8D-5 Tracking of Cracks in Fatigue Experiments Using Nonlinear Propagation of Multi-Sine Surface Acoustic Waves

Ultrasonic surface waves (SAWs) provide a sensitive means to nondestructively detect surface cracks in structures. When continuously receiving transmitted SAWs while the structure is fatigue tested, the received ultrasonic waves are modulated (due to the changing strain between the sensors or due to the opening and closing behavior of a crack that is possibly present between the sensors). The measurement of the nonlinear modulation character during the fatigue tests leads to an unambiguous interpretation of the health of the structure. In this article a procedure is proposed to measure the amplitude and frequency modulation of SAW waves during fatigue testing. Results on a bending beam test with a propagating fatigue crack show that the measured nonlinear amplitude modulation can be used to detect cracks smaller than the SAW wavelength.