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viscoplastic solids. The transformation time from a plastic like into rigid one
systemdepends onthevacuumpumpcapacity.Thesupplementaryandatthe
sametimeextraordinaryfeatureofVPPsisthatduringthetransitionprocess,
thegranularstructureis ableto memorizethe shapeofthesurroundingmold.
Although,themostimportantfactorthatinfluences themechanicalproperties
of such structuresis the underpressurevalue, the globalmaterialfeatures also
depend on the dimensions and the shape of a single grain, type of granular
material, surface roughness, filling ratio of the structure (grain volume to the
envelopevolumeratio)and the type ofloading.Thus,the preparationofsuch
a granular structure gives the possibility of controlling its global mechanical
characteristics(e.g. the Young's modulus or the proofstress) and its behavior
[7,8,9].
GenerallyVacuumPackedParticlesarenotsensitivetochangesintempera-
tureordifferenttypesofimpuritiesinsidethesystem.Thisfuturesmakespecial
granular structures useful in engineering applications where MR or ER fluids
cannot be used [10]. As it was mentioned before, properties of VPPs in some
sense canbe comparedto rheologicalfluids. We see that loosegrainsplaythe
samerolein the globalresponseof the materialaspolarizableparticles in the
fluid.Theimpactoftheelectricormagneticfieldisherereplacedbytheunder-
pressure.
Since the interest in various practical engineering application of VPP's is
recentlynoticed,theauthorsaretryingtolookforasuitablemathematicalfor-
mulationtocapturetherealbehaviorofgranularsystems.Especiallyinteresting
seemstobetheapplicationoftheconsideredgranularsystemasapartofcontrol-
labledampingstrategyforslenderobjectsvibrations[14],[15]orasasemi-active
pedestrianprotectionsystemcomponentduringcollision[16].Althoughthede-
signofVPP'sappearstobeextremelyeasy,themathematicalmodelingofsuch
systemsisquitesophisticated.
In the previous paper [11] the authors presented an initial probe to adopt
a selected rheological model for description of the uniaxial response of VPPs.
HeretheauthorsvalidatethepreviouslycalibratedGubanovmodelforamore
complexloadingconditionsi.e.stressrelaxationfollowedbyuniaxialcompression
tests.ExperimentsconductedinWarsawUniversityofTechnology,Instituteof
MachinesDesignFundamentalsconfirmedhighlynonlinearpropertiesofVPPs.
2 Experiments
Intheexperimentalresearchspeciallyperformedcylindricalspecimenshavebeen
used(Fig.1).ThesamplesweremountedintheMTS809universaltensiletesting
machine(Fig.2a).Thevacuumpump,madebyAGALabor(Fig.2c),wasused
togenerateandcontroltheinternalunderpressurevalue.Theglobalviewofthe
experimentalstandisdepictedinFig.2.
Typicalexperimentalresultsforinitiallycompressedgranularspecimenssub-
jectedafterwardstostressrelaxationtestshavebeendepictedinFig.3.
Different underpressure values result in various responses of the structure.
VPPs in a jammed state behave similarly to a viscoplastic solid body [8].
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