current techniques can be used to perform a number of dimensional
measurements. The ability to make rapid measurements without the
need for couplant or, in some cases even surface contact, makes
eddy current techniques very useful. The type of measurements that
can be made include:
thickness of thin metal sheet and foil, and of metallic coatings
on metallic and nonmetallic substrate
cross-sectional dimensions of cylindrical tubes and rods
thickness of nonmetallic coatings on metallic substrates
Corrosion Thinning of Aircraft Skins
application where the eddy current technique is commonly used
to measure material thickness is in the detection and characterization
of corrosion damage on the skins of aircraft. Eddy current techniques
can be used to do spot checks or scanners can be used to inspect
small areas. Eddy current inspection has an advantage over ultrasound
in this application because no mechanical coupling is required
to get the energy into the structure. Therefore, in multi-layered
areas of the structure like lap splices, eddy current can often
determine if corrosion thinning is present in buried layers.
Eddy current inspection has an advantage
over radiography for this application because only single sided
access is required to perform the inspection. To get a piece of
film on the back side of the aircraft skin might require removing
interior furnishings, panels, and insulation which could be very
costly. Advanced eddy current techniques are being developed that
can determine thickness changes down to about three percent of the
Thickness Measurement of Thin Conductive
Sheet, Strip and Foil
Eddy current techniques are used to measure the thickness of
hot sheet, strip and foil in rolling mills, and to measure the
amount of metal thinning that has occurred over time due to corrosion
on fuselage skins of aircraft. On the impedance plane, thickness
variations exhibit the same type of eddy current signal response
as a subsurface defect, except that the signal represents a void
of infinite size and depth. The phase rotation pattern is the
same, but the signal amplitude is greater. In the applet, the
lift-off curves for different areas of the taper wedge can be
produced by nulling the probe in air and touching it to the surface
at various locations of the tapered wedge. If a line is drawn
between the end points of the lift-off curves, a comma shaped
curve is produced. As illustrated in the second applet, this comma
shaped curve is the path that is traced on the screen when the
probe is scanned down the length of the tapered wedge so that
the entire range of thickness values are measured.
When making this measurement, it is important to keep in mind
that the depth of penetration of the eddy currents must cover
the entire range of thicknesses being measured. Typically, a frequency
is selected that produces about one standard depth of penetration
at the maximum thickness. Unfortunately, at lower frequencies,
which are often needed to get the necessary penetration, the probe
impedance is more sensitive to changes in electrical conductivity.
Thus, the effects of electrical conductivity cannot be phased
out and it is important to verify that any variations of conductivity
over the region of interest are at a sufficiently low level.
Measurement of Cross-sectional Dimensions
of Cylindrical Tubes and Rods
Dimensions of cylindrical tubes and rods can be measured with
either OD coils or internal axial coils, whichever is appropriate.
The relationship between change in impedance and change in diameter
is fairly constant, except at very low frequencies. However,
the advantages of operating at a higher normalized frequency are
twofold. First, the contribution of any conductivity change to
the impedance of the coil becomes less important and it can easily
be phased out. Second, there is an increase in measurement sensitivity
resulting from the higher value of the inductive component of
the impedance. Because of the large phase difference between the
impedance vectors corresponding to changes in fill-factor and
conductivity (and defect size), simultaneous testing for dimensions,
conductivity, and defects can be carried out.
Typical applications include measuring eccentricities of the
diameters of tubes and rods and the thickness of tube walls. Long
tubes are often tested by passing them at a constant speed through
encircling coils (generally differential) and providing a close
fit to achieve as high a fill-factor as possible.
An important application of tube-wall thickness measurement is
the detection and assessment of corrosion, both external and internal.
Internal probes must be used when the external surface is not
accessible, such as when testing pipes that are buried or supported
by brackets. Success has been achieved in measuring thickness
variations in ferromagnetic metal pipes with the remote field
Thickness Measurement of Thin Conductive
It is also possible to measure the thickness of a thin layer
of metal on a metallic substrate, provided the two metals have
widely differing electrical conductivities (i.e. silver on lead
where s= 67 and 10 MS/m, respectively).
A frequency must be selected such that there is complete eddy
current penetration of the layer, but not of the substrate itself.
The method has also been used successfully for measuring thickness
of very thin protective coatings of ferromagnetic metals (i.e.
chromium and nickel) on non-ferromagnetic metal bases.
Depending on the required degree of penetration, measurements
can be made using a single-coil probe or a transformer probe,
preferably reflection type. Small-diameter probe coils are usually
preferred since they can provide very high sensitivity and minimize
effects related to property or thickness variations in the underlying
base metal when used in combination with suitably high test frequencies.
The goal is to confine the magnetizing field, and the resulting
eddy current distribution, to just beyond the thin coating layer
and to minimize the field within the base metals.