The Use of Ultrasonic Harmonic Generation to Determine Crack Opening Conditions in Compact Tension Specimens

In 1971, Elber reported the discovery of a crack closure phenomenon that occurs with fatigue. He noted that closure of the crack planes near the crack tip can occur while the applied stress is still tensile [1]. The existence of a closure stress opens the way of defining an effective stress intensity factor, Keff, given by, Keff=(σ−σclosure)(πa)1/2 where σ is the applied tensile stress, σclosure is the crack closure stress, and a is the crack half length. The stress intensity factor is useful in correlating fatigue crack propagation data, especially after overloads [2]. The precision of Keff depends upon how well one can determine σclosure However it is difficult to experimentally determine σclosure, since conventional crack opening determination is imprecise. The purpose of this paper is to present an ultrasonic technique that shows promise as a means to accurately determine when the crack is open