Cooling rate VS temperature to establish thermographic prediction model in surface cracks in steel

Abstract

ABSTRACT:The inspection of steel welds is an important task to ensure the integrity of structures and machines. The inspection process presents two main objectives: ensure that welds meet the geometrical specifications established in the international standards, and analyze the existence of flaws. The most critical flaws to be detected during welding inspection are cracks, because their propagation under stress conditions can cause the collapse of the welded structures or the failure of the machines with welded elements. The technological requirements in nondestructive testing (NDT) for the inspection of surface cracks should not be as complex as the requirements for detection of internal cracks. However, the technique used must be efficient enough to ensure the full detection of surface defects and imperfections, including those of difficult visual detection such as little surface cracks or internal cracks open to surface. Active thermography presents an enormous detection potential for surface cracks in welding: allowing the depth estimation of the crack from the processing of the thermographic data. For this aim, the thermographic test can be designed either to evaluate the absolute temperature after heating (as a thermal stimulation of the material towards the increase in contrast between different zones of the crack) or to evaluate the cooling rate during the cooling posterior to this heating. In this contribution, a comparison between the two methods mentioned for crack evaluation is raised. The results of temperature and cooling rate for the same crack are respectively correlated with the depth data of the crack obtained from a macro-photogrammetric procedure.