The choice of the sweep-frequency rate for the probing wave in nonlinear-modulation method of crack detection in resonant samples

Vibro-acoustic modulation (VAM) was a sensitive nonlinear method used to nondestructively detect crack-like defects in materials and structures. However, the modulation intensity was strongly affected by the probing frequency, so the efficiency and reliability of VAM result remained to be enhanced. As for the efficiency, swept probing excitation has been used by researchers, with the influence of sweep rate in consideration. This paper measures and analyses the VAM results with changing sweep rate of the swept probing signal. To achieve the limiting factors for this sweep rate, the actual modulation between the sound and vibration, data acquisition and processing parameter were examined. The results demonstrated that the measured modulation intensity became inaccurate as the sweep rate grew larger, while the actual modulation between the sound and vibration changed a little. Further analysis indicated that the window length for short-time Fourier transform should be selected in a proper range, and this range became narrower as the sweep rate grew larger. Thus, there is an upper limit for the sweep rate. As for the reliability, the window length could be determined at this rate. This work can be used to select the proper sweep rate and processing parameters in VAM and further improved the reliability of this technique.