Damage localization using warped frequency transform in active structural health monitoring

A spatially distributed array of piezoelectric sensors is widely used in structural health monitoring (SHIM) for active ultrasonic methods. Owing to many advantages of delay-and-sum algorithm such as intuition, low computation cost and accurate imaging result, it is very suitable for damage localization. However, due to the dispersion effect of Lamb waves, the wave packet becomes wide and distorted as it propagates in the structure which will have a negative effect in the spatial resolution of the damage imaging. This paper presents an active imaging method using warped frequency transform (WFT) to improve the spatial resolution of the imaging result. In WFT, the warped frequency range is limited in narrowband and the central frequency is fixed to keep the wave packet undistorted. Then, the dispersion effect can be compensated and the velocity of the wave packet is converted into phase velocity which is much lower than group velocity. Due to the compensated wave packet and low wave velocity, the spatial resolution of the delay-and-sum algorithm is improved. Both single damage and dual adjacent damages are detected in the experiments. The comparisons between delay-and-sum algorithm with and without WFT are made to demonstrate the proposed method.