State of Eddy Current Inspection
current inspection is used in a variety of industries to find
defects and make measurements. One of the primary uses of eddy
current testing is for defect detection when the nature of the
defect is well understood. In general, the technique is used to
inspect a relatively small area and the probe design and test
parameters must be established with a good understanding of the
flaw that is to be detected. Since eddy currents tend to
concentrate at the surface of a material, they can only be used
to detect surface and near surface defects.
In thin materials such as tubing and sheet stock, eddy currents
can be used to measure the thickness of the material. This makes
eddy current a useful tool for detecting corrosion damage and
other damage that causes a thinning of the material. The technique
is used to make corrosion thinning measurements on aircraft skins
and in the walls of tubing used in assemblies such as heat exchangers.
Eddy current testing is also used to measure the thickness of
paints and other coatings.
Eddy currents are also affected by the electrical conductivity
and magnetic permeability of materials. Therefore, eddy current
measurements can be used to sort materials and to tell if a material
has seen high temperatures or been heat treated, which changes
the conductivity of some materials.
Eddy current equipment and probes can be purchased in a wide
variety of configurations. Eddyscopes and a conductivity tester
come packaged in very small and battery operated units for easy
portability. Computer based systems are also available that provide
easy data manipulation features for the laboratory. Signal processing
software has also been developed for trend removal, background
subtraction, and noise reduction. Impedance analyzers are also
sometimes used to allow improved quantitative eddy-current measurements.
Some laboratories have multidimensional scanning capabilities that
are used to produce images of the scan regions. A few portable
scanning systems also exist for special applications, such as scanning
regions of aircraft fuselages.
to Improve Eddy current measurements
A great deal of research continues to be done to improve eddy
current measurement techniques. A few of the these activities,
which are being conducted at Iowa State University, are described
Photoinductive Imaging (PI)
A technique known as photoinductive imaging
(PI) was pioneered at CNDE and provides a powerful, high-resolution
scanning and imaging tool. Microscopic resolution is available
using standard-sized eddy-current sensors. Development of probes
and instrumentation for photoinductive (PI) imaging is based on
the use of a medium-power (5 W nominal power) argon ion laser.
This probe provides high resolution images and has been used to
study cracks, welds, and diffusion bonds in metallic specimens.
The PI technique is being studied as a way to image local stress
variations in steel.
Pulsed Eddy Current
Research is currently being conducted on the use of a technique
called pulsed eddy current (PEC) testing. This technique can
used for the detection and quantification of corrosion and cracking
in multi-layer aluminum aircraft structures. Pulsed eddy-current
signals consist of a spectrum of frequencies meaning that, because
of the skin effect, each pulse signal contains information from
a range of depths within a given test specimen. In addition,
pulse signals are very low-frequency rich which provides excellent
depth penetration. Unlike multi-frequency approaches, the pulse-signals
lend themselves to convenient analysis. .
Measurements have been carried out both
in the laboratory and in the field. Corrosion trials have demonstrated
how material loss can be detected and quantified in multi-layer
aluminum structures. More recently, studies carried out on three
and four layer structures show the ability to locate cracks emerging
from fasteners. Pulsed eddy-current measurements have also been
applied to ferromagnetic materials. Recent work has been involved
with measuring the case depth in hardened steel samples.