Eddy current non-destructive testing (NDT) can be used for testing a range of different metal types as discussed below.
Non-Ferrous Metal Sorting
This is a test for conductivity, and a conductivity meter may be the best option for dedicated applications. It can be observed from the impedance plane diagram that the indication from a variation in conductivity is typically the same as from a crack. Both meter and impedance plane type crack detectors can be effectively used to sort similar metals with the help of an appropriate absolute probe.
The following should be kept in mind: Largely dissimilar metals may have a similar conductivity; under extremely diverse heat treatment-conditions, an alloy of one material can have the same electrical conductivity; the permissible values for similar alloys might overlap; there does not exist a direct connection between hardness and conductivity.
Impedance plan diagram
However, it would be possible to use the conductivity measurement as part of a quality control system after understanding these limitations. Although appropriate test frequencies used are in the range of 10 kHz to 2 MHz, the material thickness should be taken into account to guarantee that the depth of penetration is less than one-third of the material thickness.
Ferrous Metal Sorting
Eddy current impedance plane equipment can be used for sorting ferrous material. Unfortunately, it is not possible to generate quantitative values as the obtained reading is related to magnetic permeability, electrical conductivity, and the depth of change in material properties. Frequency range used includes 100 Hz to 10 kHz. The application of two or more frequencies provides supplementary information regarding the depth of the material properties, for example, in induction hardening.
This information has been sourced, reviewed and adapted from materials provided by Ether NDE Ltd.
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