Speckle interferometry and correlation applied to large-displacement fields

Abstract: A common problem in experimental mechanics is that when a sample is studied in e.g. a tensile test machine, the sample is often exposed to rigid body motions at the same time as small deformations occur. These two movements, the rigid body motion and the deformation, are often talked about as displacement fields. The sought deformation field is often in the micrometer range while the rigid body motions often are of millimetre or centimetre size. Therefore, it is often a problem to resolve the deformation field since it is drowned by the larger movement of the object. The displacement field can be measured with methods like speckle correlation, but the results might be of too poor accuracy to resolve the deformation field. Interferometric methods on the other hand might measure the deformation field but the rigid body motion makes the fringes disappear. In this thesis two methods are presented that makes it possible to master such measuring situations: a combination of speckle interferometry/speckle correlation and a method where the reference image is updated frequently during the experiment. Both theory and experiments are presented. For the combined speckle interferometry/speckle correlation, it is shown that the information necessary to apply the speckle correlation method is already available in interferometric recordings. With only minor changes in the calculation procedures, the speckle motion in the recordings can be determined. Interference fringes, which have disappeared due to large speckle motions, are retrieved by digital compensation for this motion. The speckle correlation technique gives the motion of the whole surface of the object so different areas of the object can move different amounts and in different directions and it is still possible to retrieve the fringes describing the deformation field. Updating the reference image during the experiment is another method used. As soon as the specimen has moved 1/10 of a speckle the reference image is updated in order to avoid speckle decorrelation. In this way the total movement of the surface is added up during the experiment and a phase map describing the displacement of the object is achieved. Finally, the magnitude of the shear in shearography is measured using speckle correlation. This allows quantitative measurements of the spatial derivative of the deformation field in shearography.

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