Evaluating defects using eddy current testing

Eddy current testing is a mainstay NDT technique and has been for decades. For testing conductive materials, ET is a reliable method for detecting pitting, cracking, and corrosion. It can also be used to detect metal hardness and the thickness of nonconductive coatings on metal parts, such as paint.

As industries demand more quality control and asset integrity management, eddy current instruments are manufactured with greater sensitivity and accuracy at detecting such faults and meeting industry demands.

“Eddy current testing is a mainstay NDT technique and has been for decades.”

How eddy current testing works

Eddy current testing is one of several electromagnetic NDT techniques engineers use. Eddy current probes consist of a wire coil in which an alternating current flows. The electrical current creates a magnetic field. The eddy current is created once the coil is brought into close proximity to the object being tested.

If the eddy current flow is interrupted, that means the instrument has detected cracking, pitting or corrosion. According to Olympus, a manufacturer of eddy current instruments, defects alter the pattern of the eddy current. The instrument then plots changes in the impedance amplitude and phase angle, which a trained operator will identify.

A single eddy current probe will measure a relatively small surface area. Eddy Current Array testing can be used to measure larger areas. With ECA, several coils are arranged in one probe assembly. According to Olympus, each coil “produces a signal relative to the phase and amplitude” to that of the object being tested. “This data is referenced to an encoded position and time and represented graphically as a C-scan image showing structures in a planar view,” Olympus noted.

Uses and advantages of eddy current

Eddy current testing is used on conductive materials, so it is an especially common and effective technique for detecting flaws in such metals as copper, aluminum, and steel.

Eddy current testing is a proven method for detecting flaws in metal structures.

Besides being used to detect defects or flaws, ECT can also measure metal thickness. This is especially useful for metal tubing. Such testing can detect corrosion or any other damage that causes metal thinning. The NDT Resource Center stated ET is especially useful for testing assemblies such as heat exchangers.

“ET is useful for determining whether a material has been subject to extreme heat.”

The NDT Resource Center noted that eddy currents are also affected by electrical conductivity and magnetic permeability. ET is useful for determining whether a material has been subject to extreme heat or has specifically been heat treated. Both can affect material conductivity.

ET offers several advantages. Results are almost instantaneous, where results can take longer with other types of testing. And tests can be conducted without having to touch the test object.

Olympus noted that ET is especially useful in such applications as:

• Weld inspection
• Conductivity testing
• Surface inspection
• Corrosion detection
• Bolt-hole inspection
• Tubing inspection

Though ET has been around for decades and remains a reliable testing method, engineers have developed ways to make it better.

According to the NDT Resource Center, research has provided breakthroughs in two areas:

• Pulsed eddy current: Using a pulsed eddy current with a spectrum of frequencies, particularly low frequencies, allows engineers to detect flaws at greater depths. Also, pulsed frequencies allow for greater analysis.
• Photoinductive imaging: This technique improves test imaging down to microscopic resolution. The combination of probes and photoinductive imaging is based on an argon-ion laser.

While eddy current testing has been used for decades, it remains an indispensable tool for detecting flaws in certain assets. And research indicates that there are new ways of improving ET, making it more precise and accurate so that it may continue to provide the testing results industries demand.

By Domenico Bellistri