Quantitative detection of subsurface defects by pulse-heating infrared thermography

The principles and methods are introduced to quantitatively detect the subsurface defect depth and size by pulse-heating infrared thermography. The subsurface defects in different metal and insulating materials were quantitatively measured by using our home-made pulse-heating infrared thermography non-destructive testing system. From our experimental results, it is shown that accurate information can be obtained during a shortly time after the surface of a target is heated by a thermal pulse. Especially for metals a fast recording of the images is required. As for mild steel, our measurements of defect depth and size were taken within one second after the pulse heating. When the defect diameter is larger than 6mm, the depth and size measurements are in good agreement with the practical values. The depth measuring errors are less than 9.1% for the range of 1.1 to 1.3mm. The sizes can be determined with an accuracy of 10% with 1.1mm defect depth. Because heat penetration in insulating material is much slower than in metal, the lateral thermal diffusion will affect the experimental results seriously, especially for a small, deep defect. For nylon with defects of 15mm and 8mm at a depth of 1.3mm, the depth measuring errors are 9.1 % and 38.5% respectively. The sizes of defects with diameters of 8mm and 4mm at the depth of 1.1mm can be measured with 8% and 25% accuracies respectively.