In situ ultrasonic measurement of crack closure

This paper describes a surface acoustic wave method for the measurement of crack closure/opening stress during fatigue testing. The dependence of surface wave signature on fatigue load reveals the crack opening/closure characteristics displayed during the fatigue cycle, from which the crack-mouth-opening and fully-crack-opening stresses are determined as functions of the number of cycles. The change of crack closure loads during fatigue life is interpreted by a simple model describing crack growth inside the pit-induced plastic yield zone. The loading-unloading hysteresis is observed and related to the movement of the crack opening/closure front across the boundary of the plastic yield zone during the crack opening-closing process. Examples are given for fatigue tests on Al 2024-T3 and Inconel 718 samples in which the surface fatigue crack initiated from a surface pit.