Research On Key Technologies Of Infrared Thermography

Infrared Thermography has been found to be a very valuable nondestructive evaluation (NDE) due to its non-contactness, rapidity, capability of imaging large area. Research on the key technologies and quantitative infrared thermography is carried through in this dissertation. The key technologies include: infrared thermography experiment and pre-processing technology of sequence images, heat conduct model and finite element analysis of infrared thermography, pulsed phase thermography (PPT), and pixel-level image fusion for infrared thermography. On the quantitative infrared thermography, the research is carried through in time and frequency domain respectively. The main contents of the dissertation are as follows:The experiment system of infrared thermography is set up and experiment studies are carried through on heating stimulator, experiment parameter, experiment condition and result. To remove the influence of uneven heating, emissivity and status of the sample surface, according to the characteristics and needs of infrared thermography, k-neighbor gauss image averaging, defect center location, rotary tracking edge detecting, and improved estimation analysis image segmentation are achieved. This lay a foundation of quantitative infrared thermography. For the first time, an empirical mode decomposition (EMD) method, which is an adaptive decomposition method, is applied to image processing of infrared thermography. Through analysis of mode function of image EMD, the characterizations of noise and uneven heating are given. Using EMD method, the noise and uneven heating are reduced at the same time and the contrast of thermal image is improved. Good result shows this method is simple and effective to overcome noise and uneven heating in infrared thermography.Experiment and finite element analysis of quantitative evaluation of defect depth and size are carried through based on the heat conduction. Firstly, the time points of beginning and end of stimulation are found out by averaging the intensity of sequence images and the base origination time point is set up for evaluation of defect depth. The relation between defect depth and sequence images contrast changing with time is analyzed. The concepts of initialization separation time point is given. The method of achieving the time points according to temperature changing with time is realized and comparison was made with maximum contrast time point. The influence of defect size, external heat stimulation intensity and duration on acquiring defect depth was analyzed. The reliability of acquiring the defect depth according to the time points above is proved. The changing rule of defect size was found and best measurement time region was given. Based on the abnormity and heat conduction of defect, full width half maximum method is improved and the measurement precision is advanced.Pulsed Phase Thermography (PPT) is a new infrared nondestructive evaluation method appeared recently. It brings abroad attention due to its advantage of strongly ability of detecting deeper defects and anti-disturbing. In this paper, The problems of FFT(fast Fourier Transform) of PPT were studied. The key factors for choosing the sampling frequency and truncation were given. This paper also studies how to change the value of sampling frequency and window width through fitting extend and interpolation methods. And, to a certain extent, the result of measurement is improved. The result of fourier transform was improved practically by combining low-pass filters and subsection curve fitting. The theory of phase blind frequency with defect depth of the quantitative pulsed phase thermography is studied through finite element analysis. Finally, the influence and error analysis of thermal diffusion on phase, amplitude and contrast image are studied .No NDT is perfect and it is the developing orientation of NDT to combine different NDTs. In this paper, for reducing noise and exactly interpretation of thermography, the pixel-level image fusion of infrared light and visible light image for infrared thermography and its application are studied. Firstly in the pixel-level image fusion, different mode image registration has been done based on feature corners. The corners and curves relative to each corner are extracted using the CSS (Curvature Scale Space) method. A new shape context descriptor for each corner is given as the criterion to match the corners. According to the purpose and need of infrared thermography, fusion methods of contrast modulation fusion , multi-scale fusion methods of Laplacian pyramid, wavelet transform and EMD, transparent fusion are studied. The experimental and application results indicate image fusion for infrared thermography can facilitate defect location and interpretation of inspection results.