Eddy Current Inspection
Back to Basics
By Joe Escobar
In the arena of nondestructive testing, there are many different processes available. From dye-penetrant to X-ray, there are numerous choices for testing aircraft structures for damage. One frequently used method is eddy current. In this article, we will discuss some of the basics of eddy current and some inspection tips to keep in mind.
Although eddy current testing may seem like relatively new technology, the fundamentals it is based on date back to the 19th century. Eddy current testing is based on electromagnetic theory that was established by the likes of Faraday, Ampere, and others in the 1800s. Faraday discovered electromagnetic induction in 1831. In fact, the first recorded use of eddy currents for NDT was reported by Hughes in 1879. He recorded changes in coil properties when they were placed in contact with metals of different conductivity and permeability. That was almost two decades before X-rays were discovered. But it wasn't until WWII that eddy current testing was used to test materials. Now, it is an accurate and widely used inspection technique.
Electromagnetic induction
Michael Faraday discovered an important characteristic of magnetic fields in the 1830s. He discovered that when the current in a conductor was changing,
such as when a switch in the circuit was opened or closed, this caused a current to flow in another wire located close to the first one. He called this phenomenon electromagnetic induction and concluded that it happened whenever the current and its associated magnetic field were changing. Electromagnetic induction forms the basis for eddy current testing.
In eddy current testing, a changing magnetic field is created on a coil through alternating current. When the coil is placed near an electrically conductive material such as aluminum, then eddy currents are induced to flow in the metal. The relationship between the energy that is used to produce the test and the resulting current and magnetic field in the metal being tested are measured. In a test material with no flaws such as corrosion or cracks, then the induced current flows unobstructed through the metal. If flaws are present, then the flow pattern of the currents is changed. These changes of patterns are detected by the test equipment.
Choosing your test coil
There are several types of test coils available for inspecting aircraft structures using eddy current. The coil used is dependent on the purpose of the test and on the shape of the part being inspected. The two most commonly used coils are surface and internal probes.
Surface probes are used when inspecting a flat surface for sub-surface corrosion or cracks at an angle to the surface. These include typical "pancake" probes for surface inspections and smaller pencil probes used for inspecting areas with odd geometries.
Internal or bobbin type coils are used to inspect the inside of bolt holes. Although 90-degree pencil probes can be used for inspecting bolt holes, it is quicker and easier to use internal probes.
Factors affecting accuracy
There are several factors that affect the accuracy of an eddy current inspection. These are testing frequency, alignment of flaws, lift-off,
and surface geometry. Let's look at some of these factors and how they affect test accuracy.
Testing frequency. The frequency used in the test is an important factor in accuracy. The frequency used depends on the material thickness, the desired penetration depth (whether suspected flaws are at the surface or below), and sensitivity required. There is an inverse relationship between penetration of eddy currents and sensitivity. The lower the frequency, the higher the penetration. Unfortunately, this also means a lower sensitivity to flaws. A compromise must be established so that sufficient penetration is achieved without sacrificing too much sensitivity.
The conductivity of the metal tested also affects testing frequency. The greater the conductivity of a material, the greater the flow of eddy currents on the surface.
Permeability, or the ease at which a material can be magnetized, also affects frequency. In fact, the depth of penetration decreases significantly with an increase in permeability. Therefore, penetration into ferrous materials like steel using practical frequencies is relatively small.
Alignment of flaws. When testing for flaws such as cracks, it is essential that the flow of eddy currents be as nearly perpendicular to the flaw as possible in order to obtain maximum response from the flaw. If the flaw is parallel to the eddy current flow, then there will be little or no distortion of the currents and the flaw can be hidden. It can be helpful to rotate the probe 90 degrees during the inspection process. This will increase the chance of detecting flaws that are not perpendicular to the eddy currents.
Lift-off. This factor can have a big influence on test accuracy. In order to get an accurate test, the probe needs to be in contact with the test area. If the probe is not against the test material surface, then an effect known as lift-off occurs. Any air between the probe and material affects the eddy currents in much the same way that defects would. This can give a false indication of a flaw. To help ensure an accurate test, always hold the probe firmly against the material. Other factors that can affect lift-off are surface irregularities, debris, and flaking paint/thick paint. Most of the eddy current testers available today feature some sort of lift-off compensation that reduces the equipment's sensitivity to lift-off, but it can only compensate for a small amount of lift-off. Large lift-off distances will affect testing accuracy, so it is important to keep the probe in close contact with the surface during testing.
Surface geometry. Different geometrical features such as curves, edges, and grooves affect test accuracy. Curves and grooves can result in a lift-off effect. In addition, whenever the test coil approaches an edge of the material being tested, the eddy currents are distorted because they cannot flow beyond the edge of the part. This distortion results in an indication known as edge effect. Edge effect limits testing near the edges of parts. The effects of curves and grooves can sometimes be removed by using a probe small enough to maintain good contact with the surface. In addition, be aware of edges and know that testing will be limited around 1/8 inch from the edge of the part.
Training
As with any nondestructive testing, there are many things to know when working with eddy current technology. You should not perform eddy current testing without proper training or without direct supervision from a certified technician. Eddy current manufacturers offer training on proper use of their equipment. There are also numerous companies that specialize in NDT training throughout the country.
The American Society for Nondestructive Testing (ASNT) is a resource for those practicing NDT methods. Covering all NDT techniques including eddy current, ASNT supports the training and certification of technicians in these technical fields. Their quarterly newsletter, The NDT Technician provides information valuable to NDT practitioners and a platform for discussion of issues relevant to their profession.
To find a NDT training company near you, you can log on to www.AMTonline.com and click on the Training Resources tab on the left navigation bar under Services.
Eddy current inspection has come a long way since its discovery in the 1800s. It has become an important part of our nondestructive testing programs. With proper use, it is a valuable testing option that can help us find defects and ensure our aircraft remain airworthy.
Additional ReSources
Advisory Circular AC 43-3A
Nondestructive Testing in Aircraft
American Society for Nondestructive Testing (ASNT)
(614) 274-6003 or (800) 222-2768
www.asnt.org
