reading this section you will be able to do
how eddy currents are used in NDT.
why NDT is so important to our society.
Nondestructive testing (NDT) means
exactly what the words say. NDT literally means testing materials
without destroying them. In other words, we can look for defects
in a variety of metallic materials using eddy currents and never
harm the material that we are testing. This is important because
if we destroy the material we are testing, it does not do much
good to test it in the first place. NDT is very important because
often the defects that we are looking for are not visible because
paint or some other coating may cover them. There might also be
defects that are so small they cannot be seen with our eyes or
any other visual method of inspection. Therefore, inspection methods
such as eddy current inspection have been developed to detect
an Eddy Current Experiment
In the experiment below you will use
eddy current testing to detect cracks in a block of metal. You
will notice that you are using a coil of wire wrapped around a
piece of iron to generate the magnetic field that caused the eddy
currents to form in the metal. In the field of NDT the coil is
called the inspection probe. The magnetic field that is generated
by the eddy currents can be detected using this same probe. We
can monitor the magnetic field being produced by these eddy currents
with an instrument called an eddyscope. If there is a change in
the magnetic field from the eddy currents, we can tell that we
have found some sort of defect in the material that we are testing.
When the instrument sees a change in the magnetic field generated
by the eddy currents, it displays a change in the signal on the
As long as the material being tested
is very uniform in every way, the eddy currents will be uniform
and consistent. If there is some defect in the material such
a crack, the eddy currents will be disturbed from their normal
circular shapes. NDT technicians use many different types
current testing equipment. Some are simple coils that are held
on a piece of metal. Others use special probes, like the one
above, that are pushed inside of tube such as those in heat exchanger
technicians on the right are performing an eddy current inspection
on the tube of a heat exchanger. Heat exchangers are used in places
like nuclear power plants. Radioactive water from the reactor
is circulated through the tubes and cooling water that will be
returned to a river or lake is circulated on the outside of the
tubes. It is very important that the radioactive water and the
cooling water do not mix. Therefore, technicians perform eddy
current inspections on the tubes to find and defects that may
be present before they become leaks in the tubing.
stands for "Nondestructive Testing."
are used to test materials and parts without harming them.
testing is just one of the methods used by technicians to find
defects before they cause problems.