ASSESSMENT OF FATIGUE BEHAVIOUR OF UHSS STEEL BUTT-WELDED JOINTS BY MEANS OF A FRACTURE MECHANICS METHODOLOGY

Advances in steel manufacturing technologies made possible the use of high-strength steel (HSS) and ultra-high strength steel (UHSS) in bridges, cranes, offshore structures, oil pipelines and automotive parts. Welding procedures had to be developed to join these materials successfully, but this is still a
major issue in mechanical design of HSS elements. Particularly in welding codes and design documents, fatigue resistance of as-welded joints is normally considered to be independent of the base material (BM) static strength. However, cyclic loaded as-welded components with high quality or post-weld
treated joints have shown improved performance when using HSS and UHSS as the base material.
The present work aims to apply a fracture mechanics methodology to the analysis of fatigue behaviour of welded joints. The approach requires estimating the driving force available for subcritical crack growth at the location of maximum stress concentration. In this regard, stress intensity factor proved to
be a sensible parameter that can account for loading scheme and local weld geometry. It can be determined by numerical modelling, which demands a change from continuum mechanics stress analysis to one that estimates fracture mechanics parameters, considering the existence of defects and cracks.
Then, the total driving force applied to the crack can be compared to its threshold for propagation, resulting in the effective driving force for crack growth.
Particularly, the effect of welding process on the fatigue behaviour of ultra-high strength steel buttwelded joints was studied. Sheets of steel S960MC and S960QL were joined with different welding techniques: Gas Metal Arc Welding (GMAW), Laser Hybrid Welding (LHW) and Electron Beam Welding (EBW). To validate the model, fatigue tests were performed with stress ratio R = 0.1, under
four points bending loading. Joints manufactured with GMAW exhibited the highest fatigue strength of the three configurations. Compared to the fatigue limit of the BM, a decrease in fatigue strength around 60% was observed in welds jointed with LHW and EBW, although the latter showed longer fatigue lives
for higher nominal stresses.
Proposed methodology allows to assess the effect of microstructure, defect size, hardness, and joint geometry resulting from each welding technique. Results conservatively describes the fatigue behaviour of each weld configuration and highlights the relative influence of all factors considered in the
assessment. Although the validated results request further studies to improve understanding of the acting mechanism, they also show the potential of welded HSS and UHSS joints compared to the standard design approaches