Eddy Current Testing: A Guide

Eddy current testing is the process of running electronic probes through the length of various types of tubes or along the surfaces of materials in order to find flaws in them.

An eddy current is a current that runs opposite to the current introduced by a probe into a conductive material.

An eddy current test (ETC) is also commonly referred to as an eddy-current test, an eddie current test (though this is a misspelling), an electromagnetic test, or an electromagnetic testing.

Using eddy-current testing, inspectors can find very small defects that might not be visible to the naked eye. 

The raw data gathered from eddy current test probes must be processed using software made for this purpose and then analyzed by trained inspectors, who know how to identify defects in ETC results.

Eddy current testing is one of several electromagnetic testing methods used for non-destructive testing (NDT), which refers to tests performed for the purposes of inspections that do not damage the material being tested.

[Eddy current testing NDT is just one of the non-destructive testing (NDT) methods that inspectors use. Learn more about NDT in this in-depth guide.]

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What Is Eddy Current Testing?

In the eddy-current testing of tubes, an inspector will run a probe through the length of a tube in order to identify tiny defects.

Eddy current tests can also help identify defects on the surface of materials, with the use of pulsed eddy current testing equipment.

Here is how the eddy current testing of tubes works:

  • Probe. An inspector starts with a probe—for example, the single-element ETC probe, which uses an alternating current. The eddy current test probe consists of a coiled conductive piece of wire.
  • Magnetic field creation. When electrified, the probe will create an alternating magnetic field.
  • Introduce the field to the object. Once the field has been created, the inspector will introduce it to the object they want to inspect by moving it through the object.
  • Create eddy currents. When the magnetic field is introduced to the object or material, it will create currents running opposite to the currents in the probe. These currents are called eddy currents.
  • Collect data. Any defects present in the material will cause a change in these eddy currents, and inspectors collect this data after introducing the probe during their eddy current testing of tubes.
  • Evaluate the data. After the data has been collected it needs to be analyzed so that defects in the object can be identified. Note that the inspector who collects the data may not always be the same inspector who analyzes it, since these two activities require different levels of training and certification.

The History of Eddy-Current Testing

The eddy current phenomenon was first observed by researcher François Arago in 1824, but it is the inventor Léon Foucault who is actually credited with its discovery.

Foucault’s discovery happened in 1855, and was based in part on research conducted by Michael Faraday, who discovered the principle of electromagnetic induction in 1831.

This principle describes the relationship between electric currents and magnetic fields, and was the result of Faraday observing that a magnetic field will pass through a conductive material in a manner that varies over time as an electric current flows through it.

eddy-current-testing-faraday-cage

A Faraday Cage, which was invented by Michael Faraday to study electromagnetism

Despite these early observations, it wasn’t until 1879 that scientist David Hughes found a potential use for eddy currents. Hughes was able to demonstrate that the properties of a coiled, conductive wire changed when it came in contact with different kinds of conductive materials.

Eddy-current testing didn’t come into mainstream use until World War II, when Professor Friedrich Förster of Germany began exploring its industrial applications.

After the war, Förster founded a company called the Foerster Group that manufactured instruments for eddy current tests, further developing the technology and expanding its potential uses.

Today, ETC is one of the most common NDT methods used by inspectors, with a well established track record for providing reliable data.

Use Cases, Types of Flaws, and Industries 

Eddy current testing is most commonly used to inspect surfaces and tubes. It is an incredibly sensitive testing method, and can identify even very small flaws or cracks in a surface or just beneath it.

On surfaces, ETC can be done with both ferromagnetic and non-ferromagnetic materials. 

In the eddy current testing of tubes, ETC can primarily only be done with non-ferromagnetic tubing.

Here are the types of flaws eddy current testing is generally used to find:

  • Cracks
  • Corrosion
  • Wear (in tubes, often due to erosion)
  • Freezing-related damage (in tubes)
  • Lack of fusion
  • Pitting
  • Wall loss / thickness loss

The types of materials eddy current testing is commonly used to inspect include:

  • Bores. Bolt hole bores, bores for in-use tubes.
  • Welds. Welded joints, nozzle welds, friction stir welds
  • Tubes. Steam generator tubing, metal tubing.

Here are the industries where inspectors most commonly use ETC:

  • Aerospace 
  • Nuclear / Power Generation
  • Manufacturing
  • Oil & Gas
  • Petrochemical
  • Transportation

Eddy Current Testing Pros and Cons

ETC allows inspectors to find defects on the surface and subsurface level of an object easily and with a high degree of accuracy—but that’s just one of the reasons inspectors commonly use it to look for defects in a material.

Here’s a list of pros and cons for ETC:

Pros

  • It is incredibly versatile in terms of accuracy and portability/ease of use).
  • Its results are highly reliable, providing data of a high quality.
  • It is highly sensitive, allowing inspectors to identify defects as small as .5mm.
  • It is effective on surfaces that have paint or some other type of coating on them.
  • It can be used on high-temperature and underwater surfaces.
  • It provides immediate data.
  • It takes a relatively short amount of preparation time to perform (i.e., not much pre-cleaning or couplant is needed).
  • It can be automated for testing uniform parts, such as boiler tubes or wheels.

Cons

  • It only works with a current.
  • ECT current always runs parallel to the surface of a material, so a defect that doesn’t come in direct contact with the current can’t be detected—and this means that some defects may go undetected.
  • It’s not ideal for inspecting large areas.
  • It’s efficacy for different depths can vary.
  • It can be subject to changes in magnetic permeability, which can make it hard to use it for inspecting parts of ferromagnetic materials.It’s also non-conductive with ferromagnetic materials, as ECT equipment is subject to permeability changes on the welds. 
  • Interpreting signals correctly can be difficult, since it may require weeding out non-relevant data points.

Electromagnetic Testing Procedure

There are several different eddy current testing methods.

Here are some of the most common ones:

1. Eddy Current Array

Eddy current array testing uses an array of electrically charged coils  to create a sensitivity profile made to identify defects in a material.

In this kind of testing, inspectors have to be careful to avoid mutual inductance between the individual coils.

2. Heat Exchanger Testing

Heat exchanger testing is one of the most popular uses for eddy currents.

In this type of testing, inspectors use eddy currents to find defects in metal tubes, providing immediate data after a single pass through with eddy current testing probes.

eddy-current-testing-heat-exchanger-tubes

Heat exchanger tubes

3. Lorentz Force ETC

Lorentz force eddy current testing is a newer NDT method that uses multiple DC magnets to try and overcome the skin effect (that is, a cancellation of a flow’s current in the center of a conductor with a corresponding reinforcement in the skin).

In addition to the use of multiple magnets, the Lorentz force eddy-current testing uses relative motion to help inspectors conduct quick, accurate eddy current tests.

4. Surface Array Testing

Surface array testing is commonly used in the aerospace industry, where it can help measure conductivity as well as corrosion / wall thickness with a high degree of accuracy.

This type of testing is very versatile, and is capable of finding defects in places that are hard to access where other inspection methods may not work.

Eddy Current Testing Equipment

There are two categories of eddy current testing equipment—probes and instruments. 

In general, probes collect the data and instruments convert that data into interpretable results.

Probes—Eddy Current Testing Equipment

1. Handheld Probeseddy-current-testing-handheld-probe

Photo credit: Zetec

Handheld probes are one of the most common types of probes used in eddy current testing. They are used in a variety of industries, and commonly come with interchangeable parts for the probe’s tip and handle.

2. Surface Array Probeseddy-current-testing-surface-array-probe

Photo credit: Zetec

Surface array probes are used in surface array testing, and commonly used to identify defects in surfaces that aren’t flat.

3. Tubing Array Probeseddy-current-testing-tubing-array-probe

Photo credit: Zetec

This probe is a type of eddy current tube testing equipment, which inspectors commonly use to collect all the data an inspector needs in a single pass through a tube.

Instruments—Eddy Current Testing Equipment

1. Eddy Current Testing Handheld Instrumenteddy-current-testing-handheld-instrument

Photo credit: Zetec

Handheld eddy current testing instruments give inspectors greater versatility in the field, providing them with a portable device for recording ETC data.

2. Eddy Current Testing Instrumenteddy-current-testing-instrument

Photo credit: Zetec

Larger eddy current testing instruments also help inspectors record ETC data. These instruments can come in both surface array and tubing configurations.

3. Modular Eddy Current Testing Unitseddy-current-testing-modular-unit

Photo credit: Zetec

A modular ETC unit is generally smaller, more portable instrument-only eddy current testing system. These units are made only for use at power plants, where they’re used to inspect condensers and steam generators.

Eddy Current Testing Standards and Codes

Leak testing is commonly used for code-based inspections.

Here are some of the more widely used leak testing codes and resources:

ASME (AMERICAN SOCIETY OF MECHANICAL ENGINEERS)

See a full list on the ASME website.

ASTM (American Society for Testing and Materials)

See a full list on the ASTM website.

ISO (International Organization for Standardization)


See a full list on the ISO website.


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