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Automatic Speckle Photography Fringe Analysis: Application To Crack Propagation And Strength Measurement

An image processing system for the automatic analysis of double exposure laser speckle photographs is described. The "Young's fringes" diffraction pattern, formed by directing a narrow laser beam through a speckle photograph, is digitised by means of a mechanically scanned linear photodiode array. The displacement vector is calculated by spectral analysis of the fringe pattern using either the Fast Fourier Transform (FFT) or the Fast Walsh Transform (FWT) algorithms. The accuracy, reliability and computation time of the two algorithms are compared as a function of fringe visibility and image resolution. The paper is illustrated by three practical applications of the image processing system. In the first, small discs of plastic bonded explosive (PBX) underwent quasi-static diametral compression (the Brazilian test). Analysis of double exposure speckle photographs recorded during the deformation revealed the strain distribution at different load levels, and the strain-to-failure of the material under test. For the second and third examples, speckle photography was used to measure the displacement fields around the tips of stationary and moving cracks. Numerical methods for calculation of the stress intensity factor from the measured displacements are presented.