Functionally Graded Hybrid Composite Engineering Using GFRP, Bamboo, and Banana Fibres for Sustainable Structural Systems

Running title: FGHC Engineering with GFRP, Bamboo and Banana Fibres DOI: 10.5281/zenodo.20373582 VELAN Ra*, RAGU RAMA SAI KRISHNAa, NAVEEN DARSHAN Ka AND JOHN PRESIN KUMAR Ab aDepartment of Mechanical Engineering, Hindustan Institute of Technology and Science, Chennai, Tamil Nadu 603 103, India bAssociate Professor, Department of Mechanical Engineering, Hindustan Institute of Technology and Science, Chennai, Tamil Nadu 603 103, India *Corresponding author. E-mail: velan0924@gmail.com; 22124026@student.hindustanuniv.ac.in ABSTRACT A functionally graded hybrid composite (FGHC) was developed using bamboo and banana natural fibres within a Glass Fibre Reinforced Polymer (GFRP) framework to address limitations inherent in conventional hybrid composite design. The laminate was fabricated via hand lay-up followed by compression moulding, with a symmetric six-layer stacking sequence [Banana/Glass/Bamboo/Bamboo/Glass/Banana]. Natural fibres were subjected to 5 wt% NaOH alkaline treatment to improve fibre–matrix interfacial bonding. Tensile testing (ASTM D638-14) yielded a mean strength of 107.0 ± 1.9 MPa with a Young’s modulus of 5.65 ± 0.04 GPa. Flexural evaluation (ASTM D790-17) recorded a peak load of 1.616 ± 0.012 kN and a flexural strength of 79.3 ± 0.6 MPa. Izod impact testing (ASTM D256-10) produced a mean impact strength of 12.0 ± 0.2 kJ m−2. All specimens exhibited progressive failure rather than catastrophic fracture, indicating effective energy dissipation. The graded architecture, in which GFRP bears primary loads, bamboo deflects cracks, and banana fibres absorb peripheral energy, yields mechanical performance superior to binary natural fibre systems. This study establishes a mechanistically justified design strategy for sustainable lightweight structural composites applicable to automotive, panel, and civil construction sectors. Keywords: functionally graded composites; hybrid reinforcement; glass fibre reinforced polymer; bamboo fibre; banana fibre NOMENCLATURE Symbol / Abbreviation Description E Young's modulus (GPa) σ Tensile stress (MPa) Vᴍ Fibre volume fraction (%) NaOH Sodium hydroxide (alkali treatment agent) GFRP Glass Fibre Reinforced Polymer FGHC Functionally Graded Hybrid Composite DGEBA Bisphenol A diglycidyl ether (epoxy resin) ASTM American Society for Testing and Materials DMA Dynamic Mechanical Analysis RTM Resin Transfer Moulding FGM Functionally Graded Material 1. INTRODUCTION Contemporary materials science is shifting substantially in response to environmental sustainability imperatives, resource efficiency demands, and the need for structural performance optimisation. Glass Fibre Reinforced Polymers (GFRPs) have been widely used in aerospace, automotive, and civil engineering applications owing to their high specific strength, corrosion resistance, and dimensional stability.1–3 However, the ecological costs associated with synthetic fibre production encompassing elevated energy consumption, resistance to biodegradation, and complex end-of-life disposal have prompted researchers worldwide to investigate sustainable reinforcement alternatives that do not compromise mechanical performance. Plant-derived natural fibres have emerged as technically and ecologically viable reinforcements for polymer matrix composites. Bamboo fibre (Bambusoideae) is characterised by a cellulose content of 60–70%, which imparts high stiffness and structural stability comparable to softwood. Banana fibre (Musa sapientum), extracted from the pseudostem, offers exceptional ductility, toughness, and energy absorption owing to its high lignin-to-cellulose ratio and multi-layered cell wall architecture.4–6 Used in isolation, natural fibres exhibit moisture uptake, poor adhesion to hydrophobic epoxy matrices, and pronounced mechanical variability collectively restricting their use in structural applications.7,8 Hybrid composite architectures, in which multiple fibre types share a common matrix, can harness the complementary properties of each constituent. The hybrid effect, originally identified by Hayashi,9 enables gains in failure strain, energy absorption, and damage tolerance that exceed rule-of-mixture predictions. Conventional hybrid composites, however, distribute fibres uniformly and without spatial optimisation, leading to suboptimal load distribution and premature damage initiation.10–12 Functionally graded structures offer a promising resolution by exploiting position-dependent property variation across the laminate thickness. Functionally graded materials (FGMs) were initially developed for thermal barrier coatings in aerospace applications.13,14 This principle was subsequently extended to fibre-reinforced polymer composites, wherein discrete or continuous gradients in fibre type, orientation, or volume fraction are engineered to match anticipated load distributions. Joshi et al.15 demonstrated that graded natural fibre composites exhibit superior energy absorption relative to uniformly reinforced counterparts. Saba et al.16 reported that strategic fibre positioning in epoxy laminates significantly improves dynamic mechanical properties. Ramesh et al.17 established that tri-phase hybrid systems outperform binary systems in both strength and toughness, provided that interfacial compatibility is addressed through surface engineering. Alkaline mercerisation using sodium hydroxide (NaOH) solutions is the most widely reported surface treatment method for natural fibres, selectively removing amorphous surface components lignin, hemicellulose, and pectin to expose crystalline cellulose microfibrils and increase available hydroxyl (–OH) bonding sites.18,19 Treatment concentrations of 4–6 wt% NaOH at ambient temperature for 1–4 hours have been reported to maximise mechanical improvement without inducing fibre degradation.20 Pickering et al.21 confirmed that 5 wt% NaOH treatment provides an optimum balance between surface reactivity enhancement and structural preservation of the fibre bundle. The present study proposes a novel FGHC architecture in which GFRP, bamboo, and banana fibre layers are positioned according to anticipated through-thickness stress gradients. The hierarchical design assigns each fibre a dedicated mechanical function: GFRP acts as the primary load-bearing phase; bamboo fibres operate as crack-arresting interlayers; and banana fibres provide peripheral energy dissipation. The laminate was manufactured via hand lay-up followed by compression moulding, with alkaline treatment applied to improve interfacial adhesion. Characterisation encompassed tensile, flexural, and Izod impact testing in compliance with relevant ASTM standards. 2. MATERIAL AND METHODS 2.1 Material System Design The composite system was designed as a tri-phase reinforcement architecture embedded within an epoxy matrix (Bisphenol A diglycidyl ether, DGEBA, combined with an amine-based hardener at a 10:1 mass ratio). Each reinforcement phase fulfils a distinct functional role within the graded laminate. E-glass woven fabric (200 g m−2, plain weave) was placed at the laminate core to serve as the primary load-bearing phase, intercepting maximum tensile and compressive stresses during in-plane loading. Short bamboo fibres (aspect ratio ≈ 40) were arranged as discontinuous reinforcement within intermediate layers to deflect and bridge propagating cracks, thereby extending fracture toughness. Woven banana fibre mats were positioned at the outer laminate surfaces, where bending-induced tensile stresses reach maximum values, capitalising on the inherent ductility and high elongation-at-break of banana cellulose. Fig. 1. Raw reinforcement materials: (a) short bamboo/banana fibre blend prepared for intermediate layer reinforcement; (b) E-glass plain-weave woven fabric (200 g m⁻²) used as the primary load-bearing phase. 2.2 Surface Engineering of Natural Fibres Both bamboo and banana fibres were subjected to alkaline surface conditioning prior to composite fabrication. Fibres were submerged in a 5 wt% NaOH aqueous solution at ambient temperature (25 ± 2 °C) for two hours, rinsed thoroughly with distilled water to eliminate residual alkali, and subsequently oven-dried at 80 °C for 24 hours. This mercerisation protocol selectively removes amorphous surface components lignin, hemicellulose, and pectin from the fibre surface. The resulting exposure of crystalline cellulose microfibrils raises surface roughness and increases the density of reactive hydroxyl (–OH) groups available for chemical interaction with the epoxy matrix, markedly improving interfacial shear strength.18,19 The governing chemical reaction for mercerisation is: Fibre–OH + NaOH → Fibre–O⁻Na⁺ + H₂O Eqn (1) wherein sodium hydroxide disrupts the hydrogen bonding network within the amorphous cellulose regions.20 Fig. 2. Alkali-treated natural fibre mat arranged on the mould surface prior to resin impregnation, illustrating uniform fibre distribution across the laminate plane. 2.3 Composite Fabrication Composite laminates were produced via the hand lay-up technique followed by compression moulding to restrict void formation. A six-layer symmetric stacking sequence was adopted: [Banana/Glass/Bamboo/Bamboo/Glass/Banana]. Each fibre layer was uniformly wetted with epoxy resin applied by a rubber roller to achieve consistent resin distribution throughout the thickness. Upon completion of stacking, the laminate assembly was placed between steel mould platens and subjected to a compression pressure of 5 MPa for 24 hours at ambient temperature to facilitate cure and fibre–matrix consolidation. Post-cure conditioning was carried out at 60 °C for four hours in a convection oven to complete crosslinking and relieve residual stresses.31,32 Fig. 3. Fabrication sequence: (a) and (b) epoxy resin mixi