Prediction of Failure Strength of Adhesive Joints Using Peel Stress and CTOA

Abstract

The failure strength of adhesively bonded joints depends on many factors such as material properties (both adhesive and adherend), specimen geometries, test environments, surface preparation procedures, etc. Among them, adhesive properties can be regarded as the most dominating factor affecting the failure strength. In this study, the failure strengths of single lap joints and double strap joints were tested for different combinations of adhesives and adherends. Both adhesive failure mode and cohesive failure mode were generated by controlling the fabrication procedure together with different material combinations. For producing a pure adhesive failure, stainless steel adherends and an epoxy-based paste adhesive (Hysol EA9394) were used. It was observed that for consistent surface treatments, peel stress distributions along the interface near the crack initiation point show similar trend at failure loads for single lap joints with different geometrical combinations for which peel stress distributions were calculated using the finite element analysis. Cohesive failures were generated using two different joints, single lap joints consisting of aluminum adherends bonded with the Hysol adhesive and double strap joints with graphite/epoxy composite adherends bonded with film adhesive FM-94. It was observed that for double strap joints with different overlap lengths, peel stress distributions at the failure initiation point show a similar magnitude. Thus, the peel stress within a critical distance can be used as a parameter capable for predicting the joint strength for above cases. However, when boundary constraint effect (specifically adhesive thickness) changes significantly peel stress distribution near the failure initiation point is not capable of predicting failure strength for cohesive failure mode. In that situation, fracture mechanics based approaches or some other approach needed to be adopted. Fracture mechanics based approach assumed that there is pre existing crack within the structure and failure initiates when the crack starts propagating. One of the suitable approach to predict failure strength (crack propagation loads) for non-linear fracture mechanics is crack tip opening angle (CTOA) based approach. Cracks with a certain length along crack initiation points can be introduce in FEA model and CTOA values at failure load can be evaluated. This type of method is shown in this paper and the length of the pre existing crack is justified. It is observed from FEA that at failure load CTOAs follows a certain trend that can be obtained using other tests method such as double cantilever beam (DCB) tests. As CTOAs follows same trend as test data it can be adopted as parameter to predict joint failure strength.