Total fatigue life prediction for welded joints based on initial and equivalent crack size determination

When the linear elastic fracture mechanics-based approaches are used to predict the fatigue life of welded joints, initial crack size is a key point, which eventually affects the accuracy of total fatigue life prediction. Meanwhile, the life prediction process under random loading is complicated. In this paper, a novel method is proposed to determine the initial crack size, which is based on the results of back-extrapolation approach. The proposed method expresses the stress intensity factor, and the boundary between crack initiation and propagation period is taken into consideration. Based on the proposed method, deterministic total fatigue life can be obtained with fewer tests and less cost. In addition, the concept of equivalent crack size and its calculation model are proposed to reduce the complexity of the calculation process of fatigue life prediction under random loading, and model uncertainty is included into the prediction model of probabilistic fatigue life based on equivalent crack size. It is feasible, which has been verified, to take the influence of stress level into account when determining the initial crack size. Meanwhile, the proposal of equivalent crack size simplifies the calculation process of probabilistic fatigue life, and the consideration of model uncertainty is more conducive to assess the safety and reliability of the materials or structures.