To construct safe and efficient structures, the fatigue life prediction and the fatigue design is important. In steel structures fatigue failure becomes apparent as a crack initiating at notches, where stresses are disproportionately high. An existing and growing crack leads to a decrease of the load-bearing-capacity of structures and can finally result in complete failure. The fatigue design is mostly done with conventional method like the nominal stress concept or the hot-spot-method, as these very general methods are implemented in the design standard. They neglect the differentiation between crack initiation and crack propagation as well as consequential effects. Further the nowadays existing improvements in terms of material and weld quality, compared to the database used to assess the FAT-categories, cannot be considered in the design.

To predict the fatigue life of steel structures more realistic, a Two-Stage-Model is developed, that follows the phenomenological distinction of crack initiation and crack propagation using a combination of analytical methods and numerical calculations. The Model uses the Strain-Life Approach to assess the initiation phase based on the cyclic material parameters, local geometry, and stress concentration. This is followed by an X-FEM based application of the Crack Propagation Approach to assess the propagation phase. Using computer-based fracture mechanics allows the determination of the stress intensity and crack growth for arbitrary notch geometries.

Using these material-based local methods, a component S-N curve can be obtained solely with the knowledge of material data and geometry. The project starts to assess the fatigue life of butt welds and the Model will be extended to welded steel structures with complex stress-strain-fields at the decisive point.

Röscher, S., & Knobloch, M. (2019a). Towards a prognosis of fatigue life using a Two‐Stage‐Model: application to butt welds. Steel Construction, 12(3), 198–208. https://doi.org/10.1002/stco.201900018

Röscher, S., & Knobloch, M. (2019b). Two-stage-model for the prognosis of fatigue life application to butt welds. In J. Jönsson (Hrsg.), Nordic Steel Copenhagen 2019: 18th to 20th September ; The 14th Nordic Steel Construction Conference (S. 597–602). https://doi.org/10.1002/cepa.1106