Laser Based Nondestructive Inspection Techniques

Abstract

The most familiar laser based optical nondestructive methods are based on the interference of the wave fronts of monochromatic light reflected from a test surface. We explore the origins, applications, advantages and limitations of these techniques with a journey through the realm of Holographic Interferometry, Electronic Speckle Pattern Interferometry and Shearography. These methods are applied to the widest range of materials from metals to composites attempting to determine the conditions of the material that contribute to failure, in for example critical parts of a structure. These techniques are inherently of fairly high sensitivity, are non-contacting, they are able to inspect a whole field in a single procedure and results are obtained at almost real time. The inspection procedure on a test piece is identical for all three techniques. An image of the object’s surface is recorded and stored in a suitable medium; subsequently the object is perturbed mildly by mechanical or thermal means which results in its surface being deformed. At this stage a second image is captured and is made to interfere with the first image resulting in an image of the surface with zebra like stripes superimposed on it. This is known as a fringe pattern and abrupt changes in direction or spacing of these fringes may indicate a subsurface condition that affects the surface deformation. Thus porosity, voids, de-bonds, cracks, de-laminations etc. are detected. These techniques have also limitations particularly with the stability of the test piece and the inspecting instrument in relation to each other. Particular attention is paid in avoiding excessive test piece motion or environmental vibration both which will exceed the stringent requirements for fringe formation. It is well established that these laser based NDT techniques have found innumerable applications in the laboratory/field/factory environment and particularly Shearography but surprisingly it does not yet have a standard, like for example an ISO International Standard, hence we are calling for the involvement in this respect by the technical committee TC 135 of the ISO.