a generalized theory of elastodynamic homogenization for periodic media 2016 international journal...
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7/26/2019 A Generalized Theory of Elastodynamic Homogenization for Periodic Media 2016 International Journal of Solids and
1/8
InternationalJournalofSolidsandStructures84(2016)139146
ContentslistsavailableatScienceDirect
International
Journal
of
Solids
and
Structures
journalhomepage:www.elsevier.com/locate/ijsolstr
A generalized theory of elastodynamic homogenizationfor
periodic
media
H.Nassar,Q.-C.He,N.Auffray
ModlisationetSimulationMulti-Echelle,Universit Paris-Est,MSMEUMR8208CNRS,5bdDescartes,77454Marne-la-Valle,France
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received13August2015
Revised
18
December
2015
Availableonline4February2016
Keywords:
Homogenization
Willistheory
Generalizedcontinua
Optical
branches
Bandgaps
a
b
s
t
r
a
c
t
For
periodically
inhomogeneous
media,
a
generalized
theory
of
elastodynamic
homogenization
is
pro-
posed
so
that
even
the
long-wavelength
and
low-frequency
asymptotic
expansions
of
the
resulting
ef-
fective
(or
macroscopic)
motion
equation
can,
approximately
but
simultaneously,
capture
all
the
acoustic
and
some
of
the
optical
branches
of
the
microscopic
dispersion
curve.
The
key
to
constructing
the
gen-
eralized
theory
resides
in
incorporating
new
kinematical
degrees
of
freedom
in
conjunction
with
rapidly
oscillating
body
forces
as
microscopic
and
macroscopic
loadings
while
satisfying
an
energetical
consis-
tency
constraint
reminiscent
of
Hill
Mandel
lemma.
By
this
constraint,
an
effective
displacement
field
is
naturally
defined
as
the
projection
of
a
microscopic
one
onto
the
dual
to
the
space
of
body
forces.
To
illustrate
these
results,
a
two-phase
string
is
studied
in
detail.
2016
Elsevier
Ltd.
All
rights
reserved.
1.Introduction
Theelastodynamichomogenizationapproachesreportedupto
nowintheliteratureareobservedtorunintodifficultieswhenbe-ingusedtomodeldynamicaleffectsoverawidefrequencyrange.
1.The classical lowest-order Long-Wavelength (LW) Low-
Frequency(LF)homogenizationapproaches(Bensoussanetal.,
1978;Sanchez-Palencia,1980)yieldahomogeneoussubstitu-
tionCauchymediumwhichmissesalldispersiveeffectsandall
internalresonances,i.e.,allopticaloscillationmodes.
2.Thehigher-orderLW-LFasymptotichomogenizationapproaches
(Andrianovetal.,2008;BoutinandAuriault,1993)leadtoef-
fectivestrain-gradientmediawhichcanmodelwelldispersive
behaviorsandsizeeffectsbutarevalidonlyneartheacoustic
branchesindependentlyoftheorderoftheasymptoticapprox-
imationsused.
3.
The
high-frequency
asymptotic
approaches
(
Antonakakis
et
al.,
2014;Boutinetal.,2014;Colquittetal.,2014;Crasteretal.,
2010;Dayaetal.,2002;Noldeetal.,2011)aresuccessful in
capturinghigh-frequencyopticalmodesbutstillvalidonlyin
thevicinityofsomefinitefrequency.
4.The high-contrast asymptotic approaches (Auriault and Bon-
net,1985;AuriaultandBoutin,2012;Smyshlyaev,2009)havea
widefrequencyvaliditydomainenglobinganinfinitenumberof
Correspondingauthor.Tel.:+33160967794.
E-mailaddress:[email protected](H.Nassar).
opticalbranches.However,thecorrespondingeffectivebehavior
iscomplexandnonlocalintime.
5.Thenon-asymptotictheoryofWillis(1997,2011)yieldsexactly
thewholedispersioncurve.Nonetheless,thedescribedeffectivefieldsareonlyrelevantforlowfrequencies(Nassaretal.,2015b;
SrivastavaandNemat-Nasser,2014).
Themainpurposeofthepresentpaperistoconstructagener-
alizedtheoryofelastodynamichomogenizationforperiodicmedia
whichimprovesthequalityoftheWilliseffectivebehaviorasan
approximationtothemicroscopicbehaviorinawaythatLW-LF
asymptoticexpansionsbecomeabletocapture,approximatelybut
simultaneously,alltheacousticandsomeoftheopticalbranches
ofthemicroscopicdispersioncurve.Toachievethispurpose,new
kinematicalDegreesOfFreedom(DOFs)aretakenintoaccountso
astodescribesomeshort-wavelengthcomponentsofthemicro-
scopicdisplacementfieldwhichbecomedominantathighfrequen-
cies.
The
new
DOFs
are
excited
by
incorporating
various
rapidly
oscillatingbodyforcesonthemicroscaleandonthemacroscale
underanenergetical consistency constrainthereaftercalledEn-
ergyEquivalencyPrinciple(EEP).TheEEPisabalancebetweenthe
microscopicandmacroscopicvirtualworksandislaterprovento
yieldageneralizedversionofthewell-knownHillMandellemma.
WithrespecttoWillistheory,weunderlinetwomajordifferences.
First,theincorporatedloadingsaremuchricherthanthoseem-
ployedbyWillis(1997,2011).Thishastheconsequenceofreduc-
ingtheerrorcommittedduringtheupscalingprocessandprovid-
inganextendedfrequencyvaliditydomain.Second,theEEPcon-
cernsvirtualworksandnottheirexpectancies.Fromthephysical
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140 H.Nassaretal./InternationalJournalofSolidsandStructures84(2016)139146
standpoint,thisleadstoacleardistinctionbetweenthemacroscale
andthemicroscaleintermsofwavelengths.Nevertheless,itshould
bepointedoutthatthegeneralizedtheorypresentedhereisby
construction limited toperiodically inhomogeneousmediawhile
Willistheoryisformallyvalidbothforperiodicallyandrandomly
inhomogeneousmedia.
Thepaperisorganizedasfollows.InSection2,werecallsome
geometricalelementsusefulfordescribingperiodicmedia,summa-
rize
the
equations
governing
the
kinematics
and
dynamics
of
them,
andsimplifytheseequationsbyusingBloch-waveexpansions.The
mainbodyofthegeneralizedtheoryispresentedinSection3.The
EEPisfirstpostulated;thespaceofadmissiblebodyforcesisthen
definedasthesetofmacroscopicallyappliedloadings;theeffec-
tivedisplacementfieldassociatedtoamicroscopicdisplacementis
obtainedbytheEEPandproventobeanimprovementoverthe
onedefinedbyWillis;theeffectivemotionequationisfinallyde-
rivedinaformalwayandaHilMandelrelationisdemonstrated.
InSection4,ananalyticalLW-LFasymptoticapproximationtothe
effectivemotionequationisgivenforaparticular1Dtwo-phase
string.Exactandapproximatedispersioncurvesareplottedand
compared. Itappears thenhow the resultingasymptoticmodel,
thoughbasedonLFexpansions,cansimultaneouslycaptureacous-
ticandopticalbrancheswhileconservingalow-orderlocalmotion
equation.
2.Preliminaries
Inthissection,somegeometricalelementsusefulforthestudy
ofperiodicmediaarerecalled.Thegoverningequationsoflinear
elasticityare recapitulated.Bloch-waveexpansionsoffieldsand
workarealsointroduced.
2.1.Geometryandperiodicity
Let bead-dimensionalinfinitebody.DefineE asthevector
spaceoftranslationsactingonthepointsof.Givendindepen-
dent
translations
(b
j
)
j=1
...
d
,
denote
byR
the
subset
ofE
obtained
byintegercombinationsofthesevectors.ThesubsetR iscalled
alattice.Then,ascalar,vectorortensorfieldhdefinedover is
saidtobeR-periodicifandonlyifitsatisfiesh(x+r)=h(x)forallpointsx andalltranslationsrR.Accordingly,hneedsbeingdefinedonlyoveraunitcell
T=
xo+r
r= dj=1
rjbj,1/2rj
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H.Nassaretal./InternationalJournalofSolidsandStructures84(2016)139146 141
WenowassumethatCand areR-periodicsothatthesolutionuk
(x)canbewrittenas
uk
(x)=uk
(x)eikx
withanR-periodicamplitude uk
(x
)(Gazaletetal.,2013).Inother
words,totheexpansion(2.2),therecorrespondsasimilarBloch-
waveexpansionofthesolutionto(2.1):
u(x)=
Tuk(x)dd
k=
Tuk(x)ei
kxdd
k, (2.3)
withuk
(x)beingthedisplacementfieldover subjectedtofk
(x).
Incontrast,exceptforahomogeneousbody,therearenosimilar
resultsforplanewaves.
Given theBloch-wave formofuk
,andsimplifying thephase
factoreik x,themotionequationbecomes,intermsof uk(x)andfk
(x),
(+ik) {
C(x):[
(+ik)suk
(x)]
}
+fk
(x)=2(x) uk
(x).
(2.4)
Last, thex-dependenciesofBloch amplitudes fordisplacements
and body forces were explicitly annotated. In what follows, all
fieldsareunderstoodtobex-dependentunlessotherwisestated.
2.4.Externalwork
Define the virtualworkofexternal body forcesf associated
withavirtualdisplacementfielduby
f
u
whereasuperscripted denotescomplexconjugation.Also,intro-ducetheaveragingoperatorby
h
= 1|
T|
T
h(x)ddx
for all R-periodic functions h. Note that is independent ofthe choiceofT and is ill-defined fornon-R-periodic functions.
PlancherelsidentityforFouriertransformandParsevalsidentity
forFourierseriesdeliverthenasimilaridentityforBloch-waveex-
pansions:
f u=(2 )d
T
fk
uk
d
dk
=(2 )d
|
T|
T
T
fk
(x) uk
(x)ddxddk. (2.5)
Consequently,indefiningtheenergeticallyequivalenteffectivebe-
havior,wecanworkwithfieldsofasingleBlochwavenumberkT andthenapplythesuperpositionprincipleeventhoughworkis
quadratic
and
not
linear.
3.
A
general
theory
Inthissection,theenergyequivalencyprinciple(EEP),thecor-
nerstoneofthepresentapproach,isfirstpostulated,givenasim-
pleformandexploitedtodefinetheeffectivedisplacementfield.
Aformalderivationoftheeffectivemotionequationisthenpre-
sented.Theeffectiveconstitutivebehaviorisnonlocalinbothspace
andtimewhichraisesquestionsaboutitsuniqueness(Fietzand
Shvets,2010;Willis,2011). Inorder toavoid thisdifficulty,we
willbeinterestedonlyintheeffectivemotionequationwhichis
unique.Nonetheless,wewillderiveexpressions forthegeneral-
izedstress,momentum,velocityandstrainmeasureswhichare,in
particular,neededfordetermininganeffectiveconstitutivelaw.
Fig.
1.
The
effective
displacement
field
D
associated
to
a
given
microscopic
one
u
is
geometrically
interpreted
as
the
projection
of
the
latter
onto
the
space
of
admissible
displacements.SpacesF andF areisomorphicand,here,aretakentobeequaluptoachangeinunits.
3.1.Energyequivalency
In classical static or quasi-static homogenization, an energy
equivalencyrelation,knownasHillMandellemma,isprovenfora
familyofboundaryconditionsprescribedonarepresentativevol-
umeelementasmacroscopic loadings.Once theboundarycon-
ditionshavebeenspecified,HillMandel lemmacanbeusedto
define, by duality, the macroscopic stress in a strain-based ap-
proachorthemacroscopicstraininastress-basedapproach.Inthe
presentformulation,admissiblebodyforcesappliedgloballytoinsteadofboundaryconditionsaretakentobemacroscopicload-
ing.Then,anEEPispostulatedsoastodualizebodyforcesand
displacements.Thisdualitywillallowustodefinethemacroscopic
displacementfield,calledD, in termsofthemicroscopiconeu,
onceadmissiblebodyforceshavebeenimposed.
LetF bethespaceofBlochamplitudes fk,involvedin(2.2),
ofadmissiblebodyforces.TheelementsofFareseenasexternal
loadingslikelytobeappliedto.Notethattheywillremainthe
sameafterthescaletransition.ThespaceFactsasaparameterof
theapproachtobeelaboratedandneedstobechosenadequately.
Next,letF,thespacedualtoF,bethespaceofBlochampli-tudes Dk
ofadmissibleeffectivedisplacementfields.Foragiven
microscopicdisplacementfieldu,thecorrespondingeffective(or
macroscopic)displacementfieldisdefinedastheuniqueadmissi-bledisplacementsuchthat
fD=
f u (3.1)
foralladmissiblevirtualbodyforcesf(i.e., fkF,forallkT).Physically,theEEP(3.1)canbeinterpretedasrequiringthattheef-
fectivedisplacementfieldDassociatedtoagivenmicroscopicfield
ubesuchthattheworkdonebyeveryadmissiblevirtualbody
forcefinthecourseofD isequaltotheonedonebyfinthe
courseofu.Geometrically,theEEP(3.1)simplymeansthatDisthe
projectionofuontothespaceofadmissibledisplacements(Fig.1).
OnthebasisoftheEEP(3.1),ageneralizedHillMandellemmawill
beproveninSection3.4.
UsingtheBlochdecomposition(2.5),theEEP(3.1)canbeequiv-
alentlywrittenintermsofBlochamplitudesas
kT, fk
F,fk
Dk
=fk
uk
. (3.2)
3.2.Effectivedisplacementfield
BearinginmindtheEEP(3.1),choosingthespaceFofadmis-
siblebodyforcesbecomesakeysteptowardelaboratingageneral-
izedtheory.ThechoiceofFdependsultimatelyonthedegreeof
accuracywithwhichDisrequiredtoapproximateu.ThebiggerF
is,thecloserDistou.Whenallbodyforcesareconsideredasad-
missible,therelation(3.1)impliesD=uandtheeffectivemediumistriviallytheoriginalone.Inwhatfollows,westudytherather
generalcaseofpracticalimportancewhereFisfinite-dimensional
andshowhowDderivesfrom(3.1)correspondingly.
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Fig.2.Plotsoftherealpartsofthemicroscopic(u),Willis(U),andgeneralized(D)Blochdisplacementamplitudesoveroneperiodfor3eigenmodes:(k=0,=1
(0))
(top),(k=/2a,=2
(/2a))(middle),(k=0,=2
(0))(bottom).Twoshapefunctionshavebeenused:aconstantandasinewave.DetailsaregiveninSection4.
3.2.4.Effectivedisplacementfieldunderinfinitescaleseparation
ItisofinteresttoexaminewhatDbecomesunderthehypoth-
esisofinfinitescaleseparation,namely,whenk
0and
0.
Itisknownthatinthiscase,tothelowestorder,thedisplacementfielddependsonlyontheslowvariable (refertoBoutinandAu-
riault,1993,forinstance).IntermsofBlochamplitudes,thismeans
that ukisconstant.Consequently,
Uk
=
uk
=uk
,
u
k
=
uk= uk=0.Therefore,thetranslationalDOFUistheonlynon-nullcomponent
ofD,tothelowestorder.Theuseofageneralizedkinematicsis
hencejustifiedonlyunderweakscaleseparationorforhighfre-
quencieswhenmicroscopicdeformationmodesbecomesignificant.
Otherwise,itisenoughtokeeptrackofUexclusivelyasintheun-
weightedWillistheory.Asamatteroffact,ithasbeenobserved
that
a
periodic
medium
was
homogenizable in
the
Willis
sense
overtheacousticandthefirstopticalbranchesonly(Nassaretal.,
2015b;SrivastavaandNemat-Nasser,2014).Forhigherfrequencies,
oneneedstousenon-uniformshapefunctions.
3.3.Effectivemotionequation
Havingspecifiedbodyforces,themotionEq.(2.4)becomes
(
+
i
k)
{
C(x
)
:[
(
+
i
k)suk
(x
)]
}
+
Fk
+
f
k
(x
)(x)
=2(x) uk
(x
),
whichneeds tobe solvedover.Since uk
isR-periodic, it is
enoughtosolvetheaboveequationoveraunitcellTunderpe-
riodicboundaryconditions.Letgkbethecorrespondingperiodic
secondorderGreenoperator.Then,
uk
(y)
=
1
T
T
gk
(y,x)
fk
(x)ddx
=
1
T
Tgk
(y,x)ddx
Fk
+
1
T
Tgk
(y,x) ddx
f
k
(3.7)
which,combinedwith(3.5),deliversthefollowingexpressionsfor
thecomponentsofthemacroscopicdisplacementfield:
Uk
=gk
(y,x) Fk
+gk
(y,x) (x) fk
,
u
k
=(y)gk
(y,x)
Fk
+(y)gk
(y,x) (x)
f
k
,(3.8)
wheremeansaveragingwithrespecttobothxandy.TheGreenoperatoroftheeffectivemediumGkisgivenbythe
lasttwoequalitieswhichcanbewrittenconciselyas
u
j
k
=G
ji
k
f
i
k
,
wherenodistinctionismadebetweentheclassicalandgeneralized
DOFs(recallthat Uk
=
dj=1
ujk
j).Invertingtheprecedingequa-tiondeliverstheeffectivemotionequationinFourierdomain:
Z
i jk
u
jk
= fik
, (3.9)
whereZk,theinverseofGk,iscalledtheeffectiveimpedance.It
dependsimplicitlyonthefrequency.BysummingoverkTandover,weobtaintheeffectivemotionequationinxandtas
Z
i j(x,t) uj(x,t)=fi(x,t),
whereZ(x,t)isanintegro-differentialoperatorand denotescon-volutionproductwithrespecttospaceandtime.Theeffectivemo-
tionequationishencenonlocal inbothspaceandtimeand in-
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144 H.Nassaretal./InternationalJournalofSolidsandStructures84(2016)139146
volveslongwavelengths(kT)only.Thisgeneralizesequation(3.28)derivedbyWillis(1997)forperiodicmedia.
3.4.Internalwork
Asmentionedearlier,itisnotessentialforachievingthemain
purposeofthepresentworktoderiveanexplicitexpressionfor
theunderlyingeffectiveconstitutive lawwhichisnotuniqueas
in thetheoryofWillis.However, it isof interesttospecifythemacroscopicstress,momentum,strainandvelocitymeasuresthat
aneffectiveconstitutivelawinvolves.Inaddition,thesegeneralized
macroscopicmeasureswillbeshowntoberelatedtotheirmicro-
scopiccounterpartsthroughanextendedHillMandelrelation.
3.4.1.Generalizedstressandmomentummeasures
Saidmeasuresaretakentobetheonesinvolvedintheeffective
motionequationwrittenasaconservationlawequivalentto(3.9).
Startingwiththemicroscopicmotionequation
(+ik) k
+Fk
+fk
=i pk
, (3.10)
where kand pkaretheBlochamplitudesofstressandmomen-tum,wetakeitsvolumeaverageoveraunitcelltoobtain,with
thehelpofthedivergencetheorem,
ik k
+Fk
=i Pk
. (3.11)
Thisisthefirsteffectivemotionequationinvolvingtheclassical
macroscopicstressandmomentummeasures:
k
k
, Pk
pk
.
Further,projectingEq.(3.10)ontothespacespannedbytheother
shapefunctions givesriseto
ik k
s
:k
+f
k
=i pk
,
where,forsimplicity,wehaveassumedthecontinuityof sothattheboundarytermvanishes.Thegeneralizedstressandmo-
mentummeasurescanbeidentifiedas
k
k
, s
k
s
:
k
, p
k
pk
.
Theadditionalmotionequationbecomesthensimply
s
k
+
i
k
k
+
f
k
=
i
p
k
. (3.12)
NotethatEqs.(3.11)and(3.12)ononehand,and(3.9)ontheother,
arerelatedtooneanotherthroughanon-uniqueeffectiveconsti-
tutive lawwhosecharacterization isbeyond thepurposeof the
presentwork(seethediscussionbyWillis,2011,2012).
In summary, the motion equations in the spacedomain are
givenby
+F
=
iP,
s
+
+f =ip . (3.13)
Theseequationsareamicromechanicalversionoftheequations
ofequilibrium phenomenologicallyderivedbyGermain(1973).
3.4.2.Generalizedstrainandvelocitymeasures
Saidmeasuresareobtainedbyduality.Thevirtualworktheo-
remcombinedwiththeEEPyieldsk
:
k
pk
vk
=
fk
u
k
=
Fk
U
k
+
f
k
u
k
,
where kandvkaretheBlochamplitudesofthestrainandveloc-ityfields,respectively,givenby
k
=(
+
ik)
s
uk
, vk
=
i uk
.
Fig.3.Unitcell.
Substitutingbodyforcesbythecorrespondingstressandmo-
mentummeasuresaccordingto(3.12)delivers
k:
k
p
k
vk
= k:(iksUk) Pk (i Uk)+
k
(i
k
u
k
)
p
k
(i
u
k
)
s
k
u
k
.
Summingoverk,andusingPlacherelsidentity,weobtainagener-
alizedversionoftheHillMandellemma:
{
:p v}
=
{
:(sU)P (iU)
+ (u)p(iu) su}
.(3.14)
Thisresult isvalid forallvirtualcouples (,p)equilibratedbyanadmissiblebodyforcefieldandforallcouples(,v)derivedfromanarbitrarydisplacementfieldu.Fromtheaboverelation,
weidentifytheclassicalmacroscopicmeasuresofstrainandve-
locityassUandiUwhilethegeneralizedonesareu ,uandiu .
Finally,theconstructedmacroscopicfieldsmeetthemostfun-damentalrequirementsforthemtobeinterpretedasstresses,mo-
menta,strainsandvelocitiessincetheyrigorouslysatisfylocalbal-
anceandcompatibilityequations.Nonetheless,howtophysically
interpretandmeasurethesequantitiesinaprecisewayultimately
dependsonthechosensetofshape functions.Moredetailsare
giveninSection4.4regardingthisaspect.
4.
An
application:
high-frequency
behavior
through
low-frequencyasymptotics
At this point, we have generalized Willis theory by using
enrichedkinematics to improve thequalityofapproximationof
amicroscopicdisplacementubyamacroscopiconeD.Thecost
is
however
the
increasing
complexity
of
the
resulting
effective
motionequation.Anumericalprocedurededicatedtotheimple-
mentationofWillis theoryorourpreviousone isquiteheavy,
theeffectivebehaviorbeingnonlocalinbothspaceandtimewith
infiniteradiiofinfluenceingeneral.
Taylorasymptoticexpansionsprovideanefficientwaytoap-
proximate thenonlocalbehaviorwitha localoneunderappro-
priateassumptionsonkand.LW-LFexpansionshavethemainadvantageofonly requiring the solutionof staticproblemsbut
presentthedisadvantageofbeinglimitedtotheLFbehavior.The
purpose of this section is to show explicitly how generalizing
Willistheorymakesitpossibletoextendthevaliditydomainof
LW-LFexpansionstohigh-frequencybehavioroverasimple1Dex-
ample.
4.1.Setting
Considertheperiodicallyinhomogeneousstringwhoseunitcell
isdepictedinFig.3,anddefinetheshapefunction
(x
)=
2
sin(
x/a)
whichdescribestherapidlyoscillatingbodyforce
f
=
F
+
q
andwillcarrythenewDOF .ThemacroscopicdisplacementDthenreads
D=U+with
U=u, =u.
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homogenization theorywiththehigh-frequencyhomogeniza-
tiontheorysuggestedbyDayaetal.(2002)andCrasteretal.
(2010)andco-workers.
5.Concludingremarks
ThroughincorporatingnewkinematicalDOFs,thepresentwork
hasproposedanelastodynamichomogenizationtheorygeneraliz-
ing
the
one
of
Willis
in
the
case
of
periodic
media
and
reduc-
ingtheerrorcommittedduringtheupscalingprocess,especially
athighfrequencies.Inordertoillustratethepotentialofthepre-
sentedtheory,ithasbeenshownthattheLW-LFasymptoticex-
pansionoftheeffectivemotionequationiscapableofsimultane-
ouslycapturing theacousticand thefirstopticalbranchof the
microscopicdispersioncurveforasimple1Dmedium.
Two problems remain open. The first concerns the effective
elastodynamicconstitutivelawproducedbythegeneralizedtheory
proposed.Inthispaper,toavoidthedifficultyrelatedtoitsnon-
uniqueness,theeffectivemotionequationhasbeendirectlytreated
andexploited.However, innumeroussituations,itisusefuland
importanttoexplicitlyknowtheeffectiveelastodynamicconstitu-
tivelaw.Thesecondproblemregardstheoptimalchoiceofshape
functionsforwhichguidingcriteriaexistbutremainincomplete.
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