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Abstract
Magnetic materials were investigated by induction thermography at induction frequencies of 80–300 kHz. The influence of an external magnetic field on the thermal crack contrast was studied. With proper orientation of the external field, the contrast increased by a factor more than three. Numerical simulations of the static and high-frequency magnetic fields and the temperature distribution were performed. Using a stationary induction field and a modulated external field in the frequency range of 0.28–36.5 Hz, lock-in thermography was realised, which shows cracks with good contrast. The technique was applied to crack detection in ferritic steel profiles.