Toward predictive damage analysis of aluminum adhesive joints through graded cohesive zone modeling: part I-mechanical response and failure mechanisms

Functionally graded adhesives (FGAs) offer a promising approach for mitigating stress concentrations in adhesively bonded joints; however, the mechanisms governing their influence on load transfer and damage evolution remain insufficiently understood. This study presents a high-fidelity three-dimensional numerical investigation of single-lap joints using a fully coupled functionally graded cohesive zone model (FG-CZM) implemented through a USDFLD subroutine. The model was validated against experimental results, accurately reproducing joint stiffness, peak load, and failure displacement. Comparative analysis of mono-modulus adhesives confirmed the classical strength-ductility tradeoff, with the ductile adhesive exhibiting approximately 30% higher load capacity than the brittle reference. In contrast, all graded configurations achieved substantial performance improvements, with average peak-load gains approaching 40% relative to Araldite® AV138. The optimal gradation attained a load capacity within 5% of the ductile Adekit® A140 while providing enhanced deformation capability. Mechanistically, the improvements originate from stress-field redistribution rather than stress amplification. Optimized gradation increased the effective shear-transfer length from approximately 50% to nearly 80% of the overlap, reduced tensile peel stresses by up to 35%, and delayed damage localization. Furthermore, graded architecture reduced the fully degraded cohesive zone by up to 40%, promoting distributed damage and enhanced fracture resistance.