Variational Method and <scp>CZM</scp> ‐Based <scp>FEM</scp> Mechanical Strength Assessment of Adhesively Bonded Hybrid Sisal‐Glass Reinforced <scp>HDPE</scp> Composite for Automobile Side Body Panel Application

ABSTRACT This study investigates the mechanical performance and debonding behavior of adhesively bonded steel‐to‐hybrid sisal–glass reinforced high‐density polyethylene (HDPE) composites for automobile side body panels, by combining experimental testing and cohesive zone modeling (CZM)‐based finite element method (FEM) simulations. Composites were produced through compression molding, with various stacking sequences (sisal–glass–glass and glass–sisal–glass), and a fiber‐to‐matrix ratio of 30:70 for HDPE. Tensile, flexural, and impact strengths were in the range of 19.8–31.9 MPa, 25.3–38.1 MPa, and 12.5–17.2 J/m, respectively, as determined by mechanical testing. Environmental tests verified the consistent performance under thermal aging and moisture exposure. The highest shear strength was obtained for adhesively bonded single‐side strap joints (ABSSSJ) when the optimal overlap length ( OL ) was 10 mm and the optimal adhesive thickness was 0.5 mm. At failure, the highest shear stress was 32 MPa and the highest peel stress was 12 MPa. The experimental force–displacement curves were accurately reproduced by CZM‐based FEM, with a verification error of ±6%–8%. According to the sensitivity analysis, joint strength was further improved by increasing the adhesive modulus to 6 GPa, while the efficiency was reduced by an excessive overlap length. By combining closed‐form stress analysis with FEM and experimental data, this study establishes a validated variational method (VM)–CZM‐based framework. These hybrid composites were confirmed as sustainable, lightweight materials for non‐structural automotive side body panels, providing a 58% weight reduction compared with steel and a 20%–25% cost advantage over glass‐fiber‐reinforced polymer (GFRP).