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 - A Transient
Eddy-current Scanner
is an advanced transient eddy-current scanning system using
probes containing Hall sensors. The system now has recognized potential for the
detection of deep corrosion and cracks in ageing aircraft fleets. There are
significant benefits to be realized from the use of transient eddy-currents in
terms of reduced inspection time and ease of acquisition and analysis of the
data. Large areas of structure incorporating multiple variations in thickness
can be scanned without the need for any probe or set-up changes. In addition,
the use of a Hall sensor rather than a coil as a magnetic field detector
improves the spatial resolution and detectability of deep defects.
can be linked to a wide variety of scanners as a plug-in to the
ANDSCAN® software (eg
SAIC UltraImageIV scanner), or alternatively as a
transient eddy-current capability in The Boeing Company
MAUS® scanner.
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Technology Transfer
This
plug-in will be commercially exploited through two licensees, depending on
territory, as follows:
United
States of America and Canada:
NDTS Inc
Introduction to Transient
Eddy-currents
Transient (or ‘pulsed’) eddy currents have been used for several years in
non-destructive testing (NDT), primarily for the detection of small cracks
emanating from fastener holes deep in the structure(1). However, that
implementation looks for asymmetry in the signal from around the fastener and
requires centering of the probe over each fastener. The technique is therefore
time-consuming for use on large numbers of fasteners. Conventional
continuous-wave eddy-current techniques are also able to detect cracks around
fasteners using symmetry effects with similar centering requirements.
For
corrosion detection, past attention has focused on conventional eddy-currents to
detect general thinning in multi-layered structures. In order to exclude
structural effects such as plate separation it is necessary to use multiple
frequencies and determine an empirical method for combining them. As the
structure changes the required frequencies change, and so does the method of
combination. Hence, for large area scanning, the need to frequently change
settings means that these scans are also very time-consuming.
Transient eddy-current methods offer significant benefits in terms of ease of
data capture and analysis with potential for greatly reduced overheads
associated with scanning large areas of structure using eddy-currents. However,
there is an urgent need for a clear presentation of the capabilities and
limitations of transient eddy-currents in terms of various inspection scenarios.
A key issue at present is to assess the capabilities of transient eddy-current
methods for the detection and characterization of corrosion. The other main
requirement is for the detection of sub-surface cracks deep in the structure,
often initiated from corrosion sites.
The transient
eddy-current scanning system () was developed at
QinetiQ Ltd, Farnborough(2–4). Additionally a collaborative program
between QinetiQ Ltd (formerly the Defense Evaluation and Research Agency, DERA),
Farnborough, UK and the Aeronautical and Maritime Research Laboratory (AMRL),
Melbourne, Australia expanded on the system's
capabilities.
Area scans can be performed using a variety of types
of scanner to digitize the probe position. Further
development work, funded by the USAF AFRL, integrated the
technology
into two large area C-scan systems (SAIC
UltraImageIV scanner and The Boeing Company
MAUS® scanner).
An
important distinguishing feature of the QinetiQ transient eddy-current systems
is that they use Hall-effect sensors to measure directly the magnetic field.
Other transient eddy-current systems have been developed at Iowa State
University in the USA(5,6)
and by workers in Canada(7). Both of these
systems originally used a coil to sense the magnetic field and therefore measure
the rate of change of field, rather than the field itself. This results in a
sensitivity to defects which is related to frequency-squared for a coil, rather
than to frequency for a field sensor, giving a relatively poorer sensitivity to
deep defects which are detected preferentially by the lowest frequency
components of the transient signal.
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Potential advantages of transient eddy currents
It is
important to realize that, whilst some advantages come from using transient
excitation of the coil, others come from the use of a Hall sensor due to its
flat frequency response and optimal spatial resolution. In addition, Hall
sensors are not restricted to transient eddy-current systems and have been used
with conventional continuous-wave systems at QinetiQ.
The
Main Advantages of Transients are:
1)
The ease of scanning large areas of complex structure without the need to
change any setup parameters,
2)
The ease of analysis of the data and ability to distinguish between
structural changes and defects,
3)
The ability to compensate during post-processing for lift-off and edge
effects,
4)
The scope for off-line post-processing, as well as real-time processing,
of the transient data,
5)
The speed of acquisition - a transient system gives equivalent
information to a swept-frequency measurement, but in about 10ms compared with a
minimum of several seconds for a swept-frequency measurement, and
6)
Lower instrumentation costs than for multi-frequency conventional
eddy-currents, for which costs increase with the number of channels required.
The
most important advantage with transient eddy-currents is the processing to
untangle the different contributions to the transient response, producing
unambiguous defect discrimination and quantitative measurements of material
thinning.
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Distinguishing defects from
structure
Measurements of
the percentage change in total thickness are possible, computed using an
algorithm described previously(4,8). An application of this algorithm
is its ability to distinguish between thickness change and other structural
effects such as plate separation (see Figure 2). Once defects have been detected
and their type distinguished it is also desirable to know their depth within the
structure. One method, involving measuring the time to the peak of the transient
can be used to produce simple time-of-flight scans, which can be related to
depth in the structure via a previous calibration. From this information, it can
be determined which layer the defect is in.
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