Comparison between fracture mechanics and local approach models for the analysis of shallow cracks

This contribution shall introduce the application of the fracture mechanics and damage mechanics on flawed specimens by comparing the methods, especially with regard to the evaluation of shallow cracks. The investigations introduced here were conducted on specimens removed from a segment of a shape-welded vessel made of 10 MnMoNi 55. Using round notched tensile specimens the required parameters were determined for the local approach model by Rousselier. Following this the local approach model was used for calculating the macroscopic failure behaviour of CT-specimens and three-point bending specimens with long and shallow cracks. The crack depth to width ratio of the CT-specimens was approx. a/W = 0.56, that of the three-point bending specimens a/W = 0.55 and a/W = 0.07. Using experimental results and numerical calculations on the basis of the damage model according to Rousselier it is possible to examine the influence of specimen geometry and a/W-ratio on the crack initiation value Ji and the crack resistance curve. Thus an evaluation of fracture mechanics methods is possible. The course of the multi-axiality of the stress state is a possibility of assessing the transferability of crack resistance curves. In calculating the multi-axiality during a running crack it is shown how the multiaxiality changes as a function of the crack propagation. (orig.)