To address the lack of adequate techniques suited for use underwater to characterize fatigue cracks in ferritic steel welds, a group of United Kingdom oil companies approached the University College London in the ‘80s to develop the alternating current field measurement (ACFM®) non-destructive testing technique.
ACFM is an electromagnetic technique that enables detecting surface-breaking cracks without removing coatings. It uses a mathematical model to predict the crack’s surface length and depth—key parameters in assessing the severity of the defect and its impact on integrity.
How it Works An ACFM probe locally induces a small, uniform current into the surface under test. The current flows perpendicular to the weld. A surface-breaking crack disturbs the current flow as it is unable to move across the non-conductive volume of the defect. Instead, the current flows under and around the defect ends.
Figure 1. Current generated by an ACFM probe disturbed by a crack.
The changes in the current flow also affect the magnetic field above the component, which can be measured by sensors in the nose of the probe (Figure 1) .
Figure 2. Typical sensor coils in nose of ACFM probe.
One of the sensors (called Bx) measures the magnetic field strength drop at the center of the defect (blue in Figure 1). It relates to the defect depth. The second sensor (called Bz) responds to the currents rotating around the defect ends (Figure 3) . As the current rotates in opposite directions at both defect ends, the Bz sensor produces a positive signal as it passes over one end and a negative signal at the other.
Figure 3. Current rotation around defect ends.
The ACFM software produces a third display (the butterfly plot) , resulting from plotting Bx and Bz sensor data against one another (Figure 4) . Time being absent from the butterfly plot, it is unaffected by scan speed variations.
Figure 4. Bx and Bz traces and butterfly plot.
A basic ACFM probe can inspect a strip 10–15mm (0.4–0.6 in) wide, centered on its sensors.
ACFM Pros and Cons ACFM’s main advantages are:
Test through coatings as thick as 5mm (0.2 in)
Size surface-breaking cracks as deep as 25mm (1 in)
Easy scan patterns to test welds
On-site calibration unnecessary
Like any other technique, ACFM has disadvantages:
Generally less sensitive to short/ shallow defects than ECT on smooth, clean surfaces
Geometry changes such as edges and corners can produce confusing signals
Sizing of defects other than linear fatigue cracks may be less accurate
Array ACFM Probes
ACFM sensors can be combined in multi-element array probes engineered to inspect wider areas in one pass. Array probe profiles can have different configurations (Figure 5) .
Figure 5. Array ACFM probe profile examples.
Array data can be displayed as trace plots or, more intuitively, as C-scans (Figure 6).
Figure 6. ACFM C-scan display.
Typical Applications ACFM is used to inspect welded connections for surface-breaking defects in various applications, above and underwater. Light and portable instruments enable deployment by rope access specialists (Figure 7) .
Figure 7. Rope-access and tank inspections with ACFM.
Array ACFM probes can be part of remotely deployed solutions, such as scanners and crawlers, used where direct access is difficult, unsafe, or costly.
Figure 8. Remote ACFM deployment examples.
Non-Ferromagnetic Materials ACFM is also used on non-ferromagnetic materials such as titanium and Inconel® with great success where deployment conditions are difficult or where there may be deeply penetrating cracks in thick wall components.
Acceptance and Standards ACFM is widely accepted by major organizations in the oil & gas, petrochemical, and marine industries, such as ABS, Lloyds Register, DNY-GL, and BV. Standards featuring ACFM are also published by ASTM, ASME, and COFREND. Author: Michael Smith