Detection of subharmonics in ultrasonic inspection of contact surfaces under compression

Detection and sizing of a closed crack in structures are difficult by conventional ultrasonic testing. One of promising alternative methods is nonlinear ultrasonic testing which detects subharmonic responses from a closed crack. This study investigates generation of subharmonics at contact surfaces of two specimens, modeling a closed crack, under ultrasonic radiation. Experiments of ultrasonic inspection were carried out by preparing different kinds of surface roughness of the specimens and also by changing compression force acting on the two contacting specimens. Our experimental results show some subharmonic responses from the contact surfaces, which can be influenced by the contact pressure and the contact surface roughness.