Abstract
At present, lock-in thermography is widely used non-destructive method for defects detection. The informative images in lock-in thermography (e.g. phasegram) are obtained after temperature signal filtering of the raw data. The postprocessing in lock-in thermography is more complex than in other active thermography methods and very important for defects detectability. In some cases the standard postprocessing can significantly decrease the quality of the temperature signal filtering, respectively decreasing defects detectability, although the parameters of lock-in thermography measurement are selected correctly. The aim of the paper is to study the quality of temperature signal filtering in lock-in thermography depending on used offline postprocessing. For this reason, a methodology based on modelling and measurements of temperature signals from infrared thermography for determination of cases, in which the temperature signal filtering quality is decreased significantly, is used and corresponded methods for correction are proposed. The results from modelling and from real lock-in thermography measurements shows that by using of the proposed methods can be avoided decreasing of temperature signal filtering quality due to improper postprocessing. In addition, the proposed methods allow same defect detectability at lower energy, induced in tested sample, which is very useful for materials that are not sufficiently heat-resistant.
Highlights
A strong interest exists towards the use of active thermography methods in non-destructive material and structural analyses [1]-[3]
The heighten demand especially of lock-in thermography is associated with the both price reduction of the infrared cameras and computing power increase of the used computer systems. This involves complicated postprocessing of thermograms resulting in more accurate quantitative evaluations
If saturation occurs at steps 2, 3 or 4, the corresponding step can be repeated at different correlation function initial phase values until a saturation disappears
Summary
A strong interest exists towards the use of active thermography methods in non-destructive material and structural analyses [1]-[3]. The heighten demand especially of lock-in thermography is associated with the both price reduction of the infrared cameras and computing power increase of the used computer systems. This involves complicated postprocessing of thermograms resulting in more accurate quantitative evaluations. Complex lock-in analyses have been provided by two alternative low pass filtering methods in the time domain (averaging) and in the frequency domain (harmonics suppression)
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