The sensitivity of J-integrals to material law variations for semi-elliptical surface cracks

In finite element analysis of cracked bodies, it is quite common to represent the material law (stress-strain curve) in a simplified form e.g. bilinear approximation. Many steels, however, have a more complex material law, which includes a perfectly plastic region before work hardening. Two dimensional finite element analyses have shown that fracture parameters, such as the J-integral, are very sensitive to variation in material law for shallow cracks, and the writers have now commenced three dimensional analyses for surface semi-elliptical cracks in plates. Some computations have been made for specimens tested experimentally by Dodds and Read [1], and the results show that what appears to be a reasonable approximation of the material law is inadequate in modelling the deformation behaviour, and that more accurate modelling of the material law, especially the plastic part immediately after yield, which may include perfectly plastic behaviour, is required for reliable results.