RECENT ADVANCES IN ACFM TECHNOLOGY FOR SURFACE

CRACK INSPECTION

Michael C. Smith1, Duncan Murray2

ASTRACT

Alternating Current Field Measurement (ACFM) is an electromagnetic inspection technique

that can detect and provide both length and depth information for surface breaking fatigue

cracks, usually without the need for coating removal. In use for over 30 years, primarily in

the offshore oil and gas sector, ACFM is approved by certification bodies including ABS,

ASTM, Lloyds and ASME. ACFM is also widely used for topside applications, with

inspection of painted or coated welded structures being the most common area. Applications

include the inspection of process plant vessels and pipework, mining equipment, road

bridges, theme park rides, airport terminals and offshore structures above the splash zone.

This article describes the principles behind ACFM, including evaluating the advantages and

limitations compared with conventional surface testing methods. A new generation of

instrumentation has recently been developed to enhance and enable topside inspections and

these are discussed, including how these are then used in a variety of case studies of real-

world inspections.

In conclusion, the ability of ACFM to produce higher quality surface inspections, compared

to conventional techniques, and the potential of short- and long-term cost savings are

presented.

INTRODUCTION

Traditionally, surface crack detection has been conducted using magnetic particle testing

(MT) on ferrous components and liquid penetrant testing (PT) on non-ferrous components.

These techniques require the removal of paint and other coatings and extensive cleaning prior

to inspection. Any defects found can only be sized for surface length, whereas the depth of a

defect is the key parameter in determining the integrity of the structure and critical when

deciding on the appropriate repair strategy. Conventional eddy current testing (ET) can be

used through thin paint coatings to detect and estimate a defect’s depth, however, most

systems create a significant increase in false calls when used on ferrous welds due to the

complex electromagnetic variations around these welds and are not able to accurately

measure defect penetration depths greater than about 5mm (0.2”) due to the sensitivity in

liftoff changes and the need for calibration on slots.

Alternating Current Field Measurement (ACFM) is an electromagnetic inspection technique

which uses certain eddy current principles but formulates them in a way which provides a

1 PhD, CEng, MBA, MInstNDT, CSWIP ACFM L3 – Eddyfi Technologies - TSC

2 MBA, ACFM PCN L2, 3.4u – Eddyfi Technologies - TSC

particular set of features and capabilities useful for inspecting welds and other metal

components for surface breaking linear defects, such as in-service fatigue cracks.

The technique was developed in the 1980s as a result of a research project in University

College London (UCL) supported by a group of UK oil companies who were keen to adopt a

method for reliable detection and depth sizing of subsea fatigue cracks on welded tubular

intersections of jackets and other offshore structures.

The performance of the system was evaluated for offshore subsea inspection alongside MT

and conventional ET as part of the rigorous ICON trials[1] and found to offer a comparable

POD with lower false calls.

ACFM has been used in a wide variety of industries which use the technique’s ability to

detect and size surface breaking fatigue cracks through thick paint and coatings and depth

size defects that penetrate up to 25mm (1 in.) into the material.

ACFM is approved by certification bodies such as ABS[2], ASTM[3], ASME[4] and Lloyds

Register[5] and has been included in training schemes administered by ABENDI, ASNT,

CSWIP and PCN.

BASICS OF ACFM THEORY

The ACFM technique induces a locally uniform current into the component under test and

measures the density of the magnetic flux over the component surface. The electric field is

typically induced at a frequency of 5 kHz which, on carbon steels, results in the currents

being confined to a thin skin in the material surface. Surface-breaking defects perturb the

induced current, and therefore also affect the magnetic flux density. ACFM measures two

components of the magnetic flux density: one offers information about the position of defect

ends (ergo the defect’s length). The second component provides the aspect ratio (and hence

depth) of the defect. The measurements combine to confirm defect presence and, with a

sizing algorithm developed from theoretical models, establish surface length and depth.

The required locally uniform electric field is induced using one or more horizontal axis

solenoids. This contrasts with conventional eddy current probes which normally use a vertical

orientation for the excitation coil. The alternating magnetic field produced by the solenoid

couples into the surface of the metal and induces a current to flow on the surface of the metal

at right angles to the magnetic field and in the plane of the metal surface. These currents flow

in closed loops, like eddy currents. However, unlike conventional eddy current inspection,

where the eddy currents flow in small circles, in ACFMT they flow in two large circular

patterns of opposite sense, and immediately below the inducer the current lines are essentially

parallel, linear and of constant density (see Figure 1).

AC Current induced in metal surface

Magnetic field

Area of approximately uniform field

Figure 1. Uniform field under the sensor area

The primary direction of induced currents is designated as the Y axis, while the direction of

the associated magnetic field in and immediately over the surface is designated as the X axis,

as shown in Figure 2. The Z axis is the direction perpendicular to the surface.

Magnetic Field (B)

Electric Field

Figure 2. ACFM coordinate system

When the component under test is devoid of defects, the current flows along the Y axis and

the magnetic field along the X axis. When testing a weld, the X direction is parallel to the

weld line and the Y direction is perpendicular to the weld line. The magnetic field’s Y and Z

components (designated By and Bz) are zero, while the X component (Bx) is proportional to

the magnitude of the electric current.

When a linear surface-breaking defect is present in the component under test orientated along

the X axis, cutting across the electric current lines, the current is forced to flow underneath

and around the defect’s ends.

Figure 3. Disturbed electric field around a linear defect and effect on Bx strength

As it flows under the defect, some of the current is forced away from the surface. This

reduces the strength of the magnetic field in the middle of the defect (blue in Figure 3). Some

of the current flows around the ends of the defect, strengthening the magnetic field at the

ends (red in Figure 3). As it does, a partly circular flow appears. This rotation around the ends

of the crack produces a measurable, non-zero Bz.

Figure 4. Rotation of current around ends of defect and effect on Bz

ACFM software produces a third trace, referred to as a butterfly plot. This plot does not

contain phase information for a single sensor, like a Lissajous display in conventional eddy

current testing, instead it is composed of the Bx and Bz sensor data, plotted against each

time

Bz

Bz curve

time

Bx

Bx curve

other. The Bx amplitude is plotted on the vertical axis and the Bz amplitude is plotted on the

horizontal axis. See Figure 5.

Figure 5. The butterfly plot from a defect produced from the Bx and Bz traces

When a defect indication is produced by the Bx and Bz sensors, the trace on the butterfly plot

produces a downward-going loop. The direction of this loop (clockwise or counterclockwise)

depends on the direction of travel of the probe.

The resulting display removes the time element from the scan, removing scan speed variation

effects. It also aids the operator in distinguishing a defect from other features which may

produce signals in one or other of the separate Bx and Bz traces.

A simple ACFM probe contains the field inducer, amplification electronics and a single pair

of Bx and Bz sensors in the probe nose (Figure 6).

Figure 6. Sensor orientation in pencil probe nose

Bz Sensor

Bx Sensor

These basic probes can inspect a narrow strip 10 and 15mm [0.4 – 0.6 in.] wide, depending

on the probe type, centered on the probe sensors. Multi-element array probes can be created

which extend the sensitive area by adding additional Bx and Bz sensors in adjacent rows.

These can be shaped for a particular geometry or made to be compliant to adapt to a surface

profile.

Figure 7. Examples of a compliant profile array probe and a custom shaped array

ACFM sizes surface breaking defects in a fundamentally different way to conventional eddy

current testing. Instead of creating an on-site calibration based on artificial flaws in a test

piece, ACFM sizing is based on a mathematical model[7] of how the induced field interacts

with the flaws. This model covers a wide range of defect lengths, depths and aspect ratios

which cannot be replicated by the few discrete flaws usually used to calibrate a conventional

ET setup. Each probe is set up and calibrated to the model at the factory and this calibration

data is stored in the ACFM software or, on more recent probes, in the probe electronics itself.

This model is contained in the ACFM software allowing the operator to simply pick discrete

points on a defect indication to produce an instant sizing for length and depth without the

need for onsite calibration.

ADVANTAGES AND DISADVANTAGES

ACFM uses a uniform input field primarily to allow comparison of signal intensities with

theoretical predictions. However, the uniform field provides other advantages, as well as

some disadvantages, compared with conventional eddy currents.

The main advantages include the following:

1. The ability to test through coatings up to 5mm (0.2 in.) thick.

2. The ability to obtain depth information on cracks up to 25mm [1 in.] deep.

3. Easy testing at material boundaries, such as welds, since the scan path is parallel to

the boundary.

4. No requirement for onsite calibration.

The main disadvantages include the following:

1. Lower sensitivity to smaller discontinuities. This reduction in sensitivity is of little

consequence on welded or rough surfaces where sensitivity would be reduced

anyway. On smooth, clean surfaces however, ACFMT is generally less sensitive to

short or shallow discontinuities than conventional eddy currents.

2. Geometry changes such as plate edges and corners can produce signals that may

confuse the technician, even though these signals do not have the same form as a

signal from a discontinuity.

RECENT ADVANCES IN ACFM INSTRUMENTATION CAPABILITY

Until recently, ACFM equipment has required a separate PC or rugged laptop to work with an

instrument and probe. This setup is useful for some applications where there is advantage for

the probe operator, instrument and computer to be separate, such as working with divers

subsea, but for many topside applications this setup could be cumbersome and restrictive. A

new generation of ACFM instruments is now available which feature integrated screens and

processing power (Figure ) to provide much greater portability and ease of use.

Figure 8. Examples of new generation ACFM topside instrumentation: Amigo2 (left) and

PACE (right)

The new generation instruments feature an increase in data resolution from 12 bits to 16 bits,

giving a much finer resolution in digitising the input signal level. This increase in resolution

is most evident when inspecting for shallow defects or inspecting through thick coatings.

Figure 9. Signal of a 10mm x 1mm (0.4 in. x 0.04 in.)defect through 5mm (0.2 in.) coating using Amigo1 - 12bits (Left) and Amigo2 - 16bits (Right)

Figure shows the improvement in signal quality and capability of the 16 bit system in

resolving reliably a defect through 5mm (0.2 in.) coating.

Older generation ACFM instruments have been limited to a single analog input where the

filtering and phase processing were conducted in analog electronics before being digitised.

The resultant of this is that all sensor coils in a probe must be multiplexed onto this one input

resulting in slow probe movement particularly with array probes where many coils need to be

multiplexed. Increasing the number of input channels reduces the amount of multiplexing that

must be done and increases speed and/or linear data resolution.

The new Amigo2 instrument, for example, features two digital input channels, where Bx and

Bz coils can be sampled simultaneously, and can support up to 8 digital inputs where high

speed array applications are required.

TYPICAL APPLICATIONS

ACFM is used most often for inspection of carbon steel welds in highly stressed and critical

areas. These are often found in challenging access areas and the ultra-portable instruments

enable rope access technicians to perform inspections removing the need for expensive

scaffolding. Figure shows an inspection performed on a painted and corroded weld on an

offshore structure by a rope access technician. The rapid inspection could be performed

without any surface preparation and enabled not only detection but also the sizing of any

flaws found for length and depth.

Figure 10. Use of ACFM for offshore weld inspection

Similar critical welds can be found in the rail industry[7]. An example is shown in Figure

where ACFM is used to inspect the welds on a rail tankcar that join the tank to the car frame.

The company has benefitted from greatly reduced overall inspection times and more reliable

detection of in-service weld defects than using their conventional methods. There is no need

to remove the paint around the weld and all the data from the inspection is recorded and

stored for audit purposes.

Figure 11. Use of ACFM on rail tankcar welds with WFMT confirmation of an ACFM

indication

ACFM is also used in the mining industry on large gear teeth, inspecting operating

temperature welds in process plant and for remote inspection of stainless steel welds in

nuclear storage pool liners[8].

Multi-element array probes cover a wider inspection area than standard probes and this,

combined with the relative tolerance to liftoff, enables ACFM to be used with remote

deployment solutions such as scanners and crawlers where access by humans directly is

difficult, unsafe or costly. These applications include subsea inspection campaigns without

the need for divers.

On carbon steel structures, magnetically attached crawlers are a useful deployment tool with

the ACFM array probe mounted onto a compliant arm which is able to keep the probe in

contact with the surface.

For remote inspection, cameras, or other means, are used to position the crawler and probe

correctly on the structure and to ensure that the probe tracks the inspection zone, such as a

weld, within the tolerance of the array.

Figure 8. Examples of subsea ACFM deployment through magnetic crawler and custom

scanner

CONCLUSION

ACFM is a reliable non-destructive testing technique for weld inspection of surface breaking

fatigue cracks through coatings. Trials and in-service experience with the new generation of

instrumentation has demonstrated that it can provide reliable detection and sizing in real

world situations and importantly can reduce inspection times and costs compared to other

electromagnetic inspection methods.

REFERENCES

[1] Dover W.D., Brennan F.P., Karé R.K., Stacey A., “Inspection Reliability for Offshore

Structures”, Proceedings of OMAE 2003 Offshore Mechanics and Arctic Engineering,

Cancun, Mexico, 8-13 June, 2003

[2] ABS, 2014, “Guide for Nondestructive Inspection of Hull Welds,” American Bureau of

Shipping, Houston, Texas.

[3] ASTM, 2017, ASTM E2261: Standard Practice for Examination of Welds using the

Alternating Current Field Measurement Technique, ASTM International, West

Conshohocken, PA.

[4] ASME, 2015, 2015 ASME Boiler & Pressure Vessel Code: V Nondestructive

Examination, “Article 15: Alternating Current Field Measurement Technique (ACFMT),”

The American Society of Mechanical Engineers, New York, NY, pp. 346–349.

[5] Lloyds Register, 2015, “Acceptance of the TSC ACFM Inspection Technique.”

[6] Lewis, A.M., Michael, D.H., Lugg, M.C. and Collins, R. Thin-skin electromagnetic fields

around surface-breaking cracks in metals. Journal of Applied Physics, Vol. 64, No. 8, 1988,

pp. 3777-3784.

[7] Topp, D and Smith M, “Application of the ACFM inspection method to rail and rail

vehicles”, Insight Vol.47 No. 6, June 2005

[8] Smith, M and Laenen C, “Inspection of nuclear storage tanks using remotely deployed

ACFMT”, Insight Vol.49 No. 1, January 2007