Threshold stress intensityThreshold stress intensity range, ΔKth, is analogous to fatigue or endurance limit. It defines the minimum cyclic loading conditions for the onset of fatigue crack growth. Unlike fatigue limit, ΔKth consists of two components, ΔKth,x and ΔKth,i. ΔKth,x represents that portion of the applied load cycle over which the fatigue crack is closed and cannot respond to varying applied stress. ΔKth,i, is analogous to the endurance limit on the S–N curve. It represents the intrinsic component of ΔKth that reflects the resistance of a fully open crack tip to the onset of extension under cyclic loading. For a long time, it was not possible to measure either of the two components in isolation. This shortcoming was recently overcome through the development of a test method that enables characterization of ΔKth,i as a function of applied stress ratio as well as load history. This paper describes the significance of the newfound mechanical property and its engineering application.