Speckle Interferometry and Correlation Applied to Large- Displacement Fields

A common problem in experimental mechanics is that when a sample is studied in, e.g. a tensile test machine, the sample is often simultaneously exposed to rigid body motions 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 rigid body motions often are of millimetre or centimetre size. Therefore, resolving the deformation field is often a problem 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 lack accuracy to resolve the deformation field. However, interferometric methods might measure the deformation field, though the rigid body motion makes the fringes disappear. This thesis presents two methods that make it possible to master such measuring situations: a combination of speckle interferometry/speckle correlation and a method where the reference image is frequently updated during the experiment. Both theory and experiments are presented. For combined speckle interferometry/speckle correlation, 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 object’s whole surface so that different areas of the object can move different amounts and in different directions, while 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 1/10th of a speckle, the reference image is updated 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, allowing quantitative measurements of the spatial derivative of the deformation field in shearography.