Volumetric and Surface Positron Annihilation Studies of Fatigue Damage Accumulation in a Steel Alloy

Fatigue crack nucleation comprises a significant portion of the fatigue life for many structures. The ability to inspect and quantify damage before significant cracks appear could improve safety while allowing components to be retired only when they have reached the end of their useful life. Positron annihilation is one technique that allows for observation of damage accumulation on a much smaller scale than traditional nondestructive evaluation techniques. As-polished and shot-peened 300M steel specimens were fatigued under constant amplitude. Tests were interrupted at several fatigue life ratios between the as-received and failed conditions, and damage was measured with positron annihilation techniques. Positron annihilation was sensitive to both induced residual stresses and fatigue damage build up, but measurement uncertainty was high and sensitivity was low. Positron annihilation presents a potentially revolutionary nondestructive evaluation technique that could give a quantitative measure of early fatigue damage.