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A simple modification of the one-dimensional expression for the thermal contrast of a layer provides a useful prediction of peak contrast temperature and contrast peak time for defects of all aspect ratios. The new analytical results have been shown to agree with numerical modelling. The thermographic nondestructive evaluation (NDE) rule-of-thumb that defects are detected if aspect ratio exceeds two is shown to have no general validity as peak contrast is found to depend critically on defect depth and absorbed excitation energy as well as defect aspect ratio. The effects of thermal diffusivity anisotropy are included in the analysis and illustrated by simulations of defect image contrast in composite materials.
Almond et al. (Tue,) studied this question.