Total Focusing Method Damage Imaging in Frequency Domain Using Laser-Ultrasonic Lamb Wave Based on Time-domain Filtering in Multi-band

A fully non-contact experimental platform for ultrasonic Lamb wave damage detection was constructed, where laser exciting and the scanning laser Doppler vibrometer were used to realize the high-resolution pickup of the Lamb wave field in the structure, which has overcome the disadvantages of low spatial resolution caused by the conventional contact Lamb wave transducer. In order to suppress the dispersion effect of broadband laser-ultrasonic signal, we proposed time-domain filtering in multi-band method based on wavelet analysis to decompose the broadband signal into multiple narrowband ones and separate the scattering signals effectively without reference signal. On this basis, the total focusing method (TFM) was used for damage imaging. However, when the traditional TFM was applied to image based on ultrasonic Lamb wave, the inherent dispersion characteristic of ultrasonic Lamb wave could lead to the miscalculation of time delay, thus reducing the imaging precision. Therefore, the frequency-domain TFM was developed by applying phase delay in the frequency domain. The logical AND was introduced to synthesize the damage imaging results of multiple narrowband signals to obtain high-precision damage imaging. Our study has shown that the method of time-domain filtering in multi-band combining with frequency-domain TFM can realize non-contact and accurate damage detection in isotropic plate structures, and it is a potential effective method for application in engineering practice.