Introducing the Kmax sensitivity concept for correlating fatigue crack growth data

Previous attempts to correlate stress ratio effects in fatigue crack growth rate data have often emphasized one important contribution, such as crack closure, without considering other mitigating factors such as K-max. In the past, correlating stress ratio effects using closure have been hampered by non-repeatable measurements of closure and contradictory results. A new analysis technique for determining Delta K-eff is based on an interpretation of crack closure as a stress redistribution (or load transfer) on a relatively compliant crack wake and is able to account for the contribution of cyclic crack-tip strain below the opening load. This results in the observation that the fatigue crack growth rate is not determined solely by Delta K-eff but also depends on K-max. It was observed that this K-max dependence takes the form of a power-law with the magnitude of the exponent being a measure of K-max sensitivity. A procedure for determining the intrinsic crack growth rate curve without the measurement or modeling of crack closure will be described. The implications for fatigue-life prediction are profound.