Human Skin-Like Composite Materials for Blast Induced Injury Mitigation

20 Pages·2017·8.35 MB·English

Article
Human Skin-Like Composite Materials for Blast
Induced Injury Mitigation
ArnabChanda1,2,*andRebeccaGraeter3
1 DepartmentofBioengineering,UniversityofPittsburgh,Pittsburgh,PA15213,USA
2 DepartmentofAerospaceEngineeringandMechanics,UniversityofAlabama,Tuscaloosa,AL35401,USA
3 DepartmentofChemicalEngineering,MississippiStateUniversity,Starkville,MS39759,USA;
[email protected]
* Correspondence:[email protected];Tel.:[phone]
(cid:1)(cid:2)(cid:3)(cid:1)(cid:4)(cid:5)(cid:6)(cid:7)(cid:8)(cid:1)
(cid:1)(cid:2)(cid:3)(cid:4)(cid:5)(cid:6)(cid:7)
Received:8June2018;Accepted:24July2018;Published:1August2018
Abstract: Armors and military grade personal protection equipment (PPE) materials to date are
bulky and are not designed to effectively mitigate blast impacts. In the current work, a human
skin-likecastablesimulantmaterialwasdevelopedanditsblastmitigationcharacteristics(interms
ofinducedstressreductionattheboneandmuscles)werecharacterizedinthepresenceofcomposite
reinforcements. ThereinforcementemployedwasKevlar129(commonlyusedinadvancedcombat
helmets),whichwasembeddedwithinthenovelskinsimulantmaterialasthematrixandusedto
coverarepresentativeextremitybasedhumanskin,muscleandbonesectionfiniteelement(FE)model.
Thecompositevariationstestedwerecontinuousandshort-fibertypes,lay-ups(0/0,90/0,and45/45
orientations)anddifferentfibervolumefractions. Fromtheanalyses,the0/0continuousfiberlay-up
withafibervolumefractioncloseto0.1(or10%)wasfoundtoreducetheblast-induceddynamic
stressesattheboneandmusclesectionsby78%and70%respectively. Thesefindingsindicatethat
thisnovelskinsimulantmaterialwithKevlar129reinforcement,withfurtherexperimentaltesting,
maypresentfutureopportunitiesinblastresistantarmorpaddingdesigning.
Keywords: blast;composite;skin;traumaticbraininjury(TBI);finiteelement
1. Introduction
Blastshavebeenamajorcauseoftraumaticinjuriesamongsoldiersinvolvedinarmedcombatsin
politicallydisturbedareassuchasIraqandAfghanistan[1]. Besidesmildtraumaticbraininjury(TBI),
severewhole-bodyinjuriesareoftenreported,whichcouldbemainlyattributedtopoorlydesigned
armors and military grade personal protection equipment (PPE). To date, realistic experimental
modelinghasbeenachallengeduetodifficultyinsimulatingblastsandethicalissuesassociatedwith
animalandhumantesting. Thishasledtoextensivecomputationaleffortsintoinvestigationofballistic
andblast-inducedmitigation[2,3]effectsofarmors,specificallyhelmetmaterialsincludingcomposite
based paddings. An axisymmetric numerical model of the head and helmet was developed by
Khaliletal.[4]tostudythehelmet’sloadresponseforashortdurationballisticimpact. Severalstudies
wereconductedbyHoofetal.[5,6]ontheimpactofballisticloadingonthepersonnelarmorsystemfor
groundtroops(PASGT)helmet,whichindicatedthatballisticimpactsmainlycausedlocalizedeffects
attheimpactarea,withminimaleffectsontherestofthehelmet. Impactofahighvelocityballistic
studiedbyBaumgartneretal.[7]revealedariskofskullfractureduetohelmetbackforcedeformation
(BFD).Aareetal.[8]investigatedhowhelmetshellstiffnessandimpactdirectioninfluenceshead
injury,andfoundthattoavoidinjury,theshelldeflectionshouldnotbemorethantheinitialdistance
betweentheheadandthehelmet. Leeetal.[9]studiedtheeffectofvaryinghelmetcushioningon
ballistic head impacts. It was concluded that the helmets with all types of cushions protect well
J.Compos.Sci.2018,2,44;doi:10.3390/jcs2030044 www.mdpi.com/journal/jcs
J.Compos.Sci.2018,2,44 2of20
againstsmallprojectiles,butpoorlyforlargeballistics. Tanetal.[10]studiedtheimpactperformance
ofadvancedcombathelmet(ACH)usingfiniteelement(FE)analysis,whichindicatedthesuperior
characteristicsoflowhardnessfoamsforballisticimpactmitigation. Lietal.[11]alsostudiedtheeffect
ofballisticimpactsonACH,andfoundthatlarge,medium,andsmallvaluesofBFDoccurreddueto
frontal,crownandlateralimpactsrespectively. Also,BFDwasfoundtodecreaseincorrespondence
withadecreaseinobliqueimpactangle. Additionally,thesimulationsindicatedthattheBFDwas
inverselyrelatedtothehelmetsizeforthesameimpactvelocity. Recently,Rodriguez-Millanetal.[12]
studiedtheeffectofstandardballisticimpactsonacombathelmet,andbasedontheexperiments,
developedacomplexandhighfidelitynumericalmodelofthehelmet. Thisnumericalmodelwas
usedtostudytheballisticlimitofthehelmet,whichcorrelatedwellwiththeexperimentalresults.
The study concluded that the computational model was biofidelic and appropriate for modeling
differentballisticstestsonthecombathelmet.
Withrespecttothestudyofblastinducedimpacts,mostinjuryincidenceshaveoccurreddueto
shockwaveinducedoverstressingofthecranium[13]. Anothermajorlocalizedoverpressurecausing
effecthasbeenidentifiedtobetheunderwasheffect,inwhichtheincomingshockwaveinteractswith
theamountofwavereflectedfromhelmets,causingpressureamplificationatdifferentheadlocations.
Ganpuleetal.[14]developeda2DFEmodeltostudytheinteractioneffectofaheadandafrontalblast
shockwaveinboththepresenceandabsenceofaprotectivehelmet. Amplifiedunderwashpressures
wererecorded,whichwasattributedtothehelmetgeometry,curvatureandgapbetweenthehead
andthehelmet. Mossetal.[15]observedthesignificantlyreducedeffectofunderwasheffectsinthe
presenceofhelmetfoampads,howeverwithaconsequentincreaseinthecouplemomentsbetween
the head and the helmet causing localized pressure buildups. Zhang et al. [16] also modeled the
effect of frontal blast loading on the head in presence of a helmet with padding. The results from
thestudyindicatedreducedpressuretransmissionstothebrain,howeveraccompaniedwithahigh
pressureunderwasheffectatthebackofthehead. Nyeinetal.[17]modeledthestressreducingeffects
ofamilitarygradehelmetandfaceshieldinfrontalblastsimulations. Significantstressreductions
were observed at the brain, however the effect of blast direction and underwash effect were not
modeled. Tofurtherinvestigatetheunderwasheffect,Moghaddametal.[18–21]developedadetailed
FE model of the head and tissues, and studied the biomechanical responses of the tissues due to
blastwavestravellingfromdifferentdirections. Threemodelswerestudied: Theunprotectedhead,
semi-protectedhead(withhelmet),andfullyprotectedheadwithhelmetandfaceshield. Fromthe
analyses, the pressure increase due to the underwash effect was found to significantly vary with
the incident blast wave direction. Also, the intracranial pressure (ICP) was found to increase due
to the underwash effect beyond the peak ICP recorded for a helmet protected head subjected to
blast. Rodriguez-Millanetal.[22]recentlydevelopedadetailedcomputationalmodelofthehuman
headincludingthevarioussectionsofthebrain(i.e.,whitematter,greymatter,ventricularsystem,
cerebellum, and brain stem), to study the contribution of advanced combat helmet parts in blast
mitigation. Theseparatehelmetpartsmodeledwerethehelmetstructure,visor,andmandibleguard.
Theresultsindicatedthattheindividualhelmetcomponentswerenoteffectiveenoughinmitigating
blastdamage,andallthehelmetcomponentstogetherwereabletoreduceblastdamageuptofive
times.Grujicicetal.[23]studiedtheblastmitigationefficiencyofacombathelmetwithelastomerbased
polyuriaasthepaddingmaterialforanumericallymodeledhead,andfoundittoperformbetterthan
theconventionalethylene-vinyl-acetate(EVA)foam. Grujicicetal.[24]alsoinvestigatedtheenergy
dissipationandshockwaveattenuationcapabilitiesofgranularmaterials. Recently,Jensonetal.[25,26]
developedasimpleFEmodeltostudytheeffectofblastonahelmetmadeofnanocompositebased
material,andcompareditsperformancewiththatoftheACH.
Besidesthenumericalanalysisofblastimpactsonthehead,veryfewcomputationalstudieshave
considered the effect of blasts on the rest of the human body such as the extremities, described in
Chandaetal.[27,28]. Also, akeyliteraturegapalsoexistsinmodelingofcompositematerialsfor
improvedblastresistanceinpartsofthebodyotherthanthehead. Besidesblastimpactmitigation,
J.Compos.Sci.2018,2,44 3of20
J. Compos. Sci. 2018, 2, x 3 of 19
ootthheerr rreeqquuiirreemmeennttss ooff ssuucchh mmaatteerriaialsls ininccluluddee lilgighht twweiegihgth, tt,atialoilroarbaibliitlyit, ya,nadn dfleflxeibxiilbitilyi t[y27[2].7 B].asBeads eodn
othnet hreecreencte ndtedveelvoeplompemntesn tosno hnuhmumana ntistsisuseu esismimulualnatnst s[2[92–93–23]2,] ,aa nnoovveel lsskkiinn ssiimmuullaanntt mmaatteerriiaall wwaass
ddeevveellooppeedd aannddr ercerceraetaetdedw withitihnina FaE FmE omdeold.eKl.e vKleavrl1a2r9 1fi2b9e frisb(etrysp (itcyaplliycaulsleyd uisnecdo minb caotmheblamt ehtes)lmweetrse)
ewmebree demdebdedindteodt hinetnoo tvheel nsokvinels simkiunl sainmtu(alasntht e(ams tahteri xmmataritxe rmiaal)tenruiaml)e nriucmalelyritcoalslyim tou lsaitmeualtawteo a-l tawyeor-
claoymepr ocsoimtepcoosviteer icnogv.eTrihnigs.c Tohmisp ocosimtepcoosviteer icnogvewriansgu wseads tuospedro ttoe cptraorteecptr eas reenptaretisveenteaxttirveem eixtytr-ebmasiteyd-
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ccoovmeprionsgitwe acsoevvearliunagt ewdafos revvaarlyuiantgedre ifnofro rvceamryeinngt tyrepiensf,olracye-mupesn,ta ntydpfiebs,e rlavyo-luupmse, farnacdt iofinbse.rS evcotliuonm2e
dfriascctuisosness. tSheectmioant e2r iadlisfcaubsrsiceast itohne amnadteterisatiln fga,barnicdattihoen FaEndm toedsetilndge, vaenlodp tmhee nFtE. Smecotdioenl d3ewvielllodpimsceunsts.
tSheectrieosnu l3t sw,fiolll ldoiwsceudssb tyhteh reecsounltcsl,u fsoilolonwsiendS beyc ttihoen c4o.nclusions in Section 4.
2. MaterialsandMethods
2. Materials and Methods
Arepresentativeextremitybasedcomputationalmodeloftheskin,muscleandbonesectionwas
A representative extremity based computational model of the skin, muscle and bone section was
developed,andcoveredwithtwolayersofacompositematerial(madeofKevlar129embeddedwithin
developed, and covered with two layers of a composite material (made of Kevlar 129 embedded
anovelelastomer-basedskinsimulantmatrixmaterial). Figure1showsthesectionofahandglove,
within a novel elastomer-based skin simulant matrix material). Figure 1 shows the section of a hand
whichcouldpossiblybereinforcedwiththetwocompositecoveringlayerstoprotecttheunderlying
glove, which could possibly be reinforced with the two composite covering layers to protect the
skin,muscleandbonesectionsfromblasts. Thevariousdimensionsusedtomodeleachofthesections
underlying skin, muscle and bone sections from blasts. The various dimensions used to model each
arepresentedinFigure1. Also,itshouldbementionedthatacurvedmodelwasselectedbecause
of the sections are presented in Figure 1. Also, it should be mentioned that a curved model was
none of the human body parts are flat, and have a certain amount of curvature everywhere [33].
selected because none of the human body parts are flat, and have a certain amount of curvature
Thiscurvaturehasbeenfoundtohaveaneffectonblastloading[34]. Thefollowingsectionsdiscuss
everywhere [33]. This curvature has been found to have an effect on blast loading [34]. The following
thefabricationandtestingoftheskinsimulantmaterial,materialcharacterization,geometricalmodel,
sections discuss the fabrication and testing of the skin simulant material, material characterization,
meshing,theloadsandboundaryconditionsusedintheFEanalyses.
geometrical model, meshing, the loads and boundary conditions used in the FE analyses.

Figure 1. Our computational model of a representative extremity based skin, muscle, and bone
Figure1.Ourcomputationalmodelofarepresentativeextremitybasedskin,muscle,andbonesection.
section. Also shown are the two layers of protective composite covering.
Alsoshownarethetwolayersofprotectivecompositecovering.
2.1. Fabrication and Testing of Skin Simulant Material
2.1. FabricationandTestingofSkinSimulantMaterial
A two-part soft elastomer material (shore hardness 30 A) from Smooth-On Inc. (Macungie, PA,
Atwo-partsoftelastomermaterial(shorehardness30A)fromSmooth-OnInc. (Macungie,PA,
USA) was mixed in 1:1 weight ratio. 20 test specimens were generated, each of which were clamped
USA)wasmixedin1:1weightratio. 20testspecimensweregenerated,eachofwhichwereclamped
onto a uniaxial tensile testing machine (from MTS Corporation, Eden Prairie, MN, USA). Each test
ontoauniaxialtensiletestingmachine(fromMTSCorporation,EdenPrairie,MN,USA).Eachtest
specimen had a 35 mm length, 9 mm width and 2.5 mm thickness after clamping (See Figure 2), and
specimen had a 35 mm length, 9 mm width and 2.5 mm thickness after clamping (See Figure 2),
a test rate of 0.4 mm/s was selected for all the tests, based on literature [35]. The average stress versus
stretch response obtained from the tensile tests (see Figure 3) were found to lie within the bounds of
the human skin mechanical properties [35].
J.Compos.Sci.2018,2,44 4of20
andatestrateof0.4mm/swasselectedforallthetests,basedonliterature[35]. Theaveragestress
versus stretch response obtained from the tensile tests (see Figure 3) were found to lie within the
boundsofthehumanskinmechanicalproperties[35].
J. Compos. Sci. 2018, 2, x 4 of 19
J. Compos. Sci. 2018, 2, x 4 of 19

Figure 2. Skin simulant matrix material during a test.
FiFgiugruer2e. 2S. kSikninsi msimulualnatnmt mataritxrimx mataerteiarliadl udruinrignag tae stet.st.

Figure 3. Stress versus stretch response of the skin simulant matrix material, lying within the bounds
FiFgiugruer3e. 3S. tSretrsessvs evresrussusst rsetrtechtchre rsepsopnosnesoef otfh tehsek sikninsi msimuluanlatnmt mataritxrimx mataerteiarli,ally, liynignwg withitihninth tehbeo buonudnsds
of the human skin mechanical properties reported in literature [35].
ofotfh tehhe uhmumanansk siknimn mecehcahnaincaiclaplr porpoeprteiretsierse preoproterdteidn ilnit elirtaetruartuer[e3 5[3].5].
The novel skin simulant material was developed for the matrix material of the composite
The novel skin simulant material was developed for the matrix material of the composite
coverings because it is as flexible as the human skin, and thus can stretch with the movement of the
coverings because it is as flexible as the human skin, and thus can stretch with the movement of the
body location where the reinforced armor is placed, without any discomfort [36]. Also, this elastomer
body location where the reinforced armor is placed, without any discomfort [36]. Also, this elastomer
material can be casted in any shape or size. Thereby, depending on the shape and curvature of the
material can be casted in any shape or size. Thereby, depending on the shape and curvature of the
body location, subject-specific armor designs can be made. Additionally, the skin simulant material
body location, subject-specific armor designs can be made. Additionally, the skin simulant material
is ilsi glhigth wt ewigehigth wt iwthit ha na ne setismtiamteadte dd ednesnitsyi tyo f o0f .000.0009019 1g /mg/mm3m. 3T. hTeh efo lfloolwloiwngin gse sceticotnio nd idsciuscsusessse tsh teh e
characterization methodology of this skin simulant matrix material, which allowed us to use it further
characterization methodology of this skin simulant matrix material, which allowed us to use it further
in the computational study.
in the computational study.
J.Compos.Sci.2018,2,44 5of20
Thenovelskinsimulantmaterialwasdevelopedforthematrixmaterialofthecompositecoverings
because it is as flexible as the human skin, and thus can stretch with the movement of the body
location where the reinforced armor is placed, without any discomfort [36]. Also, this elastomer
materialcanbecastedinanyshapeorsize. Thereby,dependingontheshapeandcurvatureofthe
bodylocation,subject-specificarmordesignscanbemade. Additionally,theskinsimulantmaterial
is light weight with an estimated density of 0.00091 g/mm3. The following section discusses the
characterizationmethodologyofthisskinsimulantmatrixmaterial,whichallowedustouseitfurther
inthecomputationalstudy.
2.2. LinearandNon-LinearMaterialModelsUsedinComputationalStudy
Elastomersaresoftmaterials,whichhavenon-linearmechanicalbehaviorundertensileloads[37].
Hyperelastic curve-fit formulations such as the Fung model, Mooney-Rivlin’s model, and Yeoh’s
model are used often to characterize the load response of soft materials [38–41]. Hyperelastic
constitutiveequationsarebasedonthedefinitionofthematerialspecificstrain-energydensityfunctions
(symbolically written as ψ) [42–48]. The strain energy density function typically depends on the
principalstretches(λ,i=1,2and3)alongtheCartesiancoordinateaxes,ortheinvariants(I,i=1,2
i i
and3)oftheCauchy-greentensor,whicharethefunctionsoftheprincipalstretches(seeEquation(1)).
Inthecurrentstudy,theYeoh’smodelwasusedtocharacterizethenon-linearbehaviorofthesoft
matrixmaterial. TheYeoh’sstrainenergydensityfunctionisgivenbyEquation(2).
3 3 3
I = ∑λ2,I = ∑ λ2λ2,I = ∏λ2 (1)
1 i 2 i j 3 i
i=1 i,j=1 i=1
ψ = ∑3 C (I −3)i+ K(J−1)2 (2)
Yeoh i0 1
2
i=1
Equation(3)presentstherelationbetweenthestress,stretchandthestrainenergydensityfunction,
for a uniaxial mechanical test. Solving simultaneously for Equations (2) and (3), the stress versus
stretch relationship is given by Equation (4). The stress versus stretch data obtained for the skin
simulant(usedasthematrixmaterial)intheprevioussectionwascharacterizedusingEquation(4).
Thevariouscurvefitparameters c , c and c wereestimated(Table1). Itshouldbementioned
10 20 30
herethatthestrengthofblastloading(orstrainrates)maysignificantlyaffecttissueortissuesimulant
properties. Thiseffecthasnotbeenconsideredinthepresentstudy,whichmaychangetheresultsto
someextent. Thisshouldbeacknowledgedasalimitationofthiswork.
∂ψ ∂ψ
σ = λ −λ ,σ = σ =0 (3)
1 1 3 2 3
∂λ ∂λ
1 3
1 2
σ =2(λ2− )(c +2c (I −3)+3c (I −3)2),d = (4)
Yeoh λ 10 20 1 30 1 i K
TheelasticitymodulusofKevlar129fibermaterial[49,50],humanskin,muscle,andbonewere
obtained from recent literature [31,47,48,51–56], and are listed in Table 1. The composite material
changesduetovaryingfibervolumefractions[57]havebeenmodeledwithintheFEsystem,using
differentfibercross-sectionsandlengths,andnotbymodifyingtheglobalmaterialpropertyofthe
compositeusingtheruleofmixtures.
J.Compos.Sci.2018,2,44 6of20
Table1.Bodysection,andcompositematerialmodelparametersusedinthenumericalstudy.
Non-LinearMaterialPropertyforSkinSimulantUsedasMatrixMaterial
c (MPa) c (MPa) c (MPa) d1,d2andd3
10 20 30
YeohModelParameters
0.946 4.948 0.015 0.225
LinearMaterialPropertiesforKevlar129Fibers
ModulusofElasticity(MPa) Poisson’sRatio
200,000 0.3
MaterialPropertiesforBodySectionsfromLiterature
LinearElasticParameters ModulusofElasticity(MPa) Poisson’sRatio
Skin 5 0.49
Muscle 100 0.46
Bone 15,000 0.2
2.3. GeometricalModel
Thehumanskincomprisesofthreelayersnamelytheepidermis,dermisandthehypodermis,
withanoverallthicknessof1to4mm[58]. Themusclelayeriswidelyvaryinginthicknessranging
fromalmostzero(inareassuchastheskull)to3cmandabove(inthighsandbiceps)[59–61]. Inour
currentwork,a2mmsectionofthebonewasmodeled,followedbya3mmmuscleand3mmskin
layers. AlltheselayersweremodeledinFEsoftwareANSYSAPDL(17.1,Ansys,Inc.,Canonsburg,
PA,USA),usingthebottom-upapproach,startingwithkeypoints,followedbycreationofsplines,
areasandvolumes. Theslightcurvaturetothemodelwasgiventorealisticallymodelasectionofthe
humanbody,whichistypicallycurvedandnotflat. Additionally,alltheadjacentlayersweremadeto
shareacommonsurface,sothatloadscantransferfromonesectiontoanotherintheFEanalyses.
Computationalmodelsoftwo-layercompositecoverings(madeofKevlar129fibers,embedded
withinanelastomer-basedskinsimulantmatrixmaterial)wasusedtocovertheskin,muscleandbone
sectionlayersagainsttheblastloading. Figure4showsthegeometricalmodelswithcontinuousfiber
basedcompositelay-upsnamely0-0,90-0,and45-45orientations. Themodelswiththe45-45lay-up
orientations(seeFigures4and5)weremodeledascubicalsectionunliketheothermodelswhichare
cuboidalinshape. Thiswasdonetosimplifythefiberlay-upforthe45-45orientationandmakeit
symmetric. However,itshouldbementionedthatthisslightdifferenceinthemodelgeometrywas
assumedtohaveaminimaleffectontheresults,duetothetypeofconstraintandloadapplication
methodology adopted for the analyses (discussed in the upcoming sections). Also, it should be
added that the composite configurations tested in this work are just the fundamental composite
configurationstestednumericallyforbetterunderstandingofthemechanicalbehavioroftheproposed
novelcompositelayersunderblastloading. Theexperimentalcompositeconfigurationsprovidedby
aramidmanufacturersmaybewidelydifferent,whichwillbetestedinfuturestudiesusingshocktubes.
J. Compos. Sci. 2018, 2, x 6 of 19
2.3. Geometrical Model
The human skin comprises of three layers namely the epidermis, dermis and the hypodermis,
with an overall thickness of 1 to 4 mm [58]. The muscle layer is widely varying in thickness ranging
from almost zero (in areas such as the skull) to 3 cm and above (in thighs and biceps) [59–61]. In our
current work, a 2 mm section of the bone was modeled, followed by a 3 mm muscle and 3 mm skin
layers. All these layers were modeled in FE software ANSYS APDL (17.1, Ansys, Inc., Canonsburg,
PA, USA), using the bottom-up approach, starting with key points, followed by creation of splines,
areas and volumes. The slight curvature to the model was given to realistically model a section of the
human body, which is typically curved and not flat. Additionally, all the adjacent layers were made
to share a common surface, so that loads can transfer from one section to another in the FE analyses.
Computational models of two-layer composite coverings (made of Kevlar 129 fibers, embedded
within an elastomer-based skin simulant matrix material) was used to cover the skin, muscle and
bone section layers against the blast loading. Figure 4 shows the geometrical models with continuous
fiber based composite lay-ups namely 0-0, 90-0, and 45-45 orientations. The models with the 45-45
lay-up orientations (see Figures 4 and 5) were modeled as cubical section unlike the other models
which are cuboidal in shape. This was done to simplify the fiber lay-up for the 45-45 orientation and
make it symmetric. However, it should be mentioned that this slight difference in the model geometry
was assumed to have a minimal effect on the results, due to the type of constraint and load application
methodology adopted for the analyses (discussed in the upcoming sections). Also, it should be added
that the composite configurations tested in this work are just the fundamental composite
configurations tested numerically for better understanding of the mechanical behavior of the
proposed novel composite layers under blast loading. The experimental composite configurations
J.Compporso.vScidi.e2d01 b8,y2 ,a4r4amid manufacturers may be widely different, which will be tested in future studies7 of20
using shock tubes.

(a) (b) (c)
Figure 4. Computational models with continuous fiber-based composite covering layers, namely: (a)
Figure 4. Computational models withcontinuous fiber-basedcomposite covering layers, namely:
0-0 (cuboidal), (b) 90-0 (cuboidal), and (c) 45-45 (cubical) lay-up orientations.
(a)0-0(cuboidal),(b)90-0(cuboidal),and(c)45-45(cubical)lay-uporientations.
Short fiber composite materials typically have a length as 100 times the fiber outer diameter [57].
SShinocret mfibosetr ocfo tmhep foibseitres marea ttehriinanlsert ythpainc athlley hhuamveana, lsehnogrtt hfibaesrs1 0m0atyi mloeosk tlhikeefi pboewrdoeur tteor tdhiea nmaketeedr [57].
Sinceemyeo. sAtlsoof, tiht eis fivbereyr sdaifrfeicuthlti nton aelrigtnh asnhotrht efibheurms iann a, csehrotaritnfi obrieernstamtiaoyn, lbouotk mliakye bpe oawchdieevratbolet hweitnha ked
advanced manufacturing techniques [62]. In the current work, for simplification of the geometry and
eye. Also,itisverydifficulttoalignshortfibersinacertainorientation,butmaybeachievablewith
to also investigate orientation effects, short fiber-based composite covering layers with 0-0, 90-0, 45-
advanced manufacturing techniques [62]. In the current work, for simplification of the geometry
45 lay-ups were modeled (see Figure 5).
andtoalsoinvestigateorientationeffects,shortfiber-basedcompositecoveringlayerswith0-0,90-0,
45-45lay-upsweremodeled(seeFigure5).
J. Compos. Sci. 2018, 2, x 7 of 19

(a) (b) (c)
Figure 5. Computational models with short fiber-based composite covering layers, namely: (a) 0-0
Figure5. Computationalmodelswithshortfiber-basedcompositecoveringlayers,namely: (a)0-0
(cuboidal), (b) 90-0 (cuboidal), and (c) 45-45 (cubical) lay-up orientations.
(cuboidal),(b)90-0(cuboidal),and(c)45-45(cubical)lay-uporientations.
In both the composite covering layers, the matrix was modeled first, followed by the fibers
I(nwbhoicthh atrhee aclsoom cuprovseidte). cTohvee friibnegrsl wayeerres p,etrhfeecmtlya tbroixndweads wmitohd theele mdafitrrisxt ,ufsoinllgo wthee dovbeyrltahpe ofipbereartsio(nw hich
arealisno AcNurSvYeSd A).PTDhLe. Afib seqrusawree creropsse-rsfeecctitolny bwoans dcehdosweni tfhort htheem fiabterrisx ouvseirn ga tchirecuolvare rclraopsso-speectriaotni,o nin
because it helps in better control of the number of mesh elements, and the mesh size [56] during the
ANSYSAPDL.Asquarecross-sectionwaschosenforthefibersoveracircularcross-section,becauseit
FE analyses. Also, the spacing between the adjacent fibers were kept same throughout.
helpsinbettercontrolofthenumberofmeshelements,andthemeshsize[56]duringtheFEanalyses.
Also,thespacingbetweentheadjacentfiberswerekeptsamethroughout.
2.4. Finite Element Modeling
2.4. FiniteIEn ltehme ecnutrrMenotd weloinrkg, the fibers were meshed first, followed by the composite skin simulant matrix
material, skin, muscle, and the bone sections [56,63–65]. Figure 6 captures the meshes of the various
Isnectthioencsu frrroemn taw 90o-r0k c,otnhteinfiuboeurss lwaye-urep mcaesseh. Tedhefi irnstte,gfroaltleodw medesbhy ist ahlesoc oshmopwons iitne Fsikgiunres i6m. ulantmatrix
material,Tskhien f,imbeurss cwlee,rea nmdesthheedb uosninegs emcatipopnesd[ 5b6ri,c6k3 –m6e5s]h. Faingdu wreit6h csaizpitnugr ecsonthtreolms seuschhe sthoaft tfhoer tvhaer ious
sectiocnrossfsr-osemctiaon9,0 -a0ll ctohne tfionuuro eudsgleasy h-auvpe cjuasste .onTeh eelienmteegntr adtievdisimone s[h56i]s. Talhsios ssmhoawrt nmienshFiinggu treech6n.ique
Thhelepefidb eurss tow oebrteaimn eas rheeadsounasbinleg nmumapbpere dof bmriecskh meleemshenatsn dforw tihteh fsibizeirns.g Tchoen ttwrool scosmupchositthea stkfion r the
cross-ssimecutliaonnt, malaltrthixe lafoyeurrs ewdegree smheasvheedj unsetxot nweitehl eam freeen ttedtriavhiseidornal [t5y6p]e. Tmheissh.s mThaer stkmine, smhuinscglet,e acnhdn ique
the bone sections were meshed using sweep mesh, starting at the common surface shared between
the second composite layer (meshed with tetrahedral elements) and the skin layer. A higher order 3D
20-node Solid 186-type quadratic tetrahedral element was chosen for the volume meshes. This robust
element type was selected as it is used widely in the industry and supports large deflection, creep,
large strains, hyperelasticity, plasticity, and stress stiffening [66].
In the computational model in Figure 6, X and Z axes were selected along and orthogonal to the
direction of fibers respectively, and Y axis was along the depth of the layers. For each layer, flexible
boundary conditions were placed on the four sides, including constraining of translation along the Y
axis (to allow the blast load to effectively transmit through the different layers similar to in case of
pure bending of rectangular plates [57]). The translation along the X and Z axes were not constrained
to allow interaction of the unit cell like computational section model with the adjacent similar sections
through shear deformation. The rotations were constrained along all axes, simulating a rigid and high
inertia skin-muscle-bone section with well fastened protective composite layers. It should be
mentioned here that if the boundary conditions associated with the edges of the studied unit cell were
modified (such as if they were deformable in all directions), the stress distribution results may
significantly change as observed previously by Chanda et al. [34]. Therefore, the results from this
analysis should not be extrapolated beyond the modeled boundary conditions. The bottom of the
bone section (which was modeled as a small representative part of the entire bone underneath) was
constrained in all degrees of freedom to simulate its strong attachment with underlying bone tissue.
Flexible contact pairs were defined to the capture the attachments of the different layers. For the
contact pair, a normal penalty stiffness of 1, penetration tolerance of 0.1, and a contact algorithm
based on augmented Lagrangian method was selected. The contact surfaces were considered as fully
J.Compos.Sci.2018,2,44 8of20
helpedustoobtainareasonablenumberofmeshelementsforthefibers. Thetwocompositeskin
simulantmatrixlayersweremeshednextwithafreetetrahedraltypemesh. Theskin,muscle,andthe
bonesectionsweremeshedusingsweepmesh,startingatthecommonsurfacesharedbetweenthe
secondcompositelayer(meshedwithtetrahedralelements)andtheskinlayer. Ahigherorder3D
20-nodeSolid186-typequadratictetrahedralelementwaschosenforthevolumemeshes. Thisrobust
elementtypewasselectedasitisusedwidelyintheindustryandsupportslargedeflection,creep,
largestrains,hyperelasticity,plasticity,andstressstiffening[66].
InthecomputationalmodelinFigure6,XandZaxeswereselectedalongandorthogonaltothe
directionoffibersrespectively,andYaxiswasalongthedepthofthelayers. Foreachlayer,flexible
boundaryconditionswereplacedonthefoursides,includingconstrainingoftranslationalongtheY
axis(toallowtheblastloadtoeffectivelytransmitthroughthedifferentlayerssimilartoincaseof
purebendingofrectangularplates[57]). ThetranslationalongtheXandZaxeswerenotconstrained
toallowinteractionoftheunitcelllikecomputationalsectionmodelwiththeadjacentsimilarsections
through shear deformation. The rotations were constrained along all axes, simulating a rigid and
highinertiaskin-muscle-bonesectionwithwellfastenedprotectivecompositelayers. Itshouldbe
mentioned here that if the boundary conditions associated with the edges of the studied unit cell
weremodified(suchasiftheyweredeformableinalldirections),thestressdistributionresultsmay
significantlychangeasobservedpreviouslybyChandaetal.[34]. Therefore, theresultsfromthis
analysisshouldnotbeextrapolatedbeyondthemodeledboundaryconditions. Thebottomofthe
bonesection(whichwasmodeledasasmallrepresentativepartoftheentireboneunderneath)was
constrainedinalldegreesoffreedomtosimulateitsstrongattachmentwithunderlyingbonetissue.
Flexible contact pairs were defined to the capture the attachments of the different layers. For the
contactpair,anormalpenaltystiffnessof1,penetrationtoleranceof0.1,andacontactalgorithmbased
J. Compos. Sci. 2018, 2, x 8 of 19
onaugmentedLagrangianmethodwasselected. Thecontactsurfaceswereconsideredasfullybonded
withnullinitialpenetrationandsurfaceoffset. Anautomaticcontactadjustmentforgapclosureand
bonded with null initial penetration and surface offset. An automatic contact adjustment for gap
pencelotrsautrieo nanrde dpuecnteitornatwioans raedduocpttieodn fwoarst haedoapntaeldy sfeosr. the analyses.

FigFuirgeu6re. M6.e sMheesshoefst hoef vthaer iovuarsiolauyse rlsayoefrtsh eofb othnee ,bmonues,c lmeaunscdles kainnds escktiino ns,eacntidonth, eancodm thpeo sciotemcpoovseirtien g
layceorsv,eorifntgh elacyoemrs,p ouft tahtieo cnoamlmpuotdateilo.nal model.
A time varying Friedlander type blast loading was applied on the top surface of the composite
AtimevaryingFriedlandertypeblastloadingwasappliedonthetopsurfaceofthecomposite
covering layer with the load steps listed in Table 2, using a dynamic analysis in ANSYS APDL [34].
coveringlayerwiththeloadstepslistedinTable2,usingadynamicanalysisinANSYSAPDL[34].
The induced von Mises stresses in different layers were quantified.
TheinducedvonMisesstressesindifferentlayerswerequantified.
Table 2. Various load steps simulating a time varying Friedlander type blast loading, used in the
computational model.
Load Step Time (ms) Pressure (MPa)
1 0 to 0.5 0
2 0.5 to 0.6 0.35
3 0.6 to 1.0 −0.05
4 1.0 to 3.0 0
3. Results and Discussion
In a primary blast impact, bone fracture and muscle rupture are the most commonly observed
phenomena, mainly during the peak blast pressure [67–69]. Thereby, the stresses at the bone and
muscle sections were estimated for the unprotected bone, muscle and skin model in the following
section. The blast-induced bone and muscle stresses due to the composite protection covering with
varying fiber length (continuous and short fiber) and lay-ups (0-0, 90-0, and 45-45 orientations) have
also quantified.
3.1. Blast Stresses in Unprotected Bone, Muscle and Skin Section Model
Figure 7a shows the mesh of the unprotected cubical model. Very high peak stresses were
observed at the center of the bone section (Figure 7b). The muscle being a soft tissue, the stresses were
observed to have a much more uniform distribution than in the bone section (Figure 7c). Additionally,
the peak stress values observed were consistent with the peak stresses observed on the bone and
muscle sections due to blast loading in previous work by Chanda et al. [34]. It should be mentioned
here that some high stresses were observed at the edges due to the choice of constraints in the FE
model (discussed in the previous section).
J.Compos.Sci.2018,2,44 9of20
Table2. VariousloadstepssimulatingatimevaryingFriedlandertypeblastloading, usedinthe
computationalmodel.
LoadStep Time(ms) Pressure(MPa)
1 0to0.5 0
2 0.5to0.6 0.35
3 0.6to1.0 −0.05
4 1.0to3.0 0
3. ResultsandDiscussion
Inaprimaryblastimpact,bonefractureandmusclerupturearethemostcommonlyobserved
phenomena, mainly during the peak blast pressure [67–69]. Thereby, the stresses at the bone and
musclesectionswereestimatedfortheunprotectedbone,muscleandskinmodelinthefollowing
section. Theblast-inducedboneandmusclestressesduetothecompositeprotectioncoveringwith
varyingfiberlength(continuousandshortfiber)andlay-ups(0-0,90-0,and45-45orientations)have
alsoquantified.
3.1. BlastStressesinUnprotectedBone,MuscleandSkinSectionModel
Figure 7a shows the mesh of the unprotected cubical model. Very high peak stresses were
observedatthecenterofthebonesection(Figure7b). Themusclebeingasofttissue,thestresseswere
observedtohaveamuchmoreuniformdistributionthaninthebonesection(Figure7c). Additionally,
the peak stress values observed were consistent with the peak stresses observed on the bone and
musclesectionsduetoblastloadinginpreviousworkbyChandaetal.[34]. Itshouldbementioned
herethatsomehighstresseswereobservedattheedgesduetothechoiceofconstraintsintheFE
model(discussedintheprevioussection).
J. Compos. Sci. 2018, 2, x 9 of 19

(a) (b)

(c)
Figure 7. (a) Bone, muscle and skin section unprotected model (without composite coverings),
Figure7.(a)Bone,muscleandskinsectionunprotectedmodel(withoutcompositecoverings),Stresses
Stresses generated at peak blast pressure at the (b) Bone section, and (c) Muscle section.
generatedatpeakblastpressureatthe(b)Bonesection,and(c)Musclesection.
3.2. Blast-Induced Bone Stresses with Protective Composite Covering Variants
The resultant peak blast stresses developed at the bone section were evaluated for the six
composite covering variants, namely the 0-0 short fiber type, 90-0 short fiber type, 45-45 short fiber
type, 0-0 continuous fiber type, 90-0 continuous fiber type, and 45-45 continuous fiber type (presented
in Figure 8). The continuous fiber based composite coverings were found to possess better blast
mitigation characteristics compared to the short fiber based composite coverings. This observation is
expected, as a higher degree of blast induced stress reduction is likely due to a higher volume fraction
of fibers present within the continuous fiber based composite coverings. Also, there may be a slight
effect of the boundary conditions selected for this study, where the continuous fibers are all
constrained at both ends along the Y axis, compared to the few short fibers (which are constrained
mostly at one end along the Y axis).

(a) (b)
J. Compos. Sci. 2018, 2, x 9 of 19

(a) (b)

(c)
J.Compos.Sci.2018,2,44 10of20
Figure 7. (a) Bone, muscle and skin section unprotected model (without composite coverings),
Stresses generated at peak blast pressure at the (b) Bone section, and (c) Muscle section.
3.2. Blast-InducedBoneStresseswithProtectiveCompositeCoveringVariants
3.2. Blast-Induced Bone Stresses with Protective Composite Covering Variants
The resultant peak blast stresses developed at the bone section were evaluated for the six
The resultant peak blast stresses developed at the bone section were evaluated for the six
compositecoveringvariants,namelythe0-0shortfibertype,90-0shortfibertype,45-45shortfiber
composite covering variants, namely the 0-0 short fiber type, 90-0 short fiber type, 45-45 short fiber
type,0-0continuousfibertype,90-0continuousfibertype,and45-45continuousfibertype(presented
type, 0-0 continuous fiber type, 90-0 continuous fiber type, and 45-45 continuous fiber type (presented
in Figure 8). The continuous fiber based composite coverings were found to possess better blast
in Figure 8). The continuous fiber based composite coverings were found to possess better blast
mitigationcharacteristicscomparedtotheshortfiberbasedcompositecoverings. Thisobservationis
mitigation characteristics compared to the short fiber based composite coverings. This observation is
expected,asahigherdegreeofblastinducedstressreductionislikelyduetoahighervolumefraction
expected, as a higher degree of blast induced stress reduction is likely due to a higher volume fraction
offiberspresentwithinthecontinuousfiberbasedcompositecoverings. Also,theremaybeaslight
of fibers present within the continuous fiber based composite coverings. Also, there may be a slight
effectoftheboundaryconditionsselectedforthisstudy,wherethecontinuousfibersareallconstrained
effect of the boundary conditions selected for this study, where the continuous fibers are all
atbothendsalongtheYaxis,comparedtothefewshortfibers(whichareconstrainedmostlyatone
constrained at both ends along the Y axis, compared to the few short fibers (which are constrained
endalongtheYaxis).
mostly at one end along the Y axis).

J. Compos. Sci. 2018, 2, x (a) (b) 10 of 19

(c) (d)

(e) (f)
Figure 8. Resultant bone stresses due to peak blast pressure, in presence of various composite
Figure 8. Resultant bone stresses due to peak blast pressure, in presence of various composite
covering variants. (a) 0-0 short fiber; (b) 90-0 short fiber; (c) 45-45 short fiber; (d) 0-0 continuous fiber;
covering variants. (a) 0-0 short fiber; (b) 90-0 short fiber; (c) 45-45 short fiber; (d) 0-0 continuous
(e) 90-0 continuous fiber; (f) 45-45 continuous fiber.
fiber;(e)90-0continuousfiber;(f)45-45continuousfiber.
The induced dynamic time varying von Mises stresses at the bone section (averaged across 10
elements at the center area) generated due to the Friedlander blast loading, in presence of the different
protective composite covering variants are captured in Figure 9. Among the three continuous fiber
based composite coverings, the 0-0 continuous variant was found to be most effective in minimizing
the resultant blast stresses at the bone section, both at the peak (seen in Figure 8) and negative blast
phases. The 90-0 was found to be the second best, followed by the 45-45 orientation composite
covering type for blast mitigation. This finding can be mainly explained with the differences in fiber
volume fraction of each configuration, as the loading in all cases is orthogonal to the fiber directions
in the composite coverings. In the 0-0 continuous variant, due to the cuboidal shape of the
computational model, the length of fibers along the Z direction (0°) are longer than the fibers along
the X direction (90°) in the 90-0 continuous variant. This possibly led to a slightly higher induced
peak bone stresses in the 90-0 continuous variant. The shortest fiber lengths are in the 45-45
continuous variant (which is cubical in shape), which may have led to significantly higher induced
peak stresses at the bone due to a low fiber volume fraction within the protective composite layers.
Among the short fiber composite cover variants, the 90-0 type led to the most significant blast
mitigation, followed by the 0-0 type and 45-45 orientations. This observation is also likely due to the
relative fiber volume fractions of the short fibers within the cuboidal 0-0 and 90-0 short fiber
composite configurations, and the cubical 45-45 short fiber composite configuration.
The degree of average blast stress mitigation at the bone section was estimated quantitatively
for the 0-0 continuous fiber-based composite covering variant, by comparing it with the unprotected
model (discussed in previous section). A significant average blast stress reduction of 78% was
observed with the covered model over the unprotected model. Additionally, the peak displacements
were also estimated (Table 3), which exhibited a similar trend to the peak stresses.