Part I: Principles of inline eddy current testing
June 10, 2009
Eddy current testing does more than detect product defects. When used with a proper monitoring system on a mill staffed by highly trained operators, it can help to optimize the mill's efficiency. The first part of this two-part article covers eddy current system principles
Editor's Note: This article, the first of a two-part series, is adapted from Richard Fisher's paper presented at Metal Matters, March 14-16, 2007, Lake Buena Vista, Fla. © 2007 by the Fabricators & Manufacturers Association Intl. (FMA).
It's no secret that quality is playing an increasingly important role in today's competitive marketplace. In fact, many tube and pipe producers are facing increasing customer demands to deliver products with zero defects. Although this may be impractical or even impossible, eddy current inspection offers a way to improve quality, reduce the likelihood of shipping faulty tube or pipe, and trim production costs. Eddy current testing can be favorable in terms of cost and service life, and it can help a producer to break into new markets. Finally, it offers a means of controlling the manufacturing process, thus reducing scrap and downtime.
The last point, process control, may be the most overlooked and most important benefit of all.
A circular magnetic field forms around a conductor whenever an electric current flows through it (see Figure 1). Forming the conductor into a loop concentrates the magnetic field through the center of the loop. If metal or any other electrically conductive item is placed in the magnetic field, a secondary current is induced to flow in the object. These secondary currents flow in circular paths, like eddies in a stream; hence the term eddy current.
If the excitation coil moves over a major flaw in the material, the impedance of the coil is slightly altered and remains so until the coil passes the flaw. This subtle change can be detected by a very sensitive voltmeter connected across the coil. This technique is useful for recognizing large or continuous defects. Commonly called an absolute coil, this setup can detect an open seam in a welded tube or pipe. However, it does not detect small defects.
Detecting small defects requires a coil that is much more sensitive but not susceptible to variations caused by temperature swings or minor changes in air gap between the coil and the workpiece, also known as liftoff. The differential coil provides a simple but elegant solution to the problem.
The differential coil is actually two coils in one (see Figure 2). The two coils oppose each other. It generates very little output when it detects signals that are common to both windings. For example, interference signals produced by liftoff, vibration, or temperature drift tend to affect each winding equally and therefore cancel each other.
A differential coil's strength is that it produces a relatively large voltage output when one winding detects something different from the other (hence its name). This allows it to detect small defects. In addition, the phase angle of the defect signal can be used to identify and classify various types of defects (see Figure 3).
Because these coils have complementary strengths, a complete system needs both an absolute and a differential coil (see Figure 4).
Another important consideration is the limited depth of penetration of eddy currents. The field weakens with depth, which reduces the sensitivity to ID defects, particularly on heavy-wall tube. The test frequency, the material's electrical conductivity, and its magnetic properties (permeability) affect the depth of penetration.
Permeability variations, present in carbon steel and 400-series stainless steels, have a tremendous effect on penetration depth but are easily rendered insignificant by magnetically saturating the material using a powerful, external DC electromagnet. However, this does add to the size and cost of the magnetizing assembly.
Some materials, such as 300-series stainless steels and nonferrous materials, do not require magnetic saturation, so initial equipment costs are lower.
Any qualified applications engineer can quickly determine the ideal test frequency and then calculate the resulting depth of penetration. A typical limit is approximately 0.320 in. (8 mm) for magnetically saturated carbon steel and 0.080 in. (2 mm) for high-conductivity, nonferrous materials such as copper and aluminum.
Types of Defects. A properly installed and calibrated eddy current system can detect the following:
However, eddy current testing has some limitations as well. Certain conditions are difficult or impossible to detect, including cold welds and some other continuous defects.