Eddy current testing is a type of non-destructive testing which is used to detect the defects in components without damaging or reducing the service life of the component.
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Principle of eddy current testing
When a coil carrying alternative current comes in a vicinity of a metallic material, due to electromagnetic induction eddy current is produced in the metallic material. If the defect is present in the material, the magnitude of induced current changes. This magnitude depends on the following factors –
- Frequency and magnitude of alternating current
- Permeability of material
- Electrical conductivity of the material
- Hardness of material
- Different types of defects in material
- Shape of material
- The relative position of a coil carrying alternative current and material
Changes in any of the above factors result in the variation in induced current. Therefore it is possible to identify the defect defective component among others by keeping the factors one and sixth constant on the basis of induced current. Change in eddy current also changes the magnetic impendence of the material/specimen and it converts into voltage and can be observed on the voltmeter.
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Due to spacing between material and coil, impendence value changes. It creates some problems for identifying the correct induced current and observed could be misled by thinking that it is a defect in the component. This problem can be overcome by using the phase and time relation of eddy current.
Types of coil uses in eddy current testing
There are two types of coil are being used in eddy current testing as follows
In absolute coil, the inductance of the test material/specimen is compared with the standard material/specimen.
These coils are used in pairs and connected in series opposite to each other. Standard material is wrapped by one coil and the material which we are testing is surrounded by another coil. If we get output zero from two coils means there is no defect in our material.
Testing coils can be flat or cylindrical. To get better sensitivity and measurement, a proper coil should be used. For flat specimens flat coils should be used and vice versa. The sensitivity of the instruments also depends upon the size of the coils. The large size of the coils reduces the sensitivity of the measuring instrument.
For testing, the alternate current frequency uses in the ranges between – 200Hz to 6 x 106
Applications of eddy current testing
- Eddy current testing is used to measure properties as magnetic permeability, electrical conductivity, condition after heat treatment, dimensions, grain size, and hardness.
- It is also used to measure coating thickness.
- Differentiate dissimilarly of metals.
- Identify differences in their structure and composition.
Advantages of eddy current testing
- Speed of eddy current testing is very fast.
- This testing is suitable for both ferrous and nonferrous components.
- Due to digitalization, permanent records of the testing can be available for a long time.
- The testing specimen does not come in direct contact with electricity.
- This testing has the flexibility to test the entire production output because of its automation.
Limitations of eddy current testing
- It requires the proper size of the coil to maintain the sensitivity of instruments.
- This method only works on metallic components.
- The shape of the coil should be similar to the shape of the testing specimen.
- As this testing is digital, the instruments require are costly.
- Instruments used in the testing should be standardized.
- Calibration of the instruments is very important.
- Both at or near the surface defects can be identified.
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In this article, we have seen the principle, applications, advantages, and limitations of the eddy current testing. Now we know how does eddy current testing works and is being used in the non-destructive testing of the material. Feel free to ask any doubt in the comment section.