Abstract
Pulse or flash thermography is a method of nondestructive evaluation that finds subsurface flaws in materials by observing a heat pulse and subsequent cooldown using a thermal camera. A fundamental constraint of pulse thermography is lateral heat diffusion that tends to blur the shapes of defects. It can be difficult to interpret the thermal image sequence from a pulse thermography test. This paper presents a model-based inversion for pulse thermography that uses the known physics of heat conduction to as a basis for representing the recorded thermal image sequence. The technique provides a means to solve for the reflectivity distribution of defects across multiple layers, such as delaminations in a composite material. The layer reflectivity distributions provide a compact and concrete representation of the thermal image sequence. The technique gives excellent interpretability and resolution with minimal noise gain. Model-based inversion is demonstrated on several carbon fiber reinforced plastic (CFRP) specimens.
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Notes
Twice because half of the injected heat flows up and half flows down.
The first few rows of x correspond to the excitation pulse on the surface, for which the depth is zero, but scaling by zero would be obviously problematic. Some sort of scaling is necessary in order to be dimensionally compatible. Because there is plenty of data to evaluate the excitation source intensity the noise level is very low and a wide range of scaling factors would be adequate. In these tests we used an effective depth value corresponding to the time t3 of the 3rd usable frame, \(z=\sqrt {\pi \alpha _{z}t_{3}}\) and scaled the column by an additional factor of 50.
Do not confuse the spatial dimension x with the reflector amplitudes being solved for x
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This research was funded by NASA Early Stage Innovation under award NNX15AD75G.
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Holland, S., Schiefelbein, B. Model-based Inversion for Pulse Thermography. Exp Mech 59, 413–426 (2019). https://doi.org/10.1007/s11340-018-00463-2
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DOI: https://doi.org/10.1007/s11340-018-00463-2