LDV and Vibrothermgraphy experiments using forward and backward frequency sweep

It was observed that the backward sweep generates a higher amplitude due to softening nonlinearity at the defect. The out-of-plane LDR frequency and shape of the defect estimated by laser Doppler vibrometry are found to be in agreement with LDR-based vibro-thermography results.

Also, a backward sweep provides more temperature rise in all cases. Therefore, LDR-based vibro-thermography using a backward sweep is found to be the most efficient method for sizing impact damage in composites.