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What is Shearography?
Shearography is an optical non-destructive test that is used to determine the contents of a surface material. The main application of this test is to inspect the quality of the composites and other materials. This test is very useful in high quality production industries such as aerospace, automobile, wind energy, and marine industries. It is also used for material research.
It’s a non-contact test with a high rate of coverage. This is an important advantage of this technique over any other.
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How Shearography is Used
The process of shearography non-destructive testing is based upon a shearography camera. The camera provides a spectral analysis of the surface of the material that needs to inspection. The function of the camera is to record the interferometric photo of the surface, and this photo is considered a unique footprint of the surface. The camera includes the roughness and the shape of the specimen too.
Now, you will need to apply some load to stress the specimen. Actually, heat is used for stressing the material, the heat supplied expands the specimen. However, if it has some weak spots (the spots that are beign tested for) then the material will expand more at the weaker spots. Now, the shearoghraphy camera will be used again. Take another interferometric photo of the expanded specimen. The interferometric footprint of the area of deformed state is now used to further studies. To find the information of the material, you will need to subtract the old photo from the new one, i.e. compare the both photos. This is how the shearography works.
To understand the whole process, the applications of the shearography are given below. With these applications, you would understand where the shearoghraphy is used, and what the limitations of shearography testing of composites are.
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Applications of Shearography
Used in the Inspection of Wind Turbine Blades
The production of the blades of wind turbine includes some complex procedures. After the production of these blades, the inspection is very difficult because of their large size. The huge size of these blades requires fast and accurate inspection, which can cover several square meters in a small time.
The technique of shearography is used to inspect the specimen of huge sizes like these blades. It detects various defects occurred during the production processes such as delamination, kissing bonding, wrinkle, and debonding. The whole instrument gives its first reading within 5 minutes.
This technology provides maximum flexibility to the operator. It is also used for inspecting the helicopter’s blades similar to wind turbine blades.
Used for Inspecting Bonded Sandwich Structures with Foam Cores and Stiff Skins
A bonded sandwich structure is a structure that consists of stiff stainless steel skins and low-density foam cores. The defects in this structure are present mainly in between skin and core. They may also present in the bondline of the structure. These defecst are responsible for the passing of ambient air through them.
Shearography is found to be very useful to inspect such structures. It is used to detect the defects, and it also estimates the depth position in stiff areas.
Used in Aerospace Industries
A high quality inspection system is required in aerospace industries. A special branch of shearography, named robotics shearography, is used in various Jet manufacturing companies today. It’s a fully automatic system that was installed for the first time in 2008 . It is capable of inspecting 1-2 square meters per minute of material, which is a pretty good speed.
The system works in a vacuum chamber. The vacuum chamber is used to excite the parts of the structure. However, it also uses heat to deform the material, if required. It can supply heat up to 3Kw to the material at a time.
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Limitations of Shearography
The Possibility of Damaging Material
The most important problem with the shearography non-destructive testing is that it may cause the failure of the material or specimen. This is because; the procedure of the testing requires deformation of the specimen. Therefore, you have to apply load or heat to it. However, if the applied load or heat is high, it may cause failure of the specimen.
The limitations of shearography testing of composites are given below.
Applicable Only for Particular Specimen
The second drawback of this test is its area of application. It can only be applied to such specimen that has surface roughness of one wavelength of light or more. Smoother surfaces will produce random interference that may tend to wrong results of inspection.
Results are Complex to Interpret
The results provided by this technique are very complex, and difficult to interpret. You will need an experiencde person to handle the results of the shearography test.
Require Perfect Lightning
The shearograhy test is based upon optical instruments. Because of this, there must be good lightning over the specimen to achieve best results.