Thermographic crack detection in ferritic steel components using inductive heating

Active dynamic thermography using inductive heating was used for investigations on different steel components from automotive and steel industry. The defects were perpendicular and slanted surface cracks. For the ferritic steel investigated in the study, the skin depth was usually smaller than the crack depth. Cracks with a depth down to 200 ?m were detected. A theoretical model for the temperature profile around a crack resulting from a given induction field was set up and compared with experimental results. Both in model and in experiment, an almost linear dependence of the defect contrast on defect depth was found up to a depth of about 0.8 mm. The dependence of the contrast as a function of the crack orientation vs. the induction field was studied. In comparison to ultrasonically excited thermography, induction heating was applicable to a larger variety of specimen shapes and showed more crack indications.