- Phase unwrapping applied to portable digital shearography
- Benchmarking Shearographic NDT for Composites
Digital Shearography is an Optical non-destructive testing technique, suitable for the inspection of manufactured components for defects. The technique employs the correlation of speckle images before and after object deformation and is particularly suited for inspecting composites for a range of defects including internal debonds and delaminations. The Department of Mechanical Engineering at the University of Cape Town has developed a portable Digital Shearography system which has the ability to perform phase stepped inspections. In addition, a software project has just been completed which complements the existing software by unwrapping the phase fringes into a displacement gradient map.
This paper describes the principle of Digital Shearography, before highlighting the development of the portable Digital Shearography system. Selected aircraft composite samples from a carbon fibre helicopter rotor blade are then subjected to phase stepped shearography inspections using the new system. The developed phase unwrapping algorithm is described and then applied to the obtained fringe patterns and the results presented. The paper is concluded with an evaluation and discussion of the complete system.
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This paper reviews Digital Shearography in its current state of development. The technique was originally proposed as a strain measurement method but has more recently found an equally important role in the field of non destructive testing. Digital Shearography, as now a day practiced in research laboratories and in industry, reveals defects beneath the surface of an object by identifying anomalies in the field of surface displacement gradients. Shearography as a non destructive testing tool has found innumerable applications involving a wide range of materials and in particular has had notable success in identifying debonds and delaminations in composite material structures.
In the face of distinct advantages over other NDT methods, such as full field view, non-contacting and real time evaluation, and proven in a vast number of applications in the laboratory/field/factory environment, surprisingly it does not yet have a standard, like for example an ISO International Standard. The objective of this paper is a call for the standardization of Digital Shearography based on the involvement of interested parties calling for the start of the process, perhaps as is suggested here, by the technical committee TC 135 of the ISO.
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