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Theory

Digital Shearography

The manner in which Shearography measures the rate at which an object deforms under stress is is as follows: The object under inspection is illuminated with a single mode laser. The light, which reflects off the object surface, is viewed through a set of shearing optics. The function of the shearing optics is to laterally shear the image of the object into two, causing the two sheared images to overlap. Due to the monochromatic nature of the laser light, the overlapped images interfere and produce a unique speckle pattern. This speckle pattern is focused onto the CCD plane of a video camera, captured and digitised by a computer. This set-up is shown schematically in the figure below.

A speckle pattern of the unstressed object is initially captured and stored in a computer as a reference image. The object is then stressed artificially by either mechanical, pressure or thermal methods, which in turn causes the object to deform. If the relative displacement between two points on the object surface changes due to the applied stress, a corresponding change in the laser beam path length occurs, causing the intensity distribution of the speckle pattern to change. By recording these subsequent speckle patterns, digitising them and comparing them to the initially stored image, a final image is produced, which consists of alternating black and white 'zebra-like' fringes. 

Mathematically the fringes can be represented by the following equation: 

The equation above indicates that the correlation fringes along which  is constant, represent lines of constant displacement rates. The spacing between adjacent fringes is a function of the displacement gradient according to: 

where:
n = no of fringes. 

This implies that for a given object surface area, an increase in displacement gradient will produce a corresponding increase in number of fringes.