Evaluating the soundness of bonding using shearography

Shearography is an optical measurement technique invented to overcome several 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 shearography, one for measuring surface displacement and the other for measuring surface displacement derivatives. This paper discusses the underlying principle 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 response of a defect to stresses. Two practical types of stressing for revelation of debonds are described – static loading using vacuum stressing, and dynamic stressing by means of vibrational excitation. In vibrational stressing, both single frequency excitation and broadband excitation can be used. While vacuum stressing is limited to detecting debonds with closed boundaries, the vibrational excitation can be used for detecting debonds with closed as well as opened boundaries. A major drawback associated with the use of single excitation frequency is the need for vibrating the test object within an appropriate frequency range, as otherwise the test data obtained will not readily reveal the debonds. This paper also describes a new method that uses multiple frequency sweep (equivalent to broadband excitation) which fosters rapid detection and unambiguous assessment of the soundness of adhesive bonding.