Review and comparison of shearography and pulsed thermography for adhesive bond evaluation

To reduce costs and facilitate automation in the automotive industry, adhesive bonding has gained popularity as a replacement for conventional mechanical fasteners such as bolts, screws, rivets, and welding. Adhesive bonding is particularly useful for bonding parts made of plastics and polymer composites, which are playing an increasing role in reducing vehicle weight. However, the adhesive bonding process is more susceptible to quality variations during manufacturing than traditional joining and fastening methods. Shearography and pulsed thermography are full-field, noncontact, nondestructive testing methods that are widely used in the aerospace industry, offering significant potential as practical tools for in-process inspection of adhesive bond quality. The two techniques are often used to address a common set of aerospace applications, e.g., delaminations or skin-to-core disbonds in composite structures. However, they are fundamentally different, based on different flaw detection mechanisms: Shearography measures the sample's mechanical response to mechanical stresses, while pulsed thermography measures the sample's thermal response to an instantaneous thermal excitation. For the convenience of potential users and readers, the authors review shearography and pulsed thermography. The potential of these techniques for inspecting adhesive bonding is demonstrated and compared.