Evaluation of the cracked lap shear test for mixed-mode fracture toughness estimation of adhesive joints

Cohesive zone models are a widely used method in adhesive joint strength predictions, but it requires the knowledge of fracture toughness of the adhesive in the different loading modes, together with the mixed-mode behaviour. The cracked lap shear test is a simple but seldom studied mixed-mode test and, as such, a deeper knowledge about its accuracy and of the available data reduction methods is required. This work consists of the experimental and numerical analysis of the cracked lap shear test to estimate the mixed-mode fracture toughness of two structural adhesives. The experimental work consisted of testing cracked lap shear specimens with the different adhesives. Numerically, cohesive zone model were used to predict the behaviour of bonded joints under mixed mode, while assessing the validity of the available data reduction methods to estimate fracture toughness. While the experimental analysis showed that the Brussat's method led to offset results compared to the other methods, the cohesive zone model analysis confirmed the inadequacy of the Brussat's method, and the suitability of the Grady and Kinloch's models for mixed-mode fracture analysis.