Alternating Current Field Measurement (ACFM) technique uses electromagnetic inspection with an alternating current to find indications within metals. The ACFM probe measures the electromagnetic field by inducing electric currents on the surface. An indication or crack is detected when these electric currents are disturbed.
ACFM can locate surface-breaking cracks and give information about the length and depth of the indication. With ACFM, the interpretation of the results can be done directly and the testing can be done through coating, paint and rust without calibration.
ACFM has multiple benefits compared to Magnetic Particle Testing (MPI) and Eddy Current Testing (ET). ACFM is seen as an advanced inspection technique similar to Eddy Current Testing, but with significantly lower amount of spurious signals. Just like MPI, ACFM can test welds in underwater conditions, but ACFM has a higher probability of detection (PoD).
Crack depth gauging works according to the Alternating Current Potential Probe Procedure. It can be used for the exact determination of the extension of surface cracks in iron or steel parts. The course of the crack to be measured along the surface must first be determined before by other NDT methods such as magnetic testing (MT) or penetrant testing (PT) and known at the start of measurement.