Non-destructive testing, NDT, is a very broad group of structural or material inspections and as the name implies, these inspections do not destroy the material/structure being examined. NDT plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. Because NDT does not permanently alter the article being inspected, it is a highly valuable technique that can save both money and time in product evaluation, troubleshooting, and research. NDT technicians and engineers define and implement tests that locate and characterize material conditions and flaws that might otherwise cause serious accidents such as, planes to crash, reactors to fail, trains to derail, pipelines to burst, and a variety of troubling events.
This concept is extended, and known as Non-Destructive Evaluation (NDE), when combined with an assessment of the significance of any defects found. However, they are both terms often used interchangeably. Some testing methods must be conducted in a laboratory setting, others may be adapted for use in the field. Several commonly employed NDT techniques and their characteristics are described below.
Eddy current testing is one of most common electromegnetic testing NDT methods. It uses induced electrical currents to detect defects. Essentially, the technique uses a coil (ECT probe) carrying an alternating current as transducer. This produces an alternating magnetic field parallel to the axis of the coil which in turn induces eddy currents in the surface of the test object. These eddy currents set up a magnetic field opposing that produced by the coil, thus changing the impedance of the coil. Interruptions in the flow of eddy currents, caused by imperfections, dimensional changes, or changes in the material’s conductive and permeability properties, are detected.
Most eddy current testing is based on the measurement of the coil impedance, though it is possible to measure the magnetic field directly. ECT has a very wide range of applications. Since ECT is electrical in nature, it is limited to conductive material. There are also physical limits to generating eddy currents and depth of penetration (skin depth). In general, this technique is used to inspect relatively small areas and is therefore better suited for inspecting areas where damage is already suspected, it nevertheless has a variety of applications: from measuring material thickness to detecting corrosion damage. In nuclear power plants, eddy-current methods provide the best in-service inspection method for steam generator tubing. The following components are required to perform Eddy Current testing: an Eddy Current tester, a remote positioning device, and an eddy current probe drive control system.
The disadvantages of this technique are that it is sensitive to lift-off, it is a point test, so scanning is required for large areas, and it is generally limited to near surface defect of conducting materials.