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.
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.