Experimental study of stress–strain and failure modes of adhesively bonded hybrid sisal–glass reinforced HDPE composite for automobile side body panel applications

This study presents the first systematic experimental evaluation of adhesively bonded steel-to-hybrid natural sisal–glass fiber reinforced high-density polyethylene composite joints for automotive side body panel applications. Single-lap tensile and lap-shear tests were performed on joints with varying fiber–matrix ratios, adhesive thicknesses (0.2–1.0 mm), overlap geometries, and surface conditions. The results show that hybrid sisal–glass reinforcement significantly improves joint strength and stiffness: the ultimate tensile strength reached 62–68 MPa, and the optimal shear strength was 10.5–11.8 MPa at adhesive thicknesses of 0.2–0.5 mm. Simulated environmental exposure (water immersion and elevated temperature) reduced shear strength by 35–45% due to interfacial degradation and matrix plasticization. Failure mode analysis revealed a shift from predominantly cohesive failures to mixed adhesive–cohesive modes under harsher conditions and varied overlap geometries. These findings establish quantitative design guidelines for lightweight, sustainable automotive body panels using recyclable sisal–glass hybrid composites. The results demonstrate that such joints can meet strength requirements while reducing weight and material costs.