Employing Thermal Contrast Definitions to Improve Defect Detection Capability in Thermography Non-Destructive Testing

Investigating the integrity of structures and equipment such as airplanes, the pipelines, the oil platforms, the bridges and the pressure vessels is an essential task in the industry. Employing a reliable and accurate methods for investigating the health of the industries equipment’s is vital to increase the health and reduce the financial hazards. Nowadays, the use of thermographic inspection technique or the inspection using infrared waves as a useful and advanced tool among the various methods of condition monitoring and nondestructive evaluations is expanding. Infrared thermography is performed with the help of thermal information analysis using non-contact thermal imaging devices. The infrared thermography is used to detect hot spots, heat losses, leaks, insulation defects, etc., so that the nondestructive men and maintenance personnel can take appropriate actions to fix the problems. In this research, a steel plate with 20 flat bottom holes (FBH) with diameters of 2 to 10 mm and distances from the test piece surface (Defect depth) ranging from 0.5 to 2 mm was used as a test sample. A flash lamp and two projectors were used as heating sources and then the thermal image sequences were recorded. It was observed that only 11 defects can be detected in the thermal raw image. In order to improve capability of thermography test, 6 techniques of Thermal contrast including absolute thermal contrast, running contrast, normalized contrast (final value), normalized contrast (maximum value), standard contrast and differentiated absolute contrast (DAC) have been applied to the of thermal raw images. Thermal contrast based techniques found to be useful in improving defect detection by increasing contrast between the image of the defect and sound area and reducing effects of non-uniform heating. Normalized contrast (final value) had the best performance in terms of increasing the number of detected defects and was able to reveal 16 defects.