Innovative NDT technique for detection of surface cracks based on ferrofluids excited with DC and AC magnetic fields

An innovative NDT technique is proposed for surface inspection of materials not necessarily magnetic or conductive, based on local magnetic field variations due to ferrofluid deposited in the cracks. The feasibility of the technique is assessed preliminarily, based on signal detectability without applied external magnetic field, and under applied DC and AC fields. The signals are quantified analytically, experimentally and numerically. In DC, detection is based on local magnetic flux density variations. In AC, detection is based on the existing phase lag between the field close to the crack and the applied field. This approach has inherent advantages: the phase lag, as opposed to the magnetic flux density, is independent of the quantity of ferrofluid in the crack and the magnitude of the applied field. The model agrees well with the tests, showing that the signal increases with the applied field strength, up to the saturation magnetization of the ferrofluid, and decreases with the distance to the crack longitudinal axis, and thus it can provide useful estimations of the signal. The proposed technique, requiring application of external fields to magnetize the ferrofluid to enhance the signal, seems promising: the model suggests that signals associated to cracks significantly smaller than the minimum detectable surface cracks for comparable classical NDT techniques are easily detectable with commercial magnetometers.