44 A. Giordano and F.-P. Chiang 4.2 Experimental Procedures The system used for the data acquisition is as shown in Fig. 4.1a. The specimen is illuminated by an expanded laser beam, and the image of the object is captured by a digital camera. The light intensity of each speckle pattern is digitized into an array of 8688 ×5792 pixels. The image processing software ImageJ [7] is then used to convert the JPG image files into 8-bit TIFF files. The 8-bit TIFF files are then loaded into an image correlation software based on Fourier transform [8] called CASI and processed. Basic processes of the technique involve data acquisition and image processing. In the data acquisition stage, two speckle patterns of the specimen, one before and one after the deformation, are captured by the camera and stored on an SD card, which is then registered into the computer. The image processing stage consists of four steps with ImageJ and CASI. First, the JPG image files are converted into 8-bit TIFF files which are then input into CASI. The three steps of the process that CASI is necessary for was described by Chen and Chiang [8]. First, an equivalent double-exposure speckle pattern is obtained by superimposing the two digital images, and it is segmented into a series of small subimages. Second, a fast Fourier transform (FFT), is applied to each subimage and “compared” with a correlation calculation. Last, a second FFT is applied to the resulting value giving rise to an impulse function whose position vector is nothing but the displacement vector experienced collectively by all the speckles contained within the subimage. Repeating the last two steps to all other subimages, we can resolve the full-field 2-D displacements. Upon completion of this process, the slope fields are generated. These slope fields are then numerically integrated to produce the deflection of the plate and numerically derived to produce the curvatures of the plate. An image of the algorithm that CASI performs is displayed in Fig. 4.2. Fig. 4.1 (a) Schematic of the data-acquisition system. (b) Arrangement for recording spatial speckles for plate bending. (c) Sample of black and white speckles
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