Shearography pair method for reliable non-destructive inspection of millimeter and submillimeter defects in fiber-reinforced composites

• Shearography pair method is developed to reduce fiber noise for defect detection. • Good inspection as shearography with stacked phase maps but faster and with less data. • It enables sub-mm defect detection (down to 0.6–0.8 mm in diameter) in composites. • These results are one of the smallest defect sizes detected with shearography Non-destructive inspection (NDI) of small-scale defects in fiber-reinforced composites is an urgent challenge to ensure the structural integrity of safety–critical components. Shearography is a non-contact and full-field optical NDI method that can be used to characterize surface strain components under loading. Thermal loading is widely used in shearography because of the advantages of being non-contact and convenient for in-service inspection. Shearography has received considerable industrial acceptance for the inspection of aerospace and marine composite structures, however its efficacies in detecting small mm and sub-mm defects have not been fully characterised. Besides, one major issue regarding shearography with thermal loading is fiber-related deformation or fiber noise, which can affect the efficacy of defect detection significantly, especially when detecting small and deep defects. In this study, the novel shearography pair method is proposed and developed to reduce fiber noise for reliable inspection of small mm and sub-mm defects in composites. The defect detection capabilities of the proposed method have been studied and compared with conventional shearography practice and with fast Fourier transform (FFT) and principal component analysis (PCA) based signal processing algorithms. The results demonstrate that the proposed shearography pair method has the advantages of less fiber noise, improved inspection results, and being faster with reduced number of datasets. It enables the detection of mm and sub-mm defects (down to 0.6–0.8 mm in diameter) in composites; these inspection results are one of the smallest defect sizes detected with shearography and reported in literature.