Multi-material adhesive joints with thick bond-lines: Crack onset and crack deflection

This study investigates the fracture onset and crack deflection in multi-material adhesive joints with thick bond-lines (approximate to 10 mm) under global mode I loading. The role of adherend-adhesive modulus-mismatch and pre-crack length are scrutinized. The parameters controlling the crack path directional stability are also discussed. Single-material (i.e. steel-steel and GFRP-GFRP) and bi-material (i.e. steel-GFRP) double-cantilever beam joints bonded with a structural epoxy adhesive are tested. The joints are modelled analytically, considering a beam on elastic-plastic foundation, to include characteristic length scales of the problem (e.g. adhesive thickness, plastic zone) and numerically using Finite Element Model. An empirical relation, in terms of geometrical and material properties of the joints, that defines the transition between non-cohesive and cohesive fracture onset is found. Above a specific pre-crack length the stress singularity at pre-crack tip rules over the stress singularity near bi-material corners, resulting in mid-adhesive thickness cohesive fracture onset. However, the cracking direction rapidly deflects out from the adhesive layer centre-line. Positive T-stress along the crack tip is found to be one of the factors for the unstable crack path.

Automated summary

This study investigates the fracture onset and crack deflection in multi-material adhesive joints with thick bond-lines under global mode I loading, analyzing the role of adherend-adhesive modulus-mismatch and pre-crack length. The research discusses the parameters controlling the crack path directional stability and the transition between non-cohesive and cohesive fracture onset. The results show that stress singularity at pre-crack tip and T-stress along the crack tip are important factors affecting crack path stability.