A failure criterion developed for crack of polyurethane adhesive bonding structures under hygrothermal aging environment

In the service process of vehicles, the performance of adhesive components is usually affected by hygrothermal aging and complex stress states for a long time. Therefore, the failure criterion of adhesive structure with practical environmental applicability is very important for fracture prevention and adhesive performance optimization. In this study, polyurethane adhesive (ISR-7008)/aluminium alloy joints were selected as the study object, and −40 ∼ 85 °C/30 ∼ 95 % RH was selected as the cyclic hygrothermal aging environment. The joints were subjected to cyclic hygrothermal aging for 0 ∼ 720 h. The quasi-static tensile tests of joints with different loading angles (0 ∼ 90°) were carried out by the improved Arcan fixture to simulate the complex stress states. The stress–strain curve, initial failure load and area of the joint were obtained, and the stress distribution of the adhesive layer was further obtained by combining the finite element method. Then, based on the ratio method, the stress components with a dimension of MPa are selected, and the equivalent stress is calculated by linear combination. The equivalent stress that is not affected by the complex stress states is selected as the failure criterion of the adhesive layer by iteration. By comparing the failure mode and the initial failure load, it is found that the ratio of 3rd invariants of the deviatoric stress J*₃ and 2nd invariants of the deviatoric stress J*₂ can predict the failure strength of the joint after hygrothermal aging. The results show that the maximal relative errors between the predicted results and the experimental results under complex stress states are 14.44 % (unaged) and 17.35 % (hygrothermal aging for 720 h). The validity of the failure criterion and the applicability of different structural joints are verified by the tensile test of single lap joints. This failure criterion provides a new idea for the failure prediction of vehicle body adhesive structure.