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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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 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.
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.