Fatigue behaviour of an epoxy adhesive under mixed-mode conditions

The aim of this study is to characterize the fatigue behaviour of a structural epoxy adhesive loaded in mixedmode conditions. To this effect, several fracture tests were carried out: Double Cantilever Beam (DCB) for pure mode I, End Notched Flexure (ENF) for pure mode II, and mixed-mode testing, using an apparatus that enables the application of mixed-mode loadings to an adhesive and with high strength steel adherends. The fracture envelope was calculated using experimental data and it was found to be compatible with a quadratic criterion. A novel quadratic law criterion was developed to fit the envelope. The fatigue behaviour was studied recurring to fatigue crack growth curves for the same types of loads studied in the quasi-static tests. A relationship between mixed-mode phase angle and the Paris law slope was found, where the existence of mode II loads was found to delay crack propagation. By performing fatigue tests at 60% and 80% of the maximum load, it was noticed that increasing the load leads to the increase of the threshold energy release rate values.