Shearography, also known as speckle pattern shearing interferometry, is a method for measuring and detecting a range of different defects on metallic and composite materials. This non-destructive inspection technique uses coherent laser light in a similar manner to holographic interferometry to create a visual representation of a test object, for non-destructive testing (NDT), strain measurement and vibration analysis.
The interferometric images produced by shearography can help detect disbonds, delaminations, impact, porosity, wrinkles or other damage. It is widely used for aerospace applications as well as for wind turbine blades, the automotive industry and for materials research.
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Shearography uses a camera, or a series of cameras, to measure the interferometric properties of a surface, using the coherent and monochromatic properties of laser light to create an image of the surface.
Firstly, a picture is taken of the surface in a neutral or unloaded state. If the surface is not completely smooth the light reflected from the surface will produce a speckle pattern which is recorded by the camera.
Once the first image has been taken, the material is stressed or excited through a mechanical load or thermal heating. This loading will cause the material to react and any defects to expand accordingly. A second interferometric photo is then taken to show the sample in its newly loaded and deformed state.
The second image will show a different speckle pattern to the first and, by subtracting it from the first image, a shearograpic fringe pattern, or shearogram, will be produced. This shearogram shows the topography of any surface defects, including cracking, disbonding, delamination, fluid ingress, porosity and wrinkling.
The characteristic black and white fringe pattern provides information about the relative deformation of the object being inspected. If the pattern is regular in appearance then there are no defect features, however if the pattern is disturbed there will be a subsurface defect.
This process not only allows an operator to identify defects, but can also be used to measure the extent of deformation in composites and metallic materials.
This technique is also known as temporal phase shift or digital shearography.
As a non-contact test with high rates of coverage there are a number of advantages offered by shearography, including:
1. Large Area Testing
Shearography allows for rapid testing of large components and parts, with a capacity to test at up to 1m² per minute.
2. Lifecycle Support
As a non-destructive inspection method, shearography offers expedited defect detection as well as being used for quality control processes and material structural integrity qualification. As a result, it can be used to support the entire lifecycle of a product, from research and development through to manufacture quality control and in-service inspection and operation.
3. Material Use
Shearography systems can be used for a wide variety of materials, including carbon fibre laminates; ceramics; composite and metal honeycombs; composite overwrapped pressure vessels; bi-metals; fibre-metal laminates; metal to metal bonds; foam cores; and rubber. The good performance on honeycomb materials is of particular note, as these are a challenge for many traditional NDT techniques.
4. Inspect Complex Structures
Shearography allows for the inspection of components irrespective of material size, shape, geometry or complexity in even the most hard-to-reach places.
5. Range of Detection
This inspection technique offers a wide range of detection capabilities for both metal and composite structures. The discontinuities that can be detected include cracked or crushed cores; cracking; disbonds; delaminations; fluid ingress; kissing bonds; porosity; wrinkles; repair defects; and impact damage. Shearography can also be used to gain structural information such as for bulkheads, overlaps, ply drops, ribs, splices and stringers.
6. Outperforms other NDT Techniques
Shearography outperforms many other NDT methods, such as acoustic emission, dye penetrant, eddy current, magnetic particle, radiography and ultrasonic testing. This is due to offering full field measurement, high sensitivity, easy visualisation, fast measurement speed and a real time display of the results.
Despite the many advantages of shearography, there are a few potential limitations:
1. Possible Material Damage
Because the procedure requires the specimen or material to be deformed between the two images being taken, there is a chance that the component could be damaged if this is not done carefully.
2. Cannot Be Performed on Smoother Surfaces
Because the test needs a surface roughness of at least one wavelength of light to be performed, shearography cannot be used on smoother surfaces. Smoother surfaces will return random interference, leading to the wrong inspection results.
3. Complexity of Interpretation
The results can be difficult to interpret, meaning that you need an experienced person to handle the results of a shearography test in order to accurately read the fringe patterns.
Shearography is primarily used for non-destructive testing for flaws in objects and materials. Because of the many advantages it offers, this method has been taken up by the automotive and aerospace industries as a recommended method for testing rubber and composite materials.
The process also finds use in the marine, defence, rail, and wind power industries, while also being used for applications including textiles and art conservation. Shearography is well used for composite materials, but its ability to inspect rubber means it’s also used to examine products like tyres.