Application and Evaluation of the Implicit gradient Model for the fatigue Assessment of welded joints with severe Imperfections

Kurzfassung

Laser-hybrid welded joints are increasingly utilized in modern engineering applications. As this technology is relatively new, the right parameters for achieving a defect free weld seam remains a challenge. Consequently, laser-hybrid welding often introduces severe imperfections into the seam, such as weld splatters, undercuts or excessive root penetrations. These surface irregularities significantly reduce the structural lifespan and ultimately lead to fatigue failure. Previous studies have shown that the point of fatigue failure often occurs in regiones with higher stress concentrations and additionally that weld imperfections tend to increase these stress concentrations near the weld. To accurately predict the fatigue behavior, it is important to establish a reliable local fatigue assessment methodology and therefore to enhance the accuracy of fatigue life predictions in explicit for laser-hybrid welded butt joints with severe imperfections. Conventional numerical methods often fail to account for irregular surface topologies, as they are typically based for idealized standard cases. To accurately predict the fatigue life of specimens with irregular surfaces, the microstructural support effect must be considered. In previous studies the microstructural support effect was applied to these welds considering the theory of critical distance, the limit of these lays in the large number of notches across the weldment. An alternative approach to incorporate the support effect is the implicit gradient model. The implicit gradient model, applied to 3D finite element models, averages the stress over a defined volume around the notch. By varying this volume, the stress concentrations on the surface are redistributed. Through the application of the implicit gradient model, a correlation between the maximum stress concentration and the experimentally observed fatigue point of failure was achieved for most of the specimens. Based on these effective stress concentrations and experimental fatigue data, an effective notch stress SN-curve for butt joints with severe imperfections was derived