Measuring the effect of environment on fatigue crack-wake plasticity in aluminum alloy 2024 using electron backscatter diffraction

Compact tension samples fatigue cracked within ultrahigh vacuum and humid air were examined using electron backscatter diffraction band contrast (i.e., pattern quality) maps obtained at the crack-wake. A protocol for relating changes in band contrast with the extent of plasticity (i.e., the cyclic plastic zone size, rcpz) as a function of environment, stress intensity range ΔK, and grain orientation is developed. Essentially, a Gaussian curve is fit to the band contrast plotted as function of distance from the crack-wake and the full width at half maximum is taken as a measure of rcpz. It is shown that the quadratic dependence between rcpz and ΔK predicted by conventional fracture mechanics theory is consistent with the observations, however, there are also strong grain-to-grain variations. Additionally, rcpz is sharply reduced in a humid environment most likely due to the combined effect of hydrogen on plasticity and decohesion.