Active thermography (AT) based on radiation heat source has been successful in locating delaminations in Carbon Fiber Reinforced Plastic (CFRP). This is due to a high contrast in thermo-physical properties that delaminations show in comparison with fibers or matrix. However for defects with much smaller contrast, such as incipient heat damage, the efficiency drops dramatically. In this paper we investigate some of the main factors which could lead to the sensitivity limitations of the AT for low contrast defect detection. The first factor in consideration is the complex surface absorption. When common halogen lamp is used as a heat source, both surface and sub-surface absorption are expected. Simulated cases of surface and sub-surface absorption show that each of them will result in different cooling rates. The complexity is also enhanced due to variations in absorption across the surface. Even within an area of one cell of the plain-woven (PW) structure in the first fiber layer a temperature difference of up to 14 degree can be achieved. The second factor is the limited rate of heat transfer transverse to the sample surface. All of these effects are expected to produce contrast arising from the incipient heat damage and may cause the ambiguity and difficulties in the defect identification with AT.
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