Theories and industrial applications of optical interferometric NDT techniques: A review

Optical techniques are based on the development of laser, imaging sensors and computer technologies with a view to providing full-field and non-contacting measurement of surface profiles and deformations. They are categorised in three groups: moir, holography and speckle-based techniques. These optical techniques have been used in material characterisation, defects detection, strain measurement, design verification/optimisation, residual stress evaluation, vibration analysis and 3D shape measurement. In this paper, the principles of these techniques are overviewed first, their advantages and disadvantages highlighted. Compared to other non-destructive testing techniques, optical techniques reveal defects in an object by assessing the response of surface deformation to defects under stressing, which is closely correlated with the strength and integrity of the object. Not only the modes of defects could be determined, the strains and stresses field associated with these modes could be quantified as well. Therefore, optical techniques have attracted increasing interest from the NDT community in the past decade, and have been applied to a range of industrial sectors to meet the demanding nondestructive testing of various engineering components and structures.