Shearography and Applications in Nondestructive Evaluation

This article reviews shearography and its applications in nondestructive testing. Shearography is a laser-based technique for full-field and non-contacting measurement of surface deformation (displacement or strains). Despite being a relative young technique, it has already received considerable industrial acceptance, in particular, for nondestructive testing. One major difference of shearography from other NDT techniques is the mechanics of revealing flaws. Shearography reveals defects in an object by identifying defect-induced deformation anomalies which are more relevant to structural weakness. Other applications of shearography include strain measurement, material characterization, residual stress evaluation, leak detection, vibration studies and 3-D shape measurement. Introduction: Shearography is an optical method for measuring surface deformation. Unlike traditional measurement techniques, shearography does not require the laborious task of mounting a large number of strain gages or transducers. It is a non-contacting method that yields full-field information about surface displacement or displacement derivatives. Shearography was developed to address several limitations of holography. Its significant advantages include (1) not requiring a reference light-beam, thus leading to simple optical setups, reduced coherence length requirement of the laser, and lax vibration isolation; and (2) direct measurement of surface strains (first-order derivatives of surface displacements). These distinct advantages have rendered shearography as a practical measurement tool that can be employed in industrial settings, and it has already gained wide industrial acceptance for nondestructive testing. For instance, the rubber industry routinely uses shearography for evaluating tires, and the aerospace industry has adopted it for nondestructive testing of aircraft structures, in particular, composite structures. Recently the technique has been extended to non-destructive evaluation of civil engineering structures. Other applications of shearography include: measurement of strains, material properties, residual stresses, 3-D shapes, vibrations, as well as leakage detection. Three versions of shearography are in existence which are based on different recording media : photographic; thermoplastic; and digital. Recently, a unified approach for practising holography and shearography was reported. In this paper, only the digital version of shearography is presented. Description of Digital Shearography : A schematic diagram of digital shearography is shown in Fig. 1. The object to be evaluated is illuminated with laser light radiating from a point-source, and it is imaged by an image-shearing camera connected to a microcomputer for recording and processing. The camera comprises a CCD image sensor, a lens and an image shearing device. The image shearing device consists of a double-refractive prism and a polarizer. The function of the image shearing device is to produce a pair of laterally displaced (sheared) images, and hence the technique is named as shearography. The principle of image-shearing using a doubly-refractive prism is illustrated in Figure 2 showing that a light beam passing through the prism is split into two angularly separated beams. Conversely, through the image-shearing device, two non-parallel beams of light scattered from two different object points become nearly collinear. Since the spatial frequency of the interference fringe pattern of two beams is proportional to the sine of the half angle between the interfering beams, two nearly collinear beams produce a very low frequency interference pattern that is comfortably resolved by a video image sensor such as CCD.