Closed crack growth monitoring using nonlinear ultrasonic imaging method

It was necessary to clarify closing mechanism of closed fatigue crack and evaluate such in high precision so as to ensure reliability of nuclear reactor and airplane. Three dimensional crack growth monitoring of closed crack depth distribution in the length direction and change in open and closed region within a crack with crack extension was conducted for closed fatigue crack created at compact tension specimen made of aluminium alloy A 7075 using such developed methods of subharmonic phased array for crack evaluation (SPACE), imaging method for closed cracks using nonlinear response of elastic waves at subharmonic frequency, and also load difference phased array (LPDA), nonlinear ultrasonic imaging method for closed cracks using subtraction of responses at different external loads. Results showed closed region within a crack was different in surface and inside and also open and closed region changed with crack extension in the three-dimensional way. (T. Tanaka)