Transient eddy-Current plug-in to ANDSCAN® or MAUS®
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Figure 1 . TRECSCAN® being used with an ANDSCAN® arm and software.
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TRECSCAN® - A Transient Eddy-current Scanner
TRECSCAN® is an advanced transient eddy-current scanning system using probes containing Hall sensors. The system now has recognised potential for the detection of deep corrosion and cracks in ageing aircraft fleets. There are significant benefits to be realised 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.
TRECSCAN 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.
This plug-in will be commercially exploited through two licensees, depending on territory, as follows:
United States of America and Canada: NDTS Inc
Rest of the World: Sonatest plc
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 characterisation 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 (TRECSCAN®) was developed at QinetiQ Ltd, Farnborough(2–4).
Additionally a collaborative programme between
QinetiQ Ltd (formerly the Defence 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 digitise the probe position. Further development
work, funded by the USAF AFRL, integrated the TRECSCAN 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.
It is important to realise 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.
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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Figure 2. Example of how a simple mapping of the transient perpendicular magnetic field (left) can be dominated by signals due to plate separation, obscuring metal thinning which can be revealed using the thickness change algorithm (right). The specimen has machined thinning in between a regular array of fasteners.
These arrays have 9 Hall sensors and TRECSCAN acquires simultaneously on all 9 elements. TRECSCAN can work at around 50 points per second on structures of less than 0.300" thickness, although the rate decreases for thick structures. With a 9-element probe this would increase to about 450 points per second, with some additional savings due to improved scanning technique. If your required lateral scan increment is 1 mm (0.040") then you could manage 1.62 sq m/hr (18 sq ft/hr). If you are prepared to accept a 2 mm (0.080") scan increment then maybe 72 sq ft/hr could be managed. 280 sq ft/hr would be achievable with a 4 mm (0.160") scan increment.
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Author: R A Smith.
Copyright © 2004 QinetiQ Ltd. All rights reserved.
Revised: January 24, 2004
