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In the case of uniaxial stress the change in intensity (Al) is:

Data Reduction and Analysis The average grey level of the unloaded image (I*) was calculated. The difference in grey levels between to the loaded and unloaded images was calculated (Al = I x - I2) . The average differences were normalized by dividing by the average intensity of the original image, I lm The normalized average differences in grey levels were plotted against the axial strain.