This paper concerns the development of ultrasonic intermodulation as a method of robustly detecting cracks in engineering components. The bispectrum signal processing technique is used to analyse the nonlinear response of a sample to continuous excitation at two frequencies. The increased nonlinearity due to defects such as fatigue cracks is detected. The technique is shown to be insensitive to the support conditions and excitation positions. The importance of the shape of the excited modes is demonstrated and suggests that global inspection can be achieved only by exciting multiple modes. This multimode approach is then applied to the detection of cracking of a steel steering actuator bracket.
Courtney, C. R. P., Drinkwater, B. W., Neild, S. A., & Wilcox, P. D. (2008). Factors affecting the ultrasonic intermodulation crack detection technique using bispectral analysis. NDT and E International, 41(3), 223-234. https://doi.org/10.1016/j.ndteint.2007.09.004