Identification of kissing defects in adhesive bonds using infrared thermography

A carbon fibre reinforced plastic (CFRP) adhesively bonded single lap joint sample is used for comparing the detection of different defect types using pulsed phase thermography (PPT). Firstly, a polytetrafluoroethene (PTFE) insert, of the type widely used to simulate defects in composite materials, was added to the bond line of the joint. Liquid layer kissing defects were simulated using silicon grease. PPT clearly identified the PTFE but not the silicon grease contamination. The PPT identified the silicon grease defect when the joint was loaded. It is postulated that kissing defects can be detected using thermography if a small load is applied to the joint, as loading opens the defect and produces a gap that provides sufficient thermal contrast for detection. Thermoelastic stress analysis (TSA) is used to validate the approach. On-site application is addressed both in terms of the load application and the use of low cost infrared (IR) detectors.