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Eddy Current Array Tutorial

Eddy Current Testing Applications and Capabilities

If you’re looking to invest in eddy current equipment, it’s important to know its applications and capabilities. In this article, we will discuss the different ways industries can use eddy current testing (ECT) to meet specific regulations and quality standards. We will also advise on the best testing methods and equipment for each application.

Rivet Inspections

Each individual eddy current coil in the probe produces a signal relative to the phase and amplitude of the structure below it. Depending on imaging capabilities of the ECT instrument, this data is referenced to an encoded position and time and represented graphically as a C-scan image. When the eddy current coils pass over a defective rivet, they generate a unique signal response. For coils that are affected by a crack initiating from the rivet hole, an amplitude change is represented in the C-scan display. For coils that detect no change, the color representation remains constant in the C-scan display.

Corrosion Detection

Corrosion detection using eddy current array (ECA) technology offers major advantages over conventional eddy current inspection methods. Each individual eddy current coil generates a unique electrical signal in relation to the structure below it. The coils can detect very small changes in material thickness, along with other parameters, and display these changes as a color-coded C-scan image. Imaging using eddy current array enables easy interpretation of the data generated from the probe coils. After it has been collected, the inspection data can be stored, transmitted, and analyzed.

Crack Detection

Eddy current crack detection equipment is divided into high-frequency devices for finding surface cracks in ferrous and nonferrous materials and low-frequency devices for finding subsurface cracks in nonferrous materials. Eddy current testing is highly sensitive when detecting surface cracks. Frequencies of the order of 2 MHz give high resolution, but the probes are small, making covering large surface areas time-consuming.

Low-frequency crack detectors require larger probes to achieve suitable coil inductances. Phase sensing circuits are also required because subsurface fields are influenced by phase changes. Frequency settings are critical for detecting subsurface cracks in nonferrous materials; aim for a range 100 Hz to 100 kHz depending on the depth of penetration.

Tube and Wire Testing

Eddy current testing can be used for detecting flaws in tubes, bars, and wires at speeds of up to 3 m/s. This is done using automated systems that eject or mark tubes or wires with known flaws. The constant test speeds and differential coils enable modulation of test signals with the speed and then filtering to remove noise. Using different coils can lead to tubes with defects passing through the eddy current system without being detected. Industries using eddy current testing for tube and wire inspections should also be aware of edge effects and extrusion defects. Because of edge effects, tube ends cannot be detected, and extrusion defects along the center of bars cannot be detected because the eddy current field is at zero intensity.  

Condenser Tube Inspection

Eddy current technology is also used for condenser tube inspection. Tube thinning is the most common defect in condensers. When a frequency known as f90 is selected on the ECT instrument, signals from thinning on the outside surface can be 90° out of phase from signals from thinning on the inside surface. The extent of thinning can be determined at test speeds of 200–300 mm/s by recording X and Y signals from the impedance diagram on a two-channel strip chart recorder.

Baffle plates that separate condenser tubes can cause problems during inspection. Tubes are usually nonmagnetic, made from stainless steel, cupro-nickel, or titanium. The baffle plates are ferrous and the permeability signal interferes with the thinning signals from between the tube and baffle plate. Certain devices are designed to overcome this problem, using two frequencies simultaneously and then mixing the two signals to remove unwanted permeability effects.

Material Sorting

Different eddy current methods can be used to distinguish certain materials. Conductivity meters can be used to sort aluminum and copper alloys, while ferrous and electromagnetic sorting bridges are used to determine which steels have been hardened and separate them. Eddy current testing methods provide a better sample of material properties because they penetrate below the surface. It’s also a very fast, efficient method.

Weld Testing

High-frequency eddy current equipment can be used to detect defects in ferrous welds. The benefit of using eddy current testing is that cracks can be detected through paint layers, which can be particularly useful if you don’t want to damage the test material in any way. The disadvantages are that permeability changes and liftoff noise from rough cap surfaces can impact the success of the inspection. Some devices are used to supplement magnetic particle inspections underwater, distinguishing strong spurious indications from toe cracks. This helps overcome certain problems with weld inspection, to an extent. 

Coating Thickness Measurement

High near-surface resolution helps make eddy current testing useful for accurately measuring the thickness of coatings on metal substrates, both metallic and paint.

Bolt Hole Inspection

Nuts, bolts, and bolt holes require significant engineering to ensure that certain specifications are met. This can be critical for health and safety in industries such as aerospace. Eddy current testing equipment can be used for multilayer bolt hole inspections to identify layers in which flaws occur. Bolt hole flaws are often small and difficult to detect, eddy current technology uses electromagnetic fields to detect very small-scale irregularities with high accuracy and speed.

How Often Should Eddy Current Testing Be Done?

Ideally, eddy current testing will occur every 3 to 5 years, however, this should be done more frequently if there is active damage occurring that should be monitored closely to ensure certain regulations and health and safety standards are met. It’s recommended that eddy current testing is included in annual inspections to ensure proper regularity.

If you’re looking for eddy current testing equipment for one of the above applications, you can find more on flaw detectors and probes. If you want to see more of this technology in action, our application notes highlight real use cases of our products. 

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