Single-axis combined shearography and digital speckle photography instrument for full surface strain characterization

Abstract

Full characterization of the surface strain requires the mea- surement of six displacement gradient components of the surface strain tensor. The out-of-plane displacement gradient component may be di- rectly measured using the full-field speckle interferometry technique of shearography, but to fully characterize the surface strain using shearog- raphy, a minimum of three illumination, or viewing, directions are re- quired. The image processing technique of digital speckle photography (DSP) is sensitive to in-plane displacement for normal collinear illumina- tion and viewing, with the displacement gradient components obtained by differentiation. A combination of shearography and digital speckle photography is used to perform full characterization of the surface strain using a single illumination and viewing direction. The increase in com- plexity compared with a standard single-channel shearography system lies predominantly in the additional image processing requirements. Digi- tal speckle photography image processing is performed using the optical flow field technique and the advantages of this technique compared with correlation are discussed. The design of the instrument is described and full surface strain measurements made with the system are presented.