Flash thermography (FT) is a well-established non-destructive testing (NDT) technique that uses a short (~msec) pulse from a flashlamp to uniformly heat the surface of a sample and interrogate its subsurface structure based on the surface temperature response, measured with an infrared (IR) camera. Heat flow into a defect-free sample is described by a 1-D diffusion model, which transitions to higher dimensions due to subsurface discontinuities. 1-D approaches can be unreliable in the vicinity of abrupt thickness change, termed as the 'transition zone,' where lateral heat flow from the thin to thick region may mask surface temperature changes due to heat flowing into the part. In this work, we quantify the uncertainty in a "subsurface step," of a steel sample heated on its front flat and smooth surface, while the thickness (L) of the plate changes with a known step size (dL) on the backside. Finite element models simulating the FT process were developed to understand the effect of sub-surface steps on the thermal diffusion and compared with experiments for 12 varying step (dL/L) combinations. The width of the transition zone was measured using the Thermographic Signal Reconstruction (TSR) method. Results indicate that the transition zone can be defined simply as a function of its geometry. Experiments confirmed that the model predictions work well under the assumption that the steps are properly distinguishable from each other. Lastly, equations to estimate the detectability of a step were developed to be used in addition to the camera's detection limits. Overall, the approach used can be extended to anisotropic materials such as composites and bonded joints to enable efficient NDT of structural components.
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