History of ET

1831: Discovery of the electromagnetic induction phenomenon by Michael Faraday (UK) and Joseph Henry (USA.)

1855: Leon Foucault demonstrates the existence of induced currents in a metallic body placed close to an electromagnetic field. These induced currents are the eddy currents, named from “eddies” that are formed when a liquid or gas flows in a circular path around obstacles.

1879: The first Eddy Current application is developed by David Hughes for material sorting.

1920’s: Eddy current devices begin to appear in the steel industry for measurements on billets, round stock, and tubing. The first eddy current thickness-measurement equipment is developed in 1926.

11950’s: Dr. F. Förster is the first to combine precise theoretical and experimental work to develop the first instrument based on the impedance plane. He is known as the father of modern Eddy Current technology.

1960’s: Better understanding of the eddy current theory is used to develop several applications, mainly for aerospace and nuclear industries.

1970’s: H.L. Libby pioneers self-balancing systems and multi-frequency inspection methods. Dodd and Deeds develop analytical and numerical models explaining the eddy current phenomenon.

1990’s: Multi-channels and multi-frequencies eddy current instruments become available. They are mainly used in the nuclear industry. Several alternative electromagnetic techniques are developed to inspect ferromagnetic material. Among them, the RFT technique for tube testing and the ACFM technique for weld inspection.

2000’s: The eddy current array technology, used in nuclear industries since several years, become more accepted for industrial in-line system and aerospace applications.

Production of a magnetic field:

• Oersted discovered that any wire carrying an alternating current produces a magnetic field around itself.
• The direction of the current relative to the magnetic field is determined with the right-hand rule:
• When the fingers of the right hand bend in the direction of the magnetic field, the thumb points in the direction of the current flow.
• When the wire is shaped into a coil, the interaction of each turn produces a global magnetic field around the coil.
• This magnetic field oscillates at the same frequency as the current injected into the coil.

Induction of eddy currents:

• When a coil is placed over a conductive part, opposed alternating currents are generated; these are the eddy currents.
• The eddy currents oscillate at the same frequency as the current injected in the coil but with a small delay; this is the phase lag.
• If a defect in the part disturbs the path of the eddy currents, it creates a local magnetic field that changes the balanced condition of the system.
• Such changes can be detected by monitoring variations of the coil impedance

Depth of penetration:

• The induction of eddy currents in a conductive material is a diffusion phenomenon with limited penetration.
• For this reason, the Eddy Current technique is only applicable to the detection of surface or near-surface defects.

Penetration range of a few millimetres, depending on the physical properties of the material

Eddy current testing:Eddy-current testing uses electromagnetic induction to detect aw in conductive materials. It is used to detect near-surface cracks and corrosion in metallic objects such as tubes and air-craft fuselage and structures. ECT is more commonly applied to nonferromagnetic materials, since in ferromagnetic materials the depth of penetration is relatively small.

ProcedureIn a standard eddy current testing a circular coil carrying current is placed proximity to the test specimen (electrically conductive). The alternating current in the coil generates changing magnetic field which interacts with test specimen and generates eddy current. Variations in the phase and magnitude of these eddy currents can be monitored using a second 'search' coil, or by measuring changes to the current owing in the primary 'excitation' coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any aws, will cause a change in eddy current ow and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard (at coil) eddy current inspection, the most widely used eddy current technique. However, eddy-current testing can detect very small cracks in or near the surface of the material, the surfaces need minimal preparation, and physically complex geometries can be investigated. The testing devices are portable, provide immediate feedback, and do not need to contact the item.

Remote field testing (RFT):It is an electromagnetic method of non-destructive testing whose main application is finding defects in steel pipes and tubes. RFT may also referred to as RFEC (remote field eddy current) or RFET (remote field electromagnetic technique). An RFT probe is moved down the inside of a pipe and is able to detect inside and outside defects with approximately equal sensitivity (although it cannot discriminate between the two). Although RFT works in nonferromagnetic materials such as copper and brass, its sister technology eddy-current testing is more effective in these materials.

Pulsed eddy current method:A method for the detection and the characterization of corrosion in multi-layer metallic structures. For this technique, a coil (or coils) is used both as field source (driven by a square wave voltage-controlled excitation), and/or as field sensor (measuring a transient response). The field sensor allows the capture of information about the condition of the area of the structure under inspection. The ability of this technique to detect corrosion hinges on the use of a transient response feature (i.e., Lift-off Point of Intersection) to infer the presence of material loss.