On the energy release rate of aged adhesive joints

In Chapter B.7 the influence of ageing to the critical energy release rate is described which was investigated experimentally for seven modifications of basic and technical epoxy and polyurethane adhesives. It is discussed that the main contribution to the critical energy release rate is provided by rate dependent deformation mechanisms altered by the (mainly reversible) influence of water. The local dissipative deformation shields the crack tip from the applied load. The amount of shielding and the measured critical energy release rate depend on the volume of the deformation zone. The deformation zone size is determined by the competition between the loading rate, the deformation rate of the adhesive, the crack propagation rate, and the thickness of the adhesive layer. Each modification of adhesive which was investigated shows its characteristic behaviour. A quantitative understanding on the basis of generic mathematical fracture mechanical models is hindered by the violation of the assumption of homogeneity and isotropy since the dissipative mechanisms probably are activated in the entire region between the adherends in the direction of thickness.