Fatigue crack propagation in some PM steels

Mechanisms of fatigue crack growth have been studied for a range of PM stee ls at relative densities of 0.90 and 1.0, for which strength, fracture toug hness, and microstructural information was also available. It is shown that the Paris exponents for steady state crack growth are between 8 and 18 whe n rho(r) is similar to 0.9 but when rho(r) is similar to 1.0 the exponents are between 2.6 and 4.0, i.e in the range typical of wrought steels (2-4). At both densities, threshold stress intensities are between 5.5 and 10.8 MP a m(1/2) when R = 0.1. Combinations of these thresholds and yield strengths are comparable with those for wrought steels. When R = 0.8, reductions in threshold to between 2.7 and 5 MPa m(1/2) are attributed to crack closure e ffects. At rho(r) = 0.90, Fe-0.5C fails by progressive rupture of sinter ne cks. Astaloy A, with 0.2%C and 0.6%C, and Distaloy AB-0.6C have smaller pla stic zone sizes and the cracks follow more difficult paths through particle s as well as necks. When rho(r) is similar to 1.0, fracture is partially by true fatigue modes and partly by cleavage, the bursts of cleavage being mo re noticeable when K-max is high.