Experimental investigation of strength and failure modes of adhesively bonded hybrid sisal glass reinforced HDPE composite for automobile side body panel applications

This study investigates the mechanical strength and failure behavior of adhesively bonded steel–hybrid sisal–glass reinforced high-density polyethylene (HDPE) composite joints (ABSSSJ) for lightweight automotive side body panel applications. Hybrid composites were fabricated by varying the fiber-to-matrix ratio (F/M%) of sisal and glass fibers in an HDPE matrix and bonding them with a structural epoxy adhesive. The effects of F/M ratio, moisture exposure, temperature, adhesive thickness, overlap geometry, and surface roughness on joint strength and failure modes were systematically evaluated through tensile and shear testing. Unlike previous studies that examine these parameters independently, this work investigates their combined influence within a single experimental framework, enabling multi-factor performance optimization. Results indicate that increasing glass fiber content significantly enhances tensile stiffness and strength, with S5G25H70 (25% glass, 5% sisal) achieving the highest tensile stress of 30.33 ± 1.12 MPa Peak shear strength (10.91 MPa) was maintained under short-term moisture exposure (2–4 h), while prolonged exposure caused hydrolytic degradation. Mechanical performance peaked at 35 °C (11.43 MPa) and decreased beyond 40 °C due to matrix softening. Optimal load transfer occurred at an overlap length of 7.5 mm (14.61 MPa), while a 0.12 mm adhesive thickness yielded the highest shear stress (9.23 MPa). Surface preparation with SF50 grit improved adhesion through enhanced mechanical interlocking. The optimal configuration (S5G25H70, 0.12 mm adhesive thickness, 7.5 mm overlap, 30–35 °C, SF50 surface) provides a balanced combination of strength, stiffness, and durability. These findings demonstrate that optimized ABSSSJ hybrid composites offer a viable and sustainable alternative for lightweight automotive body structures through controlled hybridization and adhesive joint design.