Quantitative subsurface defect evaluation by pulsed phase thermography: depth retrieval with the phase
|Authors:||Ibarra Castanedo, Clemente|
|Abstract:||Pulsed Phase Thermography (PPT) is a NonDestructive Testing and Evaluation (NDT& E) technique based on the Fourier Transform that can be thought as being the link between Pulsed Thermography, for which data acquisition is fast and simple; and Lock-In thermography, for which depth retrieval is straightforward. A new depth inversion technique using the phase obtained by PPT is proposed. The technique relies on the thermal diffusion length equation, i.e. μ=(α /π·f)½, in a similar manner as in Lock-In Thermography. The inversion problem reduces to the estimation of the blind frequency, i.e. the limiting frequency at which a defect at a particular depth presents enough phase contrast to be detected on the frequency spectra. However, an additional problem arises in PPT when trying to adequately establish the temporal parameters that will produce the desired frequency response. The decaying thermal profiles such as the ones serving as input in PPT, are non-periodic, non-band-limited functions for which, adequate sampling Δt, and truncation w(t), parameters should be selected during the signal discretization process. These parameters are both function of the depth of the defect and of the thermal properties of the specimen/defect system. A four-step methodology based on the Time-Frequency Duality of the discrete Fourier Transform is proposed to interactively determine Δt and w(t). Hence, provided that thermal data used to feed the PPT algorithm is correctly sampled and truncated, the inversion solution using the phase takes the form: z=C 1 μ, for which typical experimental C 1 values are between 1.5 and 2. Although determination of fb is not possible when working with badly sampled data, phase profiles still present a distinctive behavior that can be used for depth retrieval purposes. An apparent blind frequency f’b , can be defined as the blind frequency at a given phase threshold φd , and be used in combination with the phase delay definition for a thermal wave: φ=z /μ, and the normalized diameter, Dn=D/z, to derive an alternative expression. Depth extraction in this case requires an additional step to recover the size of the defect.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||11 April 2018|
|Collection:||Thèses et mémoires|
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