Fracture mechanisms of hybrid adhesive bonded joints: Effects of the stiffness of constituents

In this study, different single-lap hybrid joints are used to analyse the effects of the stiffness of the adherends and the adhesive on the failure mechanism. The hybrid joints include a combination of (a) different adherends: aluminium (6082 T6) and PolyPhtalamide (PPA) reinforced with 50% of glass fibre (grade HTV-5H1 from Grivory) and (b) different adhesives: epoxy-based adhesive (Loctite EA 9497) and silane-modified polymer-based adhesive (Teroson MS 9399). Six different single-lap joints are fabricated and analysed. The cohesive parameters of different adhesives against different adherends are determined respectively using single-mode coupons and validated with finite element modelling. Single-lap shear tests are conducted to understand different fracture mechanisms of the joints. Finite element (FE) models using the Cohesive Zone Method (CZM) are developed to simulate the failure of the joints and validated by the testing results. Different failure processes obtained from different hybrid joints combinations are discussed further by analysing the stress distributions along the interfaces of the joints. Finally, the relationship between the stiffness of the constituents of a hybrid adhesive joint and its failure mechanism is summarised. The load vs displacement behaviour of the single-lap joints demonstrate that the stiffness of adherends affects the maximum failure load of the joints with rigid adhesive (epoxy). However, the joint with flexible adhesive (polyurethane) is not sensitive to the stiffness of the adherends. In addition, higher shear stress distribution occurs in the interface adjacent to the adherend with lower stiffness, leading to the failure initiation at the PPA side regardless of adhesive types.