of the uses of eddy current instruments is for the measurement
of electrical conductivity. The value of the electrical conductivity
of a metal depends on several factors, such as its chemical composition
and the stress state of its crystalline structure. Therefore,
electrical conductivity information can be used for sorting metals,
checking for proper heat treatment, and inspecting for heat damage.
The technique usually involves nulling an absolute probe in air and placing the probe in contact with the sample surface.
For nonmagnetic materials, the change in impedance of the coil
can be correlated directly to the conductivity of the material.
The technique can be used to easily sort magnetic materials from
nonmagnetic materials but it is difficult to separate the conductivity
effects from the magnetic permeability effects, so conductivity measurements
are limited to nonmagnetic materials. It is important to control
factors that can affect the results such as the inspection temperature
and the part geometry. Conductivity changes with temperature so
measurements should be made at a constant temperature and adjustments
made for temperature variations when necessary. The thickness
of the specimen should generally be greater than three standard
depths of penetration. This is so the eddy currents at the back
surface of the sample are sufficiently weaker than the variations
in the specimen thickness that are not seen in the measurements.
Generally large pancake type, surface probes are used
to get a value for a relatively large sample area. The instrument
is usually setup such that a ferromagnetic material produces a
response that is nearly vertical. Then, all conductive but nonmagnetic
materials will produce a trace that moves down and to the right
as the probe is moved toward the surface. Think back to the discussion
on the impedance plane and these type of responses make sense.
Remember that inductive reactance changes are plotted along the
y-axis and resistance changes are plotted in the x-axis. Since
ferromagnetic materials will concentrate the magnetic field produced
by a coil, the inductive reactance of the coil will increase.
The effects on the signal from the magnetic permeability overshadow
the effects from conductivity since they are so much stronger.
When the probe is brought near a conductive but nonmagnetic material,
the coil's inductive reactance goes down since the magnetic field
from the eddy currents opposes the magnetic field of the coil.
The resistance in the coil increases since it takes some of the
coil's energy to generate the eddy currents and this appears as
additional resistance in the circuit. As the conductivity of the
materials being tested increases, the resistance losses will be
less and the inductive reactance changes will be greater. Therefore,
the signals will be come more vertical as the conductivity increases,
as shown in the image above.
To sort materials using an impedance plane device, the signal
from the unknown sample must be compared to a signal from a variety
of reference standards. However, there are devices available
that can be calibrated to produce a value for electrical conductivity
which can then be compared to published values of electrical conductivity
in MS/m or percent IACS (International Annealed Copper Standard).
Please be aware that the conductivity of a particular material
can vary significantly with slight variations in the chemical
composition and, thus, a conductivity range is generally provided
for a material. The conductivity range for one material may overlap
with the range of a second material of interest, so conductivity
alone can not always be used to sort materials. The electrical
conductivity values for a variety of materials can be found in
the material properties reference tables.
The following applet is based on codes for nonferrous materials
written by Back Blitz from his book, "Electrical and Magnetic
Methods of Nondestructive Testing", 2nd ed., Chapman &
Hill (1997). The applet demonstrates how an impedance plane eddy
current instrument can be used for the sorting of materials.