NASA Technical Memorandum 107532

Elevated Temperature Ballistic Impact Testing

of PBO and Kevlar Fabrics for Application

in Supersonic Jet Engine Fan

Containment Systems

J. Michael Pereira, Gary D. Roberts, and Duane M. Revilock

Lewis Research Center, Cleveland, Ohio

November 1997
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NASA Technical Memorandum 107532

Elevated Temperature Ballistic Impact Testing

of PBO and Kevlar Fabrics for Application

in Supersonic Jet Engine Fan

Containment Systems

J. Michael Pereira, Gary D. Roberts, and Duane M. Revilock

Lewis Research Center, Cleveland, Ohio

National Aeronautics and

Space Administration

Lewis Research Center

November 1997
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ELEVATED TEMPERATURE BALLISTIC IMPACT TESTING OF PBO

AND KEVLAR FABRICS FOR APPLICATION IN SUPERSONIC JET

ENGINE FAN CONTAINMENT SYSTEMS

J. Michael Pereira, Gary D. Roberts, and Duane M. Revilock, Jr.

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

SUMMARY

Ballistic impact tests were conducted on fabric made from both Poly(phenylene benzobizoxazole) (PBO)

and Kevlar 29 which were selected to be similar in weave pattern, areal density, and fiber denier. The projectiles were 2.54-cm- (1-in.-) long aluminum cylinders with a diameter of 1.27 cm (0.5 in.). The fabric specimens

were clamped on four sides in a 30.5-cm- (12-in.-) square frame. Tests on PBO were conducted at room temperature and at 260 °C (500 °F). A number of PBO specimens were aged in air at 204 and 260 °C (400 and

500 °F) before impact testing. Kevlar specimens were tested only at room temperature and with no aging. The

PBO absorbed significantly more energy than the Kevlar at both room and elevated temperatures. However,

after aging at temperatures of 204 °C (400 °F) and above, the PBO fabric lost almost all of its energy absorbing ability. It was concluded that PBO fabric is not a feasible candidate for fan containment system

applications in supersonic jet engines where operating temperatures exceed this level.

INTRODUCTION

The fan containment system in a jet engine is designed to prevent a fan blade from penetrating the engine

nacelle in the event that the blade or a portion of the blade separates from the rotor during operation. The

systems are generally classified as being "hardwall" or "softwall," depending on the means used to contain

the blade. Typical hardwall systems consist of a metal case surrounding the fan rotor; that case is thick enough

to prevent penetration. Softwall systems generally consist of an impact-resistant dry fabric wrapped around a

relatively thin metal fan case. In the latter systems the major impact resistance comes from the fabric rather

than the metal case. The systems may also include a honeycomb sandwich structure between the fabric and

the metal case to provide extra stiffness and a region for the fan blade to "nest" or be captured during the

impact event. In large turbofan engines metal containment systems can weigh as much as 250 kg (550 lb). Soft

containment systems can be significantly lighter, providing a weight savings of 20 to 25 percent (Stotler,

1979).

All softwall containment systems in current commercial jet engines use the aramid fiber Kevlar (E.I. du

Pont de Nemours and Company, Wilmington, DE). Kevlar has excellent impact properties and cutting

resistance which makes it attractive for fan containment systems in subsonic engines, as well as for other

applications such as personal body armor. However, because the properties of Kevlar degrade at elevated

temperatures, it is not a suitable material for use in supersonic engines where temperatures in the fan section

are expected to exceed 200 °C (400 °F). Recently, a relatively new fiber known as Poly(phenylene benzobizoxazole), or PBO (Dow Chemical Company, Midland, MI), has become available in small quantities.

Fabric woven from this fiber has shown good impact properties in body armor applications as well as for jet

engine turbine disk containment (Pepin, 1993). In addition to its good impact properties, the fiber has shown

good thermal stability (Denny, Goldfarb, and Solaski, 1989). The objectives of this study were to measure the

ballistic impact properties of PBO fabric at room and elevated temperatures and determine the effects of aging

on its impact performance.

METHODS

Ballistic impact tests were conducted on PBO and Kevlar 29 fabrics that were selected to be similar in

weave pattern, areal density, and fiber denier. The PBO fabric was a plain weave material (44-ends/in.,

500-denier continuous filament fibers with an areal density of 203 g/m 2 (6 oz/yd2)). The Kevlar 29 fabric was a

plain weave material (48-ends/in., 400-denier fibers with an areal density of 170 g/m 2 (5 oz/yd2)). Test

NASA TM-107532 1
specimensmeasuring38cm(15in.)squarewerecutfromthefabric.Fortheballisticstesta singlespecimen

wasclampedonthefoursidesin a30.5-cm(12-in.)squarefixtureimpactedbyaprojectileacceleratedbythe

releaseof compressedheliumin agasgun.PBOspecimensweretestedatroomtemperatureandat260°C

(500°F).TheKevlarspecimensweretestedonlyatroomtemperature. Inaddition,anumberofPBOspecimenswereagedinairat204and260°C(400and500°F)for3 and6 months,respectively. A limitednumber

of specimensweretestedafteraging.

Theprojectilesusedinthesetestswere2.54-cm-(1-in.-)long6061-T6aluminumcylinderswitha 1.27-cm

(0.5-in.)diameter.A radiusof 0.8mm(0.0032in.)wasmachinedontheedgeof theimpactingface.Thegas

gunconsistedof apressurevesselwithapressurecapacityof 10MPa(1500psi)andavolumeof 2250cc

(140in.3),connectedviaa high-speedelectricrelayvalveto astainlesssteelhollowbarrelapproximately2 m

(6ft) long(fig. l). Thebarrelhadanoutsidediameterof 2.54cm(1in.)andaninsidediameterof 1.28cm

(0.505in.).Heliumwasusedasthepropellant.Thevelocityof theprojectilepriortoimpactwasmeasuredas

it exitedthebarrelbytheinterruptionof twolaserbeamsaknowndistanceapart.

Thefabricspecimenswereheatedbyusinganarrayof quartzlampsradiatingupwardandreflectingoffof

analuminumfoil reflectortoheathebackof thespecimenasshowninfigure1.Thepost-impact velocityof

theprojectilewasmeasuredbycommercial lightscreensaftertheprojectilepenetratedthefoil reflector.It

wasassumedthatthereflectorproducedanegligiblechangein thevelocityof theprojectile.Thetemperature

of thespecimenwasmeasuredbyusinganinfraredpyrometer focusedontheimpactsideof thespecimen.

Priorto impacttesting,scanswereconductedto determinethetemperaturedistributionin thespecimens. The

temperaturewasrecordedatagridofpoints5cm(2in.)apartintheverticalandhorizontal directions. Generally,within+10 cm (+4 in.) of the center, the temperature of the specimen remained relatively uniform. At a

nominal temperature of 260 °C (500 °F) the temperature in this region was typically within _+8 °C (15 °F), or

3 percent of the nominal. Near the comers of the specimens, the temperature was as much as 30 °C (54 °F)

lower than the nominal.

The relative ballistic performance of PBO and Kevlar 29 at room temperature and PBO at an elevated

temperature was evaluated by comparing the velocity required to penetrate the fabric. This velocity was

determined by plotting the projectile exit velocity as a function of the impact velocity. An empirical relationship was fit to the data for cases where penetration occurred by making the assumption that the energy

absorbed by the fabric was independent of velocity; that is,

lmv? - 2l mY°2 = lmu2 2

(1)

ot"

Vo = _ -- U 2 (2)

where m is the mass of the projectile, vi is the impact velocity, vo is the exit velocity, and u is a constant.

When the exit velocity is zero, u corresponds to the velocity required to just penetrate the fabric vp. Values for

the penetration velocity for each of the fabric systems were determined by performing a least squares fit of

equation (2) to the impact data for that system.

While the Kevlar 29 and PBO fabric weaves were similar, it was not possible to obtain fabrics with

exactly the same areal density and fiber denier. The Kevlar 29 (with an areal density of 170 g/m 2 (5 oz/yd2))

was somewhat lighter than the PBO (with a density of 203 g/m 2 (6 oz/yd2)). Therefore, test results of the two

fabrics could not be directly compared. However, results of impact tests on Kevlar fabrics using standard

fragment simulators indicate that the energy absorbed by the fabric is proportional to the fabric areal weight

(Figucia, 1980; Cunniff, 1992). To compare the two systems, therefore, the amount of energy absorbed by the

Kevlar Eab" in the current study was scaled by the ratio of areal densities; that is,

E,abs = Pl Eabs (3)

where E_bs is the scaled energy absorbed by the Kevlar, Pl is areal density of the PBO, and P2 is the areal

density of the Kevlar. This led to the following modified empirical relationship for the Kevlar 29:

vo=_v -l.2u 2 (4)

NASA TM-107532 2
RESULTS

TheunagedPBOfabricwasableto absorbgreaterenergyatroomtemperaturethaneitherthePBOspecimensatelevatedtemperatureortheKevlar29specimensatroomtemperature(fig.2).TheelevatedtemperaturePBOperformedbetterthantheroomtemperatureKevlar.Thedatapointsfallingonthecurveinfigure2

correspondtoimpactsin whichpenetrationdidnotoccur;henceall oftheenergywasabsorbedbythefabric.

Themaximumenergyabsorbedwas132,92,and50J (97,68,and37ft-lbf),respectively, fortheroomtemperaturePBO,elevatedtemperaturePBO,androomtemperatureKevlar.Scalingof theenergyabsorbedbythe

Kevlartoaccountforitslowerarealweight(eq.(3))increasedthepredictedmaximumenergyfor anequivalentweightKevlarto 60J(45ft-lbf).

A qualitativeassessment ofhefabricspecimensindicatedthatthedamageintheKevlarfabricappeared

to bemorelocalizedthaninthePBO.InPBOspecimensin whichpenetrationhadoccurred,permanent

deformationwasevidentoverarelativelylargeareaof thespecimen.

Thegeneral natureof theresultshowninfigure2issimilarto dataintheliterature(Cunniff,1992)where

it is shownthattheenergyabsorbedbythefabricincreasesastheprojectilevelocityis increaseduntilthe

penetrationvelocityisreached, afterwhichthereis aninitialdecreaseintheabsorbedenergy.Asthepenetrationvelocityincreasesfurther,theabsorbedenergymayeitherdecreasefurther,remainconstantorbeginto

increaseagain.Asafirstapproximationforthepurposeof deducingapenetrationvelocityfromthedata,it

wasassumedthat,for caseswherepenetrationoccurred,theabsorbedenergywasindependent of theimpact

velocity.Thisassumptionwasthebasisfortheempiricalrelationshipof equation(2).

Theexitvelocityasafunctionof impactvelocityforthethreetestseriesis showninfigure3.In thisfigurethestraightlinerepresentsthelimitingcasewherenoenergyis absorbedbythefabric.Foragivenimpact

velocitytheexitvelocitywaslowestfortheroomtemperaturePBOandhighestfortheKevlar29.Thefigure

showsareasonablygoodfit betweenthedataandthesimpleempiricalfunctionof equation(2).ThepenetrationvelocityVp was taken to be the intersection of the empirical curve with the ordinate. The penetration

velocities were found to be 155, 133, and 93 m/sec (509, 436, and 305 ft/sec), respectively, for the room temperature PBO, elevated temperature PBO, and room temperature Kevlar.

The empirical curves are shown in figure 4 with the curve for Kevlar scaled to correct for the lower areal

weight. Again, in this figure the straight line represents the limiting case where no energy is absorbed by the

fabric. The scaled relationship increased the predicted penetration velocity to 102 m/sec (335 ft/sec). Also

shown in figure 4 are the results for three impact tests on aged PBO specimens, one aged for 3 months at

204 °C (400 °F), the second aged for 6 months at the same temperature, and the third aged for 6 months at

260 °C (500 °F). It is clear that there is a significant reduction in energy absorption in the aged specimens.

CONCLUSIONS

Unaged PBO has excellent impact energy absorbing characteristics. Its performance was somewhat

degraded at elevated temperature. At 260 °C (500 °F) it was able to absorb approximately 70 percent of the

energy that the room temperature fabric could absorb. However, both at room temperature and at 260 °C

(500 °F), it was significantly better than a similar weight Kevlar fabric. It should be noted that, while steps

were taken to scale the Kevlar results to a higher areal density fabric, a direct comparison of equal areal

density and fiber denier fabrics was not made because of the constraint of production specifications of each

fiber.

While PBO appears to be an excellent material for impact energy absorption, its performance is significantly reduced after aging at temperatures of 204 °C (400 °F) or above. This material, therefore, is not a

feasible candidate for a fan containment system in supersonic jet engines where temperatures exceed this

level.

REFERENCES

Cunniff, P.M.: An Analysis of the System Effects in Woven Fabrics Under Ballistic Impact. Textile Research

Journal, vol. 62, no. 9, Sept. 1992, pp. 495-509.

Denny, L.R., Goldfarb, I.J., and Soloski, E.J.: Thermal Stability of Rigid-Rod Polymers. Mater. Res. Soc. Symp.

Proc., 1989.

Figucia, F.: Energy Absorption of Kevlar Fabrics Under Ballistic Impact. U.S. Army Natick Research and

Development Command, 1980.

Pepin, J.: Fiber Reinforced Structures for Turbine Engine Fragment Containment. AIAA Paper 93-1816, June

1993.

Stotler, C.L.: Development of Advanced Lightweight Systems Containment. NASA CR-165212, 1979.

NASA TM-107532 3
Velocity

chronograph

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Projectile

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.... Fabnc specimen

Figure 1 .mlmpact test facility.

Gun barrel

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200

150

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PBO, room temperature

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0 50 100 150 200 250 300 350

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Figure 2.mEnergy absorbed by Kevlar 29 and PBO at room temperature and PBO at 260 °C

(500 °F) as a function of impact velocity.

NASA TM-107532 4
500 I I I I

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300

200

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Figure 3.mResidual velocity after impact as function of impact velocity.

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500 - PBO, 260 °C (500 °F)

Kevlar 29, scaled

O PBO, aged 260 °C (500 oF), 6 months,

room temperature impact

400 -- [] PBO, aged 204 °C (400 °F), 3 months, / --

room temperature impact

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Figure 4.mResidual velocity for aged fabrics compared with empirical results for unaged

specimens.

500

NASA TM-107532 5
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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

November 1997 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Elevated Temperature Ballistic Impact Testing of PBO and Kevlar Fabrics

for Application in Supersonic Jet Engine Fan Containment Systems

6. AUTHOR(S)

J. Michael Pereira, Gary D. Roberts, and Duane M. Revilock

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

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

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

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-537-04-26

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-10852

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM-107532

11. SUPPLEMENTARY NOTES

Responsible person, J. Michael Pereira, organization code 5910, (216) 433-6738.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Categories: 07 and 27 Distribution: Nonstandard

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

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 word#)

Ballistic impact tests were conducted on fabric made from both Poly(phenylene benzobizoxazole) (PBO) and Kevlar 29

which were selected to be similar in weave pattern, areal density, and fiber denier. The projectiles were 2.54-cm- (1-in.-)

long aluminum cylinders with a diameter of 1.27 cm (0.5 in.). The fabric specimens were clamped on four sides in a

30.5-cm- (12-in.-) square frame. Tests on PBO were conducted at room temperature and at 260 °C (500 °F). A number

of PBO specimens were aged in air at 204 and 260 °C (400 and 500 °F) before impact testing. Kevlar specimens were

tested only at room temperature and with no aging. The PBO absorbed significantly more energy than the Kevlar at both

room and elevated temperatures. However, after aging at temperatures of 204 °C (400 °F) and above, the PBO fabric lost

almost all of its energy absorbing ability. It was concluded that PBO fabric is not a feasible candidate for fan containment

system applications in supersonic jet engines where operating temperatures exceed this level.

14. SUBJECT TERMS

Fan containment; Impact; Fiber; Fabric; Ballistic

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