An eddy current is an electrical current induced in a piece of metal due to the relative motion of a nearby magnet. Any time a magnet passes a metallic object, its magnetic field induces an electric current, which swirls around near the surface of the metal like an eddy in a river. This electric current creates its own magnetic field, which opposes the motion of the magnet.
- This module is intended to present information on the NDT method of eddy current inspection.
- Eddy current inspection is one of several methods that use the principal of “electromagnetism” as the basis for conducting examinations. Several other methods such as Remote Field Testing (RFT), Flux Leakage and Barkhausen Noise also use this principle.
- Electromagnetic induction
- Generation of eddy currents
- Inspection applications
- Equipment utilized in eddy current inspection
- Reference standard
- Advantages and Limitations
- Glossary of Terms
- Eddy currents are created through a process called electromagnetic induction.
- When alternating current is applied to the conductor, such as copper wire, a magnetic field develops in and around the conductor.
- This magnetic field expands as the alternating current rises to maximum and collapses as the current is reduced to zero.
- If another electrical conductor is brought into the proximity of this changing magnetic field, the reverse effect will occur. Magnetic field cutting through the second conductor will cause an “induced” current to flow in this second conductor. Eddy currents are a form of induced currents!
Generation of Eddy Currents
Eddy currents are induced electrical currents that flow in a circular path. They get their name from “eddies” that are formed when a liquid or gas flows in a circular path around obstacles when conditions are right.
In order to generate eddy currents for an inspection a “probe” is used. Inside the probe is a length of electrical conductor which is formed into a coil.
Alternating current is allowed to flow in the coil at a frequency chosen by the technician for the type of test involved.
A dynamic expanding and collapsing magnetic field forms in and around the coil as the alternating current flows through the coil.
When an electrically conductive material is placed in the coil’s dynamic magnetic field electromagnetic, induction will occur and eddy currents will be induced in the material.
Eddy currents flowing in the material will generate their own “secondary” magnetic field which will oppose the coil’s “primary” magnetic field.
This entire electromagnetic induction process to produce eddy currents may occur from several hundred to several million times each second depending upon inspection frequency.
Eddy currents are strongest at the surface of the material and decrease in strength below the surface. The depth that the eddy currents are only 37% as strong as they are on the surface is known as the standard depth of penetration or skin depth. This depth changes with probe frequency, material conductivity and permeability
- There are three characteristics of the specimen that affect the strength of the induced eddy currents.
- The electrical conductivity of the material
- The magnetic permeability of the material
- The amount of solid material in the vicinity of the test coil.
- Information about the strength of the eddy currents within the specimen is determined by monitoring changes in voltage and/or current that occur in the coil.
- The strength of the eddy currents changes the electrical impedance (Z) of the coil.
- Impedance (Z) in an eddy current coil is the total opposition to current flow. In a coil, Z is made up of resistance (R) and inductive reactance (XL).
- Resistance – The opposition of current flow, resulting in a change of electrical energy into heat or another form of energy.
- Inductive Reactance (XL) – Resistance to AC current flow resulting from electromagnetic induction in the coil.
- Impedance (Z) – The combined opposition to current flow resulting from inductive reactance and resistance.
One of the major advantages of eddy current as an NDT tool is the variety of inspections that can be performed. The following slides depict some of the these capabilities.
Material Thickness Measurement
- Thickness measurements are possible with eddy current inspection within certain limitations.
- Only a certain amount of eddy currents can form in a given volume of material.
- Therefore, thicker materials will support more eddy currents than thinner materials.
- The strength (amount) of eddy currents can be measured and related to the material thickness.
- Eddy current inspection is often used in the aviation industries to detect material loss due to corrosion and erosion.
- Eddy current inspection is used extensively to inspect tubing at power generation and petrochemical facilities for corrosion and erosion.
Crack detection is one of the primary uses of eddy current inspection. Cracks cause a disruption in the circular flow patterns of the eddy currents and weaken their strength. This change in strength at the crack location can be detected.