A Fracture Mechanics Approach to Durability Calculations for Adhesive Joints

Effective use of adhesive bonding in automotive vehicle bodies requires analytical methods for durability, so that potential fatigue problems and unnecessary overdesign may be eliminated before the physical prototype stage and release of product with unquantified safety factors avoided. This paper describes a fracture mechanics-based method for predicting the durability of adhesive joints, based on work previously carried out at Volvo [1]. The method requires relatively modest modifications to a typical vehicle body FE mesh. Adhesive bonds are represented by bar elements around the periphery of each bond. Grid point forces from shell elements adjacent to the adhesive bond are recovered and used to determine line forces and moments at the edge of the glued flange. These forces and moments are then transferred to an analytical sandwich model of the joint. This enables approximate calculations of the strain energy release rate (or rather the equivalent J-integral) to be made for assumed small cracks at the edge of the adhesive. Analytical results are compared with calculations based on detailed FE meshes for typical test specimen geometries, and with physical test data. J-integral values calculated for different geometries and loadings are shown to be able to correlate the durability of joints, particularly at longer lives, enabling useful estimations of joint durability to be made.