Effect of thermal residual stresses on the apparent interfacial toughness of epoxy/aluminum interface

In this paper, the apparent fracture toughness of the interfaces of several epoxy-based adhesives and an aluminum substrate is experimentally measured. Double layer specimens with initial interfacial cracks are made for four-point bending tests. Thermal residual stresses exist on the interface due to the coefficient of thermal expansion (CTE) mismatch between the underfill and aluminum. Silica fillers are used to modify the CTE of the epoxy-based adhesives so that various levels of interface thermal residual stresses are achieved. It is found that the apparent interfacial toughness is significantly affected by the thermal residual stress. In general, thermal residual stress undermines the interfacial crack resistance. In some cases, the residual stress is sufficient to result in adhesive and/or cohesive failure