What is Eddycurrent testing?
Eddy-current testing is an electromagnetic testing method, which can be applied to any conductive materials. Also referred to as 'Inductive testing'.
The method is normally used for: Crack detection, corrosion and thickness measuring, material sorting, detection of heatimpacted zones, measurement of coatings, measuring electrical conductivity and metal detection.
The equipment, in principle, consists of 3 components: A generator, a testspool and a recording instrument; an ammeter (ampere meter) or oscilloscope.
The generator produces an alternating current which is send through the testspool. As the current runs through the spool, a magnetic field is created around it, the primary field. This magnetic field induces eddy currents in the test piece/material when the spool is held near it. Eddy currents in the test piece will also induce a magnetic field, the secondary field, which is opposite to the primary field. The size of the secondary field compared to the primary is dependant on the electrical and magnetic properties of the test piece.
If there is an ammeter (ampere meter) connected to the test piece, the ammeter will give a standard deviation which indicates the condition of the test piece. If the ammeter is then reset and zeroed, it will stay at 0 as the spool is moved across the test piece, provided that the eddy currents is running unhindered through the piece.
Should the currents change in value, the secondary field size will change accordingly. This change will also translate to the primary field in the spool and change the currents running through it, which can then be measured in the ammeter.
The eddy currents values is affected by changes in the test pieces electrical and magnetic properties, cracks or inclusions in the test piece and distance from the spool to the piece. These properties is what Eddy Current Testing relies on.
When it comes to crack detection, there are several advantages to using Eddy Current Testing, compared to other methods:
- Eddy current testing is done quickly as there is no pre- or post-treatment of the test piece.
- There is no direct contact between the testspool and the test piece, which means it can be applied to painted surfaces without risking damage to the objects.
- It is usable under water.
- The method is cheap in use as there is no consumable materials involved.
- Can be automated for big series of tests.
History of Eddy current testing:
In the 1775-1900 period, scientists like Coulomb, Ampere, Faraday, Ørsted, Arago, Maxwell and Kelvin made discoverys and formulated much of the knowledge we have today about magnetism and electromagnetism. This knowledge is what many of todays principles in electromagnetic, non-destructive testing, is still based upon.
In 1824 Arago discovered that, oscillations of a magnetized pendulum was greatly dampened, in close proximity to a non-magnetic, electric conductive material. In 1820, Ørsted discovered the magnetic field that is created around a conductor, with a current running through it. In the same year, Ampere discovered that two currents running opposite each other in two close-together conductors, cancels out the magnetic field. Faraday discovered the principles of electromagnetic induction in 1831.
Maxwell compiled these and other discoveries in a two-volume piece, published in 1873. Maxwell's equations for magnetism and electromagnetism is still in use as the foundation for research in these scientific fields today.
It would take quite a few years before this knowledge was applied to non-destructive testing (NDT). In the 1930's and 40's, big advancements were made however. One of the driving forces behind these advancements was the German scientist, Dr. Förster.In this period he performed many groundbreaking experiments and formulated theories for eddy current testing. Furthermore he constructed new types of equipment to perform the testing.
Since Dr. Förster constructed his first devices, the development in eddy current testing took off. Especially the period of 1975-1985, the development was immense. Equipment and devices were developed as well as the popularity of the method itself. In the mid 1980's, the first generations of microprocessor based equipment showed up on the market. These new equipments enabled storage of data; testing data, reference flaws, indications etc.