Initiation sites for cracks developed from pits in a shot-peened 12Cr martensitic stainless steel

A serial sectioning procedure was carried out on a shot-peened 12Cr martensitic stainless steel to determine the location of fatigue crack initiation sites from pits of varying depth. Multiple cracks, including fully-developed cracks (surface-breaking cracks extending beyond the pit base) and non-fully-developed cracks, were observed for pits of depth greater than 70 μm. If the longest crack at each side of the pit was assumed to be the dominant crack, most dominant cracks did not initiate from the pit base despite a propensity for macro-stress and strain localisation to the pit base. These results indicate that common assumptions about crack initiation from pits in shot-peened steel can be misleading. Knowledge of the location of crack initiation sites and how the crack develops is required to characterise the mechanical driving force for the crack prior to attainment of a fully-developed crack.