EFFECTS OF LOCAL CONSTRAINT ALONG 3-DIMENSIONAL CRACK FRONTS - A NUMERICAL AND EXPERIMENTAL INVESTIGATION E

A two-parameter characterization of the crack front state in a variety of geometries (mode I) is explored by means of three-dimensional finite element analysis, including finite strain effects. In particular the approximate J-Q theory is scrutinized and it is found that Q appears to be a good measure of the deviation in stress triaxiality ahead of a crack tip as compared to the highly constrained plane strain SSY-solution, also in cases where the crack front is relatively curved. The implications for cleavage fracture in the upper transition region are elucidated by appraising the results from an extensive experimental program, where both tension and bending type of plane specimens as well as surface cracked plate specimens had been tested. It appears that the J-Q concept together with some cleavage failure criteria, e.g. the RKR-model, can be applied locally along three-dimensional crack fronts in a structure in order to assess cleavage fracture. To the extent that one dominating cleavage fracture spot could be Located at a three-dimensional crack border, this was in general found at the position which had undergone the most critical J-Q sequence, in the light of the RKR-criteria. Microstructural features of both ductile and cleavage fracture are elucidated by a fractographical survey performed.