Relationship between crack opening behavior and crack tip scattering and diffraction of longitudinal waves

The time of flight diffraction technique has attracted attention as the most accurate discontinuity depth measurement technique in the industrial testing field. Since it utilizes a weak discontinuity tip echo, the most harmful discontinuity in industrial structures is a crack because crack closure sometimes causes an error in crack detection through time of flight diffraction measurement. However, the dependence of the quantitative behavior of the crack tip echo on crack closure for the longitudinal waves used in time of flight diffraction has not yet been investigated. In this paper, we prepared UNS A97075 (7075-T6) aluminum alloy specimens with a fatigue crack penetrating the surface and the crack tip opening displacement was controlled by the amount of load. The amplitude and spectrum of the crack tip scattered and diffracted echoes of 5 MHz ultrasonic longitudinal waves for various crack tip opening displacement values were measured. As a result, the crack tip opening displacement of 0.1 mum (3.9 X 10(-6) m.) was found to be sufficient for giving echoes with amplitude comparable to that of the echoes from artificial slits, in the time of flight diffraction measurement using a 5 MHz longitudinal wave. The minimum detectable crack tip opening displacement by the time of flight diffraction technique was found to be 20 nm (7.9 x 10-7 m.). The crack tip opening displacement for a fatigue crack can be estimated using the amplitude or the peak frequency in the spectrum of the crack tip echo.