Role of plastic work in fatigue crack propagation in metals

The plastic work required for a unit area of fatigue crack propagation U was measured by cementing tiny foil strain gages ahead of propagating fatigue cracks and recording the stress-strain curves as the crack approached. Measurements of U and plastic zone size in aluminum alloys 2024-T4, 2219-T861, 2219 overaged, and A1-6.3 wt% Cu-T4, and a binary Ni-base alloy with 7.2 wt% A1 are herein reported. The results are discussed along with previously reported measurements of U in three steels and 7050 aluminum alloy. When U is compared to the fatigue crack propagation rate at constant ΔK along with strength and modulus, the conclusion is drawn that U is one of the parameters which determines the rate of fatigue crack propagation. The relation of U to microstructure is also discussed. “Homogeneous” plastic deformation in the plastic zone ahead of the crack seems desirable.