Criteria of nonlinear fracture mechanics

Difficulties are experienced in using some of the examined fracture criteria in the elastoplastic region. The apparent fracture toughness varies in a wide range. The effect of the thickness of the specimen on cracking resistance is relatively small. In a partial case examined in this work, i.e., in fracture of the low-strength aluminum alloy with slight hardening in the conditions of the plane stress state at the crack tip, the range of applicability of certain critria is very small so that it is necessary to use the fracture criterion which does not take into account the crack length. Consequently, in this case, the two-parametric fracture criterion is more suitable for the apparent fracture toughness K^*-a/W ratio curve. This is illustrated clearly by Fig. 9 where the fracture load, i.e., the fracture criterion belonging in the group of the fracture criteria of the "intermediate state," was predicted by the Newman method. In fact, the criteria of nonlinear fracture mechanics can be used in the conditions restricted by two limiting states: fracture at linear elastic strains, and fracture as a result of the formation of a plastic hinge. The interpolated curve used in such a method is usually known as the two-parametric Newman method or the CEGB method which is the same in the present case. This is possible owing to the fact that the extent of hardening of the aluminum alloy examined in this work is only very small. Further investigations are required for predicting the critical crack growth with satisfactory accuracy.