Towards a prognosis of fatigue life using a Two-Stage-Model - Application to butt welds

Nominated for the Bernt Johansson Outstanding Paper Awards at Nordic Steel 2019
This paper presents a Two-Stage-Model for determining the fatigue life of steel structures. The model follows the distinction between crack initiation and crack propagation according to the phenomenological background. The first stage uses the local strain-life approach to predict the number of cycles for crack initiation Ni considering the cyclic material behaviour as well as the load-time history. The second stage calculates the crack propagation based on a linear-elastic fracture mechanics approach. Using X-FEM in combination with cyclic loading, an arbitrary, solution-dependent crack path is achieved to determine the number of cycles for propagation Np. The Two-Stage-Model was applied to butt welds to determine their total fatigue life Nf in terms of a proof-of-concept study. The study investigated the ability of the model to consider the influence of the weld geometry, i.e. plate thickness and weld imperfections (excess and vertical offset). The results are presented by means of S-N curves for initiation life, propagation life and total fatigue life. A comparison with experimental results provided in the literature shows that the Two-Stage-Model can reflect the weld quality of butt welds in terms of fatigue life.