Fracture tests under mixed mode I + III loading: An assessment based on the local energy

In this paper, some recent data exploring mixed mode I + III fracture of cracked polymethyl methacrylate specimens are re-analysed in terms of the local energy. The data have been obtained by using a new loading fixture proposed for exploring mixed mode I + III fracture experimentally. While a large bulk of experimental results from cracked specimens of polymethyl methacrylate under pure modes of loading, in particular under mode I loading, can be found in the literature, only very few tests have been carried out considering multiaxial static loadings, and for this reason, only limited studies have been performed on possible criteria for the fracture assessment under mixed mode I + III conditions. By taking advantage of the recent set of data on polymethyl methacrylate tested at different values of the mode mixity ratio, a criterion based on the local strain energy density previously applied by the same authors only to pure modes of loading is successfully extended here to the case of tension and torsion loadings applied in combination. Good agreement is shown to exist between the theoretical estimates and the experimental data.