Systematic assessment and validation of compliance-based crack closure measurements in fatigue

Different methods of closure measurement and issues related to the identification of closure points are examined. Based on compliance curves obtained from crack mouth clip gauges and near-tip strain gauges, a systematic assessment of closure measurements has been made by a variety of non-subjective methods. Closure results obtained from compliance curves are also compared with direct crack tip observations. For results obtained on a commercial Al alloy, AA2024-T351 plate, it is shown that curve fitting methods based on a combination of linear and quadratic functions provide particularly sensitive and consistent closure measurements. The potential separation of plane stress and plane strain closure effects that arise in real, three-dimensional specimens is demonstrated via a combination of global compliance measurements and the use of side-grooved specimens.