Measurement of All Orthogonal Components of the Displacement Field in the Volume of Scattering Materials Using Tilt Scanning Interferometry

Abstract

Finite Element Method Updating and the Virtual Fields Method are powerful techniques for elastic modulus and stiffness identification from full field measurements of displacement and strain [1]. They work at their best when provided with dense and multicomponent experimental displacements (or strain) data, i.e. when all orthogonal components of displacements (or all components of the strain tensor) are known at points closely spaced within the volume of the material under study. This reduces the chances of finding multiple solutions to the inverse problem of identification, a common limitation encountered when only surface deformations are measured to characterize damage or infer sub-surface characteristics in a material [2]. Digital speckle pattern interferometry (DSPI), a mature technique for surface deformation measurements, has been effectively extended into 3D by two fundamentally different approaches: 1) by using multiple wavelengths so that depth information can be encoded in the temporal frequency of the intensity signal, and 2) by using a monochromatic light source with multiple illumination directions by scanning the angle of the illumination beam. The former approach is known as Wavelengths Scanning Interferometry (WSI) [3], and the latter as Tilt Scanning Interferometry (TSI) [4].