Quantitative thermal imaging of aircraft structures

ABSTRACTAircraft structural integrity is a major concern for airlines and airframe manufacturers. To remain economically competitive,airlines are looking at ways to retire older aircraft, not when some fixed number of flight hours or cycles has been reached, butwhen true structural need dictates. This philosophy is known as "retirement for cause." The need to extend the life of commer-cial aircraft has increased the desire to develop nondestructive evaluation (NDE) techniques capable of detecting critical flawssuch as disbonding and corrosion. These subsurface flaws are ofmajor concern in bonded lap joints. Disbonding in such ajointcan provide an avenue for moisture to enter the structure leading to corrosion. Significant material loss due to corrosion cansubstantially reduce the structural strength, load bearing capacity and ultimately reduce the life of the structure.Recent advances in technology have spawned a number of new NDE techniques that provide quantitative information aboutflaws in aircraft structures. In a effort to develop NDE techniques with improved reliability and reduced cost, several improve-ments in large area scanning techniques have been made. A few of these techniques currently being developed are thermalimaging, acoustic emission, scanning array ultrasonics, coherent optics, radiography and magnetic field visualization.One technique in particular that has proved quite promising is infrared thermal imaging. Thermography has a number ofadvantages as an inspection technique. It is a totally noncontacting, nondestructive, imaging technology capable of scanning alarge area of the aircraft fuselage for defects in a matter ofa few seconds. Advances in fast, inexpensive image processors haveaided in the attractiveness of thermography as a NDE technique. These image processors have increased the signal to noiseratio of thermography and facilitated significant advances in post-processing. The resulting digital images can also be archivedfor later comparison with the results of subsequent inspections thus providing a means of monitoring the evolution of damagein a particular structure.The National Aeronautics and Space Administration's Langley Research Center has developed a thermal NDE systemdesigned for application to disbonding and corrosion detection in aircraft skins. By injecting a small amount of heat into thefront surface of an aircraft skin, and recording the time history of the resulting surface temperature variations using an infraredcamera, quantitative images of both bond integrity and material loss due to corrosion can be produced. This paper will presenta discussion of the development of the thermal imaging system as well as the techniques used to analyze the resulting thermalimages. The analysis techniques presented represent a significant improvement in the information available over conventionalthermal imaging due to the inclusion of data from both the heating and cooling portion of the thermal cycle.Results of laboratory experiments on fabricated disbond and material loss samples will be presented to determine the limita-tions of the system. Additionally, the results of actual aircraft inspections will be shown, which help to establish the field appli-cability for this technique. A recent application of this technology to aircraft repairs using boron /