ACFM stands for 'Alternating Current Field Measurement'.

Background

The ACFM technology was developed by TSC in the 1980's from the successful ACPD (further details here) contacting technique to provide a system for crack detection and sizing without the need for any electrical contact.

The crack sizing capability has resulted from the use of a uniform input field which allowed theoretical studies at the NDE Centre at University College London to predict crack depth from a knowledge of the surrounding a.c. electromagnetic fields.

The technique was initially developed to allow crack sizing underwater where the ACPD technique was hindered by the need for good electrical contact. However, the other advantages arising from non-contact and a uniform input current (ease of scanning, little adverse effect from material property changes or probe lift-off) meant that the technique was quickly applied to topside inspections as well, particularly on painted or coated welded structures.

Operational Principles

An ACFM sensor probe is placed on the surface to be inspected and an alternating current is induced into the surface. When no defects are present the alternating current produces a uniform magnetic field above the surface. Any defect present will perturb the current forcing it to flow around and underneath the defect; this causes the magnetic field to become non-uniform and sensors in the ACFM probe measure these field variations.

Two components of this magnetic field are measured - one provides information about the depth or aspect ratio of the defect(s), the other provides information on the positions of the ends of each defect. The two signals are used together to confirm the presence of a defect and, together with a sizing algorithm, measure its length and depth.