Use of Alternating Current Field Measurement
The ACFM technique had been used primarily for the detection of fatigue cracks in offshore structures, both on the subsea and topside structural sections. The testing applications were then extended to the testing of pressurized systems and process plants (for example, pressure vessels and pipe work). Testing trials have been carried out to prove the detection and sizing capabilities of the technique compared to the results obtained using more conventional techniques such as magnetic particle testing, eddy current and ultrasonic creeping wave. The technique was then used on the testing of nonmagnetic materials, such as stainless steel and titanium, and proved equally successful. Since then, ACFM has played a useful role in testing for nonfatigue cracks at normal and elevated temperatures. Procedures have been produced so that transverse discontinuities and those at an inclined angle to the toe can be differentiated from longitudinal discontinuities and cracks detected on the inner surface from the outer surface can be easily identified (only on certain materials and within a specific thickness range).

Because the technique has proven so versatile, ACFM is now being used on a wide number of applications. The technique has been used to test the steel structures of roller coasters and the people carrying components, with great cost benefits to the theme park owners. The ACFM technique has also been used in the testing of bridges with the use of abseilling techniques to test the more difficult access areas as well as through coatings using conventional inspectors. The combination of rope access (abseilling - without scaffolding) and the ACFM technique has also been adapted to the testing of dockside and other large cranes - once again with great economic benefits (Figure 1). An example of this is a major crane that can be tested at its most critical areas in 6 h (Raine and Laenen, 2001).

Figure 1 - Weld test on a dockside crane.

 

The oil and gas industry has not been ignored and the technique is still being used in petrochemical applications. In one recent application, $500,000 was saved by changing from traditional testing to ACFM. More examples were given in a recent paper at the ASNT ICPIIT topical in June 2001 (Raine, 2001). The ACFM technique is taking its place in the testing world as a technique which can reliably detect and size surface discontinuities with additional economic and time saving benefits.

In service testing can include many different areas of activity, including the offshore industry, public safety, petrochemical, civil and mechanical engineering. All of these sectors have their own problems in the forms of access, such as different forms of coating - from the protective to the cosmetic - and the type of materials which require testing. In the majority of cases, fatigue type cracks need to be detected. These are common in the offshore industry as well as in the civil, mechanical engineering and public safety sectors. In the petrochemical industry, environmental cracks, such as stress corrosion cracking, hydrogen sulfide cracking and stress orientated hydrogen cracks, are required to be detected.

The ACFM technique was originally developed for testing of carbon steel welds on subsea structures, which were usually nodal welds. A number of probes were developed such as a general purpose weld testing probe, a 30 degree angle probe for examining tight angle geometries and a pencil probe specially designed to examine welds that had been subjected to grinding. This was used to test the bottom of the ground toe of the weld to determine if discontinuities were present and then determine their length and depth or to confirm that the discontinuity had been removed.

During a trial organized by University College London, where samples were produced to reproduce some of the difficult geometries and access problems located in process plants, it was found that additional probes were required to gain access and detect and size the discontinuities located within the samples. A variety of mini and micropencil probes have been developed with straight and 90 degree access with increased sensitivity. In addition, it was realized that testing of short lengths of weld also created problems, in that the communication rate was too slow to produce a good representation of the weld result on the visual display screen. New software that eliminates this problem, including communication rates, has also been developed, which allows scanning speeds seven times faster than before. This allows greater presentation on the screen for shorter lengths of welds and faster scanning speeds for the testing of long lengths of weld.

The ACFM technique was also used to test structures that had been coated with protective or antifouling coatings so that the expensively applied coatings did not have to be removed and reapplied, thus avoiding costly preparation and reinstatement. The topside testing engineers also adopted the technique for the testing of process and pressurized plant, structural steelwork and crane pedestals. The system was used in conjunction with rope access teams, proving the usefulness of two man operations (Figure 2). Tests could be carried out up to 50 m (164 ft) between the ACFM probe and the ACFM unit, while the operator could be another 30 m (98 ft) from the unit.

Figure 2 - Rope access testing of a coated offshore installation.

 

Figure 3 - Mechanized ACFM testing used to test a weld on a titanium riser.

 

The technique has also been applied to testing of drill threads on casing and drill tools. A special transportable system has been produced to automatically test the drill thread ends and classify them. This provides go/no go reporting. The system is based on new ACFM array technology. A handheld probe has also been produced to test drill threads with the portable ACFM system.

New materials are being used for components and coatings on offshore structures, but the ACFM system has now been successfully applied to ferritic steels, austenitic stainless steels, aluminum, duplex, super duplex, monel, inconel and titanium (Figure 3). It has also been used to test through the following coatings:

	flame sprayed aluminum
	epoxy coating
	standard paints
	ferrite based paints
	copper coated threads.

Some tests have to be carried out when the plant is operational. ACFM has been used during tests at 253 K (-4 °F) and up to 773 K (932 °F).

Because of the above advantages, the ACFM technique has been used to test coated flare booms, epoxy coated pig traps, painted nozzle welds, pipe butt welds, pipe and saddle support welds and pressure vessel seam welds as well as conduct the above mentioned tests.

Since 1996, the ACFM technique has been further developed following customer requirements. When it was realized that the original probe coverage was not adequate, new probes were designed. When a particular client wanted to detect a particular minimum size discontinuity, the probes were redesigned and this modification has been built into production probes. Some testing companies found that the scanning speed was not fast enough when long lengths of weld required testing, or that short weld scans needed to be expanded so that they were presented on a single scan. The software and the communications rate were adapted to take these two points into consideration.

 

Results of Using Alternating Current Field Measurement
When POD trials are carried out on any technique, the technique is usually compared to magnetic particle testing as that is normally classed as the industry norm. When POD trials were carried out in the early 1990s on carbon steel welds, ACFM and magnetic particle testing gave similar results although magnetic particle testing had a greater number of false calls (Rudlin and Dover, 1993). In a trial carried out in 2000 on axle components in their dirty condition, ACFM produced a POD of 84% compared to 44% for magnetic particle testing, even though magnetic particle testing was carried out on a cleaned surface (Pollard and Lear, 2000). The axles had all been rejected for use in service. All of the discontinuities present were fatigue cracks produced during service, which had caused them to be scrapped by the overhauler using magnetic particle and ultrasonic testing techniques. The trials showed that ACFM would have rejected all of the axles, whereas magnetic particle testing would have rejected only four. The ACFM system was set up to detect discontinuities greater than 0.2 mm (8 x 10^-3 in.) deep, although the standard for the testing was to detect discontinuities 0.5 mm (0.02 in.) deep. In this case, if magnetic particle testing had been the only testing technique applied to these axles, then some axles with discontinuities present would have been returned to service with the probability that accidents would have occurred. This trial shows that magnetic particle testing should not be classed as the infallible technique it is usually made out to be and should not be used as a benchmark against which other techniques are compared.

Ultrasonic and magnetic particle testing have been used as standard testing techniques in the past for the testing of welds, and their use has been historic. As such, it has been difficult to replace their use. The results of the recent Netherlands Institute of Welding and the Programme of Assessment of NDT trials show that manual ultrasonic testing gave only a 50% detection rate from the Netherlands Institute of Welding trials and between 30 to 70% for the Programme of Assessment of NDT trials. The sizing errors were quite poor with an average length error of 8 mm (0.3 in.) with a standard deviation of 24 mm (0.9 in.). The depth measurements were worse with a slope of 0.24 and an intercept of 2.7 compared with the ideal of a slope of 1 and an intercept of 0. The overall result was that all small discontinuities were oversized and large discontinuities were undersized (McGrath, 1999). This again illustrates that a technique, which is used every day, may not be the most suitable technique for the task in hand. Some industries are still using manual ultrasonic testing for measuring the depth of surface breaking cracks when there are much more suitable techniques available.

Trials with ACFM on welds show that no discontinuities were missed below a POD of 80% for all discontinuities longer than 9 mm (0.4 in.). As well, discontinuity depth measurements were accurate to within 1 mm (0.04 in.) with a POD of 91% for all discontinuities deeper than 1 mm (0.04 in.). Even with this information, magnetic particle and manual ultrasonic testing are accepted as standard techniques in all of the conventional industries, not withstanding the fact that, when using these techniques, the surfaces have to be cleaned prior to testing and couplant or inks have to be removed after testing. This residue may be classed as hazardous waste and special disposal procedures have to be used for environmental reasons. This does not occur when using the ACFM technique.

Training schemes, which are internationally recognized (such as CSWIP, PCN and the Lloyds Register of Shipping), are in place and many technicians have now been trained at Levels I and II. A scheme similar to those mentioned has been submitted to ASNT. ACFM training approved by Lloyds Register, leading to a Lloyds Certificate, is currently offered by a major US NDT training school. The ACFM technique has been approved by certifying authorities such as Det Norske Veritas, Lloyds Register, Bureau Veritas, Germanischer Lloyds and Offshore Certification Bureau for use on offshore structures and petrochemical plants after going through trials with these organizations. Much of the ground work is done by technicians in the field who see other companies using the new techniques and inform their company engineers using the bottom up transfer of information.

However, the technician coming across ACFM for the first time sees it as just another electromagnetic technique. He is probably wondering whether or not it is eddy current. It is not eddy current. The eddy current technique uses a compact circular excitation alternating current, which produces an alternating magnetic field (the primary field). This field then produces eddy currents in the material, which in turn produce their own magnetic field (the secondary field). This field opposes and modifies the primary field. This results in a very sensitive detection capability, but also makes the technique prone to strong lift off signals and signals due to material property changes. The secondary field can be affected by variations in conductivity, permeability, part geometry, thickness and discontinuities. All of these discontinuities, including the cracks, produce display signals. Some of them are very similar and it takes a well trained operator to discriminate between a crack and a noncrack signal.

Unlike eddy current testing, the ACFM technique is generally insensitive to permeability, metallurgical changes and lift off; the new probes ensure that edges have little effect. Each probe is delivered with an individual software file, which sets up the instrument to give the best available performance. Since the technique is relatively insensitive to lift off, it can be used to test through coatings.

The ACFM technique uses a uniform electric current which is induced into the material under test - this produces a magnetic field, which, if a discontinuity is present, will be disturbed and flow around its edges. Special techniques are used to induce these electric currents and the components used are built into the probes. Small detectors or sensors are built into the probe, which measures the magnetic field disturbances. Because the alternating current field is uniform, it is possible to model the current flow and to separate the magnetic field into its three components. The X and Y components are in the plane of the material surface (Y being in the same direction as the flow of the current into the weld area and X being parallel with the weld toe). The Z component is normal to the weld toe (Figure 4).

Figure 4 - Description of magnetic field flow around a crack: the Bx sensor responds to current density and shows a reduction with crack depth; the Bz sensor responds to crack ends and shows a trough/peak as the probe passes over the crack and indicates crack length.

 

Figure 5 - Typical traces from a discontinuity with standard probe: (a) chart recorder; (b) butterfly plot.

 

When no discontinuities are present and the uniform current is flowing in the Y direction, the magnetic field is uniform in the X direction perpendicular to the current flow, while the other two components (By and Bz) are zero. The presence of a crack causes the current to flow around the ends of the discontinuity and below it. This causes a concentration of the current at the ends of the discontinuity and a reduction in the current at the deepest part of the crack. This concentration of the current results in creating peaks and troughs in the Bz and a reduction in the Bx along the length of the crack, which, when it is at its smallest value, is at the deepest part of the crack (Figure 5). The sensors in the ACFM probe measures the values of the Bx and Bz as it traverses the length of the weld - the former giving crack depth and the latter crack length. These measurements, together with software algorithms, are used to give the accurate measurements of length and depth of the discontinuity. To aid interpretation, the values of Bx and Bz components are plotted against each other and will produce a closed loop if a discontinuity is present. This is called a butterfly loop. The right and left edges of the loop give an indication of where the ends of the discontinuity are present as these are the locations of the Bz - ve and Bz + ve. These positions plotted on the weld give the estimated length of the discontinuity. The positions of the Bz - ve and Bz + ve are located just inside the true length of the discontinuity. When this length is used with the software algorithm and the Bx background and minimum values, accurate values of length and depth are produced. The magnetic field values being measured are absolute and these are used with mathematical look up tables so that there is no requirement to calibrate the ACFM instrument before use using a sample piece with artificial discontinuities.

The mathematical model look up tables have been produced using fatigue cracks rather than slots because slots do not behave the same electrically as a crack in carbon steel. As such, all of the probe files have been produced for carbon steel, but there are now probe files available for stainless steel. Thus, the length and depth of a crack in stainless steel can now be produced. If any other conducting material is being tested, then a calibration curve will have to be produced for that material using a block with discontinuities present. The depth measurements produced with the carbon steel look up tables will be compared with the actual values of the discontinuities in the block and a calibration curve will be produced. The length measurements will not be affected.

 

Accepting and Understanding Options of Alternating Current Field Measurement
Before a technician uses the ACFM technique, they should understand how the technique works. They should also understand all of the options they have with the equipment before they begin their testing. As well, they should be aware of all the trials that have been carried out to prove the technique in case a client asks questions about the validity of the test results. Examples have been given above of developments that have taken place over the last five years using the ACFM technique in an attempt to get the technique accepted more as a general technique instead of some technique which is brought out to be used on special occasions.

What does it take to get new technology accepted by old industries? Compared to ultrasonic and magnetic particle testing, any form of electromagnetic testing has always been looked on with suspicion. In this case, ACFM has had to undergo more trials and produce better results than manual ultrasonic or magnetic particle testing because of preconceived ideas. So, why don't these new technologies get used? In so many cases, when a testing company is contacted by a client who requires some welds to be tested, it is easy for the company to quote and supply a magnetic particle testing technician without thinking about the residual cost to the client. This is the very same client who has to clean the welds, arrange scaffolding and also arrange for any debris to be removed as well as recoat the welds after testing. The cost of the testing itself is quite small. In many cases, the client would rather pay more for a noninvasive testing technique which would not give him the logistics problem of carrying out magnetic particle testing. Both the client and the testing company need reeducating. Articles have been published in Materials Evaluation, papers have been given at ASNT conferences over the last five years and the equipment has been shown at a number of ASNT exhibitions in order to address this ACFM technique.

A number of companies who have heard about ACFM, seen a demonstration and then have used it in their plants have come to an economic understanding of the technique and I am sure that this also applies to other new techniques. When trying to get the technique used, the end user usually wants to know who can supply the new technique. And when speaking to the testing company, the testing company wants to know if the user company has specified the technique that they want used. The circle remains unbroken until a company becomes a product champion and specifies the technique.

 

Conclusion
Going back to the first paragraph of this paper, the 1996 paper introduced some people to ACFM as a method, which provided some of the basics for improved electromagnetic testing. Hopefully, this update will give an insight to more people into the development of a technique which could produce great economic advantages in certain industries if it was applied.

 

References
McGrath, B.A., Programme for the Assessment of NDT in Industry, AEA Technology, 1999.

Pollard, M. and A. Lear, "Evaluation of Electro-Magnetic Non-Destructive Testing Techniques," Engineering Link, Report 11363/03, December 2000.

Raine, A. and C. Laenen, "Applications Using the Alternating Current Field Measurement (ACFM) Technique, Using Rope Access," Insight, Vol. 43, No. 5, May 2001, pp. 318-321.

Raine, A., "Cost Benefit Applications Using the Alternating Current Field Measurement Testing Technique," Proceedings - ASNT's International Chemical and Petroleum Industry Inspection Technology (ICPIIT) VII Topical Conference, June 18-22, 2001, pp. 111-117.

Rudlin, J.R. and W.D. Dover, "Results of Probability of Detection Trials," IOCE Conference, Aberdeen, October 1993.