Description

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 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.