While the inherent conductivity of a material is always the same, there are internal factors that can cause what appears to be a change in the inherent conductivity
(a) Alloys.
Alloys are combinations of other metals and/or chemical elements with a base metal. Each metal or chemical element has an individual effect on the conductivity of the base metal.
The conductivity of the base metal is changed to a value related to the composition of the alloy. Thus it is possible to identify basic metals and their alloys by measuring their conductivities.
(b) Hardness.
When a metal or alloy is subjected to heat treatment (or to excessive heat during normal operation) the metal will become harder or softer depending on the material. This change in hardness is brought about by an internal change in the material which also affects the conductivity of the material. This change in conductivity can also be detected by eddy current test methods. An improper heat treatment can be detected in this manner.
(c) Temperature and Residual Stresses.
The ambient temperature and internal residual stresses of a material under test also have an effect on the conductivity of the material. These changes can also be detected by eddy current testing. An increase in the temperature of the material normally results in a decrease in the conductivity of the material. Residual stresses cause an unpredictable, but detectable, change in conductivity.
(d) Conductive Coatings.
The presence of a conductive coating on a conductive material changes the inherent conductivity of the base metal just as an alloy would. However, if the thickness of the cladding varies, the conductivity will vary. This change in thickness can be detected by eddy current testing methods.
Permeability
When an energized test coil is placed upon non-magnetised ferromagnetic material, the field is greatly intensified by the magnetic properties of the material so that a large change in the impedance of the test coil occurs.
If the magnetic field strength at various locations varies even slightly, these small variations have a large effect on the impedance of the coil. These changes in the impedance of the coil are often so large (in comparison to the changes caused by changes in conductivity or dimension) that they mask all other changes. When specimen geometry permits, this effect may be overcome by magnetizing the material to saturation using a separate coil powered by direct current. Magnetic saturation effectively eliminates any variations in the residual magnetic field due to magnetic variables, and thus allows other variations to be measured.
After testing is completed, the article must be demagnetized.
Frequency
Frequency is one of the few operator controlled variables in eddy current testing. The main use of frequency is controlling depth of penetration, density and phase of induced eddy currents. In general terms, higher frequencies are used to detect surface breaking discontinuities, and lower frequencies for sub surface testing.
Proximity
Lift off and fill factor are the terms used to describe any space that occurs between the article under test and the inspection coil. Each has an identical effect on the eddy currents. Lift off and fill factor are essentially the same thing; one is applied to surface coils and the other to encircling and internal coils.
(a) Lift Off.
When a surface coil is energized and held in air above a conductor the impedance of the coil has a certain value. As the coil is moved closer to the conductor the initial value will change when the field of the coil begins to intercept the conductor. Because the field of the coil is strongest close to the coil, the impedance value will continue to change until the coil is directly on the conductor. Conversely, once the coil is on the conductor any small variation in the separation of coil and conductor will change the impedance of the coil. The lift off effect is so pronounced that small variations in spacing can mask many indications.
(b) Fill Factor.
In an encircling coil, or an internal coil, fill factor is a measure of how well the conductor (test specimen) fits the coil. It is necessary to maintain a constant relationship between the diameter of the coil and the diameter of the conductor. Again, small changes in the diameter of the conductor can cause changes in the impedance of the coil.
This can be useful in detecting changes in the diameter of the conductor but it can also mask other indications.
Geometry
The two main factors in component geometry affecting eddy currents are thickness and edge/end effect.
(a) Thickness.
Changes in material thickness may be caused by manufactured geometry or in service corrosion/erosion. If the material thickness is less than the effective depth of penetration, any change in the material thickness will affect the eddy currents, this can be used to good effect to measure material thickness.
(b) Edge/end effect.
Eddy currents are distorted when the end, or an edge, of a part is approached with the test coil since the currents have no place to flow. The distortion results in a false
Since, to the test coil, the edge of the part looks like a very large crack or hole, there is a very strong reaction that will mask any changes due to other factors. The limit as to how close to the edge a coil can be placed is determined by the size of the coil and any shielding applied.
Probe handling
Under ideal conditions the probe coil should be presented to the test surface at a constant angle to the surface with constant lift off and pressure. Changes in probe angle, contact pressure, or the way the probe is held (hand capacitance) will cause changes to the signal from the probe.
In nondestructive testing where the majority of inspections utilize the hand held surface contact coil method, the influence of a bad probe handling technique cannot be over emphasized. The effects of probe handling can be reduced with the use of special spring loaded probes which maintain the probe at a constant angle and pressure to the surface. These are usually used where scanning is to be carried out on flat surfaces, or where conductivity or paint thickness measurements are being taken. When scanning close to changes of section (geometry effect) the use of simple probe guides will assist in good probe handling resulting in a more effective inspection.
Discontinuities
Cracks cause a distortion of the eddy current field due to the fact that the eddy currents have to flow around them. This results in an increased resistance path and a corresponding reduction in eddy current strength. Similarly corrosion causes an increased resistance within the material with a corresponding reduction in eddy current strength. In each case a meter or spot display change will result.
To ensure that inspections are carried out to a repetitive standard, reference blocks with artificial defects are used. These blocks should be of similar material specification (alloying, heat treatment, conductivity) to the component under test. By setting up the flaw detector (standardization) to give a known response from the artificial defect, an inspection can be carried out repeatedly to the same standard.
The flaw detector sensitivity settings (standardization) must be checked, at regular intervals, and as a minimum.
(a) Prior to each inspection.
(b) After each inspection.
(c) When obtaining a suspect fault indication/prior to confirming a fault indication.
Note: If it is discovered that the flaw detector setting are incorrect, all components tested since previous correct standardization was confirmed, must be re-tested.
3. INSTRUMENTATION 3.1. Principles and basic characteristics of eddy current probes
Eddy current probes are based on relatively simple principles and usually consist of an assembly containing one or more coils in a suitable configuration. The shape of the coil, its cross-section, size, and configuration are parameters that need to be considered to produce a particular probe suitable for a specific application or range of applications. This coil is energized by an alternating current of known frequency and amplitude which gives rise to the magnetic field which is also of varying type. When this coil is brought closer to a conductive test material, there is an induced voltage generated in the sample.