Standard development for Eddy Current Arrays in lieu of Magnetic Particle Testing

The technology of Eddy Current Arrays has advanced considerably in recent years. In several trials, superior performance compared to magnetic particle testing was shown. In response, ASME has published new articles on Eddy Current Array (ECA) testing in the 2019 edition of the BPV, and IIW has started a working group to develop ISO standards for the use of eddy current array testing in lieu of magnetic particle testing. Significant interest has been indicated by stake holders, including manufacturers, end-users and regulators. In the paper, the work performed in the IIW working group will be presented. This includes a literature survey on the relative performance of eddy current array testing and magnetic particle testing. In particular, the performance in terms of probability of detection and characterization will be explored. The technical background of performance differences with conventional eddy current testing, magnetic particle testing and dye penetrant testing will be evaluated. Conclusions are drawn on the most important properties to be treated in a new ISO standard in the ISO 17635 framework. The scope of the IIW working group includes a general standard, a weld testing standard and acceptance criteria. Finally, the issues discussed in standard development will be presented and the resulting formulations explained. Comparisons will be made between the ASME and IIW/ISO resolutions.


Introduction
Eddy Current technology has advanced towards testing with Arrays, just like Ultrasonic Testing.The incorporation of Eddy Current Array (ECA) technology into standards lags behind ultrasonic arrays, however, which limits implementation.The standardization efforts described in this paper mainly result from industrial user requests for standards.
ECA technology involves replacing one or more eddy current probes with one or more array probes.From a fundamental perspective, anything that can be done with an array can also be done by moving conventional probes.However, several new techniques are possible with ECA by combining the operation of multiple elements.This also requires new concepts for procedure development and implementation.

Current activity in ASME and IIW committees
This paper reports on activities in two standards bodies: committee V of the International Institute of Welding (IIW) which develops ISO standards for several NDT methods, and Committee V of the Boiler and Pressure Vessel Code of ASME which publishes its own standards.
ASME started with development of ECA standards in 2017.The first two, BPVC V Article 8, Mandatory Appendix IX and X, where published in the 2019 edition.A new Mandatory Appendix XI to the same article, on tangential field techniques (see below), will be published in the 2023 edition.NDT standard adoption in ASME is typically a twostage process where the standard is first published in BPVC V and then needs to be referred to in a construction code e.g., BPVC VIII for pressure vessels, or addressed in a code case.The second stage, a code case, is currently underway in ASME VIII div 3, giving the condition under which surface examination can be performed with Eddy Current testing.
IIW started activities in 2019 with the formation of a working group, to develop standards in the ISO 17635 framework for weld testing.This is also usually a two stage process, with the General Principles and Weld Testing standards being developed first, and Acceptance Criteria being developed once experience has been gained with the new technique.In this case, so much evidence was presented for equivalent performance of Eddy Current Array, Magnetic particle Testing (MT) and dye Penetrant Testing (PT) that it was decided to develop all three standards at once.Status, at the time of writing this paper is, that a committee draft is available.

Probability of Detection
The usual metric for performance of an NDT technique is Probability Of Detection (POD).The comparison of POD for ECA, MT and PT are of interest for the standard developments presented in this paper.While only one studies was available with head to head comparison of these three technique [1], information on the individual techniques, and comparison of conventional Eddy Current testing to MT and PT is available in several sources, including a recent meta study by the Australian Airforce [2][3] and HSE [4].Additionally, older studies were revised, including the ICON report, which studied these methods under field conditions [5].At this point, it should also be mentioned that the ACFM technique, executed with an Array, is considered a tangential field Eddy Current technique for the purpose of these standards.
Figure 1 shows the comparison of the POD of ACFM and MT for several studies [4].It can be easily seen that the POD curves are very similar.Other sources give similar results.It should be noted however that the absolute values for POD are not always in the same range.The POD in field conditions went down for all methods in the same way.It was concluded that this is mainly due to the application on which the three methods (ET, MT and PT) are performed.Surface roughness seems to be the limiting factor for POD, rather than the properties of the NDT method.
Figure 1 POD curves for HSE report [4] for MT (left) and ACFM (right) It was concluded to assume similar POD for all methods involved for the purpose of standards development.In the IIW ECA working group it was decided to develop guidance on how to adapt to difficult conditions in the standard, as ECA has some features that allow for doing so.

Techniques in Eddy Current Arrays
Several new concepts are possible with ECA techniques, resulting from using transmit and receive coils in new ways.First it is important to distinguish a number of possible array arrangement, then we will go into the use of patterns (aka topologies) from making measurements.

Staggered row arrays
The most common ECA probe design is one in which multiple rows of coils are placed next to each other, often with staggered rows to provide overlapping sensitivity areas.Figure 2 shows a typical arrangement with the sensitivity profile of the coils.

Tangential Field and Alternating Current Field Measurement array probes
For depth measurement of cracks, several techniques have been proposed which utilizes drive coils oriented tangentially to the inspection surface.Commercial names include Alternating Current Field Testing (ACFM), TECA (Tangential Eddy Current Array) and BFET (Balanced Field Electromagnetic technique).Perturbations of the eddy current induced magnetic field are monitored with one or multiple passive receive coils whose orientations are perpendicular or tangential to the inspection surface.Perturbations of the eddy current field are the result of eddy currents flowing around and underneath a surface breaking discontinuity.The secondary magnetic field created by the eddy currents can be monitored in the x, y or z directions (i.e.B-x, B-y and B-z).An example of a drivereceive coil arrangement is shown in Figure 3.Both perpendicular and tangential receive coil orientations are capable of providing detection of a surface breaking discontinuity.In addition, the tangential oriented receive coil (B-x orientation) will provide information about the depth of the discontinuity, lift off and material property variations while the receive coil oriented perpendicular to the material surface (B-z, orientation) will provide information about the length of the discontinuity as it is sensitive to the difference in eddy current rotation around the beginning and end of the discontinuity.Information gathered from the passive receive coils can be displayed on a strip chart, two-dimensional C-Scan display and associated impedance plane diagrams.

Orthogonal array probes
Very similar results compare to tangential field arrays can be achieved by using coils which have themselves an orientation.The resulting technique has been labelled orthogonal array probes.Combining the results of coils, it is possible to extract the B-x and B-y field components.An example of possible coil shapes is shown in Figure 4.

Patterns for transmission and reception
The use of patterns (aka.topologies) can enhance the sensitivity for particular situations or type of defects.Three typical patterns are listed in table 1., with some of their advantages and limitations.

Topology Coil arrangement Advantages Disadvantages
Table 1 Several common patterns (aka.topologies) with their advantages and disadvantages

Techniques and testing levels
The weld testing standard will need to include a table which shows how coil arrangements and topologies shall be combined to make a standardized testing technique, how they can be used at various testing levels to achieve the quality levels in ISO 5817.

Issues discussed in standard development
Several issues were discussed in the standard development working groups.Some of them are discussed here, with information on how they were resolved in the IIW working group.

Surface condition
Surface condition is an important variable in ECA procedure development.It will determine what coil size is feasibly; if the surface condition is rough a larger coil is needed.This in turn will influence the detectability of small flaws.ECA can test on rough surfaces, like an untreated weld cap, successfully with a suitable detection limit.In addition, surface conditions may give rise to variation in lift-off, which the instrument should have a capability to compensate for.
Issues with surface roughness are not unique to ECA.For PT a series of reference block with star shaped artificial defects were developed, and described in ISO 3452-3, which have a defined varying surface roughness and a different artificial defect size to match this surface roughness.In the standard development it is investigated to reuse this reference block design for ECA, and match the corresponding detection and acceptance levels of PT.

Influence of material properties
Magnetic permeability and electrical conductivity of the material can affect the eddy current measurements, and as such, may have an impact on the accuracy of testing.The system must be able to compensate for variations in the magnetic permeability and electrical conductivity typically found in material, by using, for example a standardization technique that measures and compensates for material properties.
Object conditions, such as non-uniform thickness of non-conductive and non-magnetic coating have an influence on the test accuracy and shall be compensated for.If a coating thickness exceeds the specified coating thickness range of the system, the testing accuracy can be significantly reduced.In addition to the capabilities of the system, the technician should be trained to recognize and deal with these conditions.

Conclusion
Development of ECA application standards in the ISO 17635 framework are well underway in an IIW working group.Several important issues, such as creating categories of ECA techniques, and defining them for the purpose of testing and quality levels, have been described.A direction for creating acceptance levels depending on surface roughness, in line with accepted approaches in PT have been identified.

Figure 2
Figure 2 Typical arrangement of coils for staggered row ECA's

Figure 3
Figure 3 Typical arrangement of coils for Tangential Field techniques

Figure 4
Figure 4 Typical coil shapes for Orthogonal array probes