Development of Digital Shearography for Dual Sensitivity Simultaneous Measurement Using Carrier Frequency Spatial Phase Shift Technology 2023-01-0068

Digital shearography has many advantages, such as full-field, non-contact, high sensitivity, and good robustness. It was widely used to measure the deformation and strain of materials, also to the application of nondestructive testing (NDT). However, most digital sherography applications can only work in one field of view per measurement, and some small defects may not be detected as a result. Multiple measurements of different fields of view are needed to solve this issue, which will increase the measurement time and cost. The difficulty in performing multiple measurements may also increase for cases where the loading is not repeatable. Therefore, a system capable of measuring dual fields of view at the same time is necessary. The carrier frequency spatial phase shift method may be a good candidate to reach this goal because it can simultaneously record phase information of multiple images, e.g. two speckle interferograms with different fields of view. It then obtains the phase information of each interferogram by separating them from the spectrum using the Fourier Transform (FT) method. The challenge of using this method is that the phase information can partially overlap on the spectrogram, resulting in a bad phase map. This paper presents a new idea for separating the phase information on the spectrogram. The new idea adjusts the shearing directions, leading to complete separation of the spectrums of the two images, e.g. one in the horizontal direction and the other in the vertical direction. The phase information of each interferogram can then be excavated by windowing the corresponding spectrum and taking the inverse Fourier Transform. In digital shearography, the phase information is directly related to gradient of surface deformation; thus, gradient of surface deformation with different fields of view, also called dual sensitivity, can be obtained. The principle of this method will be described and demonstrated by experiment results.