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Thermal wave scattering from closed, slanted cracks is investigated both theoretically and experimentally. A Born approximation calculation is carried out for the particular case of gas-cell detection for three crack angles. Good agreement is found with experimental images of both the magnitude and phase of the gas-cell signal variation for cracks fabricated in an aluminum alloy at these same angles. Good agreement is also found between theory and experiment for the frequency dependence of the thermal wave scattering from a 45° crack. It is shown theoretically and confirmed experimentally that a strictly vertical, closed crack is not observable by gas-cell detection, but is easily seen by mirage-effect (optical beam probe) detection. The results for model cracks are applied to the case of brittle fractures in solids.
Grice et al. (Tue,) studied this question.