Local Approach to Notch Depth Dependence of CTOD Results

The effect of the notch depth on the critical CTOD value at cleavage fracture initiation was analysed by the local approach. Three-point bend CTOD specimens with different notch depth extracted from a high strength steel were tested at the lower shelf temperature. The critical CTOD values showed large dependence on the notch depth. Three-dimensional elastic-plastic finite element analyses were performed to investigate the crack tip stress fields for all specimen geometries. The numerical results showed that shallow and deep notch specimens exhibit very different near tip stress fields at the same CTOD level, which leads to the specimen geometry dependence of the fracture behaviour. The analysis of the notch depth dependence of CTOD results was conducted on the basis of two formulations of the local approach. In the first formulation, cleavage fracture was assumed to be controlled by the maximum principal stress. In the second formulation, the stress tensor ahead of the crack tip was fully described in terms of all principal stresses and cleavage fracture was assumed to be controlled by an equivalent stress for mode I loading. It was shown that both formulations of the local approach lead to geometry independent parameters, especially when the stress triaxiality effect is taken into account. Prediction of the notch depth dependence of CTOD results based on the local approach presented good agreement with the experimental data, particularly when stable crack growth prior to cleavage was not significant.