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NONDESTRUCTIVE EVALUATION (NDE) OF COMPOSITE/METAL BOND INTERFACE OF A WINDTURBINE BLADE USING AN ACOUSTO-ULTRASONIC TECHNIQUE*

John H. Gieske and Mark A RumseySandia National Laboratories

P.O. Box 5800, MS 0615Albuquerque, NM 87185-0615

Abstract

An acousto-ultrasonic inspection technique wasdeveloped to evaluate the structural integrity of theepoxy bond interface between a metal insert and thefiber glass epoxy composite of a wind turbine blade.Data was generated manually as well as with a PC baseddata acquisition and display system. C-scan imagingusing a portable ultrasonic scanning system provided anarea mapping of the delamination or disbond due tofatigue testing, field operation conditions, ormanufactured conditions of a turbine blade. Comparisonof inspection data with a destructive visual examinationof the bond interface to determine the extent of thedisbond showed good agreement between the acousto-ultrasonic inspection data and visual data.

Introduction

Sandia National Laboratories has been investigatingtubular composite-to-metal lap joints.1,2As a tool todetermine the quality of the joint bonds, pulse-echoultrasonic C-scan NDI techniques have been used. Theprinciples of ultrasonic C-scan imaging with manual andautomated scanners can be found in a number ofsources.3,4The ultrasonic C-scan method was shown tobe successful in detecting and mapping incompleteadhesive fills, interface debonds and compositedelaminations in laboratory prepared and tested samples.As an extension to this work, the composite-to-metaljoints in the root section of selective wind turbine bladeswere tested. This report describes an acousto-ultrasonicvariation of the C-scan NDE technique that has beendeveloped for looking at these joints. A schematic viewof a wind turbine blade with a representative composite-to-metal joint is shown in Figure 1. Ultimately, theacousto-ultrasonic NDE technique is being developed tobe used on full-scale wind turbine blades in themanufacturing, testing and field environments.________________________* This paper is declared a work of the U.S. Governmentand is not subject to copyright protection in the UnitedStates.

Ultrasonic Inspection Techniques

An investigation of characterizing the root bond areashown in Figure 1 for defects was undertaken usingultrasonic techniques. Normal pulse-echo, through

transmission, and pitch-catch techniques wereexamined. A cross section of the root bond areashowing the geometry and propagation path ofultrasonic pulses by these techniques is displayed inFigure 2. Since the fiberglass epoxy composite wasover 1 inch thick for the wind turbine blade underexamination, considerations of ultrasonicreflections,

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scattering, and attenuation of the pulses from themultiple interfaces within the composite wereimportant.

Pulse-echo and pitch-catch techniques were investigatedwith limited success. Clear back-surface echoes fromthe bond interface① as shown in Figure 2 or from theback-surface of the steel insert at② were not distinctfrom all the echoes generated from the multipleinterfaces of the composite. In general, ambiguousresults were obtained with the pulse-echo and pitch-catch techniques. However, a through transmission (TT)technique proved to show unambiguous results whereareas of disbonds, delaminations, and voids could bemapped out and detected with certainty.

As an example, Figure 3 shows the ultrasonic C-scanresults obtained with a TT technique for a305 mmsquare test sample that had two strips of disbonds

purposely manufactured into the test piece thatconsisted of the structural components of the fiberglasscomposite bonded to a steel plate. Adequate C-scanimages of the same disbond areas could not be obtainedwith the pulse-echo or pitch-catch techniques. Theresults of Figure 3 were obtained with an automated

ultrasonic data acquisition and display system using awater bath to couple the ultrasonic energy between thetransducers and the test piece. However, waterimmersion techniques will not be appropriate for windturbine blade examinations. In addition, the blade willbe installed in a fixture or wind turbine where the insidesurface of the turbine blade will not be accessible forapplication of an ultrasonic transducer for the TTtechnique. For these reasons, an acousto-ultrasonic(AU) inspection technique was developed. The AUtechnique would approximate the detection capability ofthe TT technique as near as possible.

The AU technique developed is illustrated in Figure 4where contact transmitter and receiver ultrasonictransducers are used at the OD surface of the turbineblade. The receiver transducer is attached to the ODsurface of the steel insert inboard of the fiberglasscomposite. As illustrated in Figure 4, the ultrasonicpulse generated by the transmitter enters the composite,travels through the composite thickness, and then entersthe steel insert where it propagates down the insert andfinally arrives at the receiver transducer. The first arrivalof the pulse at the receiver is most sensitive to theenergy entering the steel insert directly below thetransmitter since the acoustic wave velocity in the steelis faster than that in the composite and very little energyis lost in the steel.

For a disbond in the epoxy adhesive interface, a crack inthe composite, or distributed damage in the composite,little if any energy enters the steel insert directly belowthe transmitter. Therefore, the first arrival signalrecorded by the receiver will be

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significantly diminished or of zero amplitude dependingon the severity of the damage in the structure below thetransmitter. As the transmitter is moved away from thereceiver, a gradient of the signal amplitude recorded bythe receiver will be observed. However, the gradientshould be uniform for a normal undamaged bladearound the circumference of the blade for a given axialseparation of the transmitter and receiver transducers.Abrupt decreases, non-uniform gradients, or very lowvalues of the recorded AU signal amplitude by thereceiver as the transmitter is translated along the axial orcircumferential directions of the insert would indicatepossible disbonding, cracking, or material degradationof the composite at the location of the transmitter.

The AU technique was implemented using the testsample of Figure 3 by attaching the receiver transducerto the bottom side of the steel plate of the sample. Thedetection of the two strip disbonds was observed by anabrupt loss of signal recorded at the stationary receiveras the transmitter passed over the disbonds. This testdemonstrated the capability of the AU technique tosimulate the IT examination and it could be used in asimilar way to assess and identify certain areas ofdamage and disbonds at the metal insert of a full-scaleturbine blade.

AU Signal Amplitude Results Obtained on WindTurbine Blade Joints

AU signal amplitude data using the technique as shownin Figure 4 was collected on 3 full-scale turbine bladeroot joints at the steel insert root bond area. The rootbond area, as shown in Figure 1, consisted of the areaincluded by 360 degrees around the cylindrical steelinsert and the entire length in the axial direction startingat the inboard edge of the fiberglass composite. Forreference points, this area was divided into squarecircumferential and axial segments as shown in Figure5. The grid of the segments that was scribed on thesurface of the blade was used as reference lines formapping the amplitude of the AU signal over the rootbond area. For the first blade early in the developmentalstages of the project, the AU signal amplitude wasrecorded manually by observing the signal height on theoscilloscope screen as the transmitter transducer wasplaced point-by-point in the area defined by the gridlines.

Manual AU Data Acquisition and Display for a WindTurbine Blade

The first AU examination was performed on a windturbine blade while it was attached to the fixture of afatigue tester. The blade had undergone a number ofhigh load fatigue cycles at the time of the AUexamination. The AU amplitude data was taken bymoving the transmitter point by point by hand along agrid line in the axial direction while the receiver wasstationary at the bottom of the grid line as shown inFigure 4. In a similar manner, axial AU amplitude datawas taken at grid lines separated in the circumferentialdirection until the entire area of the insert wasexamined.

A pair of 400 kHz 1 inch diameter contact transducers5

were used for the transmitter and receiver transducers.A 0.25inch thick RTV silicone rubber pad was bondedto the transmitter to provide coupling of the acousticenergy from the transducer into the composite. Thesurface of the composite was also coated with anultrasound gel couplant to facilitate the transfer of theacoustic energy from the transmitter as the transducerwas moved over the surface of the composite. Apulser/receiver module6 with 60 dB gain was used togenerate and amplify the acoustic signals. The receivertransducer signal amplitude was recorded from 0 to 10where 10 was full screen height on the oscilloscopescreen. Coverage of the entire steel insert area resultedin a table of AU amplitude data that contained 20 rowsfor the axial direction and 34 columns for thecircumferential direction.

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A color contour plot of the tabular data was made byentering the AU data into a Microsoft Excelspreadsheet. Figure 6 displays an equivalent gray scalemapping of the AU data over the steel insert area of theblade. The AU data in Figure 6 indicated that somedamage was present at this stage in the fatigue cycleschedule near the high and low pressure sides of theblade as referenced in the sketch of Figure5.Maximum loading occurs at the outboard end of thesteel insert during the fatigue test and during the normaloperation of the blade in these two areas. After the AUexamination was performed, the blade was fatiguetested to failure. It is noted that the failure of the bladewas expected and it did occur in these two areas of theblade.

At this stage of the project, the interpretation of the AUdata was encouraging but not conclusive. It did showthat the method had the potential of indicating early

damage in the root bond area of a steel insert of a windturbine blade during testing and could be appliedmanually using a minimum amount equipment.

Automated Data Acquisition and Correlation of AUResults with Dissection of a Turbine Blade

To gather conclusive results, a second turbine bladejoint that contained a failure of the bond was examined.A visual examination of this turbine blade showed that afailure existed at the composite-to-steel bond interfacebut the area of the disbond was unknown. This bladewas then used to obtain an AU signal amplitudemapping of the disbond and the results were comparedwith visual inspection by cutting the composite intosmall sections between grid lines and removing themfrom the steel insert.

To improve on the manual data collection processwhich was tedious and slow, an automated datacollection scheme was implemented. The improvedsystem used a PC ultrasonic data acquisition anddisplay system7 integrated with an encoded manualscanner for recording the position of the transmitter. AUdata was collected fast and efficiently in 0.25 inchincrements in both the axial and circumferentialdirections using the PC data acquisition and displaysystem. The PC manual scanner system provided adirect mapping of the AU signal amplitude by C-scancolor images of the scanned area. The number of C-scanimages enclosing the total area of the root bond wascompleted in approximately 1 hour.

For a single C-scan image of the AU amplitude data, thereceiver transducer was attached to the steel insert at thecenter segment of a group of 3 circumferentialsegments. The X-Y plot was generated by moving thetransmitter in the axial direction (X axis) and in thecircumferential direction (Y axis). As a result, a C-scanarea plot of the AU amplitude data was developed forthe group of 3 circumferential segments. To cover theentire circumferential area of the blade, a total of 6 C-scan images were made. For eachX-Y plot, the AUsignal was gated in time delay for the data acquisitionsystem to cover the entire length of the axial scan.

Figure 7(a) displays a montage of the 6 C-scan imagescollected around the entire circumference of the blade.The C-scan images of Figure 7(a) were rendered in asimple gray scale since the normal 16 color scale couldnot be reproduced in this publication. Some detail of theAU signal variations is therefore lost but an

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assessment of the disbonded area is clearly indicated inthe gray scale images. The area of Figure 7(a) shown inblack where a low AU signal amplitude was recordedindicated the area of disbond.Figure 7(b) shows the corresponding visual examinationof the bond interface of this blade by cutting thesegments of the composite and removing them from thesteel insert. The removed pieces of the composite werethen reassembled with the metal-to-composite bondinterface facing outward. A photograph of thereassembled pieces is shown in Figure 7(b) where thebond failure is shown in black due to a dark metal oxidepresent on the adhesive surface of the failed area. Acomparison of Figures 7(a) and 7(b) shows that the AUdata correlated well with the disbond visually observedafter removing the composite with the exception of twosmall areas. The area to the right of center in Figure7(a) indicated an area of low AU signal but notcompletely disbonded. It is possible that the compositewas held in compression against the metal surface so

that a so-called kissing bond existed in this small area.Also, the area just above the left eyebolt hole in Figure7(a) is shown in black because the metal path to thereceiver was obstructed by the eyebolt hole. This areadoes not show a disbond in Figure 7(b).

The steel insert area of a third blade joint was alsoexamined with the PC data acquisition and displaysystem. A manufacturing defect was suspected in thisblade but the location and extent of the defect was notknown. Figure 8(a) shows the montage of the 6 C-scansof the AU signal amplitude. The AU signal amplitudedata was processed so that a black color was assignedto signal amplitudes below a given threshold value. Theresult is shown in Figure 8(b). The area of the defect isthen clearly indicated in Figure 8(b) by the area inblack. After the AU examination was made, a partialremoval of the composite skin at the area of C-scannumber 3 showed that a large void existed in one of the

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layers in the composite and the void area correlated wellwith the C-scan image number 3.

Conclusions

From the examples shown in this paper, thedemonstrated acousto-ultrasonic method can provideuseful information of disbond, delaminations, voids, andcomposite damage in the root bond area of the steelinsert of a wind turbine blade. The method can be usedon the factory floor for quality control by detectingmanufacturing defects such as voids in the bondinterfaces of the composite as well as at the composite-to-metal interface. The method can be used as adiagnostic tool to assess the accumulative damage thatoccurs during fatigue testing by mapping changes of theAU signal amplitude during progressive stages of thetest. The scanning equipment is portable so that AUexaminations can be made in the field to assessaccumulative damage that may occur during the normalservice life of the blade.

Acknowledgment

This work was supported by the United StatesDepartment of Energy under Contract DE-AC04-94AL85000.

Sandia is a multiprogram laboratory operated by SandiaCorporation, a Lockheed Martin Company, for theUnited States Department of Energy.

References

1. Guess, T.R., Reedy, E. D., Jr., and Slavin, A. M.,1995,“Testing Composite-to-Metal Tubular LapJoints,” Journal of Composites Technology &Research Vol. 17, No. 2, pp. 117-124.

2. Reedy, E. D., Jr. And Guess, T. R.“Tubular LapJoints for Wind Turbine Applications: Test andAnalysis,”Proceedings 14th Annual Energy-Sources Technology Conference and Exhibition,Houston, TX, January 1991.

3. American Society for Nondestructive Testing(ASNT), “Nondestructive Testing Handbook,”Second Edition, Vol. 7, Ultrasonic Testing, 1991.

4. ASM International Handbook,“NondestructiveEvaluation and Quality Control,“Vol. 17, 4thPrinting, Jan. 1996.

5. Aerotech Transducers supplied by KrautkramerBranson Inc., Lewistown, PA.

6. Pulser/receiver model 5055, Panametrics Inc.,Waltham, MA.

7. Infometrics Inc. now supported by SONIX Inc.,Springfield, VA.