Microcrack nucleation, growth, coalescence and propagation in the fatigue failure of a powder metallurgy steel

Detailed quantitative micrographic data are presented for Stages I and II of a Powder Metallurgy Fe-1.5Cr-0.2Mo-0.7C steel specimen fatigued in bending with R = -1 at 24 Hz and a stress amplitude of 312 MPa. The fatigue limit was similar to 240 MPa, at which stress level no microcracks were detected in static loading. Testing was interrupted at 100 cycles and at further 29 intervals until failure after 49 900 cycles. For each arrest, surface replicas were made in the two regions where maximum stress was applied. Microcracks could nucleate below 100 cycles, when their sizes ranged from < 5 to similar to 20 mu m. Fractographic examination identified the failure-originating site, which was then associated with the crack system observed on the 'last' pre-failure micrograph. Detailed examination of the eventual failure region showed nucleation, at various cycle intervals, of 18 microcracks, their subcritical growths, arrests and coalescences with continuing cycling to form a critical crack 2.25 mm deep. Stepwise microcrack growth was probably rapid - to the next arrest or coalescence. For each (micro)crack size stress intensity factors, K(a)s, were estimated and, at the end of Stage II, for the coalesced crack, K-a reached K-1C, independently estimated to be similar to 36 MPa m(1/2).