Why eddy current test

Skilled operators looking to provide the best testing performances can select the right type of probe and determine the demand for additional frequencies and probes. There are also limitations to the depth eddy currents can reach, making ultrasonic non-destructive testing methods much more useful for depth penetration. When used together with ultrasonic methods, eddy current testing can be optimized for superior accuracy and resolution.

The primary use for eddy current testing is to determine if a material has surface or subsurface flaws. ECT provides a major advantage as it can be used to test large volumes of material rather quickly.

The two main features eddy current testing can measure are crack detection and the conductivity of the materials used. Cracks can cause disruption in the flow patterns of the eddy currents and weaken them. Conductivity can be detected because the ECT instruments used are sensitive to any changes in the materials properties. Small surface cracks and defects near the material can indicate the material is not fit for use in the application.

Skilled operators are needed to understand the instruments of the ECT testing process and detect issues with materials. In some cases, a surface finish or the roughness of the material can impact testing. In addition, it is common that eddy current testing is used in determining metal thickness, detecting thinning caused by corrosion or determining coating thickness or magnetic permeability.

Operators must have an intricate knowledge of how to select the proper probe, which fits the geometry of the part and coil to produce the proper current flow.

They must also understand the type of defect they are detecting, where it is located and its position. A skilled operator must be able to work within frequencies to achieve the most optimal resolution for testing.

With the highest density of eddy currents at the surface, sub-surface flaws demand lower frequencies to penetrate deeper into the material, but this will result in less sensitivity. Also, ferromagnetic or other highly conductive materials will demand the use of lower frequencies as well. Trainer operators know selecting the right probe that fits the geometry of the coil and size of the material is essential to conducting a proper examination.

Eddy current testing instruments must also always be calibrated to the proper standards for each test. It is imperative for the operator to identify the test piece and how it behaves under changing conditions.

Calibration involves using reference guidelines like material, shape, size and any preexisting defects such as cuts, holes and milled features. In addition, thickness measurements rely on samples of known thickness. By taking into account preexisting features on the test piece, an operator can determine what is essential when setting up the probe for the testing process.

There are also a variety of different probes that can be used, all of which offer advantages in optimizing the overall performance and results of the testing.

They are also usually larger in diameter and depend on lower frequencies for increased depth penetration. Surface probes are excellent for testing large surface areas. For those applications that require higher frequencies or even higher resolution of near surface flaws, a pencil probe may be used as it offers small diameter coil. For machined parts or materials, sometimes a bolt hole probe is used to inspect the holes in a material.

This can be operated by hand, or by machine, allowing for maximum flexibility. For other types of fastener holes, a donut probe may be used for inspection. Sliding probes which operate at higher scan rates for fastener holes can also be used. In applications with tubing, ID probes can be used for inspection and come in a large range of sizes. In addition, OD probes where the test piece is moved through the coil can be used for the exterior testing of tubes and bars.

Eddy current testing is used is such a wide variety of applications for industries that demand materials to be defect free, but some of the most common applications for ECT are in high demand for welded components or materials that need to be corrosion resistant. This magnetic field oscillates at the same frequency as the current running through the coil. When the coil approaches a conductive material, currents opposed to the ones in the coil are induced in the material — eddy currents.

A defect in the conductive material disturbs the path of eddy currents, creating a local magnetic field that changes the balance of the system.

This can be detected by measuring the changes in impedance in the coil, which is a telltale sign of the presence of defects. With time, different technologies were developed such as Pulsed Eddy Current PEC that detects flaws and corrosion in ferrous materials and Eddy Current Array ECA that uses multiples coils together to get an effective reading of a large area on a single pass.

When it comes to surface applications, the performance of any given inspection technique depends greatly on the specific conditions — mostly the types of materials and defects, but also surface conditions, cleanliness, etc. However, in most situations, the following are true:. Since the created magnetic fields penetrate the coating layer, there usually is no need to remove the coating. Eddy Current Testing ET. Magnetic testing without coating removal.

Used in these markets:. Offshore Wind. Recreational Industry.