Digital Shearography for Nondestructive Evaluation and Application in Automotive and Aerospace Industries

Digital shearograohy has demonstrated great potential in revealing defects in objects especially in detecting delaminations in composite materials. It is gaining more and more acceptance by automotive and aerospace industries in the field of nondestructive testing (NDT) of composite materials. A key optical component used in shearography is a shearing device in which the shearing amount, shearing direction and under the most cases the phase shift technique are determined and introduced. The paper displays different shearing devices. A theoretical analysis and an experimental comparison between them are demonstrated. The new measuring system of digital shearography with a capability for simultaneous measurement both in x and y directions and new concept for shearing amount, i.e. the critical shearing amount at which the measuring setup has a maximal sensitivity, are developed and introduced. The measuring sensitivity, the manner of load and illumination are discussed in details. The recent developments of digital shearography for NDT and its potentials, limitations and application are demonstrated by examples of NDT for different materials. Introduction and Background: The demanding requirements of product quality and reliability has led to the need for highly efficient NDT method that is real time, whole-field and non-contact-based. Optical method such as thermography, holography, Electronic Speckle Pattern Interferometry (ESPI) and shearography (as called Speckle Pattern Shearing Interferometry) etc. are emerging as strong candidate for new industrial NDT tools because of their virtues of being whole-field, non-contacting and noncontaminating. Of the optical techniques, shearography has already been proven to be a practical one and is gaining more and more acceptance by automotive and aerospace industries in the field of NDT of composite materials. Sheargraphy is a laser based optical measuring and testing method that is similar to holographic interferometry and ESPI. Because of a utilization of a special shearing device, shearography, however, measures a gradient of displacement, not the displacement itself as holography or ESPI does. Stains are functions of displacement gradients; thus, shearography yields strain information directly. Because defects in objects usually induce strain concentration, it is easier to reveal defects with strain anomalies than with displacement anomalies. Moreover, a rigid-body motion does not produce strain; thus shearography is insensitive to such motion. This is a significant advantage of shearography, which indicates the usefulness of shearography in typical industry operation. Although many advantages, a successful application of digital shearography for NDT in industries still depends on depth and type of defects, the type of materials, the shearing amount and direction, the manner of load and laser illumination, and so on. This paper will systemically analyze the effects of these parameters, in particular, their effects on the measuring sensitivity. The theory and methodology of recent developments of digital shearography for NDT are described. Its potentials, limitations and applications are demonstrated by examples of NDT for different materials Measurement Principles of Digital Shearography: Digital shearography is a laser measuring technique based on digital data processing, phase-shifting techniques and interferometry. According to the common sense of interferometry, two beams with an identical wavelength are required for a purpose of interference. Usually, an identical wavelength for the two beams can be obtained from one laser by using a beam splitter, such as the object beam and the reference beam used in the setups of holography and electronic speckle pattern interferometry (ESPI). A distinguishing feature of shearography is the use of a self-reference interference system. Instead of using a reference beam, shearography utilizes a shearing device to bring the light waves from two points on the object surface into one point on the image plane,