48 A. Giordano and F.-P. Chiang Fig. 4.5 Fringe patterns of curvature contours of a clamped circular polycarbonate plate under concentrated central load (a) ∂ 2w/∂x 2–field, (b) ∂ 2w/∂y 2–field, (c) ∂ 2w/∂x∂y–field 4.5 Conclusion and Discussion A digital method has been proposed to use laser speckle generate slope, curvature, and deflection contours of a bent plate. This is achieved by digitally recording the laser speckle pattern form in a plane at certain distance away from the specimen surface before and after loading the plate; and the resulting photographs are then fast Fourier transformed twice with the digital image correlation software CASI to produces the slope contours. The curvature contour is then produced by numerically differentiating the slope data, and the deflection is produced by numerically integrating the slope data. As this technique is a digital technique, the data is stored within an array, which allows for simple manipulation to obtain the stresses, twisting, and bending moment that the plate experiences due to the load. Experience has shown that a thin coat of white paint usually improves a great deal the quality of the fringe pattern. Acknowledgment The work reported in this paper was partially supported by the Office of Naval Research through award NO.-N00014-17-12873. We would like to thank Dr. R.S. Rajapakse, Director of the ONR Solid Mechanics Program, for his support over the years on the development of speckle technology.
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