Study of cyclic crack-tip opening displacement of microstructurally small fatigue crack using digital image correlation

The current work investigates the relationship between the crack growth rate (CGR) and the cyclic crack-tip opening displacement (& UDelta;CTOD) of microstructurally small fatigue cracks by using high-resolution digital image correlation (DIC). Load-controlled fatigue tests were conducted on small-scale specimens of 18%Cr body-centered cubic ferritic stainless steel. Microstructurally small fatigue crack growth was analyzed based on accurate high sample-rate measurements, starting from a sub-grain crack length up to seven times the volume-weighted grain size d(v) = 224 & mu;m. Under these experimental conditions, the high-resolution analyses reveal that variation from the otherwise linear relationship between CGR and & UDelta;CTOD on double logarithmic scale is due to the crack-tip bypassing an inhomogeneous shear strain localization zone. In this zone, & UDelta;CTOD is not able to characterize the behavior of microstructurally small fatigue cracks. Outside the shear strain localization zone, & UDelta;CTOD still is a valid crack driving force parameter.