Abstract

In vehicle structures, combinations of metallic and non-metallic materials are becoming increasingly common. Bonding has established itself in automotive engineering for joining these materials. A generally valid method for calculating the fatigue life of the adhesive joints does not yet exist. This is due to the time- and temperature-dependent material behavior, inhomogeneous stress conditions and complex damage mechanisms. Due to the high computational effort, neither transient fatigue analyses nor fine finite element meshes can be realized in industrial applications. Therefore, methods have been developed which intelligently combine different modelling and calculation approaches. The system is initially simplified, which enables a fatigue analysis to determine the cohesive technical crack initiation in all adhesive joints of an entire vehicle structure. In S/N tests, which were carried out on specimens with stress concentrations at the joint corners, a high cohesive fracture toughness was observed in the epoxy adhesive layers. The pronounced crack propagation phases lead to massive fatigue life differences between crack initiation and total failure. In order to take this finding into account, the failure criterion was extended from the crack initiation to a local cracked zone and so-called zone-S/N curves were determined. A routine for localizing and evaluating the cracked adhesive layer areas in the finite element model was developed. With the zone-S/N curves of different specimen tests it was possible to calibrate the developed evaluation method and to calculate the degree of utilization of the adhesive layer. A major advantage of this approach is that the previous fatigue analysis can be performed in the time and frequency domain. The assessment method and the correlation to tests is presented. In addition, the application on finite element models of complex vehicle structures is shown. Here it is demonstrated which advantages result from the area-wise consideration and how the number of adhesive layer areas, which must be examined in detail, can be limited.