Detection of Subsurface Defects using Active Infrared Thermography

Active infrared thermography is a widely used technique in the field of nondestructive testing. The subsurface defects produce non-uniform heat dissipation and this phenomenon is captured by an infrared camera. The phase image gives a prominent description of the defects which are not visible in the captured image sequence. The effect of non-uniform heating and the variation in surface emissivity results in noisy data acquisition. An infrared camera of mid-wave infrared range was used for the study. A frequency modulated sinusoidal signal was used for the heating of the mild steel specimen by two halogen lamps. The source control is attained using a signal generator with frequency of modulation varied from 0.01 Hz – 0.1 Hz for the duration of 100 seconds. The images were acquired at a frame rate of 20 Hz. The specimen contains six numbers of circular flat bottom holes, at different depths from the sample top surface. The temporal profile of every pixel location in the infrared image sequence carries a similar pattern of heat dissipation from the surface and requires temperature compensation. The temperature compensated data is used for the Fourier analysis and pulse compression for better visualization of the defects. The correlation image and the phase of correlation image shows promising results. Experimental setup and comparative results are discussed in detail in this paper. The results from the image enhancement method are qualitatively efficient for the detection of subsurface defects.