Reflections on identifying the real ΔKeffective in the threshold region and beyond

The use of the crack tip stress intensity factor, K, has survived almost 50 years as the key parameter correlating fatigue crack growth. As time past the range of the stress intensity, ΔK, was recognized as causing alternating plasticity at the crack tip. The threshold level for ΔK was discovered. Further, the occurrence of crack closure was noted which affected the ΔK for different load ratios, R of cyclic loading. The ASTM method of counting the linear part of the load displacement for determining ΔKopen was found to understate the ΔKeffective, which correlates data for different load ratios. One approach to adjust for this problem is the “Partial Closure Model”, where the closure only occurs away from the crack tip. Here it will be discussed that such a model leads to a universal growth law. Moreover, this law shows application in estimating the order of magnitude of crack growth life (<107 cycles) for example with very high cycle fatigue (>109 cycles). Some advances in this application will also be cited.