ENVIRONMENTALLY ASSISTED CRACK EXTENSION DURING FATIGUE CYCLING IN STEELS - SOME FRACTOGRAPHIC OBSERVATIONS

The present paper describes the relationships that existed between the extent of environmentally assisted crack (EAC) growth and the fatigue fractographic features prevalent at ambient and elevated temperatures in steels. At both ambient and elevated temperatures the extent of EA C growth was uniquely related to the amount of static failure on the f atigue fracture surface. Under ambient conditions mainly intergranular decohesion and transgranular cleavage were recorded, while at elevate d temperatures the static failure modes were fan-shaped facet formatio n and, to a much lesser extent, microvoid coalescence. A selection of models dealing with EAC growth effects was assessed and it was establi shed that Congleton's model could best describe both the crack growth effects and the fractographic feature recorded under both ambient and elevated temperature fatigue conditions, viz.: (da/dn)ENV/(da/dn)MECH = 1/(1 - fc)m where c is the area fraction of static fracture mode; an d f and m are constant. Finally, the various mechanisms which promote EAC growth were assessed and it was observed that the degree of EAC gr owth was only dependent upon the extent of static failure on the fatig ue surfaces and not on the type of static failure mode.