NASA / TM--2000-210473 00PS--63

NDE Methodologies for Composite

Flywheels Certification

George Y. Baaklini and Kevin E. Konno

Glenn Research Center, Cleveland, Ohio

Richard E. Martin

Cleveland State University, Cleveland, Ohio

Richard Thompson

University of Texas at Austin, Austin, Texas

October 2000
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NASA/TM--2000-210473 00PS-63

NDE Methodologies for Composite

Flywheels Certification

George Y. Baaklini and Kevin E. Konno

Glenn Research Center, Cleveland, Ohio

Richard E. Martin

Cleveland State University, Cleveland, Ohio

Richard Thompson

University of Texas at Austin, Austin, Texas

Prepared for the

2000 Power Systems Conference

sponsored by the Society of Automotive Engineers

San Diego, California, October 31-November 2, 2000

National Aeronautics and

Space Administration

Glenn Research Center

October 2000
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Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NDE Methodologies for Composite Flywheels Certification

George Y. Baaklini and Kevin E. Konno

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Richard E. Martin

Cleveland State University

Cleveland, Ohio 44115

Richard Thompson

University of Texas, CEM

Austin, Texas 78712

ABSTRACT

Manufacturing readiness of composite rotors and

certification of flywheels depend in part on the maturity of

nondestructive evaluation (NDE) technology for process

optimization and quality assurance, respectively.

Capabilities and limitations of x-ray-computed

tomography and radiography, as well as advanced

ultrasonics were established on NDE ring and rotor

standards with EDM notches and drilled holes. Also,

intentionally seeded delamination, tow break, and insert

of bagging material were introduced in hydroburst-rings

to study the NDE detection capabilities of such

anomalies and their effect on the damage tolerance and

safe life margins of subscale rings and rotors. Examples

of possible occurring flaws or anomalies in composite

rings as detected by NDE and validated by destructive

metallography are shown. The general NDE approach to

ensure quality of composite rotors and to help in the

certification of flywheels is briefly outlined.

INTRODUCTION

The flywheel technology for energy storage systems is

gaining broader support, more than it did for the last 25

years [1-4], because of current support from NASA and

U.S. Air Force space programs. The drive is not only to

improve energy storage systems where the state of

charge can be known but also to concurrently perform

attitude control depending on the specific application for

a given spacecraft [5]. Some of the major challenges in

the flywheel technology for the international space

station (ISS) include high-speed rotors and carbon

reinforced composite materials that can meet the safelife design requirements, reliability and durability to endof-life, flight certification, and safety constraints while

delivering the high energy density needed [6]. In this

paper, the research is limited to the NDE of composite

flywheel rotors and rings that are targeted under the

flywheel energy storage system (FESS) program [7].

These composite rotors are to be flown on the ISS

replacing a battery charge/discharge unit.

MATERIALS AND NDE METHODS AND

STANDARDS

The composite materials under investigation, the NDE

methods employed, and the NDE standards are all

described in the following sections:

MATERIALS - Composite flywheel rings and rotors were

fabricated using proprietary materials and processes.

The composite material system consists of polymer

matrix composites (PMCs), using toughened resin

systems, with added features to enhance fracture

toughness for the laminate and prevent/mitigate the

formation of cracks. This fabrication process is autoclave

based, to assist with consolidation and to reduce void

content.

The flywheel, see Fig.l, consists of a solid cylindrical

composite energy storage unit made of concentric rings

press fit (preloaded) together, operating with a rotor tip

speed approximately 835 m/s.

NDE METHODS - Several ultrasonic methods were used

for the material characterization of the composite rotors.

Through-transmission (TT) and pulse-echo (PE)

methodologies were used for flaw detection and acoustoultrasonics (AU) and ultrasonic spectroscopy (US) were

used for degradation and damage assessment before

and after fatigue or burst tests. In this paper we limit our

discussion to the TT and PE ultrasonic scanning

methods suitable for flaw detection. Several x-ray

methods were used like conventional and microfocus

radiography and 420 KeV and 3 MeV computed

NASA/TM--2000-210473 1
tomography (CT). Conventional and microfocus

radiography were used for flaw detection and composite

architecture verification. Low energy CT, 420 KeV, was

used for thin rings and thin-rim rotors whereas high

energy CT, 2-3 MeV, was used for thick composite rings

and thick-rim rotors with hubs. Conventional and

microfocus radiography were used in the 40 to 80 KeV

and 5 mA to 0.4mA, respectively.

Pulse Echo and Through Transmission Ultrasonics

Figure 2 shows the immersion ultrasonic scanning

system during a PE inspection of one composite ring. PE

is an ultrasonic method in which only one transducer is

used to send the sound into the material and then to

receive the sound after it bounces off the back surface of

the object under inspection. TT is an ultrasonic method in

which two transducers are used, one to send the sound

from one side into the object under investigation, and a

second transducer to receive the sound after it traversed

the object. This method requires access to both front

(OD) and back (ID) surfaces of objects like composite

rings and rotors without a metallic hub. PE method is

better suited for the evaluation of an object with one side

access similar to the case of a composite rotor with a

hub. Both TT and PE methods provide information on the

internal state of the material by studying the attenuation

mechanisms that take place during the traverse of sound

through the material.

X-ray Computed Tomography and Radiography - Figure

3 shows a schematic of x-ray computed tomography

setup where the object under inspection is rotated at

small increment angles, 0.2 degree, collecting many

projections from the x-ray fan beam after it attenuates

through the object and is received at the detector. These

projections are then built in the Fourier domain allowing

fast inverse Fourier algorithms to be utilized for

reconstructing the CT cross-section, called the CT slice,

information at a specific location or height. The CT slice

reveals internal mass attenuation coefficient differences

due to the presence of different material compositions or

features. These differences can be attributed to and

correlated with density differences between material

constituents. X-ray radiography on the other hand, a

simpler imaging method than CT, uses the same

principle of x-ray attenuation. In conventional and

microfocus radiography the x-ray beam is in general

conical in shape as opposed to the fan shape in CT, and

the resulting image/radiograph is a 2D shadowgram that

superimpose all information in the x-ray path. In

microfocus radiography, the spatial resolution and

contrast sensitivity are improved by comparison to

conventional radiography because the focal spot size of

the microfocus x-ray tube is on the order of 5-50 p.m

whereas that of a conventional tube is 400 pm and

above. Also, the projection mode in microfocus

radiography reduces the scattered radiation which

normally deteriorate the signal to noise ratio in the directcontact conventional-radiography mode.

NDE STANDARDS - The NDE standards were made to

establish the capabilities and limitations of the NDE

methods employed, and to use them as NDE reference

standards to ensure that the NDE methods employed

were performing to specifications every time they were

used.

Composite Ring Standard - Figure 4 is a schematic of a

typical ring standard with EDM notches ranging from

125x125 p.m to 750x1500 [am (5x5 mils to 30x60 mils)

and drilled holes ranging from 300p.m to 1600p.m (12 mil

to 1/16") in diameter. These notches and drilled holes

were 12.5 mm (1/2") deep in a 12.5 mm (1/2") wide by

12.5 mm (1/2") high composite ring.

Composite Rotor Standard - Figure 5 is a CT slice of a

multi-layered rotor standard. The hub is made of Ti-6AI4V and the rim is a multi-layered PMC. The same set of

EDM notches and drilled holes (125x125 ram, 300 p.m,

390 p.m, 500x500 p.m, 780 p.m, and 750x1500 p.m) was

machined into each of the eight composite layers. The

set was duplicated in a spiral fashion starting at the

innermost ring at the ID to the outermost one at the OD.

RESULTS AND DISCUSSION

Ring Standard - Table I shows all the NDE findings taken

on two standard rings by using ultrasonic TT and PE

methods, conventional as well as microfocus

radiography and 420KeV and 3 MeV x-ray CT methods.

The microfocus radiography images of one of the

standard ring are shown in figure 6. The ultrasonic

images are shown in figures 7 and 8. From table I

findings and figure 6 images microfocus radiography was

demonstrated to be a viable NDE technique to detect

EDM notches and drilled holes down to a 125x125 p.m

notch and 300 p.m drilled hole. Conventional radiography

was capable of detecting EDM notches down to 250x750

p.m and drilled holes down to 780 p.m in diameter.

However, conventional radiography is capable of

detecting metallic inclusion down to 125x125 p.m.

Computed tomography findings demonstrate that EDM

notches down to 125x125 !_m and drilled holes down to

300 p.m in diameter can be easily detected by 420KeV as

well as by a 3 MeV systems. The PE ultrasonic method

was found to be capable of detecting EDM notches down

to 125x125 l.u'n and drilled holes down to 300 _m in

diameter. TT method was capable of detecting EDM

notches down to 125x1000 p.m and drilled holes down to

780 pm in diameter.

Rotor Standard - All EDM notches and drilled holes were

detected in all composite layers by using 3MeV CT. The

NASA/TM--2000-210473 2
selected CT slice shown in figure 5 substantiates the

finding. This is a major result for high energy CT to

detect low-density flaw down to 125x125 tam in a thickrim rotor. This further demonstrates that x-ray CT can

play a major role in rotor flight certification. Ultrasonic

scanning results, not shown herein because of space

limitation, demonstrated 1) penetration capabilities of 4.5

cm and detection capabilities down to 750x1500 lam

(30x60 mils) in the outermost ring only for the case of 1

MHz, 2) penetration capabilities of 2 cm and detection

capabilities of the whole set in the outermost ring and

down to 500x500 _tm (20x20 mils) in the second ring

from the OD at 2.25 MHz, and 3) penetration capabilities

of 2 cm and detection capabilities of the whole set in the

first two ring from the OD.

Seeded Delamination and Baqging Materials A

delamination was created in one hydroburst ring that

spans 180 degrees of the circumference at midpoint in

the radial direction. An optical photograph of a selected

section of the delamination is shown in figure 9 where

the schematic of propagating ultrasound bouncing off the

I.D. surface of the ring is also depicted. The time domain

of the ultrasound signal highlights the presence of the

delamination, second peak, between the front surface

reflection, peak 1, and back surface reflection, peak 3.

Gating the second peak in the time domain, the

delamination, resulted in imaging the presence of the

delamination (echo coming from the wall/interface

created by the delamination) between 180° and 0° (clock

wise) as shown in the top of figure 9.

Figure 10 shows the ultrasonic PE scan of the

hydroburst ring with seeded insert of bagging material

located at 120 degree in the middle of the 4 mm thick by

12.7 mm wide ring. The insert was easily detected by PE

ultrasonics and is depicted as a black line at the 120 °-

angle location at the top of figure 10. The tow-break was

simulated by an EDM notch on the ID surface of a

hydroburst ring. The notches were easily detected

because they were comparable to the larger EDM

notches imaged in the ring standard as shown previously

in figures 6-8 and Table I.

Examples of Possible Occurring Flaws - Figure 11 shows

a large delamination in one of the rings as detected by

PE ultrasonic method. This delamination spans a 54-

degree angle near the bottom edge, and is 15 cm in

height. The optical photograph of a section of the ring as

exposed to the surface shows clear evidence of that

delamination in figure 12. Figure 13 shows a sizeable

void, about lx2 mm (40x80 mils), at the glass-composite

interface as detected by x-ray CT. The glass layer was

machined off and the low-density feature as detected by

CT was demonstrated to be a void. A selected area from

a CT slice of a composite ring is shown in figure 14(a).

The dark region, lower-density feature near the I.D.,

depicts the presence of a resin rich area. After the ring

NASAfI'M 2000-210473

was sectioned destructively and the specific location was

exposed to the surface and polished, the optical

photograph (Fig. 14b) confirms the CT findings of the

resin rich area.

General NDE Approach Toward Flywheel Certification -

The current NDE plan, to ensure quality of composite

rotors and to help in the certification of flywheels, based

on the findings in this work is to 1) perform flaw/damage

detection on 100% of the rings with P-E ultrasonics and

real-time microfocus radiography and selected slices

with computed tomography, 2) perform flaw/damage

detection on 100% of the rotor with thick computed

tomography slices and 100% of the outer shells with

5MHz P-E ultrasonics down to 2 cm deep from the OD of

the rotor, and 3) perform general c-scan evaluation with

0.5 MHz P-E ultrasonics down to 4.5 cm deep from the

OD of the rotor. Further, based on work by Baaklini et.al.,

[8], and by Harmon and Baaklini [9] not reported herein,

NDE for material characterization and degradation

assessment of mechanical properties and interfaces is to

be performed on 100% of the rotors via acoustoultrasonics and ultrasonic resonance spectroscopy.

CONCLUSION

Capabilities and limitations of x-ray-computed

tomography and radiography, as well as advanced

ultrasonics were established on nondestructive

evaluation (NDE) ring and rotor standards with EDM

notches and drilled holes. EDM notches down to 125 by

125 lam and drilled holes down to 300 _m in diameter

were detected in a thick multi-layered rotor by using xray computed tomography. At the ring level, x-ray

microfocus radiography, x-ray computed tomography,

and pulse echo ultrasonic scans were able to detect

EDM notches down to 125 by 125 I.tm and drilled holes

down to 300 lam in diameter. Intentionally seeded

delamination, tow break, and insert of bagging material

were easily detected in hydroburst rings by using the

ultrasonic pulse echo method. Possible occurring

anomalies like delamination in polymer matrix composite

layers, void in glass layers, and resin rich regions in the

composite layers, were all detected by NDE and verified

by metallography. The general NDE approach to ensure

quality of composite rotors and to help in the certification

of flywheels was briefly outlined.

REFERENCES

1. DeTersa, S.J., "Materials for Advanced Flywheel

Energy-Storage Devices," MRS Bulletin November,

1999, pp 51-55.

2. Olszewski, M. et.al ," On the Fly or Under Pressure,"

Mechanical Engineering June 1988, pp 50-58.

3. Ashley, S.," Flywheel Put a New Spin on Electric

Vehicles," Mechanical Engineering, October 1993,

pp 44-51.
4. Moritz,B.,"CompositesMaximizeEnergyStoragein

IndustrialFlywheels,"Composites, March/April1998,

pp21-24.

5. Brown,P.J.," Flywheelsin Space,"Launchspace,

September2000,pp22-25.

6. Konno,K.E., et.al.,"Certificationof High Speed

FlywheelRotorsfortheInternational SpaceStation,"

SAE2000PowerSystemsConference,SanDiego,

October31-November 2nd2000.

7. http://space-power.grc.nasa.gov/ppo/flywheel/

8. Baaklini, G.Y. et.al., " NDE for Material

Characterization in Aeronautic and Space

Applications,"presentedat the MicroMAt2000in

Berlin, Germany, April 2000. NASA TM in

publication.

9. LauraM. Harmon,GeorgeBaaklini " Ultrasonic

SpectroscopyOf FlywheelCompositeMaterial

Systems",To be presentedat the NDEfor Health

Monitoringand DiagnosticsSymposium,SPIE,

March4-8,2001,NewportBeach,CA.NASATM#in

publication.

NASA/TM--2000- 210473 4
Standard ring #1 Standard ring #2

Defect Size UT RT RT CT CT UT RT CT CT

p.m TT C M 420 3 PE M 420 3

KeV MeV KeV MeV

125x125 hd x x x x x x

125x500 x x x x x x

125x750 x x x x x x x

125x1000 x x x x x x x x

250x250 x x x x x x x x

250x500 x hd x x x x x x x

250x750 x x x x x x x x x

250x1000 x x x x x x x x x

500x500 x x x x x x x x x

500x1500 x x x x x x x x x

750x750 x x x x x x x x x

750x1500 x x x x x x x x x

390 - x x x x x x x

780 x x x x x x x x x

Cluster of x x x x x x x x x

39O

Cluster of x x x x x x x x x

78O

300 hd x x x x x x x

1170 x x x x x x x x x

1560 x x x x x x x x x

Table I. - NDE detection capabilities of EDM notches and drilled holes

in standard ring #1 and standard ring #2. (hd is short for high density

broken metallic EDM wire; UT is ultrasonic testing; TT is through

transmission; PE is pulse echo; RT is radiographic testing; C is

conventional; M is microfocus; CT is computed tomography

NASA/TM--2000-210473 5
Fig. 1- Preloaded composite rotor for the flywheel energy storage system targeting

the replacement of chemical batteries on the international space station

rotation stage

Fig. 2.- Immersion ultrasonic scanning system as applied to composite ring evaluation

NASA/TM--2000-210473 6
Fan beam

of x-rays

Object CT Slice

[ 3_ to Axis

of Rotation

//to Axis

of Rotation

Fig. 3.- Schematic of x-ray computed tomography as applied to composite rotor evaluation

\

Fig. 4- Schematic of a nondestructive evaluation ring standard with EDM notches and

drilled holes. Ring is 12.5 mm high and 12.5 mm thick, and notches and holes are 6.25

mm deep. Measurements shown on figure are in micrometers and the schematic is not

to scale. C in 397 ( c ) stands for cluster of 397 txm drilled holes.

NASA/TM--2000-210473 7
Fig. 5 - Computed tomography slice of a multi-layered rotor standard with EDM

notches and drilled holes set (125x125, 300, 390, 500x500, 780, and 750x1500 _tm) in

each layer. This set was duplicated in a spiral fashion starting at the innermost ring

near the ID and finishing at the outermost ring near the OD.

NASA/TM--2000-210473 8
i .....ii !!!iiii=i

_ _!i_i_ _ !iiii_i!i!!i _

30x60 ___ =

III ii i | IIIII IIII III :

3(1/64)

Fig. 6 - Microfocus radiography of one NDE ring standard showing the capabilities of

detection down to 125x125 _tm (5x5 mils ) notch. Measurements shown are in mils and

images shown are prints of 5 radiographs taken in direct contact with the ID of the

ring.

5x5 5x40 10x10 10x40 20x20 20x60 30x30 30x60

Defect or notch dimensions in mils

TT Scan

at 10 MHz

TT Scan

at 5 MHz

TT Scan at

2.25 MHz

TT Scan

at I MHz

PE scan

at 5 MHz

Fig. 7 - Ultrasonic scanning of one NDE ring standard showing the capabilities of

detection with through transmission (TT) and pulse echo (PE) methods. The scans of

the ring as displayed are open cylinders (rectangles) where the vertical direction

represents the height of the ring and the horizontal direction represents the

circumference from 0-359 degrees.

NASA/TM--2000-210473 9
12.5 mm

0.4 0.8 0.4 3 hole 0.8 3 hole 0.25 .125 .25x .125

cluster cluster x 0.5 x .75

X 0.5 .75

12.5 mm

0.3 1.2 1.6

Defect/EDM notches or holes in mm

Fig. 8- Ultrasonic pulse echo (5MHz) scans of one NDE ring standard showing the

capabilities of detecting 6.5mm deep EDM notches and drilled holes

o 180 °

Fig. 9- Immersion pulse echo ultrasonic scan of a 180 o delam in a hydroburst ring

NASA/TM--2000-210473 10
O

12.7mm

359 °

Transducer bouncing the ultrasound off the back

surface (I.D.) O.D. Side

I.D. Side

Optical photo showing an edge view of the

location and shape of the insert

Fig. 10- Immersion ultrasonic pulse echo scan detects the insert of bagging material

in a hydroburst ring

33

cm

0

0 o 180 ° 359 °

Fig. 11 - Ultrasonic scan showing a large delamination in one of the rings

NASA./TM--2000-210473 11
Composi s__in

Fig. 12 - Optical photograph of a section of the ring showing the delamination (Fig. 11)

as exposed to the surface

layer

Fig. 13 - Selected section from the CT scan showing a 40x80 mils void in the glass layer

Glass

Fig. 14- Selected area from a computed tomography cross section showing a resin

rich region within the composite layer (a), and an optical photograph of the resin rich

region after destructive sectioning and polishing for validation (b)

NASA/TM--2000-210473 12

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4. ltTLE AND SUBTITLE

October 2000

NDE Methodologies for Composite Flywheels Certification

6. AUTHOR(S)

Technical Memorandum

5. FUNDING NUMBERS

WU-494-29-11-00

George Y. Baaklini, Kevin E. Konno, Richard E. Martin, and Richard Thompson

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135- 3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-12468

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2000-210473

00PS-63

11. SUPPLEMENTARY NOTES

Prepared for the 2000 Power Systems Conference sponsored by the Society of Automotive Engineers, San Diego,

California, October 3 l-November 2, 2000. George Y. Baaklini and Kevin E. Konno, NASA Glenn Research Center;

Richard E. Martin, Cleveland State University, 1983 E. 24th St., Cleveland, Ohio 44115-2403; Richard Thompson,

University of Texas at Austin, Austin, Texas 78712. Responsible person, George Y. Baaklini, organization code 5920,

(216) 433-6016.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified - Unlimited

Subject Categories: 88 and 38 Distribution: Standard

Available electronically at htto://gltrs.m-c.nasa.gov/GLTR$

This publication is available from the NASA Center for AeroSpace Information. (301) 621-0390.

13. ABSTRACT (Maximum200 words)

Manufacturing readiness of composite rotors and certification of flywheels depend in part on the maturity of nondestructive evaluation (NDE) technology for process optimization and quality assurance, respectively. Capabilities and limitations of x-ray-computed tomography and radiography, as well as advanced ultrasonics were established on NDE ring and

rotor standards with EDM notches and drilled holes. Also, intentionally seeded delamination, tow break, and insert of

bagging material were introduced in hydroburst-rings to study the NDE detection capabilities of such anomalies and their

effect on the damage tolerance and safe life margins of subscale rings and rotors. Examples of possible occurring flaws or

anomalies in composite rings as detected by NDE and validated by destructive metallography are shown. The general

NDE approach to ensure quality of composite rotors and to help in the certification of flywheels is briefly outlined.

14. SUBJECT TERMS

NDE; Composites; Flywheels; Radiography; Ultrasonics; Computed tomography;

Composite rotors; Flight certification

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OF REPORT OF THIS PAGE OF ABSTRACT

Unclassified Unclassified Unclassified

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18

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A03

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