PULSED EDDY-CURRENT NDE AT IOWA STATE UNIVERSITY -RECENT PROGRESS AND RESULTS

M. J. Johnson1, J. R. Bowler2 and F. Azeem1

1 Center for Nondestructive Evaluation, Iowa State University, Ames, IA 50011Department of ElAmes, IA 50011Department of Electrical and Computer Engineering, Iowa State University,

ABSTRACT. Iowa State University's pulsed eddy-current system has been redesigned to besmaller, more sturdy and easier to install and operate. Improvements have been made to theelectronic circuits, the computer interface, the enclosure design and to the software. Changeshave resulted in a much more user-friendly system that it portable and can be installed quicklyand easily in the field. As part of an evaluation of the new system, bond-thickness measurementson specimens supplied by Cessna were carried out. Results were found to be in good agreementwith those obtained by direct measurement. Additional experiments were carried out oncorrosion specimens from SAIC Ultra Image Inc. Using data from these specimens a calibration-based approach to flaw quantification is demonstrated.

INTRODUCTION

Iowa State University's Pulsed Eddy-Current (PEC) system has recently undergonesome major revisions. The most significant of these involved a redesign of the circuitryand physical layout of the unit, the main aim being a reduction in size of the finalinstrument. A second goal was to develop a system that was easy to setup and to operate aswell as being reliable. In addition to the hardware, the software was also revised to meetthe same goals, namely ease-of-use, user friendliness and reliability. We report here onsome of these changes and on some recent results obtained using specimens from Cessnaand SAIC Ultra Image Inc.

BACKGROUND

The idea of pulsed-eddy-current NDE is now one that is fairly familiar to thoseworking in the field of eddy-currents. In recent years there have been a number of systemsemerge that implement, in one way or another, the PEC approach. The various systems fallbroadly into two categories; those utilizing Hall-element sensors[l] and those utilizinginduction coils[2][3]. From there, two sub-categories exist; instruments that drive theeddy-current coil with a controlled current pulse and those that use a voltage pulse. For themost part, Hall-sensor based instruments use the former excitation method whereasinduction-coil systems use the latter. The ISU system is based on a Hall-element sensorand utilizes a current-drive approach to excite the eddy currents.

CP657, Review of Quantitative Nondestructive Evaluation Vol. 22, ed. by D. O. Thompson and D. E. Chimenti© 2003 American Institute of Physics 0-7354-0117-9/03/S20.00

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RECENT ADVANCES

The PEC system has been redesigned over the past year. The previousCompactPCI-based approach proved to be cumbersome, expensive and difficult to setup.In the new system, data is transmitted between a computer and the PEC module over theUSB (universal serial bus). An additional set of parallel I/O lines is used to control PEChardware settings (coil current, pulse width etc). An optional scanner can be attached tothe Ethernet port. The use of USB and Ethernet ports means that the PEC system can becontrolled using a laptop computer containing only a PCMCIA digital I/O card, Figure 1.Not only have there been major hardware revisions but the software has also beenrewritten. The software acquires pulse signals from the PEC module and provides a userinterface that allows, among other things, the various PEC hardware settings to bemodified. Scans can be performed and the data saved to disk such that it can be recalled ata later date. Useful features of the new software include an automated time-slice animationfeature that generates a movie showing property variations with depth. In addition,individual pulse response signals can be displayed by clicking on the C-scan displaywindow. The new GUI (graphical user interface) is shown in Figure 2.

RESULTS

Results presented in this paper were taken from a selection of projects that wereintended to evaluate the performance of the PEC system. Firstly, results from some bondedaluminum specimens supplied by Cessna, the idea being to nondestructively measure bondthickness, are discussed. These results demonstrate the accuracy of the PEC techniquewhen looking at one-dimensional problems, i.e. problems in which variations in thestructure or properties of the specimen are long range (in the plane of the specimen)compared with the size of the probe. Secondly, PEC system performance with regard toblind corrosion inspection is discussed by presenting some results from the round-robincorrosion experiment conducted by SAIC Ultra Image Inc.

PECModule Probe

I/O USB

NotebookPC

Ethernet MotionController

FIGURE 1. Major components of the new pulsed eddy-current measurement system developed at Iowa StateUniversity under the CASK program.

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FIGURE 2. The PECScan graphical user interface (GUI). The right-hand window displays a C-scan imagederived from an array of A-scans (left-hand window).

Bond Thickness Measurement

Two bonded aluminum specimens were obtained from Cessna. The specimensconsisted of two aluminum plates (approximately 200 mm by 100 mm in size) bondedtogether using an adhesive. The two specimens were referred to as the 'typical' and'wedge' specimens. The 'typical' specimen was manufactured to have two high points inbond thickness while the 'wedge' specimen exhibited a gradually varying bond thicknessfrom one end of the plate to the other. PEC scans were carried out on the two specimens,Figure 3, and a line scan corresponding to the centerline of the 'typical' specimen wasextracted for further analysis. A simple calibration procedure based on the peak-pulsemagnitude was applied to the PEC response data and the results compared with discretemeasurements obtained using calipers, Figure 4. Similar results were obtained for the'wedge' specimen.

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200

8S\*«s

g

•3 100

100 0x~positio& (mm)

FIGURE 3. PEC C-scan images obtained from the 'typical' and 'wedge' specimens, left and rightrespectively.

120

QJi"110

t Discrete measurements—— PEC predictions

10 15y-position (cm)

20

FIGURE 4. Bond thickness measurements and PEC predictions from the 'typical' specimen at an x-positionof 50 mm. PEC data accurately maps to thickness measurements obtained using calipers.

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Corrosion Measurements

Participation in a round-robin corrosion experiment conducted by SAIC UltraImage Inc was used as part of an ongoing evaluation of the PEC system. The round-robinpackage consisted of five specimens, two of which were used for calibration purposes.Specimen 6083 was used to obtain calibration data and then the blind specimen (Specimen6081) was scanned and estimates of size and location were made for the various corrosionindications. Specimen 6083 was scanned using the pulsed eddy-current system. A-scanresponse signals were obtained from the center of each of the specimen's six flawindications. The peak amplitude and times were recorded and used to generate calibrationcurves, Figure 5.

Following calibration, Specimen 6081 was scanned, Figure 6, and A-scan responsesignals obtained from each of the flaw indications. Application of the calibrationprocedure yielded the results shown in Table 1. In Table 1, PEC estimates of material lossand the associated interface are given alongside values provided by SAIC. There is adifference in the way PEC measurements are interpreted to provide location informationand the way SAIC quote this location information. SAIC site the exact surface from whichmaterial has been lost (e.g. To2 or top of second layer) whereas the PEC calibrationprocedure only provides information on which interface the corrosion has occurred. This isnot a shortcoming of the PEC system but merely an oversight. For the most part, there isreasonable agreement between PEC predictions and SAIC provided information. There area few instances where the PEC predictions are clearly wrong but these coincide withregions containing coincident corrosion in multiple layers, which, due to calibrationlimitations, the PEC system could not deal with adequately.

100

I 7.

.a(0

XL(0s.

50-

25

10

Material loss (mil)15 20

FIGURE 5. Calibration curves obtained from Specimen #6083. Peak-pulse times falling between 70 and 80jisecs indicate Interface 1 corrosion whereas peak times between 90 and 110 jLisecs indicate Interface 2

394

200

50 100 150x-position (mm)

200 250

FIGURE 6. C-scan image generated from a scan of Specimen 6081. Flaw indications are labeled 1 through10. The four large indications forming a square about the center of the image are fasteners.

TABLE 1. Estimates of material loss and location for the corrosion indications shown in Figure 6. Actualvalues provided by SAIC are shown in the right-hand columns, interface number (where the corrosion isoccurring) is also given because this is the figure that the PEC system calibration was configured to measure.Agreement is generally good except in the cases where there is coincident corrosion in multiple layers.

Indication

12345678910

PEC PredictionsLoss(mil)

556

226105162

Interface

2122121122

SAIC ValuesLoss(mil)

3999993393

Layer

BoTVTo32

BoTBo23

BoT/Bo2To24

BoTBoT/To2

BoTBo2Bo2

Interface

1/312

1/2111122

Agreement

PoorGoodGoodPoorGoodPoorPoorGoodGoodGood

1 BoT = Bottom of top layer2 To3 = Top of third layer3 Bo2 = Bottom of second layer4 To2 = Top of second layer

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CONCLUSIONS

The new PEC system was developed to provide a portable, lightweight, high-performance pulsed eddy-current NDE tool suitable for use in the field as well as in thelaboratory. The new instrument was found to be particularly suitable for the measurementof bond thickness in adhesively bonded aluminum plates. The reason for this is that thereis a correspondence between the evolution of the time-domain probe response and thedepth of features below the specimen's surface. This correspondence is analogous to theultrasonic pulse-echo phenomena except that it is somewhat more diffuse. Quantificationof corrosion with the S AIC specimens was for the most part reasonable except for locationswhere coincident corrosion in multiple layers was taking place. It is known that pulsededdy currents can deal with this kind of coincident material loss, it is just a question ofsetting up a more advanced calibration procedure. In the near future we plan to concentrateour research efforts on calibration and more advanced signal-processing issues.

ACKNOWLEDGEMENTS

This material is based upon work supported by the Federal Aviation Administrationunder Contract #DTFA03-98-D-00008, Delivery Order #IA026 and performed at IowaState University's Center for NDE as part of the Center for Aviation Systems Reliabilityprogram.

REFERENCES

1. Bowler, J. R., J. Harrison, D. J., in Review of Progress in QNDE, Vol. 11, eds. D.O. Thompson and D. E. Chimenti, Plenum, New York, 1992, p.241.

2. Tai, C-C. and Hung, C-Y., in Review of Progress in QNDE, Vol. 21, eds. D. O.Thompson and D. E. Chimenti, Plenum, New York, 2001, p.354.

3. Giguere, J. S. R., Lepine, B. A. and Dubois, J. M. S., in Review of Progress inQNDE, Vol. 21, eds. D. O. Thompson and D. E. Chimenti, Plenum, New York,2002, p. 1968.

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