Experimental and numerical study of optimum functionally graded Aluminum/GFRP adhesive lap shear joints using Epoxy/CTBN

Failure in adhesive joints is usually the result of a non-uniform distribution of stress, which is observed with maximum values near the two ends of the overlap, and the central parts have a minor overall load-bearing contribution. In the joint of two dissimilar materials, the stress fields are asymmetric and the optimum grading of the adhesive properties will not be symmetric. The main objective of this work was to study the performance of optimum functionally graded joints (OFGJ) to maximize the ultimate loading capacity of a lap shear joint. The adherends were aluminum 7075-T6 and unidirectional glass-epoxy composite. The study has been performed considering a grading strategy based on mixtures of an epoxy resin with different values of liquid rubber to obtain the properties variations. First, the mono-adhesive joints with homogeneous bond-line and then symmetrical graded joints with equal bands were investigated. A compliance-based optimization method with the normalized shear stress distribution was used to obtain the best properties along the bond-line. The elasticplastic model and mixed-mode cohesive damage model were used to obtain the stress distribution and failure prediction, respectively. By grading the overlap symmetrically, the stress distribution has become more uniform and the failure load compared to the base specimen (with a shear strength of 6 kN), increased by 206%. Optimization of adhesive properties using normalized shear stress distribution has positive effects on modifying stress distribution, non-shear forces elimination, improvement of the load-bearing contribution, failure mode alteration, and delay on adhesive joints failure. Therefore, there is a high increase in shear load strength of optimum graded specimen by a significant amount of 299%.