The definition and measurement of crack closure

Abstract

The conventional method of measurement of closure, using compliance measurements to define K clo and ΔK eff , is reassessed from a theoretical standpoint. The method is clearly simplistic in that it assumes that closure occurs entirely at K clo , when it is now well established that cracks close gradually over a range of K during unloading. However, results obtained from this method are often very useful in explaining fatigue behaviour. This apparent anomaly is explained if we recall that ΔK may be defined in terms of the release of elastic energy. It can be shown that, by measuring a change in compliance, the observer is actually making a direct assessment of the energy change associated with the closing crack and that the conventional measure of ΔK eff is, fortuitously, a simple approximation of that energy change. However, the measurement is not precise because it omits the energy term associated with the curved portion of the unloading line, below K clo . When this term is included, a more accurate assessment of ΔK eff can be made, which we define as ΔK true . The use of ΔK true is able to resolve some anomalies which are present in experimental data, such as the variation of ΔK th, eff with R ratio.