FATIGUE FRACTURE SURFACE-FEATURES PREVALENT IN STEELS AT AMBIENT-TEMPERATURES

Assessed in this study are the relationships that were recorded betwee n the magnitude of the environmental assisted crack (EAC), growth cont ribution and the fatigue fractographic details observed in steels unde r ambient conditions. Four discrete static failure modes were observed on the fatigue surfaces; they are primarily transgranular cleavage an d intergranular decohesion with a few instances of fan-shaped growth a nd macrofacet formation. All static failure modes could be explained b y Beacham's model for hydrogen assisted cracking. It was established t hat the extent of EAC growth was uniquely related to the amount of sta tic failure mode and that the relationship agreed well with the predic tions of Congleton's model for EAC growth expressed in terms of the ra tio of crack growth rate (da/dN) in environments of humid air and sal ine solution to da/dN in dry air for a given stress intensity factor r ange DELTAK.