How to Ensure the Best Application of Eddy Current NDT Technology

Eddy current testing (ECT) is one of the mainstream nondestructive testing (NDT) techniques for metallic components and structures. Depending on the system and situation, ECT can be an excellent way to inspect conductive components for cracks, fatigue, conductivity changes, and corrosion. It allows you to determine the recommended service life of a mechanical or electrical component and ensure that it complies with relevant industry standards. Proper calibration and analysis can help you quickly and accurately identify defective components in a system and determine next steps.

Why Use Eddy Current NDT Technology?

Eddy current testing relies on electromagnetic induction. Any internal defects or variations in the electrical properties of the material alter the magnetic field produced by ECT in specific ways, allowing easy identification of cracks, signs of fatigue, or other potential flaws.

The key benefit of NDT, and specifically ECT, is that conductive materials can be inspected in situ and largely without disrupting workflow. Components can be tested during service without a coupling liquid, and tests can be executed over a large area quickly and easily, making it an ideal technology when testing for cracks, signs of fatigue, and other potential near-surface problems.

Crack Detection and Fatigue Inspection

Manufacturers need to specify accurate service lives for their products so that their customers can develop appropriate maintenance schedules, especially in mission-critical applications of eddy current testing. External mechanical damage directly to a component is usually obvious and can be identified visually. However, internal crack propagation is much more insidious as it can significantly reduce the mechanical strength below the safety margin.

When working with mechanical components with fastener holes or other discontinuities, stress becomes concentrated near the fasteners, making them a prime location for fatigue failure. Cracks can manifest near the surface of the component and can be identified with a microscope, but inspecting internal cracks used to require destroying the component and inspecting optically.

This is where the effective application of eddy current testing becomes particularly valuable. Because the component can be inspected without destroying it, you can continuously test the component in an environment that simulates service. You can examine how and where fatigue cracking accumulates in a single component during service. This type of equipment also allows multiple components to be tested in parallel, providing more data in less time.

An eddy current testing instrument with a surface array solution can significantly reduce testing time compared to pencil probes and other more limited options. Rather than testing at a specific point, this allows testing over a large surface, and the results can be visualized using a 3D map with an RGB color scale for signal intensity. This type of tester requires less training and facilitates immediate analysis of critical components.

Single Frequency vs. Multiple Frequency Conductivity Testing

The impedance measured in your eddy current probe is intimately related to a number of system parameters. These include the frequency used to power your probe, the material properties of your component, and of course, discontinuities in the material properties due to cracks or conductivity changes.

Working with a single frequency is sufficient to determine conductivity changes, but it does not provide the whole picture. Conductivity changes may be larger at particular frequencies, either due to crystal phase changes, grain growth due to annealing during service, or crack propagation within the material. Identifying these frequencies during calibration and subsequent testing allows inspectors to quickly identify particular defects and assess the usable life of components in a mechanical or electrical system.

Calibration

In any application of eddy current testing, changes in the internal structure of a material are linked directly to changes in the secondary magnetic field. As such, a signal from a reference sample is needed for comparison. And, since material properties can vary across samples, it is a good idea to test multiple virgin samples and average across all samples, as well as determine standard deviations among data at different points. Using an ECT probe with a broad testing area will give you a 3D map of impedance, as well as some variance at each point in the 3D map.

In many cases, impedance changes due to fatigue or changes in electrical properties are obvious just from looking at a readout of the signal. However, if you need to determine usable service life for a specific component, or even identify the point of failure in a component, you’ll need to look deeper into the data. This is where analyzing data from 3D maps becomes crucial, as it allows you to identify when and where the material properties in a component begin to change.

Once a component or specific material is placed into service, you can test the impedance from multiple samples over time, giving you a set of 3D maps that you can use for comparison. This allows you to statistically test changes in the impedance data for significance. This level of analysis allows you to draw solid conclusions and justify possible repairs or redesigns of your system.

Effective Application of Eddy Current Testing: Best Practices

Ensuring the best application of eddy current NDT technology requires knowing precisely when, and how, to use it most effectively. Combining the best equipment, calibration, testing practices, and analysis techniques will help streamline your NDT procedures, giving you a comprehensive view of your system without destroying critical components. If you are interested in streamlining your mechanical and electrical testing procedures, you need eddy current test equipment that is easy to use and simplifies analysis. In terms of crack and fatigue detection, conductivity testing, and calibration, high-performance portable ECT equipment provides the ideal solution.

High-performance portable equipment can be an ideal solution in a number of inspection scenarios.

You can get the most information from eddy current testing when you use the right equipment. Zetec is a global leader in nondestructive testing (NDT) solutions. We offer high-performance solutions in both eddy current and ultrasonic technologies that help clients ascertain and maintain asset integrity and product quality. Contact us today to learn more.