Fracture mechanics of adhesive joints

This chapter summarizes the basic principles and definitions of fracture mechanics and gives an overview of its application to adhesive bonding. From the macroscopic point of view the stress intensity factor or the equivalent energy release rate can be used to describe the fracture behaviour of adhesive joints as long as the conditions of small scale yielding are fulfilled. This is the case in most applications since the energy dissipating processes are restricted to the volume of the adhesive which is usually small compared to the size of structure (or sample) to be considered. It was estimated that the main contribution to the fracture toughness, described by the critical energy release rate, originates from dissipating processes around the crack tip of a propagation crack. Even for brittle adhesives this contributions seem to be orders of magnitudes larger than what should be expected from generating new surface during crack propagation (Griffith crack). Thus, it has to be expected that the volume in which energy is dissipated in the adhesive layer in most cases reaches the adherend and the dissipation is not restricted to the immediate surrounding of the crack tip. The assumptions of homogeneity and isotropy are violated and the mathematical framework of fracture mechanics cannot simply be applied to describe the local conditions in the adhesive layer which would be a key to understand the dissipative contributions of the microstructure of the adhesive to the energy release rate quantitatively. Nonetheless, fracture mechanical approaches were successfully applied in the numerical cohesive zone model to simulate the crash behaviour of adhesive joints in automotive applications. Experimental results and data with respect to the cohesive zone model are presented for two different types of toughened adhesives under mode I, mode II, and mixed mode loading. Finally the influence of the microstructure of the adhesive to the stiffness, the strength, and the critical energy release rate is shown, however without closing the missing link between quantitative description of the energy release rate and the microstructural properties.