Fatigue crack detection using guided waves nonlinear modulation

Abstract

This paper presents a fatigue crack detection technique based on nonlinear wave modulation created by mixing two ultrasonic guided waves. Two independent input signals are generated using two surface-mounted PZT transducers; a high-frequency probing signal and a low-frequency pumping signal. Corresponding guided wave responses are measured by additional PZT transducers installed on a specimen. The presence of a system nonlinearity, such as a crack formation, can provide a mechanism for nonlinear wave modulation, and create spectral sidebands around the frequency of the probing signal. A signal processing technique combining linear response subtraction (LRS) and synchronous demodulation (SD) is developed to extract the crack-induced spectral sidebands. The proposed crack detection method is successfully applied to the identification of actual fatigue cracks grown in metallic plates. Finally, the effect of the pumping and probing frequencies on the amplitude of the first spectral sideband is investigated using the first sideband spectrogram (FSS) obtained by sweeping both pumping and probing signals over specified frequency ranges.