Quantification of Fatigue Life Improvement in S550MC Weldments by HFMI Treatment Using 2D SBFM Model

In welded joint structures, most of the fatigue failure is triggered from the welded location due to the presence of a notch effect and high tensile residual stress at the weld toe resulting from the welding process. High-Frequency Mechanical Impact Treatment (HFMI) has been proven to be a well-known solution for the fatigue strength improvement of welded structures. However, only a selected grade of structural steel has been studied so far for fatigue strength improvement by the HFMI technique for specific applications. A high-strength S550MC steel grade is used for the fabrication of industrial large cranes and earth-moving equipment. There is a gap in the study of the use of the HFMI treatment to improve the fatigue performance for S550MC grade steel. This study focuses on the study of the effect of impact treatment on the fatigue performance of weldments made of S550MC steel. A numerical analysis has been carried out using the 2D Strain-Based Fracture Mechanics (SBFM) model on cruciform welded joints in as-welded and impact-treated conditions for constant amplitude (CA) and variable amplitude (VA) loading histories. Fatigue life prediction obtained for as-welded joints from the numerical analysis using the fracture mechanics model shows reasonable agreement with the CA experiment test results for the similar component geometry from the literature. The prediction from the model shows a significant improvement in fatigue strength due to HFMI treatment with assumed residual stress distribution under both CA and VA loading conditions. Therefore, the HFMI treatment technique and its applications have a strong potential for fatigue strength improvement of welded structures.