Shearography - NDT (Non Destructive Testing)
Principle
The shearography technology is an interferometric test method with which damage or defects can be identified on components under load. This measuring technique even allows detecting deformations of only a few micrometers in size. Already a relatively low component load can thus yield unambiguous measuring results, which are mandatory particularly for NDT testing.
The test object is illuminated with laser light and observed
through a CCD (Charged Coupled Device) camera equipped with the shearing optics. These shearing optics project the object image onto the camera chip twice.
When the test object deforms under load, the laser light reflected by the component changes. By superimposing an image showing the unloaded object state with an image taken in the loaded state, the change of an image point can be determined. The optimum test load type (thermal load or pressure and vacuum load) for the individual component depends on the fault type and fault depth and is ideally determined before serial testing by trial measurements using sample components.
ISIS - Shearography-NDT - Test Procedure
A measurement consists of a reference image of the object in its natural state, followed by loading the component and taking an additional image. The image processing software automatically subtracts the two images from each other to create the comparison. Depending on the test parameter settings, the reference image can be taken either before or after applying the test load.
The test load types that are typically selected are thermal and/or pressure/vacuum loads. The optimum load type depends on the component structure and material, and on the size, position and type of the faults.
The load type therefore should be verified beforehand using a sample component; it can then be optimized with regard to intensity and duration. During the test procedure, the field of view can be monitored on the display; any detected faults are represented in real-time. The actual measurement is initiated either manually or automatically. The measuring result is displayed on the screen a few seconds and can then be printed and saved.
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