Tests were conducted in translational launchers and spin pits to generate empirical data used in the design of a Kevlar shield for containing engine burst debris. Methods are given for modeling the relationship of fragment characteristics to shielding requirements. The change in relative importance of shield mounting provisions as fragment energy is increased is discussed

Bristow, R. J.

Davidson, C. D.

English

NASA Technical Reports Server

DEVELOPMENTOF FIBER SHIELDS FORENGINE CONTAINMENT
by
R. J. Bristow and C. D. Davidson
Senior Engineers
TIIE BOEINGCOMPANY
Seattle, Washington 98124
SUMMARY
A partial review is given of the progress at Boeing toward achieving a lightweight
means for containing engine burst debris. This paper describes only the empirical
work. Another paper at this meeting, by Dr. J. H. Gerstle, deals with the Boeing
theoreticalapproach. The testing described was conducted in both translational
launchers and spin pits. Empirical model development relating fragment character-
L
- istics to shielding requirements is given. The change in relative importance of
shield mounting provisions as fragment energy is increased is given.
INTRODUCTION
lhe current shield design concepts have resulted from an evolutionary development
that began in the early 1960's. Since that time, a group at Boeing has developed
shielding for a wide range of threats: meteoroids, bullets, blast, hail, rain,
• and free-falling rocks, to mention a few. In all of these efforts, it was clear
that shielding weight could be reduced if the projectile deceleration distance was
increased. For lower velocity regimes, this could be accomplished by combini0_g
the properties of high shear resistance and elasticity in tiledirection of projectile
motion. Certain fibrous materials can provide these properties.
_ Various fibrous materials have been used since the days of spears and arrows to
-?
"_ shield against projectiles. More recently, fibrous shields liavebeen used as
"flak vests". At Boeing, glass fiber blankets have been used experimentally as
blast shielding. It was natural, then, to try fiber blankets for engine containment.
t
_ The first fibers tried, glass, performed better than metallic shields but the data
L
was inconsistent. DuPont's Kevlar fabric was then tried and has developed into
today's design concept.
217 _ PAGEBLANKHOTFIl.li1_
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i 978002i25-2i 8
https://ntrs.nasa.gov/search.jsp?R=19780002136 2020-03-20T12:22:40+00:00Z
The developmentof the Kevlar shieldhas been undertakenwith a two-pronged
approach. An analyticalcomputermodel (EBCAP)was developedbased on a
theoreticalapproach,the M.I.T.Model JET 4B and other publisheddata. The
secondapproachused the empiricaldata to generatetwo empiricalmodels that
have in turn been used to solve design problems. The first empiricalmodel
(Figurel) definedthe weight of shield for various projectilesizes and the
velocitiesat which the projectileswould be contained;this at a constant
dynamicstiffness. The secondmodel relatedthe shieldmount load to mount
dynamicstiffness. Both approaches,analyticalan_ empirical,were coordinated
with the test programand are complementary.
BACKGROUND
In order to maintaincontinuity,earlierprogram resultspublishedin a previous
paper*will be summarized.
Translationaltestingconsistedof firing s_eel cubes from a smoothborecannon
into a test shield as shown schematicallyin Figure2. Figure 3 is a photograph
of the test range. The targetassembly,shown in Figure4, consistednot only
@
, of the test shield,but a seriesof thin aluminumplates. The plates,called
"witnesssheets,"'were used to determinethe residualenergy of the cube if the
test shieldwas penetrated. The cubes were launchedfrom the cannon by means of
a polycarbonatesabot as shown in Figure 5. The ballisticlimitwas found by i
plottingpenetrationversusvelocityas shown in Figure6. (Ballisticlimit is
the limitingvelocitybelow which shield penetrationwill not occur.) The figure
shows the abscissato be made up of shield layers plus numbersof witness sheets.
The slanted line shows the number of witness sheets penetratedwhen the shield
J was removed. The "S"-shapedcurve shows penetrationwith the shield in place.
The number of Kevlar layers penetratedincreasedgraduallywith velocityuntil
_, the ballisticlimit was approached. At the ballisticlimit,the penetrabilityof
the cube increasedgreatly. At a velocitya littleabove the ballisticlimit, the
numberof witness sheets penetratedwas nearlyequal to that with no shield at all.
This signifiedthat above the ballisticlimit, the shield absorbedvery little
energy. From these data, the first empiricalmodel was developed(Figure1):
* Bristow,R. J., et al, "Advancesin EngineBurst Containment,"AGARD-R-648,
presentedat the 42nd Structuresand HaterialsPanel Meeting,R_TO-AGARD,
•_ Ottawa,Canada,April 1976.
?
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]978002]25-2]9
v, ,
., 219
1978002125-220
N = A(V)2(D)314(sino)516 - B (l)
where:
N = number of shield layers (Kevlar)
V = cube velocityat ballisticlimit - fps
D = cube size - inches
" 0 = angle betweenshield surfaceand flight path
A & B = constants.
Otherareas coveredby the previouspaper includetemperatureeffectsand the
effect of spinningon fragmentpenetration. Since neitherof these areas are
pertinentto the subjectof the currentpaper, they will not be reviewed.
One of the major efforts during the lastyear was the developmp-tof ah attach-
ment load model. The load involvedwas that in the mount of a particlJ!arshield
arrangementwith a particl_lardynamicstiffness. However,the form of the model
i.
, should be generalin nature and providesa great deal of informationon shield
, design requirements. The data for the model _ere obtainedusing the _est
i arrangementshown in Figure 7. One post was calibratedto read equivalentload
at the centerlineof the shield. In order to changeeffectilemountingstiff-
ness, a seriesof nylon ropes were run throughthe shield_nds and loopedaround
the posts as shown in Figure 8. The stiffnesswas variedby changingthe length
• or diameterof the ropes. The resultingmodel was of the form
P = CV(D)3(K)I/2(N+ E)"1/2 (2)
where:
: P = peak impact load (Ib)
K = stiffnessof mount + attachments(lb/in)
• / C & E = constants.
It shouldbe pointedout that the form of the term involvingthe numberof shield
layersmay be differentfor other shieldarrangements.
_ Once an empiricaln_del like the one above has been obtained,it is often con-
, structiveto examine it in detail in order to get an insight!nto the phenomen-
ology involved. Noticethat the peak load is directlyproportionalto the
fragmentvelocityand mass. It is not too surprisingthat the load would be
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proportion-_ to the fragmentmomentum. However,the loadbeingproportional
to thesquarerootof the stiffnessis a bit surprising.Thisis becausewe
are accustomedto seeingthe loadin a springbeingproportionalto the spring
constanttimesthe deflection.
Once the relationshipbetweenpeak loadand stiffnesswas determined,it was
temptingto try to use this relationshipto get an equivalentrelationship
betweenstiffnessand ballisticlimit. The twomodels(EquationsI and 2)
showthatin one case,loadis directlyproportionalto velocity,while in the
other,the numberof shieldlayersrequiredat the ballisticlimitis propor-
tionalto the square of the velocity. Holding fragment size constant, and
equating velocity between the two models, we get a relationship showing that
" the shieldlayersrequiredshouldbe directlyproportionalto the stiffness:
N K (3)
y
The aboveequationhas not beensubstantiated.In fact,itsvalidityis
questionablebecauseEquation1 had to be simplifiedsomewhatin orderto
_" derivethe aboveequation. It is criticalto any designprocedureto knowthe
• relationshipbetweenballisticlimitand stiffness.The determinationof this "i
relationshipis currentlybeingderivedat Boeing.
The lasttermin Equation2 showsthat themagnitudeof the peakloadis a
" functionof the numberof shieldlayers:
P _ (N + const) "1/2 (4)
This equation indicates that the peak load drops with an increase in shield
layers. Thisis becausethe greatermassof shieldmaterialactsto transfer
/ the loadovera longertimeinterval.However,Equation(4)doesnot mean)
thatan increasein shieldlayerswillalwaysdecreasethe load. The stiffness
_' of the shieldisalsoa functionof the numbersof shieldlayers. The greater
the numberof shieldlayers,the greaterthe stiffnessand hence,the greater
the load. The resultis thatfor a shieldwithfew layers,the stiffnesseffect
') predominatesand an increase in shield layers will result in an increase in load.
For heavier shields, the masseffect predominates and the peak load then tapers
off with an increase in shield layers, Figure 9 showsthe change in peak load
: with changesin shield layers.
228 '_"" _'::"
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]978002]25-229
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1978002125-230
/ •
The above paragraphshave shown that stiffnes is an importantconsiderationin
any Kevlar fabric shield design. The stiffness,as viewed by the projectile,
can be written:
[K] = [ks] + [kA] (5)
where:
K = total equivalentdynamic stiffness (Ib/in)
ks = shield stiffhess (Ib/in)
kA = attachmentstiffness (Ib/in).
Algebraically,Equation (5) becomes:
ks + kA
K - (6)
ks kA
As discussed previously,the shield _tiffness (ks) depends on the number of
shield layers. Because of this, for a shield with few layers, the shield stiff- "
mess soon predominatesover the attachment stiffness. This is shown in Figure 10.
_ The four-layershield in Figure 10 results in a rapidly increasingload at low
attachment stiffnesslevels. However, the attachmentstiffness soon becomes so
high that only the shield stiffness needs to be retained in Equation (6). As
can be seen in the figure, this is also true for heavier shields except that the
• point where the attachmentstiffness can be neglectedoccurs at a higher total
attachment stiffness.
Another area receivingemphasis during the last year concerned large fragments.
Steel cubes up to 3.75 in. in size were launched in translationalaccelerators.
_ These large cubes were contained at energy levels of up to 5_I,000 in. lb.
These tests were interestingin that a new failure mode was discovered. It was
found that at high energy levels with large cubes, a tensile failure occurred at
some distance from the impact point. (Before, the normal failure had been shear-
ing or local tension around the periphery of the projectile.) However, it was
t further found that reduction of the effective shield stiffnesswould again switch
the failure mode to one of local failure at the impact point. This local failure
was deslrab]e since it occurred at a higher energy level than the tensile failure.
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5000 _E ' ''
4000 , , RS , ,'. , ,', , ,
3000
RELATIVE
LOAD - LBS
2_-
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• 0 1000 2000 3000
MOUNT + ATTACHMENT STIFFNESS. LB/IN . .
Flsure 10. - Support Stiffness Effects. ' '
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231
1978002125-232
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The abovetests,plusthe loadmodel,made it abundantlyclearthatstiffness
isa majorconsiderationin any fibrousshie]ddesign.
Inorderto checkout the resultsol)tainedin the BoeingImpactMechanics
Laboratorytranslationalaccelerators,the Navyhas beenmostcooperative
with the use of the NAPTCspinpits. Severaltestshavebeenconductedwith
14-inch-diameterrotorsat aboutone millioninchpoundsof totalenergy. The
shieldsconsistedof a lightaluminumring (onepound)witha numberof wraps
of Kevlar(varyingfrom25 to 40). Recently,a successfultestwas madewhere
a rotorwith8.7 x lO6 in lb of totalenergywas containedby a 120 layershield.
It is expectedthaton latertests,thisnumberof layerscan be significantly
:. reduced. In all cases,the spinpit testresultswerenearthoseobtained ;
• withtranslationalacceleratorswhen adjustedfor stiffnessof the system.
, FUTUREWORK
The largesttaskyet to completeis a modelrelatingballisticlimitto stiffness. ,'
Thismode]willthenbe combinedwith theearlierballisticlimitmodelto give
requiredshieldweightas a functionof fragmentsizeand velocity,angleof ,
obliquity,and overallshield/mountstiffness.Anotherareaof studyinvolves '
techniquesthatwill reducethe inherentKevlarstiffnesswithoutlosingits i ,
inherentstrength.One methodcurrentlybeingexaminedinvolveswrappingthe I
shieldin sucha mannerthatthematerialis stressedin the biasdirection. I
• Furthertestsin the spin-pitwiththe J 65 turbineare programmed;thesewill i
be usefulin confirmingthe empiricalmodelsat higherenergylevelsand will ;_
L identifytheeffectsof multi-layerconfigurations.A numberof othersmaller
studyeffortswillbe made to fill in gapsor answerquestionsremainingfrom
., // previousstudies. _
_? CONCLUSIONS "_
Boeinghas beenstudying eng;,,e burst containment as part of a comprehensive
damagemechanismsprogram. The lastthreeyearshave beendevotedto a study "
'r
of Kevlarmaterialas the basiccontainmentmedium. Modelsfor ballisticlimit
? and attachment load are available. The modelshave closely predicted the results
obtained in spin pits. The importance of overall shield stiffness has been
determined and shield designs are being worked out that will have the proper
! stiffness. Translational test energies have been pushedup to over 540,000 in lb,
i 232 ,, "
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1978002125-233
while successful spin pit tests up to 8.7 x lO6 in Ib have been made. An areal
weight of 1,7 ]b/ft2 was required for the spin pit rotor having one million
inch pounds of energy while 8 )b/ft2 was used for the 8.7 million inch pound
energy rotor. This latter shield was not optimized and a lower areal weight
is expected,
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1978002125-234
DISCUSSION
Unknown Speaker
What are the effects of moisture and temperature/time on Kevlar?
R.B. Bristow, Boeing
There is a report put out by du Pont on that sub iect, which indicates
that Kevlar strength does indeed fall off with temperature and time. However,
we were rather surprised during our tests to find that when we heated the Kevlar
targets and fired the fragments into them, we actually had a higher ballistic
limit. The reason being that the strain rate effects increased faster with
higher temperature than does the degradation of the strength. This was covered
in a previous paper that I mentioned, and is cited in my paper here as a reference.
As far as moisture goes, I can't answer that.
D. Oplinger, Army-AMMRC
I was interested in your attachment or support load dropoff. With a_.mor,
. that's usually considered to occur because the projectile shatters at a certain
speed so that it becomes blunt. It's hard to visualize what would be causing
this in the case of Kevlar.
/
R.B. Bristow, Boein 9
I haven't been able to figure it out. One reason I brought it up was that
we have many experts here and I'd like to find out what's causing it, if possible.t
I also might mention that I feel we've come a long ways with Kevlar but we're a
long way from having something that's suitable for putting on an airplane. There's
lots of design considerations that we haven't even begun to consider•
P. Gardner, Norton Co.
That projecti_ that you passed around, the large cube, had some blunted
edges on it. Was that from the impact with the Kevlar or did it fa%l on the
floor after going _rough the first test panel?
R.B. Bristow, Boeing
The steel projectile was not deformed by going through the Kevlar. We
_ fired it in tests both below and above the ballistic limit, sc we could find
that dividing line. This one has gone through the shield and struck a steel
_' plate behind, and suffered this blunting of the edges.
2_
i_ _
]978002]25-235


Development of fiber shields for engine containment

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