higher value of 27 HV in the composite reinforced with 5 wt.% fly ash filler. Higher filler content generally leads to increased hardness because more rigid particles are dispersed within the polymer matrix [28]. There is often an optimal level of fly ash filler content. Beyond this, excess fillers may lead to agglomeration, reducing uniformity and slightly affecting hardness.

Fly ash is a ceramic-like material with fine particles that are hard and stiff. When these particles are added to an epoxy matrix, they act as fillers that occupy the matrix, reducing the deformation ability under load. This improves the material’s resistance to surface indentation, leading to an increase in hardness. The presence of fly ash fillers also restricts the mobility of the polymer chains, making the composite stiffer and harder [29]. Natural fibers (like jute and sisal) contribute to strength and rigidity. When fly ash is added, it complements the fibers by further enhancing hardness, while the fibers help maintain a balance between toughness and brittleness [2,30].

6. Conclusions

The mechanical properties of NaOH-treated sabai grass composites fabricated using the hand lay-up technique with fly ash filler (1.5 wt.%, 3 wt.%, and 5 wt.%) were studied for two fiber stacking sequences: (0°/0°/0°/0°) and (0°/90°/90°/0°). Incorporating limited amounts of industrial waste (fly ash) enhanced the composites’ strength, stiffness, and durability, creating lightweight materials suitable for domestic structural applications and automotive interiors. The salient outcomes of the investigation are as follows:

The incorporation of higher weight percentages of fly ash increased the porosity and density of the composites. The laminates with a stacking sequence of (0°/90°/90°/0°) exhibited higher void percentages due to the cross-layer fiber orientation. Increased filler content also enhanced tensile strength, achieving maximum values of 63.35 MPa for the (0°/0°/0°/0°) alignment and 42.45 MPa for the (0°/90°/90°/0°) stacking sequence with 5 wt.% fly ash. Similarly, flexural strength was highest at 118.5 MPa for the (0°/0°/0°/0°) alignment and 100.3 MPa for the (0°/90°/90°/0°) configuration. The hardness of the laminate improved with the filler content, reaching 27 HV at 5 wt.%, due to the ceramic particles in fly ash providing increased resistance to indentation and improving wear resistance. Both tensile and flexural stiffness, indicative of the composite’s ability to maintain dimensional stability under load, increased with the filler content, leading to enhanced rigidity up to a certain weight percentage of filler. The higher composite strength and improved epoxy adhesion are due to the formation of Si-O-Al and Si-O-Si bonds, as revealed by FTIR and XRD analyses.

As an eco-friendly and low-cost material, sabai grass offers an alternative to synthetic reinforcements. Moreover, the development of sabai grass epoxy composite with fly ash fillers expands the materials repository, offering designers additional options for material selection.