The 6th JointThe 6th JointFAA/DoD/NASA Conference on Aging AircraftFAA/DoD/NASA Conference on Aging Aircraft

16 16 –– 19 September 200219 September 2002

Jesse A. Skramstad, NDT Solutions, Inc.Jesse A. Skramstad, NDT Solutions, Inc.

Robert A Smith, QinetiQ Ltd Robert A Smith, QinetiQ Ltd UKUK

Nancy Wood, Nancy Wood, Boeing Aircraft CompanyBoeing Aircraft Company

A PRACTICAL IMPLEMENTATION OF A PRACTICAL IMPLEMENTATION OF TRANSIENT EDDY CURRENTS FOR TRANSIENT EDDY CURRENTS FOR

CORROSION AND CRACK DETECTIONCORROSION AND CRACK DETECTION

AbstractAbstractFuture inspection techniques for aging aircraft will have to be Future inspection techniques for aging aircraft will have to be able to able to acquire data over large areas of complex structure, with numerouacquire data over large areas of complex structure, with numerous changes s changes of thickness, without the need for adjusting the acquisition parof thickness, without the need for adjusting the acquisition parameters, in ameters, in addition to being able to reliably detect defects of appropriateaddition to being able to reliably detect defects of appropriate severity. severity. Transient eddyTransient eddy--current technology is one of the few methods offering current technology is one of the few methods offering reliable defect detectability over a wide range of depths withoureliable defect detectability over a wide range of depths without the need t the need to make adjustments at acquisition time. to make adjustments at acquisition time.

While the technology exists to perform these transient eddyWhile the technology exists to perform these transient eddy--current scans, current scans, further work is required to couple it to largefurther work is required to couple it to large--area scanners currently in use area scanners currently in use with the US Air Force and Civil airlines. This poster reports prwith the US Air Force and Civil airlines. This poster reports progress on ogress on the integration of the MAUS the integration of the MAUS ®® and Ultra Image IV scanners with the and Ultra Image IV scanners with the TRECSCANTRECSCAN®® transient eddytransient eddy--current imaging system.The first scans from current imaging system.The first scans from a MAUS a MAUS ®® system using transient eddysystem using transient eddy--currents will be presented, with currents will be presented, with details of the remaining integration program, the aim of which idetails of the remaining integration program, the aim of which is to link s to link transient eddytransient eddy--currents seamlessly into these scanning systems. currents seamlessly into these scanning systems.

What is TRECSCANWhat is TRECSCAN®® ??

•• TRECSCANTRECSCAN®® is a Prototype Transient Eddy is a Prototype Transient Eddy Current Inspection SystemCurrent Inspection System

•• Unique Analysis Software that Includes:Unique Analysis Software that Includes:–– Edge Subtraction (Correction)Edge Subtraction (Correction)–– LiftLift--Off CompensationOff Compensation–– Total Thickness MeasurementTotal Thickness Measurement–– Plate Separation Effect EliminationPlate Separation Effect Elimination–– Time Domain Eddy CurrentTime Domain Eddy Current

Why Is TRECSCANWhy Is TRECSCAN®® Different Different ??•• Uses a Hall sensor as a field detector instead of Uses a Hall sensor as a field detector instead of

a coila coil•• Eddy current pulses are generated using a coil Eddy current pulses are generated using a coil

and the Magnetic Field is measured using a and the Magnetic Field is measured using a Hall sensorHall sensor

•• Measures and captures the complete Transient Measures and captures the complete Transient data setdata set

What is a Hall Sensor?What is a Hall Sensor?

In a Semi Conductive Platelet, the Hall Voltage Is Generated In a Semi Conductive Platelet, the Hall Voltage Is Generated by the Effect of an External Magnetic Field Acting by the Effect of an External Magnetic Field Acting

Perpendicularly to the Direction of the Current.Perpendicularly to the Direction of the Current.

Hall Sensor AdvantagesHall Sensor Advantages

••Hall sensors respond to a wide frequency Hall sensors respond to a wide frequency range allowing the capture of detailed transient range allowing the capture of detailed transient data sets.data sets.••Hall sensors provide direct measurement of Hall sensors provide direct measurement of the magnetic field, essential for the application the magnetic field, essential for the application of analytical theory to the transient data.of analytical theory to the transient data.••A Hall sensor as a field detector rather than a A Hall sensor as a field detector rather than a coil, improves the spatial resolution and the coil, improves the spatial resolution and the detectability of deep defects.detectability of deep defects.

Drive

Response

Drive

Response

Conventional Transient

• 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

Transient EddyTransient Eddy--Currents:Currents:

time t

Mag

netic

Fie

ld H

z Hz(air)

Hz(specimen)

HRz = Hz(specimen) - Hz(air)

∆Hz(defect)time tC

oil C

urre

nt i

input current i(t)

Hall effect sensor

i(t)

Hz(t)

Drive coil

Ferrite core

First layer

Second layerGap Corrosion

The Transient Eddy Current MethodThe Transient Eddy Current Method

Transient signals from Transient signals from structural changes at structural changes at

different depths.different depths.

Transient EddyTransient Eddy--CurrentsCurrents

Separate CSeparate C--scan images can be scan images can be produced from each timeproduced from each time--slice.slice.

TimeTime--slicesslices

Transient Data Sample StorageTransient Data Sample Storage

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 post-processing of the data after acquisition.

• 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

Advantages of Transient Eddy CurrentAdvantages of Transient Eddy Current

Post Analysis ToolsPost Analysis Tools

•• LiftLift--off compensationoff compensation•• Edge subtraction (correction)Edge subtraction (correction)•• Total thickness measurementTotal thickness measurement•• Plate separation effect eliminationPlate separation effect elimination•• Timeslice viewingTimeslice viewing•• Time domain signal processingTime domain signal processing

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.

The Transient Eddy Current Signal Includes a The Transient Eddy Current Signal Includes a Plethora of Information, Hence the Processing of Plethora of Information, Hence the Processing of

the Transient Data Set is Very Important.the Transient Data Set is Very Important.

Layers:1) Exterior Strap2) Skin Panels3) Finger Doublers4) Longeron5) Doublers

1.80 mm (0.071”)

1.14 mm (0.045”) or0.83 mm (0.032”)

1.60 mm (0.063”)

1.91 mm (0.075”) or3.18 mm (0.125”)

Tapered and stepped1.5 mm (0.060”) to4.7 mm (0.185”)

1)2)3)4)5)

Probe

Brush1)

2)

3)4) 5)

Probe Lift-Off from .063” Strap

LiftLift--Off Problem DC10 Crown SpliceOff Problem DC10 Crown Splice

Graphic Courtesy of QinetiQ Ltd..

Without L/O CompensationWithout L/O Compensation With L/O CompensationWith L/O Compensation

LiftLift--Off Compensation Off Compensation DC10 Crown SpliceDC10 Crown Splice

LiftLift--OffOff LiftLift--Off Off RemovedRemoved

DC10 Specimen Courtesy Sandia National Labs, AANC

Edge Subtraction on a DC10 Edge Subtraction on a DC10 Crown Splice JointCrown Splice Joint

With Edge SubtractionWith L/O Comp

EDM NOTCHESEDM NOTCHES

DC10 TimeDC10 Time--Slice ExamplesSlice Examples1 2

34

5

TimeTime--Slices Later in Time Represent Lower FrequenciesSlices Later in Time Represent Lower Frequencies

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.

Defect Depth Measurement Defect Depth Measurement TimeTime--ToTo--PeakPeak

Metal Loss MeasurementMetal Loss MeasurementPercentage Total ThicknessPercentage 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 ObjectivesSystem 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

ANDSCAN 2000Software

Probe: Hall Sensor

TRECSCAN

Interface Box

TRECSCAN

DLL

Original TRECSCANOriginal TRECSCAN SystemSystem

ANDSCAN 2000Software

Probe: Hall Sensor

TRECSCAN

Interface Box

TRECSCAN

DLL

MAUS IV +Digital Position

Output Card

TRECSCANTRECSCAN +MAUS IV+MAUS IV

Proof of Concept Version

TRECSCAN

DLL

Probe: Hall Sensor

MAUS IV

TRECSCAN BOARD

TRECSCANTRECSCAN /MAUS IV /MAUS IV INTEGRATIONINTEGRATION

Operator Interface Familiarity

ANDSCAN 2000Software

Probe: Hall Sensor

TRECSCAN

Interface Box

TRECSCAN

DLL

Ultra Image

Scanning System

TRECSCANTRECSCAN +Ultra Image+Ultra Image

IntegrationIntegration

ANDSCAN Operator Interface

Technology AssessmentTechnology 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 FuturePlans 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

NDT Solutions, Inc.NDT Solutions, Inc.The Boeing CompanyThe Boeing Company

QinetiQ Ltd.QinetiQ Ltd.SAIC Ultra Image InternationalSAIC Ultra Image International

Universal Technology CorporationUniversal Technology CorporationU. S. Air Force AFRLU. S. Air Force AFRL

Project Team MembersProject Team Members

With the combined corrosion detection and With the combined corrosion detection and quantification experience of these projects quantification experience of these projects

partners, we are excited, and looking forward to partners, we are excited, and looking forward to discovering the future capabilities of discovering the future capabilities of

TRECSCANTRECSCAN

References1. 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