Multi-component shearography employing four measurement channels

摘要

Shearography is a full-field non-contact optical technique usually used to measure displacement gradient. Correlation fringes sensitive to displacement gradient are generated by the correlation of speckle interferograms recorded before and after object deformation. Shearography is sensitive to a component of displacement gradient that is determined by the illumination and viewing directions and by the direction of applied shear. The sensitivity of shearography to displacement gradient is determined by the magnitude of the applied shear and by the optical wavelength. In general, to perform a multi-component measurement three components of displacement gradient are measured, by using three illumination, or viewing, directions, followed by a coordinate transformation to an orthogonal coordinate system. The coordinate transformation procedure relies on good quality data from all three measured channels. The addition of a fourth measurement channel gives duplicate measurements for all the surface displacement gradient components. A good agreement between duplicate measurements gives a high level of confidence in the measurement accuracy. A fourth measurement channel also allows the surface strain to be fully characterised when correlation fringe quality is poorer, or when one of the views is obstructed as a result of the object shape. In this paper a theoretical analysis of multi-component shearography using four measurement channels is given and the issues and benefits are discussed.