Complete deformation analysis of transparent samples using digital shearography and holography

Digital shearography is an optical technique that allows direct measurement of strain in deforming samples and detection of surface and sub-surface defects. Because of its simple set-up and insensitivity to rigid body translations, the method has gained considerable importance in nondestructive testing in an industrial environment. Despite of its advantages, however, its application remains limited to highly scattering opaque samples due to its high dependence on the quality of speckles. This study demonstrates the suitability of digital shearography in the strain analysis of acrylic glass, a transparent sample, under sub-surface compaction loading. To generate the speckles, nominal roughness is induced at the front surface of the glass. As compared to the polariscope, digital shearography is shown to be able to resolve microscopic displacements that may not be evident as birefringence. Microscopic strain induced in the sample in the order of 60 microstrains is resolved using the shearographic set-up. For a more complete investigation of the deformation in the sample, the sub-surface displacements in the material are also mapped using Fourier digital holography to complement the shearographic results. The technique can be applied to fluid flow visualization and evaluation of polymers and other transparent materials.