Bispectral analysis of ultrasonic inter-modulation data for improved defect detection

This paper describes the use of the ultrasonic inter-modulation technique for damage detection, which can offer potentially far greater sensitivity than linear acoustical methods. The specimen is excited by two continuous sinusoidal ultrasonic signals at different frequencies and the resultant vibration signal is captured for interrogation. A sensitive signal processing technique, known as bispectral analysis, is utilised which has the property of suppressing signal noise, permitting the detection of small nonlinear effects in the received vibration signal. This approach is demonstrated experimentally on samples containing fatigue cracking. A model is also presented which highlights the benefits of the bispectrum approach and potentially allows for characterisation of the nonlinearity.