Laser Vibrometer Imaging of Delamination Interaction with Lamb Waves Using a Chirp Excitation Method

Abstract

One method that has shown great potential in visualising and characterising the interaction of guided waves with damage in composites is Laser Vibrometry. A Laser Doppler Vibrometer (LDV) can be used to produce 2D wavefield images of guided Lamb waves but a single scan is very time consuming and normally multiple scans are required at various frequencies in order to determine the best input signal. This paper demonstrates the use of a chirp excitation method requiring only a single scan and a post-processing algorithm to obtain results corresponding to any narrowband signal within the frequency range of the chirp signal. The method was used on an artificially delaminated composite panel and showed that the S0 mode, dominant at higher frequencies, mainly caused mode conversions whilst the A0 mode, dominant at lower frequencies, mainly caused a change in phase and amplitude across the delaminationOne method that has shown great potential in visualising and characterising the interaction of guided waves with damage in composites is Laser Vibrometry. A Laser Doppler Vibrometer (LDV) can be used to produce 2D wavefield images of guided Lamb waves but a single scan is very time consuming and normally multiple scans are required at various frequencies in order to determine the best input signal. This paper demonstrates the use of a chirp excitation method requiring only a single scan and a post-processing algorithm to obtain results corresponding to any narrowband signal within the frequency range of the chirp signal. The method was used on an artificially delaminated composite panel and showed that the S0 mode, dominant at higher frequencies, mainly caused mode conversions whilst the A0 mode, dominant at lower frequencies, mainly caused a change in phase and amplitude across the delamination.

Analysis of barely visible impact damage severity with ultrasonic guided Lamb waves

“…A Fourier transform was therefore required to convert them to the frequency domain and then the results had to be converted back to the time domain via an inverse Fourier transform in order to visualise the Lamb waves. This method has been used in the study by Michaels et al 51 to determine the best frequency and number of cycles for mode purity using an actuator and sensor in pitch-catch configuration, the delay and sum method in the study by Michaels et al 52 to locate damage while 42 used the technique with an LDV to visualise the interaction of Lamb wave modes with delamination.…”

“…This reconstructs guided wavefield from a set of sparse measurements by employing information on the dispersion relations of the structure under consideration. To reduce the data size and the time required to scan at multiple frequencies, Dafydd and Sharif Khodaei 42,43 proposed using a chirp excitation when using an LDV and showed the results on composite panels with artificial delamination and impact damages.…”