52 A. Giordano et al. Fig. 5.1 Optical arrangement for producing an off-axis hologram variable beam splitter to distribute the power sufficiently for the object and film. Setting the power distribution to be about even on each surface of the holographic film, the film can be uncovered and the film to develop. The monomers link together and developing the film and producing the fringes on the film. After the time for the film to develop has passed, the object beam can be blocked to check the reconstruction of the object. The film is then “bleached” with white light and allows any remaining monomers to link with the polymer chains previously created. The object is then deformed, and the interference fringes are then observed. A digital camera is used to image the object through the film to capture the fringes produced by the interference of the new object with the image of the object stored on the holographic film. 5.3 Principle of the Method Fringes are formed when coherent waves interfere. Holographic interferometry fringes are associated with the phase change of the object to the image of the object stored on the holographic film. This phase change has been studied for out of plane displacement and the relationship which relates phase change, φ, wavelength, λ, the incident angle of the object beam, θ, and the out of plane deflection, w, is as follows [7]: φ = 2π λ [w(1+cosθ)] (5.1) From Eq. (5.1) we can obtain the deflection as follows: w= λ φ 2π (1+cosθ) . (5.2) 5.4 Results In this experiment, we tested the deformation of a circular disk under a central transverse load using the photopolymer film. The plate was made of polycarbonate (diameter =10.16 cm, thickness =6.35 mm) and loaded by a central deflection of 12.7 μm. A 45 mW expanded He-Ne laser beam (λ =0.6328 μm) was used to illuminate the object and film. Figure 5.2 shows a set of fringes obtained by loading the specimen to various displacements. The displacement fringes displayed in Fig. 5.2 are in good agreement with previous experiments and the plate theory developed by Timoshenko [8].
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