The formation of subsurface non-defect fatigue crack origins

Very high cycle fatigue tests have been performed for two, two-phase steel grades, one martensitic–ferritic low-alloyed steel and one martensitic–austenitic stainless steel, in order to investigate the formation of subsurface non-defect fatigue crack origins (SNDFCO). It was found that there are transitions of fatigue crack initiation from surface defects, subsurface defects like inclusions to a subsurface non-defect matrix with increasing fatigue life. The mechanisms for the formation of SNDFCO have been investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The initiation of SNDFCO is caused by a material damage process of the soft phase due to cyclic plastic deformation, and the formation of SNDFCO is a crack propagation process. The size of the SNDFCO increases and their fracture morphology changes from more ductile to faceted when the applied stress decreases.