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thumb|A vacuum shearography hood and data display unit is applied to a composite structure to check for defects. An indication of a possible defect appears as the ripple pattern on the bottom right of the data screen.
'''Shearography''' or '''Speckle pattern shearing interferometry''' is a measuring and testing method similar to holographic interferometry. It uses coherent light or coherent soundwaves<ref>[http://www.thestandard.com.hk/news_detail.asp?pp_cat=11&art_id=629&sid=4361642&con_type=1 Ng, Jeanette. "Gadget holds key to safer buildings"] ''The Standard'' September 1, 2005 {{webarchive|url=https://web.archive.org/web/20110629195705/http://www.thestandard.com.hk/news_detail.asp?pp_cat=11&art_id=629&sid=4361642&con_type=1 |date=June 29, 2011 }}</ref> 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,<ref>Garfinkel, Simson L. "Laser Lights Up Flaws" ''Christian Science Monitor'' January 20, 1989, p.12</ref> wind rotor blades, automotive, and materials research areas.<ref>[http://spie.org/x648.html?product_id=412861 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}} {{webarchive|url=https://web.archive.org/web/20100613123724/http://spie.org/x648.html?product_id=412861 |date=13 June 2010 }}</ref> Advantages of shearography are the large area testing capabilities (up to 1 m<sup>2</sup> 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.<ref>{{Cite journal | last = Hung | first = YT | title = Shearography: A new optical method for strain measurement and nondestructive testing | journal = Optical Engineering | year = 1982 | volume = 21 | issue = May/June 1982 | pages = 391–395 | doi = 10.1117/12.7972920 | bibcode = 1982OptEn..21..391H | url = http://md1.csa.com/partners/viewrecord.php?requester=gs&collection=TRD&recid=A8234227AH&q=%22Shearography%22&uid=790196028&setcookie=yes | url-access = subscription }}</ref>
==Phase-shift technology==
To increase the sensitivity of the measurement method, a real-time phase shift process is used in the sensor. This contains a stepping mirror that shifts the reference beam, which is then processed with a best fit-algorithm and presents the information in real time.
==Applications==
The main applications are in composite nondestructive testing, where typical flaws are: Disbonds, Delaminations, Wrinkles, Porosity, Foreign objects, and Impact damages.
Industries where Shearography is used are: Aerospace, Space, Boats, Wind power, Automotive, Tires, and Art conservation.<ref>MK Meybodi, I Dobrev, P Klausmeyer, EJ Harrington, C Furlong, "[http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1359093 Investigation of thermomechanical effects of lighting conditions on canvas paintings by laser shearography]", SPIE Optical Engineering+ Applications, 2012</ref>
==Inspection standards==
The methodology of shearography is standardized by ASTM International: * ASTM E2581-07, "Standard Practice for Shearography on Polymer Matrix Composites, Sandwich Core Materials and Filament Wound Pressure Vessel’s in Aerospace Applications"
The following NDT personnel certification documents contain references to shearography: * BS EN 4179:2009 * NAS 410, 2008 Rev 3 * ASNT SNT-TC-1A, 2006 edition * ASNT CP-105, 2006 edition
==References== {{Commons category|Shearography}} {{reflist}}
Category:Nondestructive testing