Message Board Thread - "Lock-in Thermography"

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Lock-in Thermography fifa_alive 1/31/2006
I'm quite new and beginning to start studying in Thermography. I have read many articles about Lockin Thermography that its Amplitude and Phase could be determined in steady-state mode. I try to understand the temperature behavior by simulation program to see the temperature response with the heat flux excitation at the surface = q0*[1+cos(2*pi*fmod*t)]. The temperature response will not be only sinusoidal, but also the increasing temperature due to the DC-term excitation. This takes a very long time to get to steady state and it is not practical in experiment. Does anyone have experience about this, please help giving me some advice. Thanks in advance.
 
Re:Lock-in Thermography jeff_brown 2/6/2006
I have had a little experience using lock-in thermography for detecting defects in FRP composites bonded to concrete. You are correct in that the time required to reach steady state makes the method rather impractical. One approach that we did have some success with was to apply a single cycle of the sinusoidal heat flux (from zero to peak and back to zero). As long as you are collecting digital thermograms containing temperature values, you can generate a temperature vs. time response for each pixel. By applying a standard sinusoidal curve fit using least squares regression you can reduce this temperature vs. time response into a phase and amplitude value. The resulting phase image is good and tends to be independent of nonuniform heating. Let me know if this makes sense or not. I may be able to direct you to some relevant literature.
 
Re:Lock-in Thermography fifa_alive 2/8/2006
Thanks a lot for your comment. I have a bit confuse about the least square regression of sinusoidal function that you mentioin to because I found that the response is not only sinusoidal, but also the increment of temperature. In this case, it would not fit well in the sinusoidal function, would it ? I would appreciate if you can give me a hint for clarification. Also, if possible, please give me a guide to the literature to which you mention. (see picture for the response i simulated from the simulation program)
 
Re:Lock-in Thermography MoviMED 6/5/2008
Hi!
Although these messages are a bit dated, I thought I would add my comments here as well.
You actually don't need to reach steady state at all when testing composite materials. We have a turnkey Lock-in Thermography system available that is compatible with FLIR A and S series cameras. (A20M, A40M, A320G, SC4000 and SC6000).
We have inspected composites of all kinds, CFRP, FRP, honeycomb etc. Depending on depth of defect, you are looking at measurement times between 10 to 120 seconds. Our system can also perform an on-line Lock-in measurement where you can see the images while its being calculated in "real-time".
Attached is a sample image of a carbon fiber sample with flat bottom holes and a large milled out area.
More info at:
http://www.movimed.com

 


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