Study on application of J-integral fracture criteria under elastic-plastic I-II mixed mode loading

The j-integral resistance characteristics of two kinds of oil drill pipe material were studied on the basis of the calculation with a large scale yield finite element code by separation of j-integral under different I-II mixed mode loading. It was shown that the fracture toughness expressed in J-integral decreased with the increasing of the mode II component. A mixed mode ductile fracture criteria, J_c=J_IC-(α-1)J_II, was then proposed. It can be applied to the material with mild or low strength, high ductility and toughness. α was a material constant, but it was influenced to some extent by the specimen size in the test range. Some explanation and discussion about the criteria and α were performed with the investigation of the process of mixed mode ductile fracture.