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Eddy Current Testing (ET)

Eddy current testing method is used for testing of the electrically conductive materials. This method is applicable in wide scale of the branches, very often used for inspections of metallurgic semiproducts during the in-service inspections of pipe type heat exchangers and in the aerospace and automotive industry. This method is one of the group called surface testing methods. It means, it is mainly used for surface discontinuities detection. The discontinuities can be detected to specific depth under surface. This method allows measuring thickness of nonconductive coating or selecting the different materials on the base of the difference in the chemical composition or heat treatment.

Physical background

Sensor is usually composed from two coil systems - actuating and receiving. High frequency current flows through the actuating - primer coil (the kHz or MHZ frequencies are usually used). The alternating magnetic field is generated in the surrounding of the actuating coil. This field induces flow of eddy current into the material. This current induces the secondary magnetic field, which operates against the primer field. Both fields together affect the receiving coil, where the voltage is induced. In case of change of the examined material (material properties change, discontinuity presence etc.) or the distance part - probe is changed, the change of parameters in the receiver coil output occurs. This change represents indication, which is consequently processed and evaluated.
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