Electronic speckle photography applied to in-plane deformation and strain field measurements

Abstract: Non-contacting measurements of deformation and strain fields are of great importance in experimental mechanics. This thesis describes the development and applications of a video-based, computerized system called electronic speckle photography that measures the in-plane deformation of a strained body. The physical restrictions that limits the performance of the system have been carefully analysed. The system determines the deformation of a characteristic pattern, somehow attached onto the object surface. A FFT based cross-correlation of subimages from the two object states of interest is calculated. Subpixel accuracy is obtained through a Fourier serie expansion of the discrete correlation surface. For a pattern to be considered characteristic it has to be completely random, be of a high contrast and high frequent enough so that a small portion of the digitized image gets typical for its position. A good example of a characteristic pattern is a laser speckle pattern, but any pattern that fulfills the requirements can be used. For a good correlated pattern the system manages to calculate each displacement vector in the field with an accuracy of 1 % of a pixel. The accuracy decreases with increased speckle size and increased speckle decorrelation. An application in which the ESP-system has succesfully been used is to measure the deformation field caused by hygroexpansion in paper. Of primary interest in experimental mechanics is the strain field. There are two possibilities to determine the in-plane strain field with the use of the ESP-system. The first is to differentiate the measured deformation field obtained using either a laser speckle pattern or a white light speckle pattern. For this to be a feasable alternative the random error in the deformation field has to be decreased even further through the use of some filter. It seems difficult to obtain better accuracies than 100 μ strain in the strain fields through the differentiation process without losses in the spatial resolution. The second possibility relies on the motion of defocused laser speckle patterns. By combining the measured deformation fields obtained from four different illumination directions, all components of the in-plane strain field can be determined with an accuracy comparable to that of an electrical strain gauge (10 μ strain with retained spatial information. In measurements using laser speckle patterns to produce reliable results, a careful analysis of the behaviour of these patterns concerning speckle displacement, decorrelation and image point-object point correspondence have to be made. A telecentric imaging system has many positive qualities concerning these parameters for use in a defocused system and is the one that has been used in the measurements.