Digital shearography for NDT: Study on a novel experiment based method for removing global deformation✰

Abstract

Fringe patterns are typically produced as the results of traditional optical Nondestructive Evaluation (NDE) employing digital shearography. Structural flaws affect the response of the surrounding material to a certain loading, resulting in unique fringe patterns that are related to the local deformation. Smaller defect information, however, can be easily lost in the fringe pattern caused by global deformation during the flaw detection procedure. In order to improve the flaw detection capabilities of this technology, it is important to streamline the inspection process and to simplify the interpretation of the results. This study proposes a practical and effective methodology that experimentally removes fringe patterns caused by global deformation in digital shearography and only retains the defect information. For shearographic Non-Destructive Testing (NDT), two sets (3 or 4 images) of temporal phase-shifted interferograms under different loads P1 and P2 are recorded. This novel approach involves recording one additional set of temporal phase-shifted interferograms at a load between P1 and P2, e.g. P1’. Two phase maps of shearograms can be generated using the phase shift technique, corresponding to the two loads Δ2 = (P1’-P1) and Δ1 = (P2-P1), respectively. Because of the nondestructive nature of the testing, the magnitude of the loads P1 and P2 is small, and the 1st derivative of global deformation in the shearing direction of the test part is assumed to be linear. Therefore, a linear coefficient C based on the two shearograms can be determined. The information from global deformation is then removed by subtracting the shearogram generated with the small load Δ2 multiplied by the correlation coefficient C from the one obtained with the relatively large load Δ1. This technique is further improved by calculating a complete surface linear coefficient Cij, which improves the detail processing of the deformation of samples with complex geometry and mechanical properties. This technique is shown to effectively assess a wide range of structures, including aluminum specimens with prepared flaws and intricate composite laminates. The experimental findings demonstrate that the suggested technique improves the non-destructive testing capabilities of digital shearography, thereby simplifying defect detection and visualization.