Low frequency induction thermography for the characterization of hidden cracks in ferromagnetic steel components

Induction thermography has been extended to the low frequency range of 300 to 2600 Hz. The electromagnetic skin depth is thereby increased in a way that in ferromagnetic steel hidden defects with 1.25 mm coverage could be detected by interaction of the induction currents with the crack. Numerical simulations of the internal current and temperature distributions were performed. Experimentally, artificial and natural cracks were studied. The influence of the electromagnetic excitation frequency was investigated. In plate-like geometries with hidden cracks starting at the backside, an inversion of the defect contrast with frequency was observed and explained by simulation.