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Determining heat intensity of a fatigue crack from measured surface temperature for vibrothermography

We propose a simulation-assisted linear-inversion method to estimate heat intensity generated at a fatigue crack during vibrothermographic nondestructive evaluation. In vibrothermography, mechanical excitation is applied to a test specimen, and the contacting crack faces generate heat which flows to the surface and can be detected by an infrared camera. The heat intensity generated at the crack is not a directly measurable quantity, and instead has to be estimated by inverting the measured crack surface temperature. This inversion is an ill-posed problem because of the inherently diffusive nature of heat flow. The proposed method in this paper attempts to estimate the crack heat intensity by minimizing the squared error between numerically modeled crack heating and the experimentally measured surface temperature of the crack. We demonstrate that with reasonable assumptions, the effect of diffusion on the heat intensity can be overcome, and it is possible to estimate the crack heat intensity. Furthermore, we reconstruct the surface temperature on two different material systems and quantify the reconstruction accuracy of the least-squares inversion algorithm.