Introduction to Eddy-current Testing(ECT)

Magnetic particle testing is an easy and reliable NDT method to detect surface and slightly sub-surface defects but can only be used to examine ferromagnetic materials and Dye penetrant testing is limited to detection of open to surface discontinuity in non-porous materials.

History of Eddy current testing:

As already discussed in our blog History of NDT, Most of the NDT methods were brought in industrial applications between the time period of World war I & II.

Similarly, much of the development of eddy current testing (ECT) as a nondestructive testing technique for industrial applications was carried out during World War II in Germany. Professor Friedrich Förster while working for the Kaiser-Wilhelm Institute (now the Kaiser Wilhelm Society) adapted eddy current technology to industrial use, developing instruments measuring conductivity and sorting mixed ferrous components.

Later   Professor Friedrich Förster founded a company in 1948, now called the Foerster Group where he made great steps in developing practical ECT instruments and marketing them

Basic Principle:

eddy current: an electrical current caused to flow in a conductor by the time or space variation, or both, of an applied magnetic field. It’s a circulating electrical current induced in a test object by an alternating magnetic field.

The term eddy current comes from analogous currents seen in water in fluid dynamics, causing localized areas of turbulence known as eddies giving rise to vortices that occur over a prolonged period.

Lenz’s law,

named after the physicist Emil Lenz who formulated it in 1834, states that the direction of the current induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field.

For example, if the North Pole of a magnet is facing a metal plate. the magnetic field generated from the eddy currents will be in the opposite direction of the initial changing magnetic field. Hence south pole is generated in the metal plate towards the north pole of the magnet.

It is a qualitative law that specifies the direction of induced current but states nothing about its magnitude.

Generation of Eddy current in a conductive material:

In the above figure, a conductive metal plate is in a magnetic field(B, green) of a magnet. Now when the Metal plate is moved in direction of V  which causes a change in the magnetic field due to which eddy current (I, RED) is generated in the metal plate.

The increasing field at the leading edge of the magnet (left) induces a counterclockwise current, which by Lenz’s law creates its own magnetic field (left blue arrow) directed up, which opposes the magnet’s field, producing a retarding force. Similarly, at the trailing edge of the magnet (right), a clockwise current and downward counter field is created (right blue arrow) also producing a retarding force.

Elements of Eddy Current System

A basic eddy current system consist:

• Test Coil: Test coil is the basic element of Eddy Current system
• Generator: used to generate an alternating current generator driving an alternating magnetic field (primary field).
• Indicator: Meters, CRT/LCD/LED and Strip recorders are types of indicators used for eddy current testing.

Types of Coils.

Eddy current test methods:

Conductivity testing:

Discontinuity Testing:

Coating/plating thickness Testing:

Industrial Application

• aerospace industry.
• petrochemical industry
• inspecting steam generator tubing in nuclear plants.
• heat exchangers tubing in power and petrochemical industries.
• inspection of partially ferromagnetic materials such as nickel alloys, duplex alloys, and thin-ferromagnetic materials such as ferritic chromium-molybdenum stainless steel.

Advantages

• High-Speed Testing.
• Accuracy.
• It can detect surface and slightly subsurface discontinuities.
• Highly Sensitive to small discontinuities.
• Accurate Thickness measurements.
• Provide Immediate indications
• Is non-contacting.
• Equipment is portable.
• Can inspect complex shapes and sizes

Limitations

• only Conductive materials can be examined
• Limited penetration in test objects
• Several variables simultaneously affect the output indication.
• Indications are qualitative, not quantitative.
• The testing of ferromagnetic materials is sometimes difficult to examine.
• A permanent record of discontinuity is often not available.
• High user Dependence
• Surface finish and surface roughness may interfere with test results
• The test surface must be accessible to the probe.
• Depth of penetration limited.
• Flaws like delaminations lie parallel to the probe coil winding and probe scan direction are undetectable

References’

• https://en.wikipedia.org/wiki/Eddy-current_testing
• https://www.sciencedirect.com/topics/chemistry/eddy-current
• Cover image: https://www.tec-science.com/material-science/material-testing/eddy-current-testing-ect/
• PTP classroom Training Handbook  Eddy Current Testing.
• http://www.geocities.ws/raobpc/Probes.html