Alternating Current - Field Measurement  

AC-FMAlternating Current - Field Measurement – is an electrical-magnetic NDT method which is a development of the AC-PD method. AC-PD measures a voltage across a defect and compares it to a calibration measurement.

Initial AC-FM systems were developed to allow sizing of defects without the need for a calibration measurement. This was achieved by taking more than one potential difference measurement and in effect measuring the electric field in the component. Hence the name change to Alternating Current – Field Measurement.

Further developments have resulted in the AC-FM systems available today. The current is now induced in the component rather than through direct contact and the magnetic field above the component is measured rather than measuring the electric field on the surface. These advances have resulted in a non-contacting technique which can measure crack size through paint and other coatings.
The crack size often is predicted by comparing the magnetic field measurements with the field predicted by mathematical models.

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