Shearography
Encyclopedia
Shearography or speckle pattern shearing interferometry is a measuring and testing method similar to holographic interferometry
. It uses coherent light or coherent
soundwaves 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, 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. Electronic speckle pattern interferometry
is one form of shearography.
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 more insensible 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.
Industries where Shearography is used are:
Holographic interferometry
Holographic interferometry is a technique which enables static and dynamic displacements of objects with optically rough surfaces to be measured to optical interferometric precision . These measurements can be applied to stress, strain and vibration analysis, as well as to non-destructive testing...
. It uses coherent light or coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
soundwaves to provide information about the quality of different materials in nondestructive testing
Nondestructive testing
Nondestructive testing or Non-destructive testing is a wide group of analysis techniques used in science and industry to evaluate the properties of a material, component or system without causing damage....
, strain
Deformation (mechanics)
Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body...
measurement, and vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
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. Electronic speckle pattern interferometry
Electronic speckle pattern interferometry
Electronic Speckle Pattern Interferometry , also known as TV Holography, is a technique which uses laser light, together with video detection, recording and processing to visualise static and dynamic displacements of components with optically rough surfaces...
is one form of shearography.
Shearing function
When a surface area is illuminated with a highly coherentCoherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
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
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
. Analogous with Electronic speckle pattern interferometry
Electronic speckle pattern interferometry
Electronic Speckle Pattern Interferometry , also known as TV Holography, is a technique which uses laser light, together with video detection, recording and processing to visualise static and dynamic displacements of components with optically rough surfaces...
(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 more insensible 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
- Impact damages
Industries where Shearography is used are:
- Aerospace
- Space
- Boats
- Wind power
- Automotive
- Tires
Inspection standards
The non-destructive testing industry is controlled by Inspection Standard Documents & Codes. Shearography is incorporated in following standard documents:- NAS 410, 2008 Rev 3
- ASNT SNT-TC-1A, 2006 edition
- ASNT CP-105, 2006 edition
- ASTM E2581 -07, "Standard Practice for Shearography on Polymer Matrix Composites, Sandwich Core Materials and Filament Wound Pressure Wessel’s in Aerospace Applications"