Noncontact fatigue crack detection using nonlinear wave modulation spectroscopy

Abstract

Nonlinear wave modulation spectroscopy (NWMS) has been used to evaluate nonlinear acoustic signature of fatigue cracks in materials and thus to get an idea about the degree of material nonlinearity. It is done by generating ultrasonic waves at two different frequencies and measuring their modulation. The choice of two distinct frequencies plays a significant role in NWMS for different structures. In this paper, instead of using signals at two distinct frequencies, only one broadband pulse signal is used as the driving signal, which can be generated by a laser beam. This driving signal generates multi-frequency responses as different resonance frequency modes and/or Lamb wave modes, generated in a plate-like structure. Nonlinear wave modulation occurs among these frequencies when material nonlinearity exists. It increases the sideband energy and the number of peaks in the spectral plots. These two features, namely sideband energy ratio (SER) and sideband peak number (SPN), are extracted from the spectral plots to measure the material nonlinearity caused by fatigue cracks. The noncontact laser system has been built for NWMS measurement by integrating and synchronizing a Q-switched Nd:YAG laser for ultrasonic wave generation and a laser Doppler vibrometer for ultrasonic wave detection. The proposed modified NWMS technique with the noncontact laser system has been successfully used for the identification of metallic plates with fatigue cracks.