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Measurement of surface displacements and strains by the double aperture speckle shearing camer Brdicko, Jan

Abstract

In the testing of materials, structures and structural components it is often desired to determine the surface displacement and strain fields due to some external loading. Numerous optical techniques have been developed for this purpose and successfully used in particular applications. Unfortunately, when the surface deformation is quite large, as is usually the case in practical testing, most of these methods fail and only a few suitable optical interferometric techniques will work. Two of the recently developed techniques that seem to work are based on laser speckle interferometry. The first technique was described in 1972 by Duffy [l] who showed that a Double Aperture Speckle Camera (DASC) is suitable for measurement of a reasonably large in-plane displacement having its direction parallel to the line connecting the two apertures of the camera. A second technique was described in 1973 by Hung [3] . He showed that a Double Aperture Speckle Shearing Camera (DASSC) may be used to measure both the in and out-of-plane strains of planar surfaces. Duffy has not considered the fringe formation by DASC due to the displacement normal to the surface and the displacement normal to the line connecting the two apertures of DASC. Hung, in turn, has not considered the effect on fringe formation of either the in and out-of-plane displacements, or the in-plane strain, which is the partial derivative w,v (see Fig. 3.11 for the definition of w,y). Because of the great potential of DASC and DASSC stemming from their ability to measure displacements and strains over many orders of magnitude, a considerable effort was made to determine the fringe formation of the two cameras due to all displacements and strains occuring in a general deformation of a specimen surface. The theoretical analysis of models of DASC and DASSC was performed and resulted in two "new" equations describing the fringe formation by these cameras. The equations take into account the effect of all displacements , and strains on the fringe formation; in addition, the equations are "symmetric" and the equation governing DASSC reduces to the one governing DASC for the lateral shear set equal to zero. The accuracy of these equations was then verified by a number of simple experiments. Various ways of using the two cameras were proposed so that the unknown displacements and strains in the specimen surface may be calculated from the least number of fringe patterns. Computer programs based on these proposed methods were written and used in several experiments. In all instances the actual and the calculated displacements and strains agreed quite well.

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