Inspection of Damage on Laminated Plates: Thermography vs Shearography

This work aims to compare the performance of multi-damage identification on a laminated plate, using the non-destructive inspection methods: digital shearography and thermography. Low-energy impacts created two internal damages on a carbon-fibre-reinforced polymer (CFRP) plate, producing barely visible impact damage (BVID). A harmonic and transient thermal load was applied according to the method used to reveal internal damage. The phase maps were acquired by using the digital shearography system during the cooling stage of the plate after applying a pulsed thermal load. Afterwards, these maps were post-processed to enhance the phase variations indicative of internal damage, thereby making their identification clearer. The lock-in and pulsed thermography methods involve the acquisition of several thermograms over time. In lock-in thermography, thermograms are acquired during harmonic thermal loading, whereas in pulsed thermography, they are acquired during the cooling stage after applying a thermal load. Damage identification was performed based on the analysis of the phase, processed using Fast Fourier Transform (FFT) algorithm, on the analysis of the filtered thermograms, processed through Principal Components Analysis (PCA) method. The results from the digital shearography reveal the locations of the two impacts and the extension of internal damage. However, PCA images from thermography only show the extent of higher-energy impact damage, while lower-energy impact damage is barely visible. The findings in this work indicate that digital shearography is a more suitable method than thermography for identifying internal damage in thin laminated plates, due to its higher sensitivity, easier setup, and faster measurement time.