A PRACTICAL IMPLEMENTATION OF TRANSIENT EDDY CURRENTS FOR CORROSION AND CRACK DETECTION Jesse A. Skramstad, NDT Solutions, Inc. Robert A Smith, QinetiQ Ltd UK Nancy Wood, Boeing Aircraft Company The 6th Joint FAA/DoD/NASA Conference on Aging Aircraft 16 19 September 2002
Abstract Future inspection techniques for aging aircraft will have to be able to acquire data over large areas of complex structure, with numerous s changes of thickness, without the need for adjusting the acquisition parameters, ameters, in addition to being able to reliably detect defects of appropriate severity. Transient eddy-current technology is one of the few methods offering reliable defect detectability over a wide range of depths without t the need to make adjustments at acquisition time. While the technology exists to perform these transient eddy-current scans, further work is required to couple it to large-area area scanners currently in use with the US Air Force and Civil airlines. This poster reports progress on the integration of the MAUS and Ultra Image IV scanners with the TRECSCAN transient eddy-current imaging system.the first scans from a MAUS system using transient eddy-currents will be presented, with details of the remaining integration program, the aim of which is to link transient eddy-currents seamlessly into these scanning systems.
What is TRECSCAN? TRECSCAN is a Prototype Transient Eddy Current Inspection System Unique Analysis Software that Includes: Edge Subtraction (Correction) Lift-Off Compensation Total Thickness Measurement Plate Separation Effect Elimination Time Domain Eddy Current
Why Is TRECSCAN Different? Uses a Hall sensor as a field detector instead of a coil Eddy current pulses are generated using a coil and the Magnetic Field is measured using a Hall sensor Measures and captures the complete Transient data set
What is a Hall Sensor? In a Semi Conductive Platelet, the Hall Voltage Is Generated by the Effect of an External Magnetic Field Acting Perpendicularly to the Direction of the Current.
Hall Sensor Advantages Hall sensors respond to a wide frequency range allowing the capture of detailed transient data sets. Hall sensors provide direct measurement of the magnetic field, essential for the application of analytical theory to the transient data. A A Hall sensor as a field detector rather than a coil, improves the spatial resolution and the detectability of deep defects.
Transient Eddy-Currents: Digitise time-domain response to coil-current reversal Use Hall sensor to measure transient magnetic field directly Scan to measure transient as a function of position, produce images, and store full transient data set Drive Drive Response Response Conventional Transient
The Transient Eddy Current Method Coil Current i input current i(t) time t i(t) H z (t) Magnetic Field Hz H z (air) H z (specimen) H z (defect) time t H R z = H z (specimen) - H z (air) Ferrite core Drive coil Hall effect sensor First layer Gap Second layer Corrosion
Transient Eddy-Currents Transient signals from structural changes at different depths.
Transient Data Sample Storage Time-slices Separate C-scan C images can be produced from each time-slice. TRECSCAN stores the transient response at an exponentially distributed sequence of time points, to exploit the fact that the transient response varies much more slowly later in time. This time-slice data is saved at each probe position, permitting postprocessing of the data after acquisition.
Advantages of Transient Eddy Current Full frequency-range is captured in a data set Large areas of structure with multiple variations in thickness can be scanned without the need for probe or set-up changes The use of a Hall sensor as a field detector improves the spatial resolution and the detectability of deep defects. Structure variations can be optimised during analysis Advanced post-processing analysis tools
Post Analysis Tools Lift-off compensation Edge subtraction (correction) Total thickness measurement Plate separation effect elimination Timeslice viewing Time domain signal processing
The Transient Eddy Current Signal Includes a Plethora of Information, Hence the Processing of the Transient Data Set is Very Important. TRECSCAN includes unique analytical software that untangles the different contributing responses in the transient data set as a post process analysis. This software includes: Lift-off compensation The lift-off compensation algorithm eliminates signals from variations in probe lift-off caused by variable gap in multiple-layered structures. Edge subtraction The edge subtraction algorithm helps remove signals due to edges and other systematic variations in substructures.
Lift-Off Problem DC10 Crown Splice 1.80 mm (0.071 ) 3) 2) 1.14 mm (0.045 ) or 0.83 mm (0.032 ) 1) 1.60 mm (0.063 ) 4) 5) 1.91 mm (0.075 ) or 3.18 mm (0.125 ) Probe Tapered and stepped 1.5 mm (0.060 ) to 4.7 mm (0.185 ) Brush 1) 2) 3) 4) 5) Layers: 1) Exterior Strap 2) Skin Panels 3) Finger Doublers 4) Longeron 5) Doublers Probe Lift-Off from.063 Strap Graphic Courtesy of QinetiQ Ltd.
DC10 Specimen Courtesy Sandia National Labs, AANC Lift-Off Compensation DC10 Crown Splice Lift-Off Lift-Off Removed Without L/O Compensation With L/O Compensation
Edge Subtraction on a DC10 Crown Splice Joint EDM NOTCHES With L/O Comp With Edge Subtraction
DC10 Time-Slice Examples 1 2 3 4 5 Time-Slices Later in Time Represent Lower Frequencies
Defect Depth Measurement Time-To To-Peak A method for measuring the time to the peak of a balanced transient can be used to produce simple time-of-flight scans, which can be related to depth in the structure. For a given defect type the Time-To-Peak value should increase with defect depth. A calibration is required in order to calculate unknown defect depths.
Metal Loss Measurement Percentage Total Thickness Metal loss measurement in TRECSCAN is accomplished using an analytical method that calculates percentage change in total metal thickness relative to the balance point. This method gives a percentage material loss that does not need to be calibrated, provided certain criteria are met from the assumptions of the underlying theory.
System Integration Objectives Integrate TRECSCAN into the MAUS IV C-scan system Couple TRECSCAN with the Ultra Image C-scan system Evaluate C-scan data collection techniques to demonstrate production environment inspection capabilities
Original TRECSCAN System TRECSCAN DLL ANDSCAN 2000 Software TRECSCAN Interface Box Probe: Hall Sensor
TRECSCAN +MAUS IV TRECSCAN DLL ANDSCAN 2000 Software TRECSCAN Interface Box MAUS IV + Digital Position Output Card Proof of Concept Version Probe: Hall Sensor
TRECSCAN /MAUS IV INTEGRATION TRECSCAN DLL MAUS IV TRECSCAN BOARD Operator Interface Familiarity Probe: Hall Sensor
TRECSCAN +Ultra Image Integration TRECSCAN DLL ANDSCAN 2000 Software TRECSCAN Interface Box Ultra Image Scanning System ANDSCAN Operator Interface Probe: Hall Sensor
Technology Assessment Conduct experiments to access the range of capabilities for the TRECSCAN transient eddy current system Special consideration being given to the detection and characterization of cracks and corrosion located on typical Air Force aluminum aircraft skin/structure: Back side of the first layer Top side of the second layer. Back side of the second layer and deeper
Plans for the Future Focus on completing the system integration Conduct extensive system testing Demonstrate System Capabilities in Real World Trials Develop and Implement Aircraft/Problem Specific Inspection Procedures
Project Team Members NDT Solutions, Inc. The Boeing Company QinetiQ Ltd. SAIC Ultra Image International Universal Technology Corporation U. S. Air Force AFRL With the combined corrosion detection and quantification experience of these projects partners, we are excited, and looking forward to discovering the future capabilities of TRECSCAN
References 1. D. J. Harrison, Eddy-current inspection using Hall sensors and transient excitation, Defence Research Agency Technical Report DRA/SMC/TR941008, DRA Farnborough, UK, (1994). 2. D. J. Harrison, in Nondestructive Testing of Materials, Studies in Applied Electromagnetics and Mechanics, Vol 8, eds. R. Collins, W. D. Dover, J.R. Bowler and K. Miya, (IOS Press, Amsterdam, 1995), pp. 115-124. 3. S.K. Burke, G.R. Hugo, and D.J. Harrison, in Review of Progress in QNDE, Vol 17A, eds. D. O. Thompson and D. E. Chimenti, (Plenum, New York, 1998), pp. 307-314. 4. G. R. Hugo and D. J. Harrison, in Review of Progress in QNDE, Vol 18B, eds. D. O. Thompson and D. E. Chimenti, (Kluwer Academic/Plenum Publishers, 1999), pp. 1401-1408. 5. R A Smith and G R Hugo, "Transient eddy-current NDE for aging aircraft - Capabilities and limitations", Proc 4th Joint DoD/FAA/NASA Conf on Aging Aircraft, St Louis, May 2000. 6. R A Smith and G R Hugo, "Transient eddy-current NDE for ageing aircraft - Capabilities and limitations", Insight - The Journal of The British Institute of NDT, Vol 43, No 1, pp 14-20, 2001. 7. D J Harrison, "The characterisation of cylindrical eddy-current probes in terms of their spatial frequency spectra". IEE Proceedings; Science, Measurement and Technology (SMT), Special Issue on Non-Destructive Testing and Evaluation, Vol 148, No 4, July 2001. 8. R A Smith and G R Hugo, "Deep corrosion and crack detection in aging aircraft using transient eddy-current NDE", Proc 5th Joint NASA/FAA/DoD Conf on Aging Aircraft, Orlando, Sept 2001. ANDSCAN and TRECSCAN are a Registered Trademarks of QinetiQ Ltd, in the United Kingdom. MAUS is a Registered Trademark of The Boeing Company