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    Eddy Current Testing is a method of non-destructive testing, or NDT, that utilizes the process of electromagnetic induction for the evaluation and measurement of conductive materials without causing damage. Eddy current testing, or ECT, is used primarily in the detection of surface and subsurface flaws such as cracks in conductive materials like metal, and it is often used for applications in both aerospace and manufacturing.

In addition, eddy current testing can be useful in determining needed measurements and identifying corrosion resistance, as well as determining conductivity, metal hardness and some thermal properties of the material.

Unlike other methods of non-destructive testing, Eddy Current Testing does not require the use of liquids, and it is an excellent method of checking a metal’s surface structure and finding flaws. Its use is only effective on a limited range of materials that conduct electricity. While eddy-current testing is a proven method for conductive materials, other materials such as the surfaces of plastics cannot be tested with this method. To complement what the process offers, in some cases eddy current testing will be conducted in correspondence with ultrasonic testing methods, with ECT providing the surface testing and ultrasonic methods penetrating the material for increased depth.

Throughout this brief guide to eddy current testing,we will examine its history, how it works, its function and the many advantages the process offers. We will look at its common applications and some of its limitations in industrial use.

The History of Eddy Current Testing

Eddy current testing has its roots in the early 1800s when the famed English scientist, Michael Faraday, first discovered the physical process of electromagnetic induction. In 1831, Faraday discovered that when a closed path in which an electric current can circulate is used in tandem with a time-varying magnetic field passing through a conductor, an electric current will flow through it.

Electromagnetic induction, or magnetic induction, is a powerful force, and it is the principle that drives our electric motors and generators. Electromagnetic induction has a wide range of technological applications throughout history. This is the same method used for our induction cooking tools, induction welding, charging and sealing methods as well.

Eddy Current Testing utilizes this physical process. Conductors moving through a uniform magnetic field, or time-varying field, will induce currents. Eddy currents flow in closed loops within conductors, and they were first discovered shortly after Faraday’s early work. However, in most applications, these are often undesirable eddy currents as they can dissipate energy in the resistance of the conductor. To mitigate the impacts of these undesirable eddy currents, inductive coils in electronics are often used to minimize flow.

Friedrich Forster, the one who had adapted eddy current technology during the war, went on to found the Forster Group, which made advancements in developing ECT technologies. Today, eddy current testing is widely used and is an incredibly useful method of detective flaws in the surfaces of conductive materials. ECT is also indispensable when conducting thickness and conductivity measurements.

How Eddy Current Testing Works

Relying on the process of electromagnetic induction, an alternating current flows through a wire coil and produces a fluctuating magnetic field in an eddy current probe. The testing procedure uses an ECT probe, which is a coil of conductive wire that is excited by the alternating electric current. This causes the wire coil to produce an alternating magnetic field. As the field oscillates at the same frequency of the current running through the coil, currents opposite of these, or eddy currents, are induced into the conductive testing material.

These variations in electrical conductivity are used to test the object by detecting the presence of defects. If the eddy currents prompt a change in both phase and amplitude, it means a defect is present. This process is measured in the coil and can indicate the sign of surface and subsurface flaws of the conductive material.

The way the electrons behave when the probe is placed closely to the testing material is similar to water in a stream. As the eddy currents and their magnetic field flow through the metal testing material, they will continually interact with the coil and its magnetic field through induction. Changes in the thickness of the metal or surface cracks, for example, will interrupt this pattern and the field.

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