FATIGUE CRACK INITIATION AND SHORT CRACK GROWTH IN SINTERED Fe-1.5Cr-0.2Mo-0.7C STEEL

Fatigue properties of PM Fe-1.5Cr-0.2Mo-0.7C steel were determined in bending at 24Hz with R=-1 and the fatigue strength found to be ~ 240 MPa, at which stress level no microcracks were detected in static loading. Microcracks, whose sizes ranged from <5 to ~20 microns, could nucleate below 100 cycles. Stress amplitudes, S, of 260 MPa were selected to investigate the progressing failure mechanism; specimens fatigued for 1000, 5000, 15000, 24000 and 79300 [failure] cycles were examined in detail by replica microscopy in the regions where maximum stress was applied. Subsequently, fractographic examination identified the failureoriginating site, which could be associated with the crack system[s] observed on the “last” pre-failure replica micrograph. Detailed examination showed microcrack nucleation, subcritical growth and coalescence with continuing cycling to ~ 0.5 mm size. This subcritical crack extended conventionally to the critical size of ~ 3 mm, when the then estimated stress intensity factor, Ka, appeared to reach K1C, independently estimated to be ~36 MPa.m 1/2 , and thus the presented crack growth sequence illustrates good correlation between experimental data and the theoretical analysis.