Synthetic generation of vibroacoustic modulation signals for structural health monitoring

The vibroacoustic modulation (VAM) is a nonlinear ultrasonic testing method that utilizes the modulation of a high-frequency/low-amplitude probe wave with a low-frequency/high-amplitude pumping vibration, resulting in high sensitivities to damages in the structure. However, applying the method outside of the laboratory as a structural health monitoring system for actual structures is challenging, since the amplitude and frequency of the ambient vibrations – which is ideally utilized as pumping vibration – fluctuates over time. To circumvent this, we present a synthetic generation of the VAM signal using only the probe measurements , acquired at two (or more) stress levels of the structure when a steady state is reached. We could show that only 16 values (8 measured values of only two stress levels with a sampling frequency of 1/20 of the Nyquist frequency) are required to generate the often calculated Modulation Index with a mean deviation of 0.97% to the expected dynamic measured values for glass fiber reinforced composites and 1.86% for the aluminum specimens, which is negligible compared to a typical increase of the Modulation Index of 10–30 dB in case of severe damage. Even undersampled measurements at each stress level can be used without sacrificing accuracy, which reduces the sensing requirements for the sensor nodes. Moreover, this method decouples VAM from the actual need for a constant recurrent frequency and amplitude of the natural vibration in order to reliably compare measurements throughout the lifetime. Hence, this work aims to open the possibility of ultimately applying VAM to assess the structural health of complex structures.