Plastic deformation around a fatigue crack near threshold in 3%SiFe

Abstract

Fatigue crack propagation tests were conducted on 3%SiFe polycrystals at different stress ratios, and the plastic deformation around fatigue cracks revealed by the etch pit technique is discussed quantitatively in combination with the measurement of crack closure. In the high rate region, region B, the maximum plastic zone size was predictable from the fracture mechanics equation where the friction stress was substituted for the monotonic yield stress. Near the threshold, in region A, it tended to be smaller than that given by the equation established in region B. The increase in the crack closure was remarkable in region A. The size of the highly strained region correlated better with the effective stress intensity range than with the total stress intensity range in both regions A and B. The crack propagation rate was expressed as a power function of the highly strained zone size, and the threshold condition corresponded to the constant size of the highly strained region.