Diffusion-compensated correlation analysis of frequency-modulated thermal signal for quantitative infrared thermography

Abstract

Application of a broadband modulated excitation signal and its cross-correlation with the corresponding thermal response, so-called thermal wave radar or pulse compression thermography, is a well-known technique in active infrared thermography. The technique benefits from both the broadband spectrum and the pulse compression efficiency of excitation signal, providing lag and phase quantities which can be used for characterization of defects. However, the significant distortion of the thermal response due to heat diffusion makes the cross-correlation analysis inefficient, particularly for the detection of deep defects or the inspection of a material with low thermal diffusivity. To tackle this issue, diffusion-compensated correlation analysis (DCCA) of thermal signal is proposed for quantitative infrared thermography, using a frequency-modulated sweep signal as excitation waveform. DCCA is based on the correlation of the measured thermal response with a library of template thermal responses calculated using the 1D analytical solution of the heat diffusion problem. DCCA can then more reliably resolve the thermal response in the presence of measurement noise, and depending on the definition of the library, directly map it to the corresponding depth or diffusivity. Two inspection modes are studied: (i) transmission of heat from defects which act as sub-surface heat sources (e.g. vibrothermography), and (ii) reflection of an externally applied heat flux from sub-surface defects (e.g. optical infrared thermography in reflection mode). Outperformance of DCCA to thermal wave radar is analytically substantiated and verified by finite element simulation on a carbon fibre reinforced polymer plate. Furthermore, the technique is experimentally studied for thermographic inspection of a carbon fiber reinforced polymer (CFRP) coupon including artificial defects, and the advantages as well as limitations of the technique are discussed.