Relations between crack opening behavior and crack tip diffraction of longitudinal wave

TOFD method has attracted attention as the most accurate crack depth measurement technique in industrial inspection field. Since this method utilizes the crack tip ultrasonic diffraction echo and the amplitude of this echo is weak, enhancement of S/N ratio of received signal is required for accurate and reliable measurement. The most harmful defect for industrial structures is a crack and a crack closure sometimes causes failure in nondestructive crack detection by TOFD method. However, quantitative behavior of crack tip diffraction echo depending on crack closure for longitudinal wave used in TOFD method have not been investigated yet. In this paper, we prepared 7075-T6 aluminum alloy specimens with a penetrating surface fatigue crack by three point bending test. During the fatigue test, maximum applied load Kmax was reduced gradually according to the crack extension to control the maximum stress intensity factor to be constant. Using the specimen with a closed fatigue crack, crack tip opening displacement (CTOD) was controlled by loading within Kmax. The amplitude of the crack tip diffraction echo of 5 MHz ultrasonic longitudinal wave depending on CTOD was measured. Using the obtained relation as a calibration curve, the minimum CTOD required for stable TOFD measurement of fatigue crack was estimated to be 0.1 μm.