Computational analysis of mechanical behavior and progressive damage in adhesively bonded steel hybrid composite joints

This study computationally investigates the mechanical behavior, stress distribution, and progressive damage evolution of adhesively bonded steel–hybrid sisal–glass-reinforced high-density polyethylene (HDPE) single-side strap joints using a cohesive zone model (CZM) integrated into a finite element framework. Unlike previous studies that primarily considered isolated material or geometric parameters, the present work systematically evaluates the coupled influence of adhesive thickness, adhesive elastic modulus, fracture toughness, hybrid fiber composition, stacking sequence, temperature, moisture exposure, and surface condition within a unified computational framework. We employed a bilinear traction–separation law to simulate adhesive damage initiation and propagation, calibrating cohesive parameters using experimentally measured tensile and lap-shear responses. The numerical model accurately predicted stress localization near overlap-free edges and reproduced experimentally observed failure locations. Peak peel and shear stresses reached 12.3 MPa and 18.4 MPa, respectively, at the overlap ends for the optimum adhesive configuration. Increasing adhesive thickness from 0.30 mm to 1.00 mm reduced peak peel stress by approximately 29%, although excessive thickness increased joint compliance. Increasing cohesive fracture energy from 0.25 to 1.00 kJ/m² significantly delayed crack propagation and enhanced ultimate joint strength. Hybrid reinforcement also improved the predicted tensile strength of the composite adherend from 41.8 MPa for unreinforced HDPE to 62.3 MPa for the glass–sisal–glass laminate. Model predictions agreed well with experimental measurements, with deviations below 8% for peak load and failure displacement. The present results demonstrate the capability of a validated CZM-based finite element approach to quantify stress redistribution, progressive interfacial damage, and the influence of environmental degradation under the investigated loading conditions. The findings offer quantitative insight into the mechanical behavior of adhesively bonded hybrid composite joints, establishing a computational framework that supports future optimization of lightweight composite structures.