Eddy Current Testing – The Best Non-Destructive Testing For You?

Most people are not familiar with Eddy Current or Eddy Current Testing but ECT has become a star in the field of non-destructive testing. Eddy Current Testing is the use of electromagnetic testing to find leaks and identify surface and sub-surface flaws in conductive materials. It is also used to examine non-ferrous tubing in condenser and heat exchangers.

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History of Eddy Current Testing, and Where It All Started

Eddy Current Testing, also known as ECT and not to be confused with Electroconvulsive therapy, is testing that started or derived from electromagnetism. François Arago is credited with observing eddy currents in 1824 but it was in 1855 when French physicist Léon Foucault was actually credited with the discovery.

 

There was very little use for ECT or development in the field of Eddy Current until WWII. A German professor named Friedrich Förster started to look at Eddy-Currents for industrial use. Friedrich started developing coils, testing conductivity and measured out ferrous materials, all to more accurately detect flaws in conductive materials.

 

Friedrich would ultimately go on to found the Foerster Group and continue development of ECT and other non-destructive testing techniques. The Foerster Group developed practical instruments used to carry out ECT testing for the masses and today it is a widely used and accepted technique for NDT.

The Basics of Eddy Current Testing and How They Apply to Today’s Testing

The most basic form of the ECT principal uses a single coil that is excited using alternating electrical currents. When the wire is excited, it produces an electromagnetic field around the coil. The electromagnetic field oscillates (spins) at the same frequency that ran through the coil. Introducing the coil to conductive material will create currents that are opposed to the ones in the coil and these currents are Eddy Currents.

 

ECT uses Eddy Currents to produce an electromagnetic field using coils and detection instruments. When everything is normal and a conductive material is introduced to the magnetic field (normally rings) they stay circling around the coil like a stream.

 

If the conductive material has faults, breaks, or cracks, these streams start to jump off the typical Eddy-current magnetic field and causes the voltage that you are measuring change. These variants can be measured for fault detection in conductive materials allowing us to detect faults even if the eye cannot see them.

Eddy Current Testing by American Efficiency Services

To break everything down simpler, the coils produce a voltage that will be at a higher and different level than when introduced to Eddy-Currents. When Eddy-Currents are introduced the voltage steadies and will remain at the same level.

 

A fault or break stops the Eddy-Current’s oscillation and the voltage will spike back to the original levels before the Eddy-currents were introduced. The conductive material will not upset or change the spinning of Eddy-Currents but faults, cracks, and corrosion will so when the voltage jumps it typically means there is a fault in the material being tested.

 

Eddy-Current Testing is great for checking pipe’s surface area, checking for faults inside pipes, remote testing of carbon steel pipes, carbon steel weld inspections, and for clad thickness. Outside of our industry, the most common use of Eddy Current is in metal detectors.

American Efficiency Services employee checking eddy currents

The Benefits of Eddy Current Testing Compared to Other Non-Destructive Testing

  1. In favorable conditions, Eddy Current Testing can test fault in pipes up to 0.5 millimeters. With the proper equipment, the right type of conductive material, and properly trained professional, this test is extremely accurate at pinpointing even the smallest of faults.
  2. It can detect through coated materials. While other types of testing can be hindered or simply not work through paint, protective coatings, rust, and other materials, Eddy Current Testing is still accurate on coated materials up to 5 millimeters thick.
  3. No need to the clean surface area. Along the same lines as coated material, soils, uneven coatings, and rust will not affect the testing and therefore do not need to be cleaned for an accurate test.
  4. Testing is mobile and lightweight. Unlike other NDTs, Eddy-Current equipment is relatively small and portable. A proper Eddy-Current professional will have no problem coming to job sites, businesses, or even homes with the equipment and can test smaller areas compared to other non-destructive testing options.
  5. Testing through layers is no problem. Most Eddy Current professionals claim accurate testing through ten layers. We have found that we can still get accurate testing through 14 different layers without any interference due to the uniqueness of Eddy-Currents.
  6. For those looking for commercial testing, Eddy Current Testing can be automated using uniform parts. The testing is very quick and accurate. If you want to ensure quality production of conductive parts, this test can be streamlined with incredible accuracy.
  7. Lastly, this test provides accurate conductivity measurements. Instruments that are dedicated to conductivity testing use Eddy-Current. If you are not sure the material of the piping or metal in your wall you can have an Eddy Current test ran. The test will give you a conductivity reading and accurately tell you the materials in your wall.
American Efficiency Services employee doing eddy current tests

The Disadvantages Compared to Other Non-Destructive Testing

  1. Eddy-Current testing can only be done on conductive materials. If there is no electrical current that flows through the material then Eddy-Currents are not present and without Eddy-Currents, testing is not possible.
  2. It will not detect flaws parallel with the surface. During testing, the flaws must interrupt the Eddy-current waves to register through testing tools. Eddy-Currents run parallel with the surface so any flaw that runs parallel on the surface will not interfere with the currents and therefore, will not register on the detection tools.
  3. Magnetic Permutations cause testing complications. Ferromagnetic material or any small changes in permeability can have effects on Eddy-currents. While testing is not impossible under such conditions, it makes it very difficult so the testing of welds and other ferromagnetic material are less accurate.
  4. Eddy-Current Testing is best suited for small areas with simple geometries. Large spaces are not the best area for this type of testing. While it is not impossible to do, it requires large expensive equipment and is less accurate than when done on a smaller scale.
  5. Also, different geometries make Eddy-Current Testing difficult. A trained professional will be needed to differentiate flaws from deflecting signals caused by different geometries.
  6. There is no real permanent record of testing. During automation of Eddy-Current tests, a print out might be available but only during automation testing. Manual testing will not have any record or lasting reports from the testing.
  7. Lastly, Eddy Current testing requires training and certifications. It is not something an unskilled person will be able to immediately pick up. Professionals are needed to read the testing material, differentiate from deflecting signals and faults, and be able to interpret data correctly on the fly and on-site.

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