Eddy Current Testing
The most basic form of electromagnetic testing(eddy current testing, ECT), mainly used to inspect non-ferromagnetic materials. It can however, be used to detect surface-breaking defects in ferrous materials. This technique is suitable for detecting and sizing metal discontinuities such as corrosion, erosion, wear, pitting, baffle cuts, wall loss, and cracks. Eddy Current testing is based on measuring changes in probe impedance as the probe passes over the defects.
ECT is commonly used in tube testing (heat exchangers) and for surface applications (turbines, aircraft).
Examples of Non-Ferromagnetic Materials
These should be tested using the ECT method which will provide a high inspection speed and detect-ability.
- Stainless Steel
- Cu-Ni alloys
How it Works
A single element probe consisting of a copper wire is excited with an alternating electrical current, which then produces a magnetic field around itself in the direction ascertained by the right-hand rule. The magnetic field fluctuates at the same frequency as the current running through the coil. When placed near a conductive material, currents opposed to the ones in the coil are induced in the material called (“eddy currents”). When these eddy currents pass any defects that change the eddy current flow they also affect the impedance of the coils in the probe.
- Great for detecting corrosion, wear, large porosity & cracking
- No contact necessary
- Minimal preparation of the surface to be inspected
- Inspection of boilers, feed-water heaters, air coolers, heat exchangers
- Excellent at detecting surface-breaking, near-surface and far-surface defects.
Common probe types (Conventional eddy current testing)
Eddy current probes are available in a large variety of shapes and sizes. Eddy current probes are classified by the configuration and mode of operation of the test coils.
- Absolute probes
Absolute probes generally have a single test coil that is used to generate the eddy currents and sense changes in the eddy current field. AC is passed through the coil and this sets up an expanding and collapsing magnetic field in and around the coil. When the probe is positioned next to a conductive material, the changing magnetic field generates eddy currents within the material. Absolute coils can be used for flaw detection, conductivity measurements, liftoff measurements and thickness measurements.
- Differential probes
Differential probes have two active coils usually wound in opposition, when the two coils are over an area free from flaws, there is no differential signal developed between the coils since they are both inspecting identical material. However, when one coil is over a defect and the other is over good material, a differential signal is produced. They have the advantage of being very sensitive to defects yet relatively insensitive to slowly varying properties such as gradual dimensional or temperature variations.
- Reflection probes (transmit-receive)
Reflection probes have two coils similar to a differential probe, but one coil is used to excite the eddy currents and the other is used to sense changes in the test material. Often Referred to as driver/pickup probes. The advantage of reflection probes is that the driver and pickup coils can be separately optimized for their intended purpose. The driver coil can be made so as to produce a strong and uniform flux field in the vicinity of the pickup coil, while the pickup coil can be made very small so that it will be sensitive to very small defects.