Dear Researchers,
Greetings,
I am computing the phase angle information for different layers of coating thickness. For this i am considering the thermal data obtained from the Pulsed thermography and Lock-in thermography. I have used the Fourier transform to compute the phase angle in order to correlate the phase angle with the coating thickness. Surprisingly, it is found that the magnitude of phase angle changes with respect to number of harmonic for both the pulsed and lock-in thermography. The phase angle increases with the coating thickness for all the harmonic in pulsed thermography but in case of lock-in thermography it first increases for the 2 number of harmonic and start decreases for all the other.
What could be the possible reason for it?
Regards,
Shrestha Ranjit.
Without knowing the actual experimental set-up, in terms of time and frequencies, the following is an initial and incomplete response. One must be clear about the reasoned and experimental differences between pulse thermography (PT) and lockedin thermography (LT). Those differences will deliver harmonic variances using Fourier transforms. Your results are not at great variance from those of other researchers. (Noting here, in my view, that thermal waves are predominantly anharmonic.) In other words, a researcher comparing the two techniques for the IR method would need to expect a difference in the results using Fourier transforms because the resolving, or resolved, frequencies are not captured in precisely the same range over precisely the same time frame. One could reduce the variances through "tweaking" the time (pulse duration and time residency) and frequencies, post IRT image capture, but this could distort the interpretation from the results if one expects a point by point comparison of the techniques. Pulsed Phase Thermography (PPT) resolves some, but not all, variances by generating longer time-resident thermal waves more nearly represented in LT, while maintaining the thermal stresses from PT for clarifying defects through thermal concentrations in a defect area. LT detection of thermal waves on a surface under inspection will show an initial increase and then decay, or decrease, as the thermal waves reach the surface then diffuse further below the surface. This is a reasonable result considering IR passive and active thermography are primarily surface and near surface detection technologies.
Possibly, the following PDF file link will help your analysis, if you have not already reviewed the article: http://www.mdpi.com/1424-8220/16/5/743/pdf, Experimental Evaluation of Pulsed Thermography,. Lock-in Thermography and Vibrothermography on Foreign Object Defect (FOD) in CFRP.
Dear Don Bryant Hargis,
Thank you very much for your response.
Regards,
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