Improving Damage Identification by Shearography Phase Map Analysis

This paper presents a novel method for identifying multiple internal damages in laminated composite plates using filtered phase maps acquired by digital shearography (DS). The technique analyzes interference phases acquired at fixed intervals during the cooling phase after a pulsed thermal load. By correlating consecutive interference phases, a phase map is generated for each step. These maps are then processed: first, a band-pass filter isolates the small phase deviations caused by internal damage; second, the filtered maps are coherently summed to enhance the damage-induced signal against background noise significantly. The effectiveness of the method is demonstrated on a carbon-fiber-reinforced polymer (CFRP) plate containing two internal damages from low-energy impacts. Results show that the filtered phase maps enable superior identification of both the location and size of the damage compared to standard post-processing techniques. Furthermore, generating phase maps between small time intervals minimizes speckle decorrelation, and the subsequent summation step effectively mitigates random distortions from air convection and external vibrations, yielding filtered maps with a significantly better signal-to-noise ratio (SNR). The influence of the shearing direction on damage detectability is also investigated. Overall, this approach provides a robust and enhanced evaluation of damage in composite structures.