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In-situ ultrasonic monitoring of crack growth under static and dynamic loading conditions

Successful in-situ monitoring of crack initiation and growth is a necessary prerequisite for applying ultrasonic methods to structural health monitoring. For conventional ultrasonic testing methods, a focused beam may be used to directly image the crack tip; however, this method is difficult to apply during fatigue testing because of access limitations and couplant contamination issues. However, ultrasonic sensors can be permanently attached to a specimen to detect signal changes due to crack initiation and growth if the wave path is properly directed through the area of critical defect formation. The dynamics of cracks opening and closing during the fatigue process modulate the amplitude of ultrasonic waves propagating across these crack interfaces. Thus, even very small cracks can be reliably detected using permanently mounted sensors if the ultrasonic response can be measured as a function of load. A methodology is presented here that uses this behavior to detect and monitor crack formation and growth. This methodology may also be applied to structures subjected to unknown dynamic loads by using the ultrasonic signal to both estimate the instantaneous dynamic load and interrogate the integrity of the structure. Essential to the success of this method is an initial calibration on the undamaged structure where ultrasonic response is measured as a function of known static load. Results are presented from several aluminum specimens undergoing low cycle fatigue tests, and the dynamic loading results are shown to be comparable to the static ones in terms of the response of the ultrasonic signal to crack progression.