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Shearography
Shearography or speckle pattern shearing interferometry is a measuring and testing method similar to holographic interferometry. It uses coherent light or coherent soundwaves〔(Ng, Jeanette. "Gadget holds key to safer buildings" ''The Standard'' September 1, 2005 ) 〕 to provide information about the quality of different materials in nondestructive testing, strain measurement, and vibration analysis. Shearography is extensively used in production and development in aerospace,〔Garfinkel, Simson L. "Laser Lights Up Flaws" ''Christian Science Monitor'' January 20, 1989, p.12〕 wind rotor blades, automotive, and materials research areas.〔(Steinchen, Wolfgang and Lianxiang Yang. ''Digital Shearography: Theory and Application of Digital Speckle Pattern Shearing Interferometry'' SPIE Press: 1 January 2003 ) ISBN 978-0-8194-4110-2 〕 Advantages of shearography are the large area testing capabilities (up to 1 m² per minute), non-contact properties, its relative insensitivity to environmental disturbances, and its good performance on honeycomb materials, which is a big challenge for traditional nondestructive testing methods.
==Shearing function==
When a surface area is illuminated with a highly coherent laser light, a stochastical interference pattern is created. This interference pattern is called a speckle, and is projected on a rigid camera's CCD chip. Analogous with Electronic speckle pattern interferometry (ESPI), to obtain results from the speckle we need to compare it with a known reference light. Shearography uses the test object itself as the known reference; it shears the image so a double image is created. The superposition of the two images, a shear image, represents the surface of the test object at this unloaded state. This makes the method much less sensitive to external vibrations and noise. By applying a small load, the material will deform. A nonuniform material quality will generate a nonuniform movement of the surface of the test object. A new shearing image is recorded at the loaded state and is compared with the sheared image before load. If a flaw is present, it will be seen.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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