28 JOB KNOWLEDGE
EDDY CURRENT TESTING JOB KNOWLEDGE 123
The conductivity of the material affects the depth of penetration with a greater flow of eddy current at the surface in high conductivity metals and a subsequent decrease in penetration in metals such as copper and aluminium.
Permeability is the ease with which a material can be magnetised. The greater the permeability the smaller will be the depth of penetration. ’Non-magnetic’ metals such as austenitic stainless steels, aluminium and copper have very low permeability whereas the ferritic steels have a magnetic permeability several hundred times greater.
The depth of penetration may be varied by changing the frequency of the alternation current – the lower the frequency the greater is the depth of penetration. Unfortunately, as the frequency is decreased to give this greater penetration the defect detection sensitivity is also reduced. There is therefore, for each test, an optimum frequency to give the required depth of penetration and sensitivity.
Eddy current testing is an inspection method that can be used for a variety of purposes including the detection of cracks and corrosion, material and coating thickness measurement, material identification and, in certain materials, heat treatment condition. The process relies upon a material characteristic known as electro-magnetic induction. When an alternating current is passed through a conductor – a copper coil for example – an alternating magnetic field is developed around the coil, the field expanding and contracting as the alternating current rises and falls. If the coil is then brought close to another electrical conductor the fluctuating magnetic field surrounding the coil permeates the material and induces a circulating or eddy current to flow in the conductor. This eddy current, in its turn, develops its own magnetic field. This ‘secondary’ magnetic field opposes the ‘primary’ magnetic field and thus affects the current and voltage flowing in the coil. Any changes in the conductivity of the material being examined such as near surface defects or differences in thickness will affect the magnitude of the eddy current and this change can be detected using either the primary coil or a second detector coil. This forms the basis of the eddy current inspection technique.
As with any inspection method there are both advantages and disadvantages to eddy current testing. The method can be used only on conductive materials and, although all metals can be inspected, the depth of penetration of the eddy currents varies. Eddy current density is higher and defect sensitivity greatest at the surface and decreases with depth, the rate of the decrease depending on the “conductivity” and “permeability” of the metal.
WELDING WORLD MAGAZINE | ISSUE 06 | DECEMBER 2015
A parameter known as the “standard depth of penetration”, taken as the depth at which the eddy current value has reduced to 37% of that at the surface, can be calculated from the magnetic permeability, the metal’s conductivity and the frequency of the alternating current in the probe. The standard depth of penetration is generally regarded as the criterion by which the efficiency of detection can be judged, although changes in the eddy current can be detected at depths of up to three times this figure. A simple calculation may be used to select the optimum probe frequency. For any particular inspection the accuracy of the measurement of defect size, material thickness, heat treatment condition etc. is largely determined by the design of the coil (or coils) used in the examination whilst detection capability is also determined by material properties and the equipment characteristics. The selection of the probe is therefore critical for accurate results. Some inspections involve sweeping through multiple frequencies to optimize results, or inspection with multiple coils to obtain the best resolution and penetration required to detect all possible flaws. It is always important to select the right probe for each application in order to optimize test performance. The eddy current operator is therefore faced with a material whose conductivity and permeability are physical properties and outside of the operator’s control. The parameters that can be selected are probe size, probe type and frequency of the alternating current, the selection depending upon the test requirements i.e crack detection, corrosion depth, coating thickness, heat treatment condition etc. Some equipment is designed to operate using multiple frequencies or with multiple probes in order to optimize the test performance and achieve the best detection performance and depth of penetration. The results are displayed either as a digital read-out for the more simple examinations such as thickness measurements or displayed
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