Mechanical,fatigue and creep behavior of varying angle stacked bambus hampi hybrid fiber and nelumbonucifera cellulose toughened vinyl ester composites

Raw materials

The materials used for composite fabrication included vinyl ester resin, Methyl ethyl ketone peroxide (MEKP) as a catalyst, and cobalt naphthenate as a hardener, procured from Atul Ltd, Gujarat, India. Cellulose extracted from Nelumbonucifera stems, used as the filler, was obtained from Merck India, Mumbai. Bambushampi mat fiber, serving as the primary reinforcement, was sourced from Kanpur Fibers Pvt. Ltd, Kanpur, India. The bamboo fibers possess low density (∼1.2-1.4 g/cm³), good tensile strength, and a lignocellulosic structure that supports reinforcement in polymer composites. For silane surface treatment, 3-APTMS and ethanol were purchased from TCI Chemicals India Pvt. Ltd, Mumbai, India. Prior to fabrication, the fibers were cleaned, dried, and subjected to silane treatment to improve fiber–matrix adhesion. The composites were fabricated using a hand lay-up process, followed by curing at ambient conditions to obtain stable composite laminates.

Cellulose extraction from Nelumbo nucifera

The cellulose are extracted from waste biomass by following below procedure, and their process are represent diagrammatic flow chart format in Figure 1. Cellulose was extracted from Nelumbo nucifera (lotus stem) by first collecting, washing, and oven-drying the stems, which were then ground into a fine powder. The powder was dewaxed using an ethanol–benzene mixture to remove oils and resins, followed by alkali treatment with sodium hydroxide at elevated temperature to eliminate lignin. The residue was then bleached with an acidic sodium chlorite solution to remove hemicellulose and any remaining lignin, repeating the step as needed until white fibers were obtained. ¹² Finally, the purified cellulose was thoroughly washed to neutral pH and oven-dried, yielding fibrous cellulose suitable for composite fabrication.OPEN IN VIEWER

Silane treatment

Silane modification was employed to enhance interfacial adhesion between Bambus hampifiber, lotus stem cellulose, and the vinyl ester matrix represented as Figure 2. A 2 wt.% 3-APTMS solution in ethanol was prepared and stirred for 15 min, followed by immersion of the fibers and cellulose for 20 min. ¹³ Excess silane was removed by filtration, and the treated materials were oven-dried at 120°C for 25 min. FT-Raman spectra (Figure 3) confirmed functionalization, with the emergence of N-H peaks and attenuation of O-H signals, indicating successful grafting of reactive groups that improve fiber-matrix interfacial compatibility and stress transfer.OPEN IN VIEWER

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Figure 3.

Aspects of FT-Raman analysis of cellulose treated with silane.

Preparation of composites

The prepared surface modified fiber and filler are reinforced into matrix using hand layup method to fabricate composite. After matrix preparation using accelerator, promoter, and varying concentration of filler are reinforced represented in Table 1. Thereafter, the hardener is mixed into the resin matrix, to speed up the curing process. Further, in this study the fiber is stacked in different orientation [0°/45°/90° Bambus hampi] with alternating 45° and 90° fiber orientations) to enhance the performance of the composite material. After preparing the mold and matrix, the fiber is stacked and matrix reinforcement are binding. To ensure void free composite, the degassing process is taken, then the prepared composite are allowed to curing at room temperature for 1 day.

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Table 1.

Matrix reinforcement composition based on samples.

Samples	Matrix (Vol. %)	Fiber (Vol. %)	Filler (Vol. %)	Surface modification
M	100	-	-	-
MC1	59	40	1	-
MC2	58	40	2	-
MC3	56	40	4	-
MC4	59	40	1	Treated
MC5	58	40	2	Treated
MC6	56	40	4	Treated

Mechanical strength of the composites

The results indicate that both fiber stacking sequence and silane surface treatment critically affect the mechanical performance of the composites. Alternating fiber orientations (90°/45°) facilitated improved stress distribution and load transfer, enhancing tensile, flexural, impact, and hardness properties. This improvement occurs because multidirectional fiber alignment allows the applied load to be shared across fibers oriented in different directions, preventing stress concentration along a single axis and enabling more uniform load bearing within the composite. Mechanical performance increased with filler content up to an optimum level, beyond which slight reductions occurred due to agglomeration. ¹⁴ The addition of fibers and fillers increases the load-bearing capacity of the composite because the stiff reinforcement phases possess higher modulus than the polymer matrix; under applied loading, stresses are transferred from the matrix to the fibers through interfacial shear stresses, allowing the fibers to carry a larger portion of the load and thereby increasing strength and stiffness.

The reference matrix (M) exhibited low tensile, flexural, and impact strengths alongside moderate hardness, reflecting its brittleness and limited crosslink density. Incorporation of 1–2 vol.% untreated cellulose and Bambushampi fiber (MC1, MC2) markedly improved performance, attributed to stronger fiber–matrix adhesion and optimized stacking, with tensile, flexural, and impact properties increasing substantially over the reference. ¹⁵ This enhancement arises because the introduced fibers act as reinforcement bridges within the matrix, restricting polymer chain mobility and resisting deformation when external loads are applied. The fibers also serve as crack arresters, inhibiting crack initiation and propagation by redistributing stresses around the reinforcement, thereby improving load sharing between the matrix and the fibers. Surface-treated composites (MC4, MC5) showed further enhancements, demonstrating higher tensile, flexural, and impact responses, with improvements of 3–7% in tensile strength, ∼3–4% in flexural strength, and ∼12–13% in impact strength compared to untreated samples. The silane treatment improves interfacial bonding by forming chemical linkages between the hydroxyl groups present on natural fibers and the polymer matrix, strengthening the interface and enabling more efficient stress transfer during loading, which allows the fibers to fully participate in carrying the applied load. At 4 vol.% loading, both untreated (MC3) and treated (MC6) composites outperformed the reference, although filler agglomeration began to limit further gains; silane treatment mitigated clustering, sustaining efficient stress transfer. ¹⁶ Agglomeration reduces reinforcement efficiency because clustered particles create localized stress concentration regions that interrupt uniform load distribution; however, surface treatment improves dispersion and maintains effective interfacial contact, thereby preserving the load transfer mechanism. Hardness followed a similar trend, with untreated composites (MC1-MC3) improving over the reference and treated composites (MC4-MC6) achieving the highest Shore-D values, reflecting enhanced crosslinking and reduced polymer chain mobility due to better fiber–matrix interaction and optimized orientation.^17,18,19 The presence of rigid fibers and improved interfacial bonding restricts localized plastic deformation of the matrix, increasing resistance to indentation and thus improving hardness. Figure 5 presents these mechanical properties graphically.OPEN IN VIEWER

Figure 6 (a)–(c) presents optical microscopic images of the fractured surfaces of mechanically tested composites, highlighting their morphology and failure characteristics. At lower magnification, the reference sample (M) under tensile loading (Figure 6(a)) shows a relatively smooth and brittle fracture surface. This behaviour is associated with the vinyl ester resin’s low stiffness and limited cross-link density, which promotes rapid crack propagation with minimal resistance and reduced energy absorption during fracture. A more favourable morphology is observed in Figure 6(b), where the filler appears more uniformly dispersed within silane treated composite systems (MC5). The optical image shows improved fiber alignment and a relatively homogeneous matrix region, indicating better fiber-matrix reaction. This uniform distribution promotes improved stress transfer and mechanical interlocking, contributing to enhanced stiffness and load-bearing capacity during mechanical loading. Finally, Figure 6(c) reveals regions of filler clustering at high loadings, in composite MC3. These agglomerated zones appear as dense particle-rich areas that can act as stress concentration sites, accelerating crack initiation and propagation under mechanical loads. However, in silane treated composites the dispersion appears more uniform with fewer visible clusters, indicating improved fiber-matrix adhesion and reduced agglomeration, which ultimately contributes to more consistent mechanical performance.OPEN IN VIEWER

The behavior of cyclic loading

Composites treated with vinyl ester show fatigue life numbers in Figure 7. At 25% UTS, the fatigue life value is 17,531, at 50% UTS it’s 16,226, and at 75% UTS it’s 16,938 for the reference sample M, which contains just vinyl ester resin. Poor stress distribution and increased brittleness are likely consequences of an inadequate resin-to-fiber ratio, which explains why the decline is 50% rather than 25%. In the absence of reinforcement, the matrix alone bears the cyclic load, leading to localized stress concentration and rapid microcrack initiation and propagation within the brittle polymer network. Since the vinyl ester matrix has limited resistance to cyclic deformation, repeated loading causes progressive damage accumulation, resulting in premature fatigue failure. Figure 7 cyclic load bearing behaviour of all composite samples. Similarly, at 25% UTS, the fatigue life counts of the untreated samples MC1, MC2, and MC3, which contain both fiber and filler, are 27,028, 28,362, and 27,999, respectively. At 50% UTS, the fatigue life counts are 26,996, 27,955, and 27,068. At 75% UTS, the fatigue life counts are 25,831, 26,061, and 24,541, respectively. Because of the enhanced loading of fibers and fillers and the effective transmission of stress, these samples show the best fatigue resistance at 25% UTS. The presence of fibers and fillers improves the cyclic load-bearing capacity because the reinforcement phases possess higher stiffness and strength than the matrix, allowing them to carry a significant portion of the applied load. During cyclic loading, stresses are transferred from the matrix to the fibers through interfacial shear, enabling efficient load sharing and reducing the magnitude of stress experienced by the matrix. In addition, the fibers act as barriers to crack propagation by bridging microcracks and redistributing stresses around damaged regions, which delays crack growth and enhances fatigue resistance. Although reinforcement efficiency is marginally diminished, moderate fatigue performance is preserved when the load is increased to 50% UTS because the fibers still participate in load transfer and help maintain structural integrity. At 75% UTS, nevertheless, the matrix takes center stage, making the composite brittle and reducing its fatigue life; this is especially true for samples that have untreated reinforcement, where weaker interfacial bonding can lead to fiber pull-out and interfacial debonding, reducing the efficiency of stress transfer under high cyclic stresses.OPEN IN VIEWER

The fatigue life values of the treated samples MC4, MC5, and MC6 are 28,822, 29,324, and 29,115 at 25% UTS, 28,441, 28,955, and 28,654 at 50% UTS, and 27,978, 28,553, and 27,901 at 75% UTS, depending on the value. This improvement is because the resin and reinforcement are more firmly adhered to one another after being treated with silane, which improves the fiber-matrix interfacial bonding. The silane molecules form chemical linkages between the hydroxyl groups present on natural fibers and the polymer matrix, creating a stronger interface that facilitates efficient stress transfer during cyclic loading. As a result, the applied load is more uniformly distributed between the matrix and the reinforcing fibers, reducing interfacial debonding and preventing premature crack initiation. The improved interfacial adhesion also limits fiber pull-out and enhances crack bridging capability, which slows crack propagation and significantly extends fatigue life. Therefore, during cyclic stress, the treated composites show improved load transfer and postponed crack start. ²⁰

Creep resistance

Figure 8 Represents creep resistance features of the samples in graphical way. The pure vinyl ester matrix (M) exhibited the highest creep strain due to resin softening under prolonged loading. Incorporation of cellulose and Bambus hampifiber reduced creep deformation, with MC1 and MC3 showing progressively lower strains at 5000, 10,000, and 15,000 s, indicating improved long-term dimensional stability and load resistance. ²¹ OPEN IN VIEWER

By creating a physical barrier, fillers and fibers limit the stress-induced mobility of polymer chains, which in turn slows the rate of deformation. As a result, the reinforced matrix has better creep resistance than the unreinforced one. As a result of the treated fiber and filler particles, samples MC4, MC5, and MC6 show even more improvement. With creep values of 0.0060, 0.0049, and 0.0033 at 5000 s, 0.0246, 0.0219, and 0.0206 at 10,000 s, and 0.0517, 0.0459, and 0.0432 at 15,000 s, respectively, these samples display dramatically reduced creep. Chemical treatment improves stress transmission by strengthening the link between the matrix and reinforcement at the interface, which in turn decreases micro-slippage. This improves the composites’ creep resistance and drastically reduces the amount of deformation that can occur over time.