Eddy current testing is the NDT method that works on the principle of electromagnetic induction to find out and quantify the flaws and cracks in conductive materials.

Principle of Eddy Current Testing

A magnetic field will be created, when alternating current (AC) passes through a conductor, and it expands and shrinks as the AC reaches its maximum and minimum. A current can be induced in another conductor if it is brought near to the alternating magnetic field. These currents which flow in a circular path are called Eddy currents. The name was derived from the term ‘eddies’ that is a whirly motion of liquid or gas countering the main current.

The use of Eddy currents in testing

Now, Eddy currents are widely used in industries to detect faults and make measurements. Due to the property of eddy currents that limits its spread mainly over the surface of a material, they are usually used to detect flaws on the periphery or in the vicinity of the surface. While testing, if there is any defect, the field will change due to the presence of defects causing changes to the coil’s impedance which are displayed on the screen of the eddy-current flaw detector instrument. In situations when the nature of the defect is clear, Eddy current testing is the primary option to detect and quantify it. Generally it is used to inspect a relatively small area and in such cases, the probe design and test parameters must be set up with a good understanding of the defect that is to be detected.

Eddy current testing is a non-contact method, useful in finding the corrosion damages that lead to thinning of a material. So it is used to measure the thickness of thin materials such as sheet stock and tubing. It is very helpful in determining the thickness of paints and other coatings. Another use of Eddy current testing is that it can be used to predict the changes in the conductivity of some materials, when they are put in high temperatures and given heat treatment. They also assist in sorting materials.

Testing Instruments

The testing devices are portable and capable of giving quick results. A wide variety of those instruments are available in the market. Computer based systems are also in offer which provide easy data manipulation features for the laboratory. Some laboratories use scanning in multi-dimensions that can produce images and thus give a more precise result of the test.