Influence of the bainitic microstructure on short fatigue crack growth in HSS steel butt-welded joints

Fatigue failure of welds generally starts from weld toes mostly due to geometrical stress concentration effect. Nevertheless, the bainitic HAZ microstructure at the weld toe plays an important role in short fatigue crack initiation and growth. Short crack kinetics is assessed on arc-welded joints of HSS steel tested in repeated tensile loading. Crack growth is monitored by the means of replica technique. Bainitic lath packets and prior austenitic grain boundaries act as major microstructural barriers which slow down short crack growth rates especially in the very early stage. Crack growth is satisfactorily modelled according to Brown's laws for short crack propagtion stage and to a Paris-type law for long crack propagation stage. The predicted S-NR curve and conventional fatigue limit are in good agreement with experimental data. Moreover, the model is used to predict a S-Ni curve in respect of the Microstructurally Short Crack threshold condition.