Eddy current testing (ECT), or eddy current inspection, is a non-destructive testing inspection method that uses electromagnetism to detect surface and near surface defects in materials. The application of eddy current testing works with conductive materials by using an alternating current that is passed through a coil to create a changing magnetic field around the specially designed coil. When the charged coil is placed close to a test surface the magnetic field permeates the material, changing the magnetic field and creating local electrical ‘eddy currents’ in the test surface.
These eddy currents create their own magnetic field that opposes and modifies the coil’s magnetic field, which changes the electrical properties of the test coil. Electrical conductivity or permeability variations and the presence of discontinuities within the test material all cause a change in the eddy currents and a corresponding change in phases and amplitude of the measured current. By measuring the changing phase and magnitude of the current flowing through either the primary excitation coil or separate receiver coils, it is possible to determine the condition of the test material. These changes can be displayed either individually by a needle meter or in combination on a 2D impedance plane display for easy interpretation.
Variants of ECT include pulsed eddy current (PEC) testing, which uses low-frequency, pulsed signals, allowing it to penetrate deeper and pass through insulating barriers. Eddy current array (ECA) testing uses a series of single eddy current elements in a row, allowing for larger areas to be tested in a single pass, while pulsed eddy current array (PECA) is a combination of PEC and ECA.
Eddy current tests are sensitive to small cracks as well as capable of investigating physically complex geometries, providing immediate feedback without the need for couplants.
Applications
ECT is a portable technique that can be used for measuring electrical conductivity and coating thickness in addition to its use for detecting surface breaking and near-surface defects. It is primarily used by industry for crack and corrosion detection but, as a conductive technique, ECT can also measure the thickness of non-conductive coatings and be used for material identification and heat treatment condition inspection. Because it is a non-contact technique, ECT can be used to inspect surfaces at high temperatures, such as heat exchangers.