<title>Thermal characterization of defects in aircraft structures via spatially controlled heat application</title>

Recent advances in thermal imaging technology have spawned a number of new thermal NDE techniques that provide quantitative information about flaws in aircraft structures. Thermography has a number of advantages as an inspection technique. It is a totally noncontacting, nondestructive, imaging technology capable of inspecting a large area in a matter of a few seconds. The development of fast, inexpensive image processors have aided in the attractiveness of thermography as an NDE technique. These image processors have increased the signal to noise ratio of thermography and facilitated significant advances in post-processing. The resulting digital images enable archival records for comparison with later inspections thus providing a means of monitoring the evolution of damage in a particular structure. The National Aeronautics and Space Administration's Langley Research Center has developed a thermal NDE technique designed to image a number of potential flaws in aircraft structures. The technique involves injecting a small, spatially controlled heat flux into the outer surface of an aircraft. Images of fatigue cracking, bond integrity and material loss due to corrosion are generated from measurements of the induced surface temperature variations. This paper will present a discussion of the development of the thermal imaging system as well as the techniques used to analyze the resulting thermal images. Spatial tailoring of the heat coupled with the analysis techniques represent a significant improvement in the delectability of flaws over conventional thermal imaging. Results of laboratory experiments on fabricated crack, disbond and material loss samples will be presented to demonstrate the capabilities of the technique. An integral part of the development of this technology is the use of analytic and computational modeling. The experimental results will be compared with these models to demonstrate the utility of such an approach. The use of spatially controlled heat application has been shown to be beneficial for the detection of a number of types of flaws in both single layer and laminated structures. A fixed linear heat source has been used to detect such flaws as fatigue cracks 1. Additionally, Maldague 2 described a system where a sample moving at a constant velocity passes a fixed linear heat source and is observed by an infrared imager. This technique has been shown to be effective in the detection of disbonding in laminated samples. This technique has certain advantages over conventional time dependent thermal imaging where the heat source, imager and sample are fixed, but the heating and inspection times are varied. Presented here are results from a system where heat is injected into a stationary sample by a linear heat source moving at a constant velocity. Images of the temperature variations induced in the sample are made with an infrared imager following, at the same velocity as the heat source, but separated by a fixed distance. One of the advantages of this type of system is the inspection speeds possible. This paper will present results of laboratory experiments on fabricated samples, representative of aircraft structures, as well as on actual aircraft structures that demonstrate the potential capabilities and advantages for this technique. The theoretical basis for this technique will also be presented.