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Principles and Application Scope of Eddy Current Testing
Release Date:
2022/04/02
Nondestructive testing is a method that utilizes the principle of electromagnetic induction to nondestructively evaluate certain properties of conductive materials and workpieces or to detect defects by monitoring changes in the eddy currents induced within the test object.
In industrial production, eddy current testing is one of the primary methods for quality control of various metallic materials and a limited number of non-metallic materials (such as graphite and carbon-fiber composites) and their products.
Compared with other nondestructive testing methods, eddy current testing is more easily automated and delivers particularly high inspection performance for tubular, bar, and wire products.
Eddy Current Testing ：
Eddy currents arise when a conductor is placed in a time-varying magnetic field. The time-varying magnetic field induces a swirling electric field, which exerts a force on the free charges within the conductor, causing them to move and thereby generating eddy currents.
Eddy current testing (ET) is based on Faraday’s law of electromagnetic induction: an alternating current is passed through the inspection coil, generating an alternating magnetic field that is perpendicular to the workpiece.
When the detection coil is brought close to the test workpiece, eddy currents are induced on the workpiece surface, generating a magnetic field that opposes the original magnetic field and partially cancels it, thereby causing changes in the resistance and inductance of the detection coil.
If a metallic workpiece contains defects, the intensity and distribution of the eddy current field will be altered, leading to changes in the coil impedance; detecting these changes allows for the determination of defect presence.
The development of microelectronics and computer technology, along with the adoption of various signal-processing techniques, has led to significant advances in eddy-current testing transducers, eddy-current signal-processing methods, and eddy-current testing instrumentation.
Characteristics of Eddy Current Testing :
I. Advantages
1. During inspection, the coil does not need to contact the workpiece, nor is a coupling medium required, resulting in rapid inspection speeds.
2. It exhibits very high detection sensitivity for defects on or near the surface of workpieces and maintains good linear indication within a certain range, making it suitable for quality management and control.
3. Inspection can be performed under high-temperature conditions, in narrow areas of the workpiece, and on the walls of deep holes (including pipe walls).
4. Capable of measuring the thickness of metallic coatings or non-metallic coatings.
5. Non-metallic materials that can be tested for eddy current induction, such as graphite, etc.
6. The detection signal is an electrical signal, which can be digitally processed, facilitating storage, reproduction, and data comparison and analysis.
II. Disadvantages
1. The test object must be a conductive material and is only suitable for detecting surface defects in metals.
2. Detection depth and detection sensitivity are mutually contradictory; when performing ET on a given material, a comprehensive assessment must be made based on the material properties, surface condition, and inspection standards, followed by the establishment of an appropriate inspection plan and technical parameters.
3. When ET is performed using a through-type coil, the precise circumferential location of the defect cannot be determined.
4. Rotating-probe ET can achieve localization, but the inspection speed is slow.
III. Applications of Eddy Current Testing in High-End Sectors Across Various Industries
1. Aviation and Aerospace
Eddy current testing has been widely employed for the inspection of metallic components in the aerospace and aviation industries. To ensure the safety of aircraft operations, it is essential to conduct regular in-service inspections of relevant parts.
Eddy current testing is commonly used to detect surface and subsurface defects in aeroengine blades, bolts, internal cracks in bolt holes, multi-layer structures of aircraft, landing gear, wheel hubs, and beneath aluminum skin, as well as to inspect welds in wing joints for defects.
During inspection, it can effectively suppress interference signals caused by probe vibration and material non-uniformity. Metal magnetic memory testing technology can be used to diagnose stress concentration areas or early damage in the aforementioned components.
2. Power and Petrochemicals
Eddy current testing is employed for in-service and pre-service inspection of nonferrous and ferrous metal pipelines—such as copper tubes, titanium tubes, stainless steel tubes, and boiler waterwall tubes—in power plants (thermal and nuclear) and the petrochemical industry (oilfields, refineries, and chemical plants).
It can reliably detect intergranular corrosion, wall-thickness reduction, and external-wall wear in pipelines, and effectively suppress interference signals from support plates and tube sheets during inspection.
In addition, eddy current testing is employed for the inspection of components such as the central bore of steam turbine rotors, engine blades, sucker rods, drill pipes, bolts, and bolt holes; acoustic pulse testing technology is suitable for the rapid inspection of various metallic and non-metallic pipelines; and metal magnetic memory testing is used for the early diagnosis of damage in ferromagnetic components of in-service equipment.
3. Metallurgy, Machinery
Eddy current testing is used for both in-line and out-of-line flaw detection of various metallic tubes, bars, wires, and filaments.
During flaw detection, it can simultaneously address both long defects—such as longitudinal through-cracks and slowly developing defects—and short, small defects (e.g., through-holes); it effectively suppresses certain interference signals during both in-line and out-of-line pipeline inspections, such as material non-uniformity and vibration; and it exhibits high sensitivity for detecting defects on both the inner and outer walls of metallic pipelines.
It can also be used for blending and sorting of mechanical components, evaluation of carburizing depth and heat treatment conditions, hardness measurement, and more.
4. Nuclear Energy and Military Industry
Eddy current testing is used for the inspection of metallic pipelines, such as nuclear fuel rods, titanium tubes, and threaded tubes.
Pre-service and in-service inspection of gun barrels, missile launchers, artillery shell bases, and cartridge cases used in military weaponry, as well as aircraft engine blades, wings, landing gear, and wheel hubs.
Metal magnetic memory technology is used for the early diagnosis of metallic structural components in armored vehicles, naval vessels, and other applications.
Low-frequency electromagnetic fields and magnetic flux leakage techniques are used for quality control of ferromagnetic materials and welds in applications such as decks and oil storage tanks.
Research and development of post-vortex flow detection technology includes:
Improve the design theory of transducers and develop eddy-current testing transducers with superior performance; investigate eddy-current localization techniques and three-dimensional imaging methods for determining defect size, shape, location, and depth; research and promote long-range eddy-current testing technology; and further study eddy-current inspection techniques for detecting the propagation and cracking of surface fatigue cracks in metallic materials, as well as machining and grinding burns and residual stresses.
Conclusion:
The application of this technology in nondestructive testing is bound to achieve widespread adoption.
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