Simultaneous orthogonal shearography using two pairs of parallel slits and a tetra-split lens

Abstract

There are a growing number of industrial applications for shearography, especially in non-destructive testing of composite materials. Its tolerance to hostile environments is probably the most relevant success factor for industrial applications. The introduction of carrier fringes for phase recovery, replacing temporal phase shift techniques, allows measurements with a single exposure for each loading state, making shearography even more robust. Some defects are better evident when the shearing direction is horizontal. Others are best viewed with a vertical shearing direction. Thus, it is desirable to inspect in both directions. Fortunately, the literature reports some configurations capable of using carrier fringes to perform simultaneous measurements in more than one shearing direction. These systems work well, but since they use a mask with circular holes, they require a high amount of light. An alternative is the use of a pair of oblong slits, which increases the amount of light captured. This paper extends the use of oblong slit pairs for simultaneous measurement in two orthogonal shearing directions. Two pairs of parallel oblong slits are arranged orthogonally on the system mask. Although the Fourier transform of the resulting signal at the camera sensor contains the desired signals as well as a crosstalk between the slit pairs. However, the mask can be designed in such a way that the orthogonal signals are separable. To promote shearing in orthogonal directions, a polymeric lens was sectioned, forming four quadrants of 90°. The quadrants were radially displaced and arranged so that each oblong slit covers only one quadrant of the lens. As a result, four laterally shifted images are formed, two of them with horizontal shearing and the other two with vertical shearing. A proof-of-concept prototype was developed and the first results were evaluated by the authors in the laboratory environment and are presented in this article. The paper also shows details of the resulting Fourier spectrum and the strategy used to separate the orthogonal components and recover the phase using carrier fringes. The performance of the prototype met expectations and validated the concept. Future works involve miniaturization of the prototype and tests in industrial environments.