CYCLIC STRESS-STRAIN RESPONSE AND LOW-CYCLE FATIGUE LIFE IN METALLIC MATERIALS

The cyclic stress-strain response and the test method-dependent characteristics of metallic materials were discussed from the viewpoint of such metallurgical factors as the motion of screw dislocations, the formation of strain-induced martensite and dynamic strain ageing. Furthermore, their low-cycle fatigue life was examined in terms of the total strain and the plastic strain, and the existence of a unified total strain-based life curve common to the various steel materials was suggested. The plastic strain-based low-cycle fatigue life was successfully estimated through surface crack growth analysis, which was made using the elastic-plastic fracture mechanics; the inflection appearing in the log Δεp-logNf relationship of the high strength-low ductility materials as well as the Coffin-Manson law embodied in the ductile materials was satisfactorily described through the crack growth analysis. Finally, it was shown that the equivalent stress and plastic strains generated at the notch root were predominant factors in governing the crack initiation life of the grooved cylinder components.