Thermographic cameras use infrared sensors that detect incident infrared radiation that can then be converted to temperature values. This results in a map of temperature values, also known as thermograms. Inspected parts are usually in thermal equilibrium and therefore require the use of a heat source to produce a thermal contrast between the defective region of interest and the background material. This is known as active thermography. Several options exist in delivering the required heat source including laser heating, light sources (halogen light, infrared light, etc.), flash, and induction heating (eddy current thermography). It is noteworthy that commercial systems exist for eddy current, pulsed and lock-in thermography.
In this case study, pulsed thermographic inspection was performed using a 1 kW photographic flash. This is mainly due to the versatility of the flash – it is portable and easy to use yet can heat up a relatively large area. The infrared camera used was an FLIR A6751 SC and the data was recorded at 125 Hz at full window (640 x 512 pixels) from just before the flash to a few seconds after the flash. In pulsed thermography, the infrared camera will observe the variation in surface temperature of the material as the thermal wave from the heat source propagates through the part. The presence of defects will alter the heat transfer local to the region and hence appear as a region with different temperature distribution, thus allowing sub-surface defects to be detected.
A sample containing realistic defects was obtained from a previous TWI collaborative research project X-Scan (x-scan.eu). The 6 mm thick butt-welded carbon steel plate contained various types of surface and sub-surface defects. The plate was coated with a layer of paint.