Pulsed thermography for non-destructive evaluation and damage growth monitoring of bonded repairs

Military and civilian aircraft fleets require effective, robust, quick and reliable means of repairing aircraft structures that have experienced battle, environmental or operational damage (e.g., fatigue and corrosion). Common repair schemes use metallic or composite patches that are either mechanically fastened or adhesively bonded to the area of concern. The advantages of adhesive bonding over mechanical fastening of patch repair include the reduction of stress concentration associated with over-sizing of existing holes or machining of additional holes in the structure and improved aerodynamic profiles. However, the primary disadvantage of an adhesively bonded repair is the difficulty in identifying if the patch is properly bonded to the structure and whether it is still capable of carrying the required loads. Depending on the application, a loss of load-bearing capability of the bonded repair could lead to a reduction in the fatigue life or even a premature catastrophic failure of the structure. In this paper, pulsed thermography, an infrared non-destructive evaluation (NDE) technique, is proposed for the detection of disbond and monitoring of disbond growth in bonded graphite repairs. Correlated results with ultrasonic pulse-echo c-scan inspections and destructive testing show good disbond detection capability with an accuracy similar to that of ultrasonic inspection.