Effect of Bond Thickness on the Fracture Toughness of Adhesive Joints and Its Mechanism

Abstract

Examining the effect of bond thickness on the fracture toughness of adhesive joints is a common issue; however, the mechanism of this effect has not been elucidated. Damage zones around crack tips in adhesive joints with several bond thicknesses were observed using an optical microscope. Interfacial damage zones were observed in cases of bond thickness of less than 0.7mm. Specially, large interfacial damage zones grew in an adhesive layer of 0.3 mm bond thickness, which showed maximum fracture toughness. The stress shielding effect of the interfacial damage zone was expected to increase the fracture toughness of this adhesive joint. Analyses of damage around crack tips in adhesive joints were performed using the finite element method (FEM) inconjunction with Gurson's model. The result of the FEM analyses showed the increase of stress and damage in cases of thinner adhesive layers, which means a monotonic decrease of fracture toughness accompanied decreased bond thickness. Then, artificial damage along the interface between an adhesive layer and adherends was introduced in the FEM analyses. These analyses showed the maximum fracture toughness of an adhesive joint with a bond layer of about 0.3 mm, and the results confirmed the stress shielding effect of the interfacial damage zone