Experimental Investigation and Multi-Objective Optimization of Waste Granite Filled Luffa and Sisal Fiber Reinforced Epoxy Hybrid Composites Using GRA and RSM

This study investigated the effect of granite filler on the mechanical and water absorption properties of luffa/sisal fiber-reinforced hybrid epoxy-based composites. Although luffa and sisal offer advantages including biodegradability, low density, and renewability, their moisture sensitivity and poor compatibility with polymer matrices often limit their applications. To address these challenges, the fibers were alkali-treated, hybridized, and incorporated into an epoxy matrix containing varying amounts of granite filler. A Taguchi L16 orthogonal array was used to assess the influence of fiber and filler content, and Grey Relational Analysis (GRA) and Response Surface Methodology (RSM) were used for multi-response optimization and predictive modeling. The results demonstrated that increasing granite filler content significantly enhanced both the mechanical performance and water resistance of the composites. The optimum formulation, comprising 10 wt.% luffa fiber and 10 wt.% granite filler, exhibited the highest compressive strength (89.1 MPa) and flexural strength (54.8 MPa). Maximum impact energy absorption of 4.1 J was achieved at 17.5 wt.% luffa fiber, and 5 wt.% granite filler. Water absorption increased with luffa content, reaching a maximum of 14.6%; however, the inclusion of 10 wt.% granite filler reduced moisture uptake to 8%. GRA identified the 10 wt.% luffa with 10 wt.% granite filler as the optimal formulation, and experimental validation showed close agreement with RSM predictions. SEM analysis revealed improved fiber–matrix interfacial bonding, effective fiber encapsulation, and reduced void formation. These findings highlight the potential of combining natural fibers with waste granite particulates to develop eco-friendly, high-performance composites suitable for automotive, packaging, and low-load structural applications.