Detection of flaws in ferromagnetic samples based on low frequency eddy current imaging

Abstract

We report a nondestructive method to detect flaws in a ferromagnetic plate based on eddy current imaging. The inductive pickup in our previous Dahle-type probe is replaced by a thin orthogonal flux gate driven by a 1 MHz ac current. Samples are 2-mm-thick steel plates (SS400) having a 0.4-mm-wide EDM slot on the backside with various slot depths (0.4, 1.0, 1.2, and 1.6 mm). These samples were line scanned by the probe with the lift-off about 0.01 mm. The excitation field inducing the eddy current in the sample was 40 Hz–2 kHz. An interesting thing to note is the relationship between the slot depth and the excitation frequency at which peaks in the phase response locate, that is, the deeper the flaw position is, the lower the excitation frequency of peak locates. The resolution in depth profiling is evaluated to be better than 0.2 mm.