Evaluating the soundness of bonding using shearography

Shearography is an optical measurement technique invented to overcome sever al limitations of holography. One distinct advantage is that it alleviates the stringent environmental stability demanded by holography, rendering the technique practical for industrial applications. There are two modes of sh earograpby, one for measuring surface displacement and the other for measur ing surface displacement derivatives. This paper discusses the underlying p rinciple of shearography and its application in nondestructive testing, in particular, of laminated composite structures. In flaw characterization, a thin-plate model is used for back-calculating the shape, size and location of debonds. Shearographic nondestructive testing relies on measuring the re sponse of a defect to stresses. Two practical types of stressing for revela tion of debonds are described - static loading using vacuum stressing, and dynamic stressing by means of vibrational excitation. In vibrational stress ing, both single frequency excitation and broadband excitation can be used. While vacuum stressing is limited to detecting debonds with closed boundar ies, the vibrational excitation can be used for detecting debonds with clos ed as well as opened boundaries. A major drawback associated with the use o f single excitation frequency is the need for vibrating the test object wit hin an appropriate frequency range, as otherwise the test data obtained wil l not readily reveal the debonds. This paper also describes a new method th at uses multiple frequency sweep (equivalent to broadband excitation) which fosters rapid detection and unambiguous assessment of the soundness of adh esive bonding. (C) 2000 Elsevier Science Ltd. All rights reserved.