The detection of fatigue crack in thick steel with phase array ultrasonics

Abstract

In this study, the application of phase array ultrasonic sensors attached to the edge of thick steel place is studied numerically and experimentally. The dynamic numerical models are built to test different excitation frequencies and their sensitivities to different crack lengths at the sensing points. The frequencies tested are 400 kHz and 300 kHz, while the crack lengths assessed are 0 to 10 mm. In general, damage index related to defect size and mode is calculated by comparing signals from damaged structure to signals from the undamaged structure. New damage index is introduced in this study by using wave modes detected by each sensor in array as the surface wave and the reflected wave from the fatigue crack. Experimental studies are performed using the modified steel compact tension geometry instrumented with in-situ phasedarray ultrasonic sensors and an optical microscope to measure the crack growth per fatigue cycle. The optical microscope images are analyzed with image processing to determine the crack length and the fatigue cycle relationship. The fatigue loading is paused every 1 mm crack length and the ultrasonic measurement is performed. It is validated that the nearest receiver to the transmitter has the highest sensitivity to crack due to the best separation between surface wave and reflected wave.