20.2 Principles of Laser Shearography Principles of a digital shearography system that uses a Michelson configuration is illustrated in Fig. 20.1. In conventional Michelson interferometers, two mirrors are arranged perpendicular to each other, and therefore the reflected beams going to the CCD sensor are collinear. In a shearography system, one of the mirrors is tilted, which is called shearing mirror. As the shearing mirror is rotated, two identical, but displaced, images are recorded by the CCD camera. The two images are combined coherently, producing an interferometric speckle pattern at the CCD sensor, called shearogram [1–4]. If the surface of interest undergoes deformation, the optical path length of the incident light changes, and the optical phase difference due to this deformation is characterized by fringe-locus function, Ω(x, y). For near parallel illumination-observation conditions, out-of-plane gradient of displacement along horizontal and vertical axes are calculated by δw δx ¼ λΩ 4πΔx , ð20:1Þ δw δy ¼ λΩ 4πΔy , ð20:2Þ where Ωis the fringe-locus function, λ is the laser wavelength, and Δx and Δy are the magnitude of the shear in horizontal and vertical axes, respectively [1–4]. 20.3 Prediction of Shearographic Fringes by FEM and Analytical Solutions To improve shearographic imaging in Non-Destructive Testing (NDT), the process is simulated and in-turn, optical phase maps and fringe patterns corresponding to the gradients of displacement are predicted. This allows engineers to find an optimal setup and loading conditions that provide maximum accuracy with minimal effort and inspection time. Fig. 20.1 Schematic of the optical configuration of the shearography system based on a Michelson interferometer [1–4] 150 X. Chen et al.
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