Shearography or Speckle pattern shearing interferometry is a measuring and testing method similar to
holographic interferometry. It uses
coherent light
In physics, two wave sources are coherent if their frequency and waveform are identical. Coherence is an ideal property of waves that enables stationary (i.e., temporally or spatially constant) interference. It contains several distinct concepts, ...
or
coherent
Coherence, coherency, or coherent may refer to the following:
Physics
* Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference
* Coherence (units of measurement), a deriv ...
soundwaves to provide information about the quality of different materials in
nondestructive testing
Nondestructive testing (NDT) is any of a wide group of analysis techniques used in science and technology industry to evaluate the properties of a material, component or system without causing damage.
The terms nondestructive examination (NDE), n ...
,
strain
Strain may refer to:
Science and technology
* Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes
* Strain (chemistry), a chemical stress of a molecule
* Strain (injury), an injury to a mu ...
measurement, and
vibration
Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, su ...
analysis. Shearography is extensively used in production and development in aerospace, wind rotor blades, automotive, and materials research areas. 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
Coherence, coherency, or coherent may refer to the following:
Physics
* Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference
* Coherence (units of measurement), a deriv ...
laser light
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The firs ...
, 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.
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.
[MK Meybodi, I Dobrev, P Klausmeyer, EJ Harrington, C Furlong,]
Investigation of thermomechanical effects of lighting conditions on canvas paintings by laser shearography
,SPIE Optical Engineering+ Applications, 2012
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
{{reflist
Nondestructive testing