Non-destructive evaluation of bonded structures with lock-in thermography

Lock-in thermography is employed for non-destructive evaluation of several types of bonded structures, which are commonly encountered in industrial applications. Specimens were fabricated to simulate: adhesively bonded aluminium joints, which are commonly used in aeronautical and automotive fields; bonds between pipes of poly(vinyl chloride) (PVC) employed in the transport of liquids (sewage systems); and bonds between plates of Plexiglas which are widely used in the manufacturing of aquaria. Amongst bonded structures, the composite materials are very important, which are generally made of carbon, glass, or Kevlar(TM) aramid fibers and epoxy resin, and which find application in many industrial fields, especially the aeronautical industry, because of their higher strength and lower weight as compared to metallic materials. It is known that surface plasma treatment of a material improves its adhesion, but it is also known that this treatment will degrade over time if the material is not bonded immediately. Thus, to assure quality, any bonded system should be monitored by the most effective non-destructive technique. To obtain information about the ability of lock-in thermography to assess the performance of such plasma treatment, several specimens were fabricated from either composites (carbon, or Kevlar(TM) fabric layers plus epoxy resin), or glass plates with and without surface plasma treatment before bonding. In addition, a sample was obtained from a piece of a typical insulated wall of refrigerator vehicles, which actually is a sandwich of polyurethane foam between two plates of fiberglass. The results obtained show that lock-in thermography is a useful tool for non-destructive evaluation of bonded structures.