Effect of non-magnetic inclusions in magnetic specimens on defect detection sensitivity using active infrared thermography

Abstract

We study the effect of non-magnetic inclusions in the defect regions on defect detection sensitivity using alternating magnetic field assisted infrared thermography. The effect of inclusions on the resulting surface temperature profiles around the defect regions are monitored using infrared thermography under the magnetic excitation. Four mild steel specimens with simulated rectangular slots of depths 8.0, 3.3, 3.0 and 5.0mm, filled with three different non-magnetic inclusions, viz. clay, grease and wax are studied. Under an alternating magnetic field excitation, the induced eddy current in the mild steel specimens produces Joule’s heating on the surfaces, which is monitored in a non-contact way. As the non-magnetic inclusions act as a thermal insulator to the alternating magnetization induced heating, a clear thermal contrast at the defect boundaries is seen. The defect regions are clearly discernible from the thermal images and defect widths are estimated from the horizontal temperature profiles. It is observed that the temperature difference between the defect and defect-free regions initially decreases with time up to a certain time (called inversion time) and beyond that the temperature difference increases with time for clay and grease filled defects. The peak temperature difference between the defect and defect-free regions decreases with defect depth due to the magnetic flux leakage from the defect regions. The normalized temperature decay rate, determined from the blind sides of the specimens, is found to decrease with the defect depth. The sensitivity of the depth estimation procedure is higher for inclusions with lower thermal diffusivity values. This study shows the efficacy of low frequency alternating magnetic field induced heating procedure for the detection of defects filled with non-magnetic inclusions in magnetic specimens using active infrared thermography.