Cork is a material that has many characteristics, for instance, light weight, elasticity, insulation against heat, impermeability for liquid, and so forth. There are two types of cork, the natural and the agglomerated corks. In the present paper, compression tests of the natural and the agglomerated cork specimens were carried out. The compression test were done in various directions. Compressive stress was measured by a original compression apparatus, and stress-strain curves were obtained in various directions of the cork specimens. In the natural cork, there are differences between the radial and the non-radial direction. The recovery of dimensions after compression was also studied in respective directions. The structure of the deformed surface was observed by a scanning electron microscope

Abe, Takeji

Sato, Takashi

English

Okayama University Digital Information Repository

Observation of Microscopic Deformation Behavior of Cork

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering, Okayama University, Vo1.37, No.2, pp.5-12, March 2003Observation of Microscopic Deformation Behavior of CorkTakeji ABE and Takashi SATOGraduate School ofNatural Science and Technology, Okayama University,Tsushima-Naka 3-1-1, Okayama, 700-8530, JapanCork. is a material that has many characteristics, for instance, light weight, elasticity,insulation against heat, impermeability for liquid, and so forth. There are two types ofcork, thenatural and the agglomerated corks. In the present paper, compression tests ofthe natural andthe agglomerated cork specimens were carried out. The compression test were done in variousdirections. Compressive stress was measured by a original compression apparatus, andstress-s1rain curves were obtained in various directions of the cork specimens. In the naturalcork, there are differences between the radial and the non-radial direction. The recovery ofdimensions after compression was also studied in respective directions. The structure of thedeformed surface was observed by a scanning electron microscope.1 . IntroductionAlthough the quantity of the production of cork inJapan is reducing, Okayama is one of the mostimportant region for the production of cork. Cork isoriginally planted around the Mediterranean sea andimported into Japan and changed its shape for variouscommercial application. Cork is light and elastichaving good properties such as good impermeability,shielding ability of sound The unique characteristicsof cork is resulted from the microscopic shape andstructure of cork cells. Namely, the structure of corkis composed ofthe small cells and closed voids.The basic s1IUcture of the natural cork is studied byLJ. Gibson and M.P. Ashby [1,2] and M.E. Rosa etaI. [3]. However, little attention has been paid on theagglomerated cork which is widely used for variousengineering products. The agglomerated cork isproduced by dividing the natural cork into smallparticles and combined them together using the bondmaterial.The relation between the mechanical behavior ofthecork and its microstructure does not seem to have beenfully clarified yet. In the present study, the s1IUctureE-mail: t-abe@p04.oninet.ne.jp5and the defonnation behavior ofthe agglomerated corkare investigated and compared with those ofthe naturalcork using the scanning electron microscope and thecompression testing equipment made for the presentpurpose. The strain recovery after defonnation is alsostudied.2. Experimental Procedures2 . 1 SpecimenIn the present study, both the natural cork specimenand the agglomerated cork specimen were preparedThe chemical components ofcork is shown in Table I.The agglomerated cork is composed of the particles ofthe natural cork of the size 4 "-'8mm, and pastedtogether with the bond material of epoxy resin. Thedensity of the bond is about Iglcm3, and the weightpercent is 7wtJ/o of the agglomerated cork. The shapeof the compressive cork specimen used in the presentstudy is cubic with the edge length of 10 rom [4]. Thes1IUcture and the characteristic of cork are anisotropic.The natural cork has three different directions shown inFig. I, that is, the radial, the tangential and the axial6 Takeji ABE and Takashi SATOTable I Chemical components ofcork (wfllo)Suberin 40Cerin,friedelin and wax 5Lignin 27SeUulose 12Tannin 6Glycerin 6Ash (Na, K, Mg) 4direction of the original cork tree. The agglomeratedcork has different characteristi in the compression axisduring the manufacturing process and in theperpendicular direction to the compression axis.The specimen was cut by the micro-<:utting machineor the cutter knife. The average densities of thenatural and the agglomerated cork are 155 kglm3 and353kglm3, respectively.tree axisaxial directiontangential direction m~I~I~~ radial~;::.:.;..~~ l---;'-.,A..~'-..AJ-I..__ sectiontangentials tionradial direFig. 1 Corle tree and microscopic structure ofcork [1]MEM.FAC.ENG.OKA.UNL Vo1.37, 02The deformation characteristic of the bond materiaJis examined separately. The compressive specimen ofthe cylindrical shape of the size 30 mm,p diameterand 30 mm height is made from the epoxy resin andcut by the NC milling machine. The average densityof the bond material used in the present study is 1081kglmJ .2. 2 Compression testThe compression test of the cork specimen wasperformed with the specially made small equipment,which can be used in SEM as shown in Fig. 2. Thestrain t was calculated from the displacement measuredby the micrometer. The conventional strain is usedthroughout the present paper.Th maximwn compressive strain wa<; -0,8 for thenatura1 cork and -0.7 for the agglomcratlXl COli<.respectively. The compression test of the hond materialwas done by the compression testing machine with thmaximum load 980 kN (Tokyo Kouki Co.).Fig. 2 Compression apparatus for cork specimen usedinSEM'The stress during compression test was measured bythe load cell and the amplifier (NEC Sanei Co.) . Thecross section A was measured at every deformationstep of the compressive strain -0.05. The averagedvalue of the ross sections of the hoth surfaces wasadopted. The cork is visco-elastic material and hencethe stress changes with time. In the present study, thestress after 10 minutes after compression was used.The measurement of the recovery of the specimenMarch 2003 Microscopic Deformation of Cork 7-0.3 and -0.7 for one minute. Then, the compressiveload was removed and the recovery ofdimension ofthespecimen was measured in the three directions.3. Experimental Results andDiscussion2. 3 Observation ofsurfaceThe surface observation was done with the naturaltype scanning electron microscope (SEM) S-3500Ntype made by Hitachi Co. The central part of the areaof 3 X 3 mm was observed. As the cork isnon-conducting material, low acceleration voltage of15 kV was adopted for the SEM observation. Thecorresponding current is about 25 J1 A for the naturalcork, while about 47 J1 A for the agglomerated cork.3. 1 Stress-strain relationFig. 3 (a) shows the averaged stress-strainrelation measured in various directions of thenatural cork and the agglomerated cork. Fig.3 (b) shows the relation for the bond material.The stress of the cork increases rapidly whenthe cell collapse with the applied strain.The stress-stress relation of the natural cork isslightly higher in the radial direction than those in theaxial and the tangential directions [2]. The stress of theagglomerated cork is higher in the tangential directionto the compression at the early stage of compression,which may be due to the influence ofthe residual strainduring the manufacturing process.•• Radial• Axial6 Tangential• "• Q••Q• • Q "• • • • l!• • !! D• !! !! l! l!Q !!-0.2 -0.4 -0.6 -0.8Strain Eot:l3.-------------------.Fig. 4 Stress-strain curves for compressive andperpendicular directions ofagglomerated cork-0.8-0.6-0.2 -0.4Strain E(a) Natural and agglomerated cork3.--------------r--------.---,I--Natural cork :---- -Agglomerated cork !,,,,I,.,,,/.."....'.-~ ..,4'"""-Ie~2t:lfI)fI)1uJ:l~","0150.---------------,--------,-0.8-0.6o -0.2 -0.4Strain EFig. 5 Stress-strain curves ofnatural cork withvarious density in radial direction3• Compressive0 Perpendicular to•-compressive 0Ie~2 0•0t:l•0fI) 0B1 0 •0 •~ 0 •0 •0 • ••-0.8-0.2 -0.4 -0.6Strain E(b) Bond materialFig. 3 Averaged stress-strain relationso-eu~ 1008 Takeji ABE and Takashi SATO MEM.FAC.ENGOK.. NL VoL37. No.23.--------------~ 3• (J =330 !6 p =340...-.'il p=350..s• p =360~2 !I::l 1en•en 1g.tu:lItt•0 -0.2 -0.4 -0.6 -0.8Strain f-0.8-0.6-0.2 -0.4Strain f• p =140IJ =150• IJ =160 •6••.~• f),! Q ••• t!!!lttoo(a) Radial compression ofnatwal cork (b) Agglomerated cork in compressive directionFig. 6 Stress-strain curves of cork with various densityE = -0.2Axial E = -0.4LTangentiatI' =-0.6I200 j.J. mIFig.7 SEM images ofradial section ofnatural cork (compressive direction is horizontal)larch 2003 Microscopic Deformation of Cork 9[==0E =-0.4E =-0.22001-1 mIRadialLTangentialFig.8 SEM images ofaxial section ofnatural corkThe effects of lhe density p of the cork specimenare shown in Figs. 6 and 7 for the natural and theagglomerated cork., respectively. It is seen from thesefigures that the stress increases with the increase ofdensity.3 . 2 Microscopic observation of surfaceprofileThe microscopic surface profiles of the corkobserved with the natural SEM is shown inFigs. 7 "'"' 9. The direction of the compression ishorizontal direction. Fig. 7 shows the cross section tothe radial direction where lhe hexagonal honeycombstructure is observed. Fig. 8 shows the cross sectionperpendicular to the axial direction, and Fig. 9 showsthe cross section perpendicular to the tangentialdirection. 'The latter two shows the structure such thatlayers of bricks. It is seen in all cross sections that thcell wall defonns with the applied compressive strainand finally collapse. These behavior is resembled tothose oflhe aluminum hexagonal structure [5,6].The surface profile of the agglomerated corkis shown in Fig. 10, where the cork grains arebonded together. Inhomogeneous deformationis also seen in the agglomerated cork. Thereare some voids between the cork grains. Theweight percent of the bond material is about7wt % ( volume fraction is about 2.3% ).J0 Takej i ABE and Taka hi, SATO MEM.FAC.E GOK 1. VoL37, No.2AxialLdial-0 £= -0.4200 pmFig.9 SEM images oftangentiaJ section ofnarural corkE = -0.21 mmFig. 10 SEM images of agglomerated cork3 . 3 Strain recovery after compressionFig. 11 shows the strain recovery after compressionfor the natural and the agglomerated corle Fig. 12shows the averaged strain recovery after the appliedcompressive strnin of 0.3 and 0.7. In the first case,recovery is almost finished after about 14days, but for70010 compression the deformation is not completelyrecovered after unloading. The strain recovery in thecompressive direction is large for the agglomeratedcork.The directional dependence of the strain recovery isas follows. The strnin recovery of the natural cork isrelatively isotropic. The strnin recovery in theperpendicular direction is rather anisotropic, which isconsidered to be related to the microscopic structure ofthe natural cork as shown in Fig. 13.March 2003 Microscopic Deformation of Cork 11Time (min)10 100 10000 0 ill -v0• i•• Ii.Ii....Ii.Ii..........0.-----.,....---.........--.-----.....----.,..,.,.....,o E = - 0.3 Compressive direction• E = - 0.3 Perpendicular directionIi. E = - 0.7 Compressive direction... E = - 0.7 Perpendicular direction-0.3 '-- ---=- ----J-0.2" -0.1Time (min)10 100 1000I• •• •• 0 ~ 600...... Ii.Ii.... Ii.Ii.Ii.o E = -0.3 Natural cork• E = -0.3 Agglomerated corkIi. E = -0.7 Natural cork... E = -0.7 Agglomerated cork-0.3 '-- ....::..=... ----1..;; -0.1t::·asJ:t00-0.2 I--Fig. II Recovery ofdimensions aftercompression in compressive directionFig. 14 Recovery of dimensions ofagglomerated cork in compressivedirectionFig. 15 Recovery of dimensions ofagglomerated cork in tangentialdirection to compressive axis0.100 E = - 0.3 Natural cork• E = - 0.3 Agglomerated cork0.08 - Ii. E = - 0.7 Natural cork 0.1 0 I-" ... E = - 0.7 Agglomerated cork'"t:: 0.06 0.08·as ...J:t ......00 0.04 '"Ii. ....5 0.06Ii. Ii.Ii. II. ~0.02 00 0.04•0 e e e0"10 100 1000 0.02Time (min) 0Fig. 12 Recovery ofdimensions in tangentialdirection to compressive axis0.100 E =- 0.3 Radial• E = - 0.3 Tangential0.08 Ii. E = - 0.7 Radial... E = - 0.7 Tangential'" 0.06 ...t:: ......'iii ....b ...00 0.040.02 l • • • •Ii. Ii. Ii. Ii.0 Q 0 0 0 n10 100 1000Time (min)o E = - 0.3 Compressive direction• E = - 0.3 Perpendicular directionIi. E = - 0.7 Compressive direction... E = - 0.7 Perpendicular directionIi. Ii.Ii.• Ii.• • • Q0 0 0 0 n10 100 1000Time (min)Fig. 13 Recovery ofdimensions in tangentialdirection to compressive axis foraxial compression ofnatural cork12 Takeji ABE and Takashi SATOIn 1he case of the agglomerated cork, the strainrecovery is large in the compressive direction than thatin the perpendicular direction as shown in Figs. 14 and15.When cork is compared with metallic materials, thenatural cork corresponds to single crystal, while theagglomerated cork corresponds to the polycrystallinemetals. Similarly, the mechanical behavior of theagglomerated cork is more isotropic than that of thenatural cork. Furthermore, as pointed out previously,the microscopic structure of the natural cork resemblesto the aluminum honeycomb structure, and itsmechanical behavior is closely related to themicrosb:ucture.4. Conclusion1he compression test of both natural andagglomerated cork was done and the stress-strainrelation was measured. The change in microscopicsurfilce profile during compression was observed byscanning electron microscope. The strain recoveryafter compression was also studied. The effects ofthematerial anisotropy and the material density on thestress-strain relation and the strain recovery wereinvestigated. The main results obtained are asfollows.( 1 ) The obtained stress-strain relation showsincrease in stress with the density of cork for both thenatural and the agglomerated cork.( 2 ) The structure of cork observed by SEM iscellular with different shape for the cross sections ofthedifferent direction. The voids of cork structuredisappear at the strain of -0.6, where the stress increaserapidly.( 3 ) The strain recovery is high for heavilydeformed cork.The authors are indebted for Uchiyama Industrial Co.for 1he supply ofthe cork material as well as the relatedinformation. The authors are also thankful toTechnical Assistant T. Sato of Mechanical EngineeringDeparbnent ofOkayama University for his cooperationin preparing the compressive apparatus.MEM.FAC.ENG.OKA.UNI. Vo1.37, No.2References[1] LJ. Gibson and M.E Ashby, Cellular Solids -Structure and Properties -,2nd ed., CambridgeU.P. (1997).[2] LJ.Gibson, K.E.Easterling, and M.FAshby: Proc.R Soc. Lond. A377 (1981) 99-117[3] M.E.Rosa, M.A.Fortes, : 1. Mater. Sci. 23 (1988)879-885[4] B.R Kim, A. MOO, A. Sugiyama and T. Okano,Report of Agriculture Field of Tokyo Univ., 82(1990) 199-217[5] M.FAshby : Metall. Trans. A. 14A (1983)1755-1769[6] A-EBastawros, H.Bart-Smith, A.G.Evans, : J.Mech. Phys. Solids. 48 (2000) 301-322

Observation of Microscopic Deformation Behavior of Cork

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