A gradient elasticity theory for second-grade materials and higher order inertia
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Agradientelasticitytheoryforsecond-gradematerialsandhigherorderinertiaCastrenzePolizzottoUniversitàdiPalermo,DipartimentodiIngegneriaCivile,AmbientaleeAerospaziale,VialedelleScienze,90128Palermo,Italy
articleinfoArticlehistory:Received4November2011Receivedinrevisedform23March2012Availableonline26April2012
Keywords:GradientelasticityHigherorderinertiaContinuumthermodynamicsDynamicsWavedispersion
abstractSecond-gradeelasticmaterialsfeaturedbyafreeenergydependingonthestrainandthestraingradient,andakineticenergydependingonthevelocityandthevelocitygradient,areaddressed.Aninertialenergybalanceprincipleandavirtualworkprincipleforinertialactionsareenvisionedtoenrichthesetoftradi-tionaltheoreticaltoolsofthermodynamicsandcontinuummechanics.Thestatevariablesincludethebodymomentumandthesurfacemomentum,relatedtothevelocityinanonstandardway,aswellastheconcomitantmass-accelerationsandinertialforces,whichdointerveneintothemotionequationsandintotheforceboundaryconditions.Theboundarytractionisthesumoftwoparts,i.e.theCauchytractionandtheGurtin–Murdochtraction,whereasthetractionboundaryconditionexhibitsthetypicalformatoftheequilibriumequationofamaterialsurface(asknownfromtheprinciplesofsurfacemechanics)wherebytheGurtin–Murdochtraction(incorporatingtheinertialsurfaceforce)playstheroleofappliedsurfacialforcedensity.Thebody’sboundarysurfaceconstitutesathinboundarylayerwhichisinglobalequilibriumunderalltheexternalforcesappliedonit,afeaturethatmakesitpossibletoexploitthetractionCauchytheoremwithinsecond-gradematerials.Thismeansthatasecond-gradematerialisformedupbytwosub-systems,thatis,thebulkmaterialoperatingasaclassicalCauchycontinuum,andthethinboundarylayeroperatingasaGurtin–Murdochmaterialsurface.Theclassicallinearandangularmomentumtheoremsaresuitablyextendedforhigherorderinertia,fromwhichthelocalmotionequa-tionsandthemomentequilibriumequations(stresssymmetry)canbederived.Foranisotropicmaterialfeaturedbyfourconstants,i.e.theLaméconstantsandtwolengthscaleparameters(Aifantismodel),thedynamicevolutionproblemischaracterizedbyaHamilton-typevariationalprincipleandasolutionuniquenesstheorem.Closed-formsolutionsofthewavedispersionanalysisproblemforbeammodelsarepresentedandcomparedwithknownresultsfromtheliterature.Thepaperindicatesacorrectther-modynamicallyconsistentwaytotakeintoaccounthigherorderinertiaeffectswithincontinuummechanics.Ó2012ElsevierLtd.Allrightsreserved.
1.IntroductionTheelasticmaterialsconsideredinthepresentpaperbelongtotheclassofgeneralizedpolarandnonpolarmaterialsstudiedbyTruesdellandToupin(1960),Toupin(1962),Mindlin(1964,1965),MindlinandEshel(1968),GreenandRivlin(1964),andthemicropolarmaterialsaddressedbyEringen(1966).However,forsimplicityofexposition,weshalllimitourselvestoconsideringsecond-gradematerials,thatis,thefirststraingradientmaterialsaddressedbyMindlin(1964),MindlinandEshel(1968).Morepre-cisely,weshallfollowtheso-calledForm-IIformulationbythelat-terauthors,wherebythehigherorderstraintensorisdefinedasthefirstgradientofthestandard(secondorder)straintensor,andtheresultingstresstensorsexhibitsomeusefulsymmetryproperties(tobespecifiedshortly).Theinterestforthisclassofmaterialsstemsfromthepossibilityofassociatingtothemhigherorderinertiaeffects,thatis,theeffectsproducedoversuchamate-
rialwheneveritisinmotionwhilethekineticenergydependsonthevelocitygradient.Thiscombinationmakesitpossibletodis-pensewithstrainsingularitiesatsharpcracktipsandtocapturesomesizeeffectswithinthematerialdynamicbehavior,typicallythewavedispersionphenomenamanifestedbyrealmaterialsaspolymerfoams,high-thoughnessceramics,high-strengthmetalalloys,porousmaterialsandthelike,Mindlin(1964),Papargyri-Beskouetal.(2009)andAskesandAifantis(2011).HigherorderinertiaeffectswereconsideredinapaperbyMindlin(1964)dealingwithelasticmaterialswithmicrostructure,inwhichtheHamiltonprincipleisemployedtoderivetherelevantforcebalanceequationsandthematerialconstitutiveequations.Onsettingequaltozerotherelativemotionofthemicrostructurewithrespecttothecontinuum,Mindlin’stheorycanbeshowntoreduceitselftoanelasticitytheoryinwhichthestrainenergyde-pendsonthestrainandthestraingradient,andthekineticenergydependsonthevelocityandthevelocitygradient.Mindlin(1964)showedtheimportanceofthevelocitygradientforthemotionequationstobeabletocapturewavedispersionphenomena
0020-7683/$-seefrontmatterÓ2012ElsevierLtd.Allrightsreserved.http://dx.doi.org/10.1016/j.ijsolstr.2012.04.019
E-mailaddress:castrenze.polizzotto@unipa.it