Alternating current field measurement (ACFM)

This series of articles has been put together by the Practitioner Committee to describe the principles of different NDT methods and techniques as an introduction to practitioners...

Alternating current field measurement (ACFM) is an electromagnetic technique used for the detection of surface-breaking defects in metals without the need for the removal of any non-conductive protective barrier coatings ^[1]. It has been shown to offer comparative sensitivity to eddy current testing with fewer false positives when deployed on carbon steel welds, with the added advantage of being able to quickly and easily estimate both the length and depth of any defects detected.

• Ferrous and non-ferrous welds (subsea and topside);
• Threads;
• Axles;
• Storage tanks; and
• High-temperature inspections.

A cored induction coil is used to generate a locally uniform current in the surface of the material under test and two sensor coils are used to measure the magnetic flux associated with this current flow. One coil, referred to as the Bx coil, is used to provide information about the depth and aspect ratio of the defect. It measures the flux density parallel with the component surface and the received signal is a function of the current density at the surface. The second coil, or Bz coil, provides information on the length of any defects by measuring the flux density perpendicular to the test surface. The Bz signal is proportional to the curvature of the electric field in the surface ^[2,3,6].

In the presence of no defects, the current flow under the sensor coils is uniform, producing a consistent Bx signal and no Bz signal. Surface-breaking defects, such as fatigue cracks, alter the current flow at the surface. In general, the current will flow around the defect tips, resulting in curvature of the electric field. This produces a bipolar Bz response. Current density is reduced through the defect centre as the current flows under the defect. This is observed as a reduction in the Bx signal.

It is the difference between the background Bx signal and the minimum Bx signal that contains information about the depth of the defect and the physical distance between the two Bz signals that is used to estimate the length. This data is processed by an algorithm to calculate the true defect size. 

The Bx and Bz signals are also plotted one against the other to produce a third signal, called the butterfly plot. Discontinuities produce characteristic loops, which are used in conjunction with the Bx and Bz signals to confirm the presence of, further analyse and classify any suspected defects. 

Array probes containing large numbers of sensors can be used where larger areas need to be inspected with fewer individual scans. They may be implemented either manually or robotically. Array probes are routinely deployed via remotely operated vehicles (ROVs) for the inspection of subsea assets. Array probes can be encoded so that positional data is also collected. Twin-field arrays offer advantages for the inspection of non-magnetic materials^[7]. 

The mathematical model used within the sizing algorithm is based on an ideal semi-elliptical fatigue crack. As such, the morphology of the crack can have an effect on the accuracy of the result obtained. Simple fatigue cracks are more accurately assessed than networks of branched cracks, such as stress corrosion cracking and hydrogen-induced cracking.

The currents induced in the ACFM technique are confined to a thin layer at the surface, so that only defects lying within this layer can perturb the current and thus be detected. The skin depth is so low that ACFM is not a reliable method for the detection of subsurface defects.

ACFM generally has no problem working through rust, surface oxides or other low-conductivity layers. ACFM also works through thin uniform metal coatings such as zinc, even if the crack does not penetrate the coating. However, some problems can occur with non-uniform metal coatings, such as manually applied flame-sprayed aluminium. Such coatings produce strong background variations in the ACFM signal due to differences in coating thickness. These can easily mask signals from any defects that are present^[6]. 

The ACFM technique is recognised by many technical authorities, including the American Society of Mechanical Engineers (ASME), ASTM International, Bureau Veritas, the French Confederation for Non-Destructive Testing (COFREND), the International Organization for Standardization (ISO), Lloyd’s Register and the National Association of Corrosion Engineers International (NACE)^[8].

Certification available

• PCN Certification: PCN/Gen Appendix C3.1B Iss 4 Rev C details the specific requirements for the certification of personnel in ACFM testing of ferritic welds and is available on the BINDT website^[7].
• CSWIP Certification: Details of Level 1 and Level 2 certification are available through the TWI website^[4].
• Employer-based: Where appropriate and accepted by the customer and third-parties, employer-based certification based on the recommendations of documents including SNT-TC-1A is available.

References