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encapsulation.Theenvelopeisconnectedtothevacuumpumpthroughavalve.
Afterfillingtheformwithgranularmaterial,aninternalunderpressureisgener-
ated,causingthestiffeningofthestructure.Atanappropriatevalueofunderpres-
sure,thebaggygranularmediabecomesasolidlikestructuralmaterial,capableof
transferringrelativelyhighstresses.Themechanicalpropertiesofsuchstructures,
obtainedonthebasisofaloosemateriallocatedinatightspace,areinfluenced
bymanyfactors.Themostimportantfactoristheunderpressurevalue,butthe
globalmaterialfeaturesdependalsoonthedimensionsandtheshapeofasingle
grain,thetypeofgranularmaterial,surfaceroughness,fillingratioofthestructure
(grainvolumetotheenvelopevolumeratio)andtypeofloading.Thus,theprepa-
rationofsuchagranularstructuresubmittedtointernalunderpressuregivesusthe
possibilityofcontrollingitsglobalmechanicalfeatures,forexample,theYoung's
modulusortheproofstress.
Thenextbenefitofthesegranularconcretesistheopportunitytobotharbi-
trarilyandpracticallymorphtheformoftheirexternalshapes,accordingtothe
temporarydemandsofadesigner.Recentlyaninterestingpracticalapplication
of the special granularstructure was found in universal robotic gripper which
utilizedthejammingofgranularmaterial.Grippingandholdingofobjectsare
animportanttaskfordifferenttypesofmanipulators.Thedevelopmentofuni-
versalgrippersabletopickupunfamiliarobjectsofvariousshapesandfeatures
is still challenging. Most popular solutions are based on multifingered hands.
Such an approach results in additional diculties mainly related to hardware
and software complexities. In [4] authors introduce a completely different ap-
proach to a universal gripper. Individual finger-like devices are replaced by a
granularsystem.Inthissystem,whengranularmaterialispressedontoatarget
object,thematerialsoftlyflowsarounditandconformstoitsshape.Thanksto
generating a partial vacuum inside a granularsystem, granular core contracts
andhardensquicklytopinchandholdtheobject.Themainadvantageofsuch
anapproachisomittingadditionalsensoryfeedback.Theoperatingprincipleis
simple and involves the ability of granular materials to transition between an
unjammed,deformablestateandajammedstatewithsolidlikerigidity.
Theyieldbehaviourofpolymers,themainpartofinvestigatedgranularstruc-
tures,whichisthemainparameterinvestigatedingranularstructuresisstrongly
nonlinear.Furthermore,thisbehaviourisaffectedbymanyfactorssuchastem-
peratureandstrainrate.Inthelastthreedecades,agreatdealofefforthasbeen
madetoadvancetheviscoplasticmodellingofpolymers.Theviscoplasticmodels
developed to representthe particular polymers behaviour,can be divided into
physicallybasedmodelsandphenomenologicallybasedmodels.
Thephysicalapproachisbasedonanidealizeddescriptionofthemicrostruc-
ture [5]. The one-dimensionalconstitutive equationof Haward-Thackray[6] is
thefirstphysicalmodelwhichwaslaterextendedtothethree-dimensional[7,8,9].
The problemcanalso be described phenomenologicallywith regardto the ba-
sicprincipleofthermodynamicsofcontinuousmediaandbydefining adequate
internal variables. According to these phenomenological considerations, mod-
els developed for metals can be used also for the description of the nonlinear
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