Application of common-path speckle interferometer with unlimited minimal shearing amount to characterization of irregularly shaped notch

Non-destructive evaluation (NDE) techniques are widely used to detect and characterize different defects in engineering structures. However, the highly sensitive real-time quantitative imaging of irregularly shaped defects in large-area metallic structures remains challenging if conventional NDE approaches are used. In this paper, a speckle interferometer with unlimited minimal shearing amount – based on a simple imaging setup without the need to tilt any optical components – is explored for the application of detection and characterization of complex-shaped defects in metallic plates. This technology is able to directly yield strain measurements, allowing the defects to be detected by identifying induced strain concentrations under stress loading. Acoustic waves are used as stress loading for this imaging system with the advantage of a deeper penetration depth and a larger area of inspection as compared to conventional loading methods, e.g. thermal loading. The spatial carrier technique is introduced to quantitatively reconstruct the phase distribution, which contains information of the strain measurement from a single recording. To achieve the near real-time imaging of defects with relative insensitivity against noise, the image pairs are continuously recorded and correlated using the sequential subtraction method. The performance of the imaging system was tested with experimental data. The results demonstrate that the imaging system can generate high-quality images to provide good detection and characterization properties regarding irregularly shaped notches as well as accurate sizing of different notch dimensions. This work introduces a promising practical NDE tool for the near real-time, non-contacting and quantitative detection of subsurface defects with complex geometries in large-area metallic structures.