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 (ΔCTOD) 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 μm. Under these experimental conditions, the high‐resolution analyses reveal that variation from the otherwise linear relationship between CGR and ΔCTOD on double logarithmic scale is due to the crack‐tip bypassing an inhomogeneous shear strain localization zone. In this zone, ΔCTOD is not able to characterize the behavior of microstructurally small fatigue cracks. Outside the shear strain localization zone, ΔCTOD still is a valid crack driving force parameter.