Alternating Current Field Measurement (ACFM) SNT-TC-1A-Training as per Company Written Practice
 
The Alternating Current Field Measurement (ACFM) technique is a non-contact electromagnetic technique capable of both detecting and sizing (length and depth) defects in metals. The basis of the technique is that an alternating current flows in a thin skin near the surface of any conductor. By introducing a remote uniform current into an area of the component under test, when there are no defects present, the electrical current will be undisturbed. If a crack is present the current flows around the ends and down the faces of the crack.
 
The current flowing in the surface has an associated magnetic field above the surface and this magnetic field will also be disturbed if the current is disturbed by a defect. The ACFM method involves the measurement of this magnetic field. The magnitude of any disturbances in the field can then be related back to the size of defect causing them by the use of mathematical models. In its simplest form, ACFM involves the use of a single hand-held probe which contains the field induction and the field measurement sensors. This is then connected to an ACFM instrument which is, in turn, controlled by a PC which provides data displays and recording.

As the technique requires no electrical contact with the surface, it can be used to inspect through paint and coatings. The technique is widely used for weld and thread inspection and for subsea inspection of offshore platforms. It can also be used on both magnetic and non-magnetic components
and at elevated temperatures in excess of 600°C.
 
Array probes containing large numbers of sensors can be deployed either manually or robotically, typically in situations where larger areas need to be inspected or where ‘pick and place’ deployment is preferable to probe scanning. ACFM array systems have been incorporated into automated inspection systems to give simple PASS/FAIL reporting, thus avoiding the need for skilled operators.

The ACFM method should only be applied to surface-breaking defects when used on carbon steels but is suitable for subsurface flaws in some non-magnetic materials