Research Article The Excellent Mechanical Properties of...
7
Research Article The Excellent Mechanical Properties of Cork: A Novel Approach through the Analysis of Contact Stress Antonio Díaz-Parralejo, 1 Eduardo M. Cuerda-Correa, 2 Antonio Macías-García, 1 José Sánchez-González, 1 and M. Ángeles Díaz-Díez 1 1 Department of Mechanical, Energetic and Materials Engineering, School of Industrial Engineering, University of Extremadura, Avenida de Elvas, s/n, 06071 Badajoz, Spain 2 Department of Organic and Inorganic Chemistry, Faculty of Sciences, University of Extremadura, Avenida de Elvas, s/n, 06071 Badajoz, Spain Correspondence should be addressed to Eduardo M. Cuerda-Correa; [email protected] Received 18 February 2014; Accepted 30 March 2014; Published 7 May 2014 Academic Editors: P. Mandracci, Y. Sun, and R. A. Varin Copyright © 2014 Antonio D´ ıaz-Parralejo et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In many technological applications of cork, this biomaterial is under strongly localized contact stresses, which largely differ from the homogeneous distribution of stresses of the typical uniaxial compression tests. Indentation tests constitute an excellent form of determining the behavior of the materials under localized stresses. In the present study, the applicability of Hertzian and Brinell indentation tests to the evaluation of the mechanical properties of cork is tested. One of the main conclusions of the study is that the elastic anisotropy of the material is related to the anisotropic structure of the different sections cut from a cork sample, a clear difference between the back tangential section and the other sections being observed. 1. Introduction Cellular solids are materials possessing cellular microstruc- tures,which are seen in natural materials such as wood, cork, sponge, cancellous bone, and coral [1]. is kind of materials usually exhibit excellent energy absorption characteristics under compression. us, the study of the deformation behavior of cellular solids under compression has received a great deal of attention [2, 3]. Cork is a widely used material due to its excellent mechanical properties, low density, impermeability, thermal and acoustic insulation, and so forth. Examples of its use are as the stopper in bottles of quality wine, floorings, and wall coverings, and so forth. Despite its qualities for such a diversity of applications, there has been a certain unjustifiable lethargy in scientific attention to this material. ere have however been some recent studies published on the structure and mechanical properties of cellular solids in general [4, 5], and on the mechanical properties of cork in particular [6–8], which have helped to better understand the unique properties of this material. In this connection, cork and its derivatives have been used as new biosources of chemicals [9] and as the precursor for the preparation of cation exchangers [10] and, more recently, for the preparation of activated carbons for the removal of pollutants from solution [11]. In many technological applications, cork is subjected to strong contact stresses localized in small zones of the material, not to the homogeneous distribution of stresses that are involved in uniaxial compressive strength tests. e mechanical behavior of other materials such as foams of different types has been tested [2]. Nevertheless, there have been no studies on the mechanical response of the material under such conditions. In this regard, indentation tests are an excellent form of assessing the damage and behavior of materials under localized stresses [12]. Among the different methods (Hertzian, Vickers, Brinell, etc.), the Hertzian indentation test is particularly interesting because it allows one to control the contact pressure on the material Hindawi Publishing Corporation ISRN Materials Science Volume 2014, Article ID 898439, 6 pages http://dx.doi.org/10.1155/2014/898439
Transcript of Research Article The Excellent Mechanical Properties of...
Research ArticleThe Excellent Mechanical Properties of Cork A Novel Approachthrough the Analysis of Contact Stress
Antonio Diacuteaz-Parralejo1 Eduardo M Cuerda-Correa2 Antonio Maciacuteas-Garciacutea1
Joseacute Saacutenchez-Gonzaacutelez1 and M Aacutengeles Diacuteaz-Diacuteez1
1 Department of Mechanical Energetic and Materials Engineering School of Industrial Engineering University of ExtremaduraAvenida de Elvas sn 06071 Badajoz Spain
2Department of Organic and Inorganic Chemistry Faculty of Sciences University of Extremadura Avenida de Elvassn 06071 Badajoz Spain
Correspondence should be addressed to Eduardo M Cuerda-Correa emccunexes
Received 18 February 2014 Accepted 30 March 2014 Published 7 May 2014
Academic Editors P Mandracci Y Sun and R A Varin
Copyright copy 2014 Antonio Dıaz-Parralejo et alThis is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited
In many technological applications of cork this biomaterial is under strongly localized contact stresses which largely differ fromthe homogeneous distribution of stresses of the typical uniaxial compression tests Indentation tests constitute an excellent formof determining the behavior of the materials under localized stresses In the present study the applicability of Hertzian and Brinellindentation tests to the evaluation of the mechanical properties of cork is tested One of the main conclusions of the study is thatthe elastic anisotropy of the material is related to the anisotropic structure of the different sections cut from a cork sample a cleardifference between the back tangential section and the other sections being observed
1 Introduction
Cellular solids are materials possessing cellular microstruc-tureswhich are seen in natural materials such as wood corksponge cancellous bone and coral [1] This kind of materialsusually exhibit excellent energy absorption characteristicsunder compression Thus the study of the deformationbehavior of cellular solids under compression has received agreat deal of attention [2 3]
Cork is a widely used material due to its excellentmechanical properties low density impermeability thermaland acoustic insulation and so forth Examples of its useare as the stopper in bottles of quality wine floorings andwall coverings and so forth Despite its qualities for such adiversity of applications there has been a certain unjustifiablelethargy in scientific attention to this material There havehowever been some recent studies published on the structureand mechanical properties of cellular solids in general [4 5]and on the mechanical properties of cork in particular [6ndash8]
which have helped to better understand the unique propertiesof this material In this connection cork and its derivativeshave been used as new biosources of chemicals [9] and as theprecursor for the preparation of cation exchangers [10] andmore recently for the preparation of activated carbons for theremoval of pollutants from solution [11]
In many technological applications cork is subjectedto strong contact stresses localized in small zones of thematerial not to the homogeneous distribution of stressesthat are involved in uniaxial compressive strength testsThe mechanical behavior of other materials such as foamsof different types has been tested [2] Nevertheless therehave been no studies on the mechanical response of thematerial under such conditions In this regard indentationtests are an excellent form of assessing the damage andbehavior of materials under localized stresses [12] Amongthe different methods (Hertzian Vickers Brinell etc) theHertzian indentation test is particularly interesting becauseit allows one to control the contact pressure on the material
Hindawi Publishing CorporationISRN Materials ScienceVolume 2014 Article ID 898439 6 pageshttpdxdoiorg1011552014898439
2 ISRNMaterials Science
CorkPhellogen Axial Tangential
Radial
Radial Back
Transverse
Belly
Sections
Figure 1 Diagram showing the different sections and directions of a piece of cork relative to its position on the tree Also indicated above isthe morphology of the cells of each section
from very low values (elastic andor viscoelastic response) upto values high enough to generate irreversible damage in thematerial
The Hertzian test involves the application of a load 119875onto the surface of the material using spherical indenters(usually made of a hard material such as tungsten carbideWC)This test is traditionally used to investigate the plasticityof metallic materials and more recently to study the fractureof ceramics [13ndash15] In the present study we will also demon-strate its utility in investigating the mechanical properties ofcork
Cork is a natural product of the cork oak (Quercus suberL) Cork cells are generated by the activity of phellogen[16] They are stacked in columns with axes parallel to theradial direction of the tree (Figure 1) The ldquoradialrdquo andldquotransverserdquo sections are arranged in columns parallel tothe radial direction The ldquotangentialrdquo sections are arrangedperpendicular to the radial direction of the tree and have ahoneycomb-like morphology The size of the cells and theresulting thickness of each cork layer vary according to theconditions of the season in which they were formed [17] Theranges of these dimensions are as follows height 30ndash40120583medges of the bases 13ndash15 120583m cell wall thickness 1-2120583m andlayer thickness 200ndash3000120583m
2 Materials and Experimental Procedure
21 Preparation of Samples Samples of cork were suppliedby the Institute for the Promotion of Cork Wood andCoal (ICMC Spain) An automatic cutoff machine (StruersAccutom-50) with a diamond wheel was used to cut regularparallelepiped specimens (see Figure 1) For the ldquobackrdquotangential section the zone of outer crust (about 5mm thick)was first removed and for the ldquobellyrdquo tangential section thelayer (about 2mm thick) in contact with the inner bark of thetree trunk was removed
22 Mechanical Tests The tests were performed on a univer-sal testing machine (Instron Model 1122) applying loads inthe range from 0 to 300N using spherical tungsten carbide(WC) indenters of radii 55 and 127mm The speed of
application of the load was 005mmmin After several trials(gold sputtering onto the sample spraying with ink etc) itwas decided that the best approach to defining the regionof contact during indentation was to cover the WC ballwith ink The contact radius was measured under opticalmicroscopy using a digital comparator clock coupled to themobile turntable
221 Hertzian Indentation Test In Hertzian indentation testthe stress field scaleswith the contact pressure119901
0 also known
as the indentation stress [13]
1199010=119875
1205871198862 (1)
where 119875 is the indentation load and 119886 the radius of thecircle of contact (Figure 2) In accordance with the principleof geometric similarity the strain field scales with the ratio119886119903 where 119903 is the radius of the sphere This ratio is calledthe indentation strain The experimental determination of119886 for each indentation load and indenter radius 119903 allowsone to obtain the indentation stress-strain curve 119901
0(119886119903)
which is characteristic of each material and independent ofthe radius of the indenter Figure 2 shows a generic stress-strain indentation curve The linear part corresponds to theelastic contact domain Beyond a certain threshold of stressthe response is no longer linear indicating the onset ofirreversible processes that generate some level of damage tothe material [13] Under elastic contact conditions one hasthat
1199010=3119864119886
4120587119896119903 (2)
where 119864 is materialrsquos Youngrsquos modulus and 119896 is the dimen-sionless constant
119896 =9
16[(1 minus ]2) + (1 minus ]10158402)]
119864
1198641015840 (3)
where ] ]1015840 119864 and 1198641015840 are the Poisson ratios and Youngrsquosmoduli of the specimen and indenter respectively Thusknowing the elastic constants of the indenter one can
ISRNMaterials Science 3
Sphere
Specimen
P
r
2a
p
105Y
0
Hertz
ar
Figure 2 Diagram of a Hertzian indentation test (left) and typical indentation stress-strain curve (right) 119884 is the yield stress of an uniaxialtest
estimate the value of 119864 from the linear segment of theexperimental indentation stress-strain curve According tothe Tresca-Guest criterion which is reasonably acceptable forductilematerials subjected to situations of high shear stressesinelastic deformation begins at a point of the solid where themaximum tangential stress reaches the value
120591119898=119884
2(4)
with 119884 being the yield stressIn Hertzian elastic contact theory the shear stress is
maximum on the load axis at a depth of 05119886 with this valuebeing approximately
120591119898= 047119901
0 (5)
Thus from (4) one finally has
119901119884= 105119884 (6)
222 Brinell Indentation Tests In a Brinell test hardenedsteel spheres are used to produce an imprint on the surface ofthe tested material The diameter of the imprint is measuredand the hardness (HBN) of the material is calculated fromthe relationship between the applied load and the area of theimprint The area can be substituted by the diameter of theimprint with the final expression being
HBN = 119875
(1205872)119863 [119863 minus radic(1198632 minus 1198892)]
(7)
where 119875 is the applied load 119863 is the diameter of the balland 119889 is the diameter of the imprint For the results fromdifferent materials to be comparable the applied loads mustbe proportional to the square of the diameter of the ball usedfor the imprints that is
119875 = 1198701198632 (8)
The coefficient 119870 depends on the type of material to testbeing greater for hardmaterials and smaller for softmaterials
50
40
30
20
10
0
00 05 10 15 20 25
Load
(N)
Depth (mm)
TETBTR or RD
Figure 3 Load-unload curves corresponding to the differentsections of a cork specimen
A further constraint that must be satisfied to avoid bendingin the specimen is that the diameter of the imprint must bewithin the range 1198634 lt 119889 lt 1198632 that is approximately 119889 =0375119863
3 Results and Discussion
31 Load-Displacement Curves Figure 3 shows the experi-mental load-displacement curves corresponding to differentsectionsThese curves were obtained from each of the inden-tations carried out to determine the Hertzian indentationstress-strain curve One observes that the tangential sectioncorresponding to the ldquobackrdquo of the sample (TE) is weakerthan that corresponding to the ldquobellyrdquo (TB) In addition theTE section is weaker than the transverse (TR) and radial(RD) sections although the differences depend on the depth
4 ISRNMaterials Science
TE section
TR or RD section
(a)
TR or RD section
TE section
(b)
Figure 4 SEM micrographs of the TE and the TR or RD sections (a) before and (b) after applying a contact load of 100N
at which the tests were performed with respect to the TEsection
In general a large deformation of the specimen atrelatively low loads is observed In the unloading curveone observes an initial slow recovery which increases as theload is released although the energy stored by the sampleis still relatively high at total unloading Above a certainvalue (between 6 and 10N) the load curve deviates from amore or less linear behaviour showing increasing resistanceto deformationThis change is probably due to the progressivecollapse of cells with increasing contact pressure
The same effect can be seen in Figure 4 which shows thecontact deformations of different sections of the same samplewith a load of 100N There is a notable difference betweenthe deformation and damage in the back tangential section(TE) and the other sections The TE section shows greaterrecovery and less damage after applying the contact stress asis clearly observable in the figure in the cell walls This factis in accordance with other results previously reported in theliterature [18]
32 Indentation Stress-Strain Curves Figure 5 shows theexperimental indentation stress-strain curves correspondingto the different sections of a given sample The resultsshow a first linear zone up to a certain value of 119901
0that
depends on the section being tested While the results for thesections TR RD and TB are practically indistinguishable thecurve corresponding to the TE section is clearly unlike therest reflecting the relative weakness of this section Youngrsquosmoduli calculated from the linear zone of the curves arelisted in Table 1 The means of these elastic constants forthe samples tested are 72 plusmn 05MPa for the TE section and18plusmn10MPa for the rest of the sectionsThis elastic anisotropy
14
12
10
08
06
04
02
00
08060402
TE
HB (TB TR RD)
HB (TE)
TBTR or RD
00
p0
(MPa
)
ar
Figure 5 Indentation stress-strain curves for the TE TB TR or RDsections of a cork sample
is a reflection of the anisotropic structure of cork itselfTheTEsection is weaker than TB because it is the outer tangentiallayer Indeed the innermost cells are formed after the moreexternal cells and as they grow therefore have to push thelatter outwards This results in relatively greater density ofthe cell walls in the TB section and consequently a greaterresistance to elastic deformation This kind of behavior hasbeen reported by other researchers [19 20] that have studiedthe hyperelastic or viscoelastic behavior of cork and itsderivatives
ISRNMaterials Science 5
Table 1 Youngrsquos modulus (119864) yield stress (119884) and Brinell hardness (HBN) of the different sections of some cork samples
Sample 119864 (MPa) 119884 (MPa) HBN (MPa)TE TB RD and TR TE TB RD and TR TE TB RD and TR
M1 70 plusmn 05 180 plusmn 10 050 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M2 66 plusmn 05 178 plusmn 10 050 plusmn 005 078 plusmn 005 085 plusmn 005 120 plusmn 005
M3 82 plusmn 05 184 plusmn 10 052 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M4 72 plusmn 05 180 plusmn 10 052 plusmn 005 080 plusmn 005 082 plusmn 005 120 plusmn 005
33 Determination of the Yield Stress The yield stress ofthe cork samples was obtained from the critical indentationstress for the onset of plastic deformation (6) The estimationof 119901119884from the point at which the indentation stress-strain
curve deviates from linearity involves a noticeable difficultyWe therefore used a method which consists of determining119901119884from the load 119875
119884 corresponding to the first detectable
residual impression in the test surface and the correspondingcontact radius 119886 To this end a series of tests were conductedusing an indenter of known radius (119903 = 555mm) andincreasing loads and then observing the specimen under anoptical microscope to identify the smallest load at which aresidual impression was observableThe values calculated forthe yield stress in the various sections and samples are alsogiven in Table 1Themean values are 05plusmn005MPa for the TEsections and 08 plusmn 005MPa for the rest (TB RD and TR) Itshould be mentioned that the value obtained by this methodis actually a slightly high estimate of the yield stress as themeasured value of 119875
119884may be in excess Indeed for loads
somewhat smaller than 119875119884 plastic deformation may occur
but be located in such a small region that it does not give riseto a residual impression detectable under optical microscopy
34 Determination of the Brinell Hardness Index To deter-mine the Brinell hardness index one must first fix the testingconstant 119870 to use depending on the nature of the materialunder test As reference some of the usual constants used toevaluate hardness in industrialmaterials are119870 = 30 for steels119870 = 10 for Cu brasses and bronzes 119870 = 5 for light alloys119870 = 25 for Sn and Pb and so forth Values of 125 and 05are also used for very soft metals or alloys In the case of amaterial such as cork it is necessary to gradually decreasethe value of the constant until the relationship between theapplied force and the diameter of the sphere used producesan imprint that is within the limits established for the Brinelltest that is 1198634 lt 119889 lt 1198632 In this way we determined thevalue used to be119870 = 156sdot10minus2The results using this constantare listed in Table 1 One observes that the Brinell hardnessin the TE sections varies around 082 plusmn 005MPa and around115plusmn005MPa in the other sections (TB RD and TR)Thesevalues are included in the plots of Figure 5 One sees that theyare reasonably consistent with the Hertzian indentation testsIndeed Hertzian indentation stress surpassed the Brinellhardness values of each section because there is no constraintin the Hertzian tests on the diameters of the imprints whichcan even exceed the upper bound of the Brinell imprints thatis 119889 gt 1198632
4 Conclusions
In this study Hertzian and Brinell indentation tests wereused to evaluate the mechanical properties of corkThe mainconclusions and implications to be drawn from the study areas follows
(i) The methodological approach is well suited to thecharacterization of the elastic and plastic behaviourof soft materials of diverse nature (cellular materials)
(ii) The observed elastic anisotropy is closely related tothe anisotropic structure of the different sections ofcorkThe ldquobackrdquo tangential section is weaker than theother sections of the material
(iii) The specimens presented large deformations at rela-tively low stresses consistent with the low value of thematerialrsquos elastic modulus The materialrsquos elastic zonewas relatively small (low values of the yield stress) butwell defined
(iv) Finally we wish to note that Hertzian and Brinellindentation testing would seem to be well suited notonly to characterize mechanically such materials ascork but also to evaluate the intrinsic quality of thesematerials
Conflict of Interests
The authors declare that they have no affiliations with orinvolvement in any organization or entity with any financialinterest (such as honoraria educational grants participationin speakers bureaus membership employment consultan-cies stock ownership or other equity interests and experttestimony or patent-licensing arrangements) or nonfinan-cial interest (such as personal or professional relationshipsaffiliations knowledge or beliefs) in the subject of matter ormaterials discussed in this paper Consequently the authorsdeclare that there is no conflict of interests regarding thepublication of this paper
Acknowledgments
The authors thank Dr A Pajares and Dr A L Ortiz forthe fruitful discussion and the Institute for the Promotionof Cork Wood and Coal (ICMC) Junta de Extremadura(Spain) for supplying the cork samples
6 ISRNMaterials Science
References
[1] Y Takahashi D Okumura and N Ohno ldquoYield and bucklingbehavior of Kelvin open-cell foams subjected to uniaxial com-pressionrdquo International Journal of Mechanical Sciences vol 52no 2 pp 377ndash385 2010
[2] C P Gameiro and J Cirne ldquoDynamic axial crushing of shortto long circular aluminium tubes with agglomerate cork fillerrdquoInternational Journal of Mechanical Sciences vol 49 no 9 pp1029ndash1037 2007
[3] S Banerjee and A Bhaskar ldquoThe applicability of the effectivemedium theory to the dynamics of cellular beamsrdquo Interna-tional Journal of Mechanical Sciences vol 51 no 8 pp 598ndash6082009
[4] M F Ashby ldquoThe mechanical properties of cellular solidsrdquoMetallurgical and Materials Transactions A vol 14 no 9 pp1755ndash1769 1983
[5] S P Silva M A Sabino E M Fernandas V M Correlo LF Boesel and R L Reis ldquoCork properties capabilities andapplicationsrdquo International Materials Reviews vol 50 no 6 pp345ndash365 2005
[6] H Pereira M E Rosa andM A Fortes ldquoThe cellular structureof cork from Quercus suber Lrdquo IAWA Bulletin vol 8 pp 213ndash218 1987
[7] M Emilia Rosa and M A Fortes ldquoStress relaxation and creepof corkrdquo Journal of Materials Science vol 23 no 1 pp 35ndash421988
[8] J F Mano ldquoCreep-recovery behaviour of corkrdquo MaterialsLetters vol 61 no 11-12 pp 2473ndash2477 2007
[9] N Cordeiro NM Belgacem A Gandini and C P Neto ldquoCorksuberin as a new source of chemicals 2 Crystallinity thermaland rheological propertiesrdquo Bioresource Technology vol 63 no2 pp 153ndash158 1998
[10] C V Calahorro A B Garcia C P Barrera M J B Garcia andM G Corzo ldquoCation exchangers prepared from cork wastesrdquoBioresource Technology vol 44 no 3 pp 229ndash233 1993
[11] A SMestre J Pires JM FNogueira J B Parra A P Carvalhoand C O Ania ldquoWaste-derived activated carbons for removalof ibuprofen from solution role of surface chemistry and porestructurerdquo Bioresource Technology vol 100 no 5 pp 1720ndash17262009
[12] M Wilsea K L Johnson and M F Ashby ldquoIndentation offoamed plasticsrdquo International Journal of Mechanical Sciencesvol 17 no 7 pp 457ndash460 1975
[13] B Lawn and R Wilshaw ldquoIndentation fracture principles andapplicationsrdquo Journal of Materials Science vol 10 no 6 pp1049ndash1081 1975
[14] B R Lawn N P Padture H Cai and F Guiberteau ldquoMakingceramics lsquoDuctilersquordquo Science vol 263 no 5150 pp 1114ndash1116 1994
[15] Y G Jung I M Peterson A Pajares and B R Lawn ldquoContactdamage resistance and strength degradation of glass-infiltratedalumina and spinel ceramicsrdquo Journal of Dental Research vol78 no 3 pp 804ndash814 1999
[16] M A Fortes and M E Rosa ldquoDensidade da cortica factoresque a influenciamrdquo Cortica vol 593 pp 65ndash69 1988
[17] J V Natividade Subericultura Direccao Geral dos ServicosFlorestais e Aquicolas Lisbon Portugal 1990
[18] F Ben Abdallah R Ben Cheikh M Baklouti Z Denchevand A M Cunha ldquoCharacterization of composite materialsbased on PP-cork blendsrdquo Journal of Reinforced Plastics andComposites vol 25 no 14 pp 1499ndash1506 2006
[19] P J Antunes G R Dias A T Coelho F Rebelo and T PereiraldquoHyperelastic modelling of cork-polyurethane gel compositesnon-linear FEA implementation in 3D foot modelrdquo MaterialsScience Forum vol 587-588 pp 700ndash705 2008
[20] J F Mano ldquoThe viscoelastic properties of corkrdquo Journal ofMaterials Science vol 37 no 2 pp 257ndash263 2002
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Journal ofNanomaterials
2 ISRNMaterials Science
CorkPhellogen Axial Tangential
Radial
Radial Back
Transverse
Belly
Sections
Figure 1 Diagram showing the different sections and directions of a piece of cork relative to its position on the tree Also indicated above isthe morphology of the cells of each section
from very low values (elastic andor viscoelastic response) upto values high enough to generate irreversible damage in thematerial
The Hertzian test involves the application of a load 119875onto the surface of the material using spherical indenters(usually made of a hard material such as tungsten carbideWC)This test is traditionally used to investigate the plasticityof metallic materials and more recently to study the fractureof ceramics [13ndash15] In the present study we will also demon-strate its utility in investigating the mechanical properties ofcork
Cork is a natural product of the cork oak (Quercus suberL) Cork cells are generated by the activity of phellogen[16] They are stacked in columns with axes parallel to theradial direction of the tree (Figure 1) The ldquoradialrdquo andldquotransverserdquo sections are arranged in columns parallel tothe radial direction The ldquotangentialrdquo sections are arrangedperpendicular to the radial direction of the tree and have ahoneycomb-like morphology The size of the cells and theresulting thickness of each cork layer vary according to theconditions of the season in which they were formed [17] Theranges of these dimensions are as follows height 30ndash40120583medges of the bases 13ndash15 120583m cell wall thickness 1-2120583m andlayer thickness 200ndash3000120583m
2 Materials and Experimental Procedure
21 Preparation of Samples Samples of cork were suppliedby the Institute for the Promotion of Cork Wood andCoal (ICMC Spain) An automatic cutoff machine (StruersAccutom-50) with a diamond wheel was used to cut regularparallelepiped specimens (see Figure 1) For the ldquobackrdquotangential section the zone of outer crust (about 5mm thick)was first removed and for the ldquobellyrdquo tangential section thelayer (about 2mm thick) in contact with the inner bark of thetree trunk was removed
22 Mechanical Tests The tests were performed on a univer-sal testing machine (Instron Model 1122) applying loads inthe range from 0 to 300N using spherical tungsten carbide(WC) indenters of radii 55 and 127mm The speed of
application of the load was 005mmmin After several trials(gold sputtering onto the sample spraying with ink etc) itwas decided that the best approach to defining the regionof contact during indentation was to cover the WC ballwith ink The contact radius was measured under opticalmicroscopy using a digital comparator clock coupled to themobile turntable
221 Hertzian Indentation Test In Hertzian indentation testthe stress field scaleswith the contact pressure119901
0 also known
as the indentation stress [13]
1199010=119875
1205871198862 (1)
where 119875 is the indentation load and 119886 the radius of thecircle of contact (Figure 2) In accordance with the principleof geometric similarity the strain field scales with the ratio119886119903 where 119903 is the radius of the sphere This ratio is calledthe indentation strain The experimental determination of119886 for each indentation load and indenter radius 119903 allowsone to obtain the indentation stress-strain curve 119901
0(119886119903)
which is characteristic of each material and independent ofthe radius of the indenter Figure 2 shows a generic stress-strain indentation curve The linear part corresponds to theelastic contact domain Beyond a certain threshold of stressthe response is no longer linear indicating the onset ofirreversible processes that generate some level of damage tothe material [13] Under elastic contact conditions one hasthat
1199010=3119864119886
4120587119896119903 (2)
where 119864 is materialrsquos Youngrsquos modulus and 119896 is the dimen-sionless constant
119896 =9
16[(1 minus ]2) + (1 minus ]10158402)]
119864
1198641015840 (3)
where ] ]1015840 119864 and 1198641015840 are the Poisson ratios and Youngrsquosmoduli of the specimen and indenter respectively Thusknowing the elastic constants of the indenter one can
ISRNMaterials Science 3
Sphere
Specimen
P
r
2a
p
105Y
0
Hertz
ar
Figure 2 Diagram of a Hertzian indentation test (left) and typical indentation stress-strain curve (right) 119884 is the yield stress of an uniaxialtest
estimate the value of 119864 from the linear segment of theexperimental indentation stress-strain curve According tothe Tresca-Guest criterion which is reasonably acceptable forductilematerials subjected to situations of high shear stressesinelastic deformation begins at a point of the solid where themaximum tangential stress reaches the value
120591119898=119884
2(4)
with 119884 being the yield stressIn Hertzian elastic contact theory the shear stress is
maximum on the load axis at a depth of 05119886 with this valuebeing approximately
120591119898= 047119901
0 (5)
Thus from (4) one finally has
119901119884= 105119884 (6)
222 Brinell Indentation Tests In a Brinell test hardenedsteel spheres are used to produce an imprint on the surface ofthe tested material The diameter of the imprint is measuredand the hardness (HBN) of the material is calculated fromthe relationship between the applied load and the area of theimprint The area can be substituted by the diameter of theimprint with the final expression being
HBN = 119875
(1205872)119863 [119863 minus radic(1198632 minus 1198892)]
(7)
where 119875 is the applied load 119863 is the diameter of the balland 119889 is the diameter of the imprint For the results fromdifferent materials to be comparable the applied loads mustbe proportional to the square of the diameter of the ball usedfor the imprints that is
119875 = 1198701198632 (8)
The coefficient 119870 depends on the type of material to testbeing greater for hardmaterials and smaller for softmaterials
50
40
30
20
10
0
00 05 10 15 20 25
Load
(N)
Depth (mm)
TETBTR or RD
Figure 3 Load-unload curves corresponding to the differentsections of a cork specimen
A further constraint that must be satisfied to avoid bendingin the specimen is that the diameter of the imprint must bewithin the range 1198634 lt 119889 lt 1198632 that is approximately 119889 =0375119863
3 Results and Discussion
31 Load-Displacement Curves Figure 3 shows the experi-mental load-displacement curves corresponding to differentsectionsThese curves were obtained from each of the inden-tations carried out to determine the Hertzian indentationstress-strain curve One observes that the tangential sectioncorresponding to the ldquobackrdquo of the sample (TE) is weakerthan that corresponding to the ldquobellyrdquo (TB) In addition theTE section is weaker than the transverse (TR) and radial(RD) sections although the differences depend on the depth
4 ISRNMaterials Science
TE section
TR or RD section
(a)
TR or RD section
TE section
(b)
Figure 4 SEM micrographs of the TE and the TR or RD sections (a) before and (b) after applying a contact load of 100N
at which the tests were performed with respect to the TEsection
In general a large deformation of the specimen atrelatively low loads is observed In the unloading curveone observes an initial slow recovery which increases as theload is released although the energy stored by the sampleis still relatively high at total unloading Above a certainvalue (between 6 and 10N) the load curve deviates from amore or less linear behaviour showing increasing resistanceto deformationThis change is probably due to the progressivecollapse of cells with increasing contact pressure
The same effect can be seen in Figure 4 which shows thecontact deformations of different sections of the same samplewith a load of 100N There is a notable difference betweenthe deformation and damage in the back tangential section(TE) and the other sections The TE section shows greaterrecovery and less damage after applying the contact stress asis clearly observable in the figure in the cell walls This factis in accordance with other results previously reported in theliterature [18]
32 Indentation Stress-Strain Curves Figure 5 shows theexperimental indentation stress-strain curves correspondingto the different sections of a given sample The resultsshow a first linear zone up to a certain value of 119901
0that
depends on the section being tested While the results for thesections TR RD and TB are practically indistinguishable thecurve corresponding to the TE section is clearly unlike therest reflecting the relative weakness of this section Youngrsquosmoduli calculated from the linear zone of the curves arelisted in Table 1 The means of these elastic constants forthe samples tested are 72 plusmn 05MPa for the TE section and18plusmn10MPa for the rest of the sectionsThis elastic anisotropy
14
12
10
08
06
04
02
00
08060402
TE
HB (TB TR RD)
HB (TE)
TBTR or RD
00
p0
(MPa
)
ar
Figure 5 Indentation stress-strain curves for the TE TB TR or RDsections of a cork sample
is a reflection of the anisotropic structure of cork itselfTheTEsection is weaker than TB because it is the outer tangentiallayer Indeed the innermost cells are formed after the moreexternal cells and as they grow therefore have to push thelatter outwards This results in relatively greater density ofthe cell walls in the TB section and consequently a greaterresistance to elastic deformation This kind of behavior hasbeen reported by other researchers [19 20] that have studiedthe hyperelastic or viscoelastic behavior of cork and itsderivatives
ISRNMaterials Science 5
Table 1 Youngrsquos modulus (119864) yield stress (119884) and Brinell hardness (HBN) of the different sections of some cork samples
Sample 119864 (MPa) 119884 (MPa) HBN (MPa)TE TB RD and TR TE TB RD and TR TE TB RD and TR
M1 70 plusmn 05 180 plusmn 10 050 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M2 66 plusmn 05 178 plusmn 10 050 plusmn 005 078 plusmn 005 085 plusmn 005 120 plusmn 005
M3 82 plusmn 05 184 plusmn 10 052 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M4 72 plusmn 05 180 plusmn 10 052 plusmn 005 080 plusmn 005 082 plusmn 005 120 plusmn 005
33 Determination of the Yield Stress The yield stress ofthe cork samples was obtained from the critical indentationstress for the onset of plastic deformation (6) The estimationof 119901119884from the point at which the indentation stress-strain
curve deviates from linearity involves a noticeable difficultyWe therefore used a method which consists of determining119901119884from the load 119875
119884 corresponding to the first detectable
residual impression in the test surface and the correspondingcontact radius 119886 To this end a series of tests were conductedusing an indenter of known radius (119903 = 555mm) andincreasing loads and then observing the specimen under anoptical microscope to identify the smallest load at which aresidual impression was observableThe values calculated forthe yield stress in the various sections and samples are alsogiven in Table 1Themean values are 05plusmn005MPa for the TEsections and 08 plusmn 005MPa for the rest (TB RD and TR) Itshould be mentioned that the value obtained by this methodis actually a slightly high estimate of the yield stress as themeasured value of 119875
119884may be in excess Indeed for loads
somewhat smaller than 119875119884 plastic deformation may occur
but be located in such a small region that it does not give riseto a residual impression detectable under optical microscopy
34 Determination of the Brinell Hardness Index To deter-mine the Brinell hardness index one must first fix the testingconstant 119870 to use depending on the nature of the materialunder test As reference some of the usual constants used toevaluate hardness in industrialmaterials are119870 = 30 for steels119870 = 10 for Cu brasses and bronzes 119870 = 5 for light alloys119870 = 25 for Sn and Pb and so forth Values of 125 and 05are also used for very soft metals or alloys In the case of amaterial such as cork it is necessary to gradually decreasethe value of the constant until the relationship between theapplied force and the diameter of the sphere used producesan imprint that is within the limits established for the Brinelltest that is 1198634 lt 119889 lt 1198632 In this way we determined thevalue used to be119870 = 156sdot10minus2The results using this constantare listed in Table 1 One observes that the Brinell hardnessin the TE sections varies around 082 plusmn 005MPa and around115plusmn005MPa in the other sections (TB RD and TR)Thesevalues are included in the plots of Figure 5 One sees that theyare reasonably consistent with the Hertzian indentation testsIndeed Hertzian indentation stress surpassed the Brinellhardness values of each section because there is no constraintin the Hertzian tests on the diameters of the imprints whichcan even exceed the upper bound of the Brinell imprints thatis 119889 gt 1198632
4 Conclusions
In this study Hertzian and Brinell indentation tests wereused to evaluate the mechanical properties of corkThe mainconclusions and implications to be drawn from the study areas follows
(i) The methodological approach is well suited to thecharacterization of the elastic and plastic behaviourof soft materials of diverse nature (cellular materials)
(ii) The observed elastic anisotropy is closely related tothe anisotropic structure of the different sections ofcorkThe ldquobackrdquo tangential section is weaker than theother sections of the material
(iii) The specimens presented large deformations at rela-tively low stresses consistent with the low value of thematerialrsquos elastic modulus The materialrsquos elastic zonewas relatively small (low values of the yield stress) butwell defined
(iv) Finally we wish to note that Hertzian and Brinellindentation testing would seem to be well suited notonly to characterize mechanically such materials ascork but also to evaluate the intrinsic quality of thesematerials
Conflict of Interests
The authors declare that they have no affiliations with orinvolvement in any organization or entity with any financialinterest (such as honoraria educational grants participationin speakers bureaus membership employment consultan-cies stock ownership or other equity interests and experttestimony or patent-licensing arrangements) or nonfinan-cial interest (such as personal or professional relationshipsaffiliations knowledge or beliefs) in the subject of matter ormaterials discussed in this paper Consequently the authorsdeclare that there is no conflict of interests regarding thepublication of this paper
Acknowledgments
The authors thank Dr A Pajares and Dr A L Ortiz forthe fruitful discussion and the Institute for the Promotionof Cork Wood and Coal (ICMC) Junta de Extremadura(Spain) for supplying the cork samples
6 ISRNMaterials Science
References
[1] Y Takahashi D Okumura and N Ohno ldquoYield and bucklingbehavior of Kelvin open-cell foams subjected to uniaxial com-pressionrdquo International Journal of Mechanical Sciences vol 52no 2 pp 377ndash385 2010
[2] C P Gameiro and J Cirne ldquoDynamic axial crushing of shortto long circular aluminium tubes with agglomerate cork fillerrdquoInternational Journal of Mechanical Sciences vol 49 no 9 pp1029ndash1037 2007
[3] S Banerjee and A Bhaskar ldquoThe applicability of the effectivemedium theory to the dynamics of cellular beamsrdquo Interna-tional Journal of Mechanical Sciences vol 51 no 8 pp 598ndash6082009
[4] M F Ashby ldquoThe mechanical properties of cellular solidsrdquoMetallurgical and Materials Transactions A vol 14 no 9 pp1755ndash1769 1983
[5] S P Silva M A Sabino E M Fernandas V M Correlo LF Boesel and R L Reis ldquoCork properties capabilities andapplicationsrdquo International Materials Reviews vol 50 no 6 pp345ndash365 2005
[6] H Pereira M E Rosa andM A Fortes ldquoThe cellular structureof cork from Quercus suber Lrdquo IAWA Bulletin vol 8 pp 213ndash218 1987
[7] M Emilia Rosa and M A Fortes ldquoStress relaxation and creepof corkrdquo Journal of Materials Science vol 23 no 1 pp 35ndash421988
[8] J F Mano ldquoCreep-recovery behaviour of corkrdquo MaterialsLetters vol 61 no 11-12 pp 2473ndash2477 2007
[9] N Cordeiro NM Belgacem A Gandini and C P Neto ldquoCorksuberin as a new source of chemicals 2 Crystallinity thermaland rheological propertiesrdquo Bioresource Technology vol 63 no2 pp 153ndash158 1998
[10] C V Calahorro A B Garcia C P Barrera M J B Garcia andM G Corzo ldquoCation exchangers prepared from cork wastesrdquoBioresource Technology vol 44 no 3 pp 229ndash233 1993
[11] A SMestre J Pires JM FNogueira J B Parra A P Carvalhoand C O Ania ldquoWaste-derived activated carbons for removalof ibuprofen from solution role of surface chemistry and porestructurerdquo Bioresource Technology vol 100 no 5 pp 1720ndash17262009
[12] M Wilsea K L Johnson and M F Ashby ldquoIndentation offoamed plasticsrdquo International Journal of Mechanical Sciencesvol 17 no 7 pp 457ndash460 1975
[13] B Lawn and R Wilshaw ldquoIndentation fracture principles andapplicationsrdquo Journal of Materials Science vol 10 no 6 pp1049ndash1081 1975
[14] B R Lawn N P Padture H Cai and F Guiberteau ldquoMakingceramics lsquoDuctilersquordquo Science vol 263 no 5150 pp 1114ndash1116 1994
[15] Y G Jung I M Peterson A Pajares and B R Lawn ldquoContactdamage resistance and strength degradation of glass-infiltratedalumina and spinel ceramicsrdquo Journal of Dental Research vol78 no 3 pp 804ndash814 1999
[16] M A Fortes and M E Rosa ldquoDensidade da cortica factoresque a influenciamrdquo Cortica vol 593 pp 65ndash69 1988
[17] J V Natividade Subericultura Direccao Geral dos ServicosFlorestais e Aquicolas Lisbon Portugal 1990
[18] F Ben Abdallah R Ben Cheikh M Baklouti Z Denchevand A M Cunha ldquoCharacterization of composite materialsbased on PP-cork blendsrdquo Journal of Reinforced Plastics andComposites vol 25 no 14 pp 1499ndash1506 2006
[19] P J Antunes G R Dias A T Coelho F Rebelo and T PereiraldquoHyperelastic modelling of cork-polyurethane gel compositesnon-linear FEA implementation in 3D foot modelrdquo MaterialsScience Forum vol 587-588 pp 700ndash705 2008
[20] J F Mano ldquoThe viscoelastic properties of corkrdquo Journal ofMaterials Science vol 37 no 2 pp 257ndash263 2002
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
ISRNMaterials Science 3
Sphere
Specimen
P
r
2a
p
105Y
0
Hertz
ar
Figure 2 Diagram of a Hertzian indentation test (left) and typical indentation stress-strain curve (right) 119884 is the yield stress of an uniaxialtest
estimate the value of 119864 from the linear segment of theexperimental indentation stress-strain curve According tothe Tresca-Guest criterion which is reasonably acceptable forductilematerials subjected to situations of high shear stressesinelastic deformation begins at a point of the solid where themaximum tangential stress reaches the value
120591119898=119884
2(4)
with 119884 being the yield stressIn Hertzian elastic contact theory the shear stress is
maximum on the load axis at a depth of 05119886 with this valuebeing approximately
120591119898= 047119901
0 (5)
Thus from (4) one finally has
119901119884= 105119884 (6)
222 Brinell Indentation Tests In a Brinell test hardenedsteel spheres are used to produce an imprint on the surface ofthe tested material The diameter of the imprint is measuredand the hardness (HBN) of the material is calculated fromthe relationship between the applied load and the area of theimprint The area can be substituted by the diameter of theimprint with the final expression being
HBN = 119875
(1205872)119863 [119863 minus radic(1198632 minus 1198892)]
(7)
where 119875 is the applied load 119863 is the diameter of the balland 119889 is the diameter of the imprint For the results fromdifferent materials to be comparable the applied loads mustbe proportional to the square of the diameter of the ball usedfor the imprints that is
119875 = 1198701198632 (8)
The coefficient 119870 depends on the type of material to testbeing greater for hardmaterials and smaller for softmaterials
50
40
30
20
10
0
00 05 10 15 20 25
Load
(N)
Depth (mm)
TETBTR or RD
Figure 3 Load-unload curves corresponding to the differentsections of a cork specimen
A further constraint that must be satisfied to avoid bendingin the specimen is that the diameter of the imprint must bewithin the range 1198634 lt 119889 lt 1198632 that is approximately 119889 =0375119863
3 Results and Discussion
31 Load-Displacement Curves Figure 3 shows the experi-mental load-displacement curves corresponding to differentsectionsThese curves were obtained from each of the inden-tations carried out to determine the Hertzian indentationstress-strain curve One observes that the tangential sectioncorresponding to the ldquobackrdquo of the sample (TE) is weakerthan that corresponding to the ldquobellyrdquo (TB) In addition theTE section is weaker than the transverse (TR) and radial(RD) sections although the differences depend on the depth
4 ISRNMaterials Science
TE section
TR or RD section
(a)
TR or RD section
TE section
(b)
Figure 4 SEM micrographs of the TE and the TR or RD sections (a) before and (b) after applying a contact load of 100N
at which the tests were performed with respect to the TEsection
In general a large deformation of the specimen atrelatively low loads is observed In the unloading curveone observes an initial slow recovery which increases as theload is released although the energy stored by the sampleis still relatively high at total unloading Above a certainvalue (between 6 and 10N) the load curve deviates from amore or less linear behaviour showing increasing resistanceto deformationThis change is probably due to the progressivecollapse of cells with increasing contact pressure
The same effect can be seen in Figure 4 which shows thecontact deformations of different sections of the same samplewith a load of 100N There is a notable difference betweenthe deformation and damage in the back tangential section(TE) and the other sections The TE section shows greaterrecovery and less damage after applying the contact stress asis clearly observable in the figure in the cell walls This factis in accordance with other results previously reported in theliterature [18]
32 Indentation Stress-Strain Curves Figure 5 shows theexperimental indentation stress-strain curves correspondingto the different sections of a given sample The resultsshow a first linear zone up to a certain value of 119901
0that
depends on the section being tested While the results for thesections TR RD and TB are practically indistinguishable thecurve corresponding to the TE section is clearly unlike therest reflecting the relative weakness of this section Youngrsquosmoduli calculated from the linear zone of the curves arelisted in Table 1 The means of these elastic constants forthe samples tested are 72 plusmn 05MPa for the TE section and18plusmn10MPa for the rest of the sectionsThis elastic anisotropy
14
12
10
08
06
04
02
00
08060402
TE
HB (TB TR RD)
HB (TE)
TBTR or RD
00
p0
(MPa
)
ar
Figure 5 Indentation stress-strain curves for the TE TB TR or RDsections of a cork sample
is a reflection of the anisotropic structure of cork itselfTheTEsection is weaker than TB because it is the outer tangentiallayer Indeed the innermost cells are formed after the moreexternal cells and as they grow therefore have to push thelatter outwards This results in relatively greater density ofthe cell walls in the TB section and consequently a greaterresistance to elastic deformation This kind of behavior hasbeen reported by other researchers [19 20] that have studiedthe hyperelastic or viscoelastic behavior of cork and itsderivatives
ISRNMaterials Science 5
Table 1 Youngrsquos modulus (119864) yield stress (119884) and Brinell hardness (HBN) of the different sections of some cork samples
Sample 119864 (MPa) 119884 (MPa) HBN (MPa)TE TB RD and TR TE TB RD and TR TE TB RD and TR
M1 70 plusmn 05 180 plusmn 10 050 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M2 66 plusmn 05 178 plusmn 10 050 plusmn 005 078 plusmn 005 085 plusmn 005 120 plusmn 005
M3 82 plusmn 05 184 plusmn 10 052 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M4 72 plusmn 05 180 plusmn 10 052 plusmn 005 080 plusmn 005 082 plusmn 005 120 plusmn 005
33 Determination of the Yield Stress The yield stress ofthe cork samples was obtained from the critical indentationstress for the onset of plastic deformation (6) The estimationof 119901119884from the point at which the indentation stress-strain
curve deviates from linearity involves a noticeable difficultyWe therefore used a method which consists of determining119901119884from the load 119875
119884 corresponding to the first detectable
residual impression in the test surface and the correspondingcontact radius 119886 To this end a series of tests were conductedusing an indenter of known radius (119903 = 555mm) andincreasing loads and then observing the specimen under anoptical microscope to identify the smallest load at which aresidual impression was observableThe values calculated forthe yield stress in the various sections and samples are alsogiven in Table 1Themean values are 05plusmn005MPa for the TEsections and 08 plusmn 005MPa for the rest (TB RD and TR) Itshould be mentioned that the value obtained by this methodis actually a slightly high estimate of the yield stress as themeasured value of 119875
119884may be in excess Indeed for loads
somewhat smaller than 119875119884 plastic deformation may occur
but be located in such a small region that it does not give riseto a residual impression detectable under optical microscopy
34 Determination of the Brinell Hardness Index To deter-mine the Brinell hardness index one must first fix the testingconstant 119870 to use depending on the nature of the materialunder test As reference some of the usual constants used toevaluate hardness in industrialmaterials are119870 = 30 for steels119870 = 10 for Cu brasses and bronzes 119870 = 5 for light alloys119870 = 25 for Sn and Pb and so forth Values of 125 and 05are also used for very soft metals or alloys In the case of amaterial such as cork it is necessary to gradually decreasethe value of the constant until the relationship between theapplied force and the diameter of the sphere used producesan imprint that is within the limits established for the Brinelltest that is 1198634 lt 119889 lt 1198632 In this way we determined thevalue used to be119870 = 156sdot10minus2The results using this constantare listed in Table 1 One observes that the Brinell hardnessin the TE sections varies around 082 plusmn 005MPa and around115plusmn005MPa in the other sections (TB RD and TR)Thesevalues are included in the plots of Figure 5 One sees that theyare reasonably consistent with the Hertzian indentation testsIndeed Hertzian indentation stress surpassed the Brinellhardness values of each section because there is no constraintin the Hertzian tests on the diameters of the imprints whichcan even exceed the upper bound of the Brinell imprints thatis 119889 gt 1198632
4 Conclusions
In this study Hertzian and Brinell indentation tests wereused to evaluate the mechanical properties of corkThe mainconclusions and implications to be drawn from the study areas follows
(i) The methodological approach is well suited to thecharacterization of the elastic and plastic behaviourof soft materials of diverse nature (cellular materials)
(ii) The observed elastic anisotropy is closely related tothe anisotropic structure of the different sections ofcorkThe ldquobackrdquo tangential section is weaker than theother sections of the material
(iii) The specimens presented large deformations at rela-tively low stresses consistent with the low value of thematerialrsquos elastic modulus The materialrsquos elastic zonewas relatively small (low values of the yield stress) butwell defined
(iv) Finally we wish to note that Hertzian and Brinellindentation testing would seem to be well suited notonly to characterize mechanically such materials ascork but also to evaluate the intrinsic quality of thesematerials
Conflict of Interests
The authors declare that they have no affiliations with orinvolvement in any organization or entity with any financialinterest (such as honoraria educational grants participationin speakers bureaus membership employment consultan-cies stock ownership or other equity interests and experttestimony or patent-licensing arrangements) or nonfinan-cial interest (such as personal or professional relationshipsaffiliations knowledge or beliefs) in the subject of matter ormaterials discussed in this paper Consequently the authorsdeclare that there is no conflict of interests regarding thepublication of this paper
Acknowledgments
The authors thank Dr A Pajares and Dr A L Ortiz forthe fruitful discussion and the Institute for the Promotionof Cork Wood and Coal (ICMC) Junta de Extremadura(Spain) for supplying the cork samples
6 ISRNMaterials Science
References
[1] Y Takahashi D Okumura and N Ohno ldquoYield and bucklingbehavior of Kelvin open-cell foams subjected to uniaxial com-pressionrdquo International Journal of Mechanical Sciences vol 52no 2 pp 377ndash385 2010
[2] C P Gameiro and J Cirne ldquoDynamic axial crushing of shortto long circular aluminium tubes with agglomerate cork fillerrdquoInternational Journal of Mechanical Sciences vol 49 no 9 pp1029ndash1037 2007
[3] S Banerjee and A Bhaskar ldquoThe applicability of the effectivemedium theory to the dynamics of cellular beamsrdquo Interna-tional Journal of Mechanical Sciences vol 51 no 8 pp 598ndash6082009
[4] M F Ashby ldquoThe mechanical properties of cellular solidsrdquoMetallurgical and Materials Transactions A vol 14 no 9 pp1755ndash1769 1983
[5] S P Silva M A Sabino E M Fernandas V M Correlo LF Boesel and R L Reis ldquoCork properties capabilities andapplicationsrdquo International Materials Reviews vol 50 no 6 pp345ndash365 2005
[6] H Pereira M E Rosa andM A Fortes ldquoThe cellular structureof cork from Quercus suber Lrdquo IAWA Bulletin vol 8 pp 213ndash218 1987
[7] M Emilia Rosa and M A Fortes ldquoStress relaxation and creepof corkrdquo Journal of Materials Science vol 23 no 1 pp 35ndash421988
[8] J F Mano ldquoCreep-recovery behaviour of corkrdquo MaterialsLetters vol 61 no 11-12 pp 2473ndash2477 2007
[9] N Cordeiro NM Belgacem A Gandini and C P Neto ldquoCorksuberin as a new source of chemicals 2 Crystallinity thermaland rheological propertiesrdquo Bioresource Technology vol 63 no2 pp 153ndash158 1998
[10] C V Calahorro A B Garcia C P Barrera M J B Garcia andM G Corzo ldquoCation exchangers prepared from cork wastesrdquoBioresource Technology vol 44 no 3 pp 229ndash233 1993
[11] A SMestre J Pires JM FNogueira J B Parra A P Carvalhoand C O Ania ldquoWaste-derived activated carbons for removalof ibuprofen from solution role of surface chemistry and porestructurerdquo Bioresource Technology vol 100 no 5 pp 1720ndash17262009
[12] M Wilsea K L Johnson and M F Ashby ldquoIndentation offoamed plasticsrdquo International Journal of Mechanical Sciencesvol 17 no 7 pp 457ndash460 1975
[13] B Lawn and R Wilshaw ldquoIndentation fracture principles andapplicationsrdquo Journal of Materials Science vol 10 no 6 pp1049ndash1081 1975
[14] B R Lawn N P Padture H Cai and F Guiberteau ldquoMakingceramics lsquoDuctilersquordquo Science vol 263 no 5150 pp 1114ndash1116 1994
[15] Y G Jung I M Peterson A Pajares and B R Lawn ldquoContactdamage resistance and strength degradation of glass-infiltratedalumina and spinel ceramicsrdquo Journal of Dental Research vol78 no 3 pp 804ndash814 1999
[16] M A Fortes and M E Rosa ldquoDensidade da cortica factoresque a influenciamrdquo Cortica vol 593 pp 65ndash69 1988
[17] J V Natividade Subericultura Direccao Geral dos ServicosFlorestais e Aquicolas Lisbon Portugal 1990
[18] F Ben Abdallah R Ben Cheikh M Baklouti Z Denchevand A M Cunha ldquoCharacterization of composite materialsbased on PP-cork blendsrdquo Journal of Reinforced Plastics andComposites vol 25 no 14 pp 1499ndash1506 2006
[19] P J Antunes G R Dias A T Coelho F Rebelo and T PereiraldquoHyperelastic modelling of cork-polyurethane gel compositesnon-linear FEA implementation in 3D foot modelrdquo MaterialsScience Forum vol 587-588 pp 700ndash705 2008
[20] J F Mano ldquoThe viscoelastic properties of corkrdquo Journal ofMaterials Science vol 37 no 2 pp 257ndash263 2002
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
4 ISRNMaterials Science
TE section
TR or RD section
(a)
TR or RD section
TE section
(b)
Figure 4 SEM micrographs of the TE and the TR or RD sections (a) before and (b) after applying a contact load of 100N
at which the tests were performed with respect to the TEsection
In general a large deformation of the specimen atrelatively low loads is observed In the unloading curveone observes an initial slow recovery which increases as theload is released although the energy stored by the sampleis still relatively high at total unloading Above a certainvalue (between 6 and 10N) the load curve deviates from amore or less linear behaviour showing increasing resistanceto deformationThis change is probably due to the progressivecollapse of cells with increasing contact pressure
The same effect can be seen in Figure 4 which shows thecontact deformations of different sections of the same samplewith a load of 100N There is a notable difference betweenthe deformation and damage in the back tangential section(TE) and the other sections The TE section shows greaterrecovery and less damage after applying the contact stress asis clearly observable in the figure in the cell walls This factis in accordance with other results previously reported in theliterature [18]
32 Indentation Stress-Strain Curves Figure 5 shows theexperimental indentation stress-strain curves correspondingto the different sections of a given sample The resultsshow a first linear zone up to a certain value of 119901
0that
depends on the section being tested While the results for thesections TR RD and TB are practically indistinguishable thecurve corresponding to the TE section is clearly unlike therest reflecting the relative weakness of this section Youngrsquosmoduli calculated from the linear zone of the curves arelisted in Table 1 The means of these elastic constants forthe samples tested are 72 plusmn 05MPa for the TE section and18plusmn10MPa for the rest of the sectionsThis elastic anisotropy
14
12
10
08
06
04
02
00
08060402
TE
HB (TB TR RD)
HB (TE)
TBTR or RD
00
p0
(MPa
)
ar
Figure 5 Indentation stress-strain curves for the TE TB TR or RDsections of a cork sample
is a reflection of the anisotropic structure of cork itselfTheTEsection is weaker than TB because it is the outer tangentiallayer Indeed the innermost cells are formed after the moreexternal cells and as they grow therefore have to push thelatter outwards This results in relatively greater density ofthe cell walls in the TB section and consequently a greaterresistance to elastic deformation This kind of behavior hasbeen reported by other researchers [19 20] that have studiedthe hyperelastic or viscoelastic behavior of cork and itsderivatives
ISRNMaterials Science 5
Table 1 Youngrsquos modulus (119864) yield stress (119884) and Brinell hardness (HBN) of the different sections of some cork samples
Sample 119864 (MPa) 119884 (MPa) HBN (MPa)TE TB RD and TR TE TB RD and TR TE TB RD and TR
M1 70 plusmn 05 180 plusmn 10 050 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M2 66 plusmn 05 178 plusmn 10 050 plusmn 005 078 plusmn 005 085 plusmn 005 120 plusmn 005
M3 82 plusmn 05 184 plusmn 10 052 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M4 72 plusmn 05 180 plusmn 10 052 plusmn 005 080 plusmn 005 082 plusmn 005 120 plusmn 005
33 Determination of the Yield Stress The yield stress ofthe cork samples was obtained from the critical indentationstress for the onset of plastic deformation (6) The estimationof 119901119884from the point at which the indentation stress-strain
curve deviates from linearity involves a noticeable difficultyWe therefore used a method which consists of determining119901119884from the load 119875
119884 corresponding to the first detectable
residual impression in the test surface and the correspondingcontact radius 119886 To this end a series of tests were conductedusing an indenter of known radius (119903 = 555mm) andincreasing loads and then observing the specimen under anoptical microscope to identify the smallest load at which aresidual impression was observableThe values calculated forthe yield stress in the various sections and samples are alsogiven in Table 1Themean values are 05plusmn005MPa for the TEsections and 08 plusmn 005MPa for the rest (TB RD and TR) Itshould be mentioned that the value obtained by this methodis actually a slightly high estimate of the yield stress as themeasured value of 119875
119884may be in excess Indeed for loads
somewhat smaller than 119875119884 plastic deformation may occur
but be located in such a small region that it does not give riseto a residual impression detectable under optical microscopy
34 Determination of the Brinell Hardness Index To deter-mine the Brinell hardness index one must first fix the testingconstant 119870 to use depending on the nature of the materialunder test As reference some of the usual constants used toevaluate hardness in industrialmaterials are119870 = 30 for steels119870 = 10 for Cu brasses and bronzes 119870 = 5 for light alloys119870 = 25 for Sn and Pb and so forth Values of 125 and 05are also used for very soft metals or alloys In the case of amaterial such as cork it is necessary to gradually decreasethe value of the constant until the relationship between theapplied force and the diameter of the sphere used producesan imprint that is within the limits established for the Brinelltest that is 1198634 lt 119889 lt 1198632 In this way we determined thevalue used to be119870 = 156sdot10minus2The results using this constantare listed in Table 1 One observes that the Brinell hardnessin the TE sections varies around 082 plusmn 005MPa and around115plusmn005MPa in the other sections (TB RD and TR)Thesevalues are included in the plots of Figure 5 One sees that theyare reasonably consistent with the Hertzian indentation testsIndeed Hertzian indentation stress surpassed the Brinellhardness values of each section because there is no constraintin the Hertzian tests on the diameters of the imprints whichcan even exceed the upper bound of the Brinell imprints thatis 119889 gt 1198632
4 Conclusions
In this study Hertzian and Brinell indentation tests wereused to evaluate the mechanical properties of corkThe mainconclusions and implications to be drawn from the study areas follows
(i) The methodological approach is well suited to thecharacterization of the elastic and plastic behaviourof soft materials of diverse nature (cellular materials)
(ii) The observed elastic anisotropy is closely related tothe anisotropic structure of the different sections ofcorkThe ldquobackrdquo tangential section is weaker than theother sections of the material
(iii) The specimens presented large deformations at rela-tively low stresses consistent with the low value of thematerialrsquos elastic modulus The materialrsquos elastic zonewas relatively small (low values of the yield stress) butwell defined
(iv) Finally we wish to note that Hertzian and Brinellindentation testing would seem to be well suited notonly to characterize mechanically such materials ascork but also to evaluate the intrinsic quality of thesematerials
Conflict of Interests
The authors declare that they have no affiliations with orinvolvement in any organization or entity with any financialinterest (such as honoraria educational grants participationin speakers bureaus membership employment consultan-cies stock ownership or other equity interests and experttestimony or patent-licensing arrangements) or nonfinan-cial interest (such as personal or professional relationshipsaffiliations knowledge or beliefs) in the subject of matter ormaterials discussed in this paper Consequently the authorsdeclare that there is no conflict of interests regarding thepublication of this paper
Acknowledgments
The authors thank Dr A Pajares and Dr A L Ortiz forthe fruitful discussion and the Institute for the Promotionof Cork Wood and Coal (ICMC) Junta de Extremadura(Spain) for supplying the cork samples
6 ISRNMaterials Science
References
[1] Y Takahashi D Okumura and N Ohno ldquoYield and bucklingbehavior of Kelvin open-cell foams subjected to uniaxial com-pressionrdquo International Journal of Mechanical Sciences vol 52no 2 pp 377ndash385 2010
[2] C P Gameiro and J Cirne ldquoDynamic axial crushing of shortto long circular aluminium tubes with agglomerate cork fillerrdquoInternational Journal of Mechanical Sciences vol 49 no 9 pp1029ndash1037 2007
[3] S Banerjee and A Bhaskar ldquoThe applicability of the effectivemedium theory to the dynamics of cellular beamsrdquo Interna-tional Journal of Mechanical Sciences vol 51 no 8 pp 598ndash6082009
[4] M F Ashby ldquoThe mechanical properties of cellular solidsrdquoMetallurgical and Materials Transactions A vol 14 no 9 pp1755ndash1769 1983
[5] S P Silva M A Sabino E M Fernandas V M Correlo LF Boesel and R L Reis ldquoCork properties capabilities andapplicationsrdquo International Materials Reviews vol 50 no 6 pp345ndash365 2005
[6] H Pereira M E Rosa andM A Fortes ldquoThe cellular structureof cork from Quercus suber Lrdquo IAWA Bulletin vol 8 pp 213ndash218 1987
[7] M Emilia Rosa and M A Fortes ldquoStress relaxation and creepof corkrdquo Journal of Materials Science vol 23 no 1 pp 35ndash421988
[8] J F Mano ldquoCreep-recovery behaviour of corkrdquo MaterialsLetters vol 61 no 11-12 pp 2473ndash2477 2007
[9] N Cordeiro NM Belgacem A Gandini and C P Neto ldquoCorksuberin as a new source of chemicals 2 Crystallinity thermaland rheological propertiesrdquo Bioresource Technology vol 63 no2 pp 153ndash158 1998
[10] C V Calahorro A B Garcia C P Barrera M J B Garcia andM G Corzo ldquoCation exchangers prepared from cork wastesrdquoBioresource Technology vol 44 no 3 pp 229ndash233 1993
[11] A SMestre J Pires JM FNogueira J B Parra A P Carvalhoand C O Ania ldquoWaste-derived activated carbons for removalof ibuprofen from solution role of surface chemistry and porestructurerdquo Bioresource Technology vol 100 no 5 pp 1720ndash17262009
[12] M Wilsea K L Johnson and M F Ashby ldquoIndentation offoamed plasticsrdquo International Journal of Mechanical Sciencesvol 17 no 7 pp 457ndash460 1975
[13] B Lawn and R Wilshaw ldquoIndentation fracture principles andapplicationsrdquo Journal of Materials Science vol 10 no 6 pp1049ndash1081 1975
[14] B R Lawn N P Padture H Cai and F Guiberteau ldquoMakingceramics lsquoDuctilersquordquo Science vol 263 no 5150 pp 1114ndash1116 1994
[15] Y G Jung I M Peterson A Pajares and B R Lawn ldquoContactdamage resistance and strength degradation of glass-infiltratedalumina and spinel ceramicsrdquo Journal of Dental Research vol78 no 3 pp 804ndash814 1999
[16] M A Fortes and M E Rosa ldquoDensidade da cortica factoresque a influenciamrdquo Cortica vol 593 pp 65ndash69 1988
[17] J V Natividade Subericultura Direccao Geral dos ServicosFlorestais e Aquicolas Lisbon Portugal 1990
[18] F Ben Abdallah R Ben Cheikh M Baklouti Z Denchevand A M Cunha ldquoCharacterization of composite materialsbased on PP-cork blendsrdquo Journal of Reinforced Plastics andComposites vol 25 no 14 pp 1499ndash1506 2006
[19] P J Antunes G R Dias A T Coelho F Rebelo and T PereiraldquoHyperelastic modelling of cork-polyurethane gel compositesnon-linear FEA implementation in 3D foot modelrdquo MaterialsScience Forum vol 587-588 pp 700ndash705 2008
[20] J F Mano ldquoThe viscoelastic properties of corkrdquo Journal ofMaterials Science vol 37 no 2 pp 257ndash263 2002
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
ISRNMaterials Science 5
Table 1 Youngrsquos modulus (119864) yield stress (119884) and Brinell hardness (HBN) of the different sections of some cork samples
Sample 119864 (MPa) 119884 (MPa) HBN (MPa)TE TB RD and TR TE TB RD and TR TE TB RD and TR
M1 70 plusmn 05 180 plusmn 10 050 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M2 66 plusmn 05 178 plusmn 10 050 plusmn 005 078 plusmn 005 085 plusmn 005 120 plusmn 005
M3 82 plusmn 05 184 plusmn 10 052 plusmn 005 080 plusmn 005 080 plusmn 005 110 plusmn 005
M4 72 plusmn 05 180 plusmn 10 052 plusmn 005 080 plusmn 005 082 plusmn 005 120 plusmn 005
33 Determination of the Yield Stress The yield stress ofthe cork samples was obtained from the critical indentationstress for the onset of plastic deformation (6) The estimationof 119901119884from the point at which the indentation stress-strain
curve deviates from linearity involves a noticeable difficultyWe therefore used a method which consists of determining119901119884from the load 119875
119884 corresponding to the first detectable
residual impression in the test surface and the correspondingcontact radius 119886 To this end a series of tests were conductedusing an indenter of known radius (119903 = 555mm) andincreasing loads and then observing the specimen under anoptical microscope to identify the smallest load at which aresidual impression was observableThe values calculated forthe yield stress in the various sections and samples are alsogiven in Table 1Themean values are 05plusmn005MPa for the TEsections and 08 plusmn 005MPa for the rest (TB RD and TR) Itshould be mentioned that the value obtained by this methodis actually a slightly high estimate of the yield stress as themeasured value of 119875
119884may be in excess Indeed for loads
somewhat smaller than 119875119884 plastic deformation may occur
but be located in such a small region that it does not give riseto a residual impression detectable under optical microscopy
34 Determination of the Brinell Hardness Index To deter-mine the Brinell hardness index one must first fix the testingconstant 119870 to use depending on the nature of the materialunder test As reference some of the usual constants used toevaluate hardness in industrialmaterials are119870 = 30 for steels119870 = 10 for Cu brasses and bronzes 119870 = 5 for light alloys119870 = 25 for Sn and Pb and so forth Values of 125 and 05are also used for very soft metals or alloys In the case of amaterial such as cork it is necessary to gradually decreasethe value of the constant until the relationship between theapplied force and the diameter of the sphere used producesan imprint that is within the limits established for the Brinelltest that is 1198634 lt 119889 lt 1198632 In this way we determined thevalue used to be119870 = 156sdot10minus2The results using this constantare listed in Table 1 One observes that the Brinell hardnessin the TE sections varies around 082 plusmn 005MPa and around115plusmn005MPa in the other sections (TB RD and TR)Thesevalues are included in the plots of Figure 5 One sees that theyare reasonably consistent with the Hertzian indentation testsIndeed Hertzian indentation stress surpassed the Brinellhardness values of each section because there is no constraintin the Hertzian tests on the diameters of the imprints whichcan even exceed the upper bound of the Brinell imprints thatis 119889 gt 1198632
4 Conclusions
In this study Hertzian and Brinell indentation tests wereused to evaluate the mechanical properties of corkThe mainconclusions and implications to be drawn from the study areas follows
(i) The methodological approach is well suited to thecharacterization of the elastic and plastic behaviourof soft materials of diverse nature (cellular materials)
(ii) The observed elastic anisotropy is closely related tothe anisotropic structure of the different sections ofcorkThe ldquobackrdquo tangential section is weaker than theother sections of the material
(iii) The specimens presented large deformations at rela-tively low stresses consistent with the low value of thematerialrsquos elastic modulus The materialrsquos elastic zonewas relatively small (low values of the yield stress) butwell defined
(iv) Finally we wish to note that Hertzian and Brinellindentation testing would seem to be well suited notonly to characterize mechanically such materials ascork but also to evaluate the intrinsic quality of thesematerials
Conflict of Interests
The authors declare that they have no affiliations with orinvolvement in any organization or entity with any financialinterest (such as honoraria educational grants participationin speakers bureaus membership employment consultan-cies stock ownership or other equity interests and experttestimony or patent-licensing arrangements) or nonfinan-cial interest (such as personal or professional relationshipsaffiliations knowledge or beliefs) in the subject of matter ormaterials discussed in this paper Consequently the authorsdeclare that there is no conflict of interests regarding thepublication of this paper
Acknowledgments
The authors thank Dr A Pajares and Dr A L Ortiz forthe fruitful discussion and the Institute for the Promotionof Cork Wood and Coal (ICMC) Junta de Extremadura(Spain) for supplying the cork samples
6 ISRNMaterials Science
References
[1] Y Takahashi D Okumura and N Ohno ldquoYield and bucklingbehavior of Kelvin open-cell foams subjected to uniaxial com-pressionrdquo International Journal of Mechanical Sciences vol 52no 2 pp 377ndash385 2010
[2] C P Gameiro and J Cirne ldquoDynamic axial crushing of shortto long circular aluminium tubes with agglomerate cork fillerrdquoInternational Journal of Mechanical Sciences vol 49 no 9 pp1029ndash1037 2007
[3] S Banerjee and A Bhaskar ldquoThe applicability of the effectivemedium theory to the dynamics of cellular beamsrdquo Interna-tional Journal of Mechanical Sciences vol 51 no 8 pp 598ndash6082009
[4] M F Ashby ldquoThe mechanical properties of cellular solidsrdquoMetallurgical and Materials Transactions A vol 14 no 9 pp1755ndash1769 1983
[5] S P Silva M A Sabino E M Fernandas V M Correlo LF Boesel and R L Reis ldquoCork properties capabilities andapplicationsrdquo International Materials Reviews vol 50 no 6 pp345ndash365 2005
[6] H Pereira M E Rosa andM A Fortes ldquoThe cellular structureof cork from Quercus suber Lrdquo IAWA Bulletin vol 8 pp 213ndash218 1987
[7] M Emilia Rosa and M A Fortes ldquoStress relaxation and creepof corkrdquo Journal of Materials Science vol 23 no 1 pp 35ndash421988
[8] J F Mano ldquoCreep-recovery behaviour of corkrdquo MaterialsLetters vol 61 no 11-12 pp 2473ndash2477 2007
[9] N Cordeiro NM Belgacem A Gandini and C P Neto ldquoCorksuberin as a new source of chemicals 2 Crystallinity thermaland rheological propertiesrdquo Bioresource Technology vol 63 no2 pp 153ndash158 1998
[10] C V Calahorro A B Garcia C P Barrera M J B Garcia andM G Corzo ldquoCation exchangers prepared from cork wastesrdquoBioresource Technology vol 44 no 3 pp 229ndash233 1993
[11] A SMestre J Pires JM FNogueira J B Parra A P Carvalhoand C O Ania ldquoWaste-derived activated carbons for removalof ibuprofen from solution role of surface chemistry and porestructurerdquo Bioresource Technology vol 100 no 5 pp 1720ndash17262009
[12] M Wilsea K L Johnson and M F Ashby ldquoIndentation offoamed plasticsrdquo International Journal of Mechanical Sciencesvol 17 no 7 pp 457ndash460 1975
[13] B Lawn and R Wilshaw ldquoIndentation fracture principles andapplicationsrdquo Journal of Materials Science vol 10 no 6 pp1049ndash1081 1975
[14] B R Lawn N P Padture H Cai and F Guiberteau ldquoMakingceramics lsquoDuctilersquordquo Science vol 263 no 5150 pp 1114ndash1116 1994
[15] Y G Jung I M Peterson A Pajares and B R Lawn ldquoContactdamage resistance and strength degradation of glass-infiltratedalumina and spinel ceramicsrdquo Journal of Dental Research vol78 no 3 pp 804ndash814 1999
[16] M A Fortes and M E Rosa ldquoDensidade da cortica factoresque a influenciamrdquo Cortica vol 593 pp 65ndash69 1988
[17] J V Natividade Subericultura Direccao Geral dos ServicosFlorestais e Aquicolas Lisbon Portugal 1990
[18] F Ben Abdallah R Ben Cheikh M Baklouti Z Denchevand A M Cunha ldquoCharacterization of composite materialsbased on PP-cork blendsrdquo Journal of Reinforced Plastics andComposites vol 25 no 14 pp 1499ndash1506 2006
[19] P J Antunes G R Dias A T Coelho F Rebelo and T PereiraldquoHyperelastic modelling of cork-polyurethane gel compositesnon-linear FEA implementation in 3D foot modelrdquo MaterialsScience Forum vol 587-588 pp 700ndash705 2008
[20] J F Mano ldquoThe viscoelastic properties of corkrdquo Journal ofMaterials Science vol 37 no 2 pp 257ndash263 2002
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
6 ISRNMaterials Science
References
[1] Y Takahashi D Okumura and N Ohno ldquoYield and bucklingbehavior of Kelvin open-cell foams subjected to uniaxial com-pressionrdquo International Journal of Mechanical Sciences vol 52no 2 pp 377ndash385 2010
[2] C P Gameiro and J Cirne ldquoDynamic axial crushing of shortto long circular aluminium tubes with agglomerate cork fillerrdquoInternational Journal of Mechanical Sciences vol 49 no 9 pp1029ndash1037 2007
[3] S Banerjee and A Bhaskar ldquoThe applicability of the effectivemedium theory to the dynamics of cellular beamsrdquo Interna-tional Journal of Mechanical Sciences vol 51 no 8 pp 598ndash6082009
[4] M F Ashby ldquoThe mechanical properties of cellular solidsrdquoMetallurgical and Materials Transactions A vol 14 no 9 pp1755ndash1769 1983
[5] S P Silva M A Sabino E M Fernandas V M Correlo LF Boesel and R L Reis ldquoCork properties capabilities andapplicationsrdquo International Materials Reviews vol 50 no 6 pp345ndash365 2005
[6] H Pereira M E Rosa andM A Fortes ldquoThe cellular structureof cork from Quercus suber Lrdquo IAWA Bulletin vol 8 pp 213ndash218 1987
[7] M Emilia Rosa and M A Fortes ldquoStress relaxation and creepof corkrdquo Journal of Materials Science vol 23 no 1 pp 35ndash421988
[8] J F Mano ldquoCreep-recovery behaviour of corkrdquo MaterialsLetters vol 61 no 11-12 pp 2473ndash2477 2007
[9] N Cordeiro NM Belgacem A Gandini and C P Neto ldquoCorksuberin as a new source of chemicals 2 Crystallinity thermaland rheological propertiesrdquo Bioresource Technology vol 63 no2 pp 153ndash158 1998
[10] C V Calahorro A B Garcia C P Barrera M J B Garcia andM G Corzo ldquoCation exchangers prepared from cork wastesrdquoBioresource Technology vol 44 no 3 pp 229ndash233 1993
[11] A SMestre J Pires JM FNogueira J B Parra A P Carvalhoand C O Ania ldquoWaste-derived activated carbons for removalof ibuprofen from solution role of surface chemistry and porestructurerdquo Bioresource Technology vol 100 no 5 pp 1720ndash17262009
[12] M Wilsea K L Johnson and M F Ashby ldquoIndentation offoamed plasticsrdquo International Journal of Mechanical Sciencesvol 17 no 7 pp 457ndash460 1975
[13] B Lawn and R Wilshaw ldquoIndentation fracture principles andapplicationsrdquo Journal of Materials Science vol 10 no 6 pp1049ndash1081 1975
[14] B R Lawn N P Padture H Cai and F Guiberteau ldquoMakingceramics lsquoDuctilersquordquo Science vol 263 no 5150 pp 1114ndash1116 1994
[15] Y G Jung I M Peterson A Pajares and B R Lawn ldquoContactdamage resistance and strength degradation of glass-infiltratedalumina and spinel ceramicsrdquo Journal of Dental Research vol78 no 3 pp 804ndash814 1999
[16] M A Fortes and M E Rosa ldquoDensidade da cortica factoresque a influenciamrdquo Cortica vol 593 pp 65ndash69 1988
[17] J V Natividade Subericultura Direccao Geral dos ServicosFlorestais e Aquicolas Lisbon Portugal 1990
[18] F Ben Abdallah R Ben Cheikh M Baklouti Z Denchevand A M Cunha ldquoCharacterization of composite materialsbased on PP-cork blendsrdquo Journal of Reinforced Plastics andComposites vol 25 no 14 pp 1499ndash1506 2006
[19] P J Antunes G R Dias A T Coelho F Rebelo and T PereiraldquoHyperelastic modelling of cork-polyurethane gel compositesnon-linear FEA implementation in 3D foot modelrdquo MaterialsScience Forum vol 587-588 pp 700ndash705 2008
[20] J F Mano ldquoThe viscoelastic properties of corkrdquo Journal ofMaterials Science vol 37 no 2 pp 257ndash263 2002
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
Submit your manuscripts athttpwwwhindawicom
ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CorrosionInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Polymer ScienceInternational Journal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CeramicsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CompositesJournal of
NanoparticlesJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
International Journal of
Biomaterials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
NanoscienceJournal of
TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Journal of
NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal of
CrystallographyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
CoatingsJournal of
Advances in
Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Smart Materials Research
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
MetallurgyJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
BioMed Research International
MaterialsJournal of
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Nano
materials
Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014
Journal ofNanomaterials
