Experimental Analysis of Mechanical Behaviour and Damage Development Mechanisms of PVC Foams in Static Tests

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1 ISSN MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol. 10, No Experimentl Anlysis of Mechnicl Behviour nd Dmge Development Mechnisms of PVC Foms in Sttic Tests I. Gimenez 1, M.-K. Frooq 2, A. El Mhi 2, A. Kondrts 3, M. Assrr 2 1 Technology Trnsfer Center of Le Mns, rue Thlès de Milet, Le Mns, Frnce 2 Institute of Acoustics nd Mechnics, University of Le Mns, Avenue Olivier Messien, Le Mns cedex 9, Frnce 3 Interntionl Studies Centre, Kuns University of Technology, Doneličio 73, LT-3006 Kuns, Lithuni Received 23 Octoer 2003; ccepted 16 Decemer 2003 Cellulr foms re eing used incresingly s core mterils in conjunction with high strength skins, to produce strong, stiff, nd light weight sndwich structures for erospce, mrine nd trnsport industry. Due to their higher impct resistnce nd energy soring cpility, cellulr foms re eing extensively used in utomoile pplictions. This pper presents the results of experimentl investigtion of fom density effect on mechnicl ehviour nd dmge development tht induce the rupture of PVC foms in sttic tests. The experimentl investigtion ws conducted using compression, flexurl, indenttion nd sher tests for the foms of four densities. The dmge mechnisms nd properties of different fom densities were evluted in monotonic loding tests nd were compred. The otined results show the etter performnces of the fom with higher density. The strength of fom mteril increses with the increse of its density. The complete dmge of the test specimens is cused y deteriortion of the PVC cells. The mode of dmge depends on the density of mteril nd the type of loding. Keywords: fom, compression, indenttion, sher, flexurl. 1. INTRODUCTION * In the context of sndwich composite mterils, the mteril consists of high modulus reinforcing fires emedded in low modulus polymeric mtrix onded to low density core mteril lminte. Lminte nd core mterils re non-homogeneous nd nisotropic, therefore properties will vry throughout the entire structure. Mechnicl properties of sndwich composite depend on the properties of constituent lminte nd core mterils. Composite fce sheets fil s result of n interction mong mtrix crcks, fire frcture, delmintion, etc. Mechnicl properties of lmintes strongly depend upon the quntity, orienttion of the fire nd type of the mtrix. In the core it depends upon the thickness, density nd cell structure of the core. Therefore, it is necessry to understnd the ehviour of constituents of sndwich in detils [1 5]. Fom cores in sndwich components cn e used s cost-reducing production id nd for structurl pplictions. Fomed plstics lso referred to s cellulr or expnded plstics hve higher flexurl modulus. They chieve higher lod-ering cpcity per unit in weight, s well s higher energy storge nd energy dissiption cpcities. Exmples of commonly produced fomed plstics re polyurethne, PVC, polystyrene, polypropylene, epoxy, phenol-formldehyde, cellulose cette, silicone, etc. It is virtully possile to produce every thermoplstic nd thermoset polymer in cellulr form. Fomed plstics cn e clssified ccording to the nture of the cells into closed-cell nd open-cell types. Ech individul cell in closed-cell type of fom, more or less sphericl in shpe, is completely enclosed y wll of * Corresponding uthor. Tel.: ; fx: E-mil ddress: Aderrhim.Elmhi@univ-lemns.fr. (A. El Mhi) plstic, while in n open-cell type of fom the individul cells re interconnected s in sponge. Free expnsion during cell formtion usully produces open-cell foms. Closed-cell foms re produced in processes, where some pressure is mintined during the cell formtion stge. Close cell foms cn sor more energy thn open cell foms ecuse of entrpped gs within the cell, which cts s medium of energy sorption. During compression loding, the entrpped gs inside the cell cn compress s either isothermlly or diticlly depending on compression rte. Fomed plstics re produced in wide rnge of densities. The shpe, size nd distriution of cells cn e regulr or highly inhomogeneous, depending on the prticulr mteril nd dopted foming process. Accordingly, polymer foms my e homogeneous, with uniform cellulr morphology throughout, or they my e structurlly nisotropic. Foming of plstics cn e chieved in severl wys. One possiility is to crete gses inside the mss of the polymer. Once the polymer hs een expnded, the cellulr structure must e stilised rpidly. The expnsion is crried out ove the melting point, nd the fom is then immeditely cooled elow the melting point (such process is referred to s physicl stilistion) if the polymer is thermoplstic. Otherwise, chemicl stilistion cn e performed. Gs cn e whipped into solution of the plstic s low temperture oiling liquid or incorported in the plstic mix nd then voltilised y het. Cron dioxide gs cn e produced within the plstic mss y chemicl rection, or other gses (e.g., nitrogen, ir) cn e dissolved in the plstic melt under the pressure nd then llowed to expnd y reducing the pressure s the melt is extruded. The typicl process of unixil ehviour of cellulr mterils is descried in [6 7]. Typicl stress strin 34

2 digrms for cellulr mterils in tension nd compression re given. In compression, fter the initil elstic response nd «yield» plteu, the mteril suddenly stiffens due to densifiction of the mteril fter pure collpse. In unixil tension, the cell wlls lign themselves in the direction of tension, which cuses recovery of stiffness fter certin mount of non-liner deformtion. The tensile nd compressive stress strin digrms re typiclly symmetric. In the following sections experimentl studies of fom core ehviour under different loding conditions re presented. 2. EXPERIMENTAL PROCEDURE 2.1. Mterils Fom specimens of 60, 80, 100 nd 200 kg/m 3 densities with 15 mm of thickness were used for compression, indenttion, sher, nd three point ending in sttic tests. The series of the specimens were cut from fom the pnels for ech type of the test. The dimensions nd shpe of the test specimens depend on the type of the tests nd resonle compromise etween their lod nd strength. 2.2 The Experimentl Set-up The experimentl tests were crried out on stndrd hydrulic mchine INSTRON 8516 used for sttic nd ftigue tests. The force ws mesured y lod cell. The displcement ws mesured y liner displcement trnsducer (LVDT).The tests were crried out with lod cell of ±100 kn. The mximum displcement of the ctutor is ±75 mm in frequency rnge up to 100 Hz. The mchine is interfced with designed for controlling nd dt cquisition computer. Compression tests were crried out ccording to the stndrd ASTM C 365. Sttic indenttion tests were crried out ccording to the stndrd ASTM C 364. Sher tests were crried out ccording to the stndrd ASTM C 273. Three-point ending tests were crried out ccording to the stndrd ASTM D 790. The lower supports were positioned on ril llowing the vrition of the spn length etween supports. 3. RESULTS AND DISCUSSION 3.1. Sttic test Requirement for sttic testing of structurl elements with vrious modes of dmge nd different modes of loding should e defined in the eginning of testing procedure to provide necessry informtion for ftigue testing. Results from the sttic tests were used to design the ftigue experiments. Initil numer of sttic tests were performed to otin the vlues for F u (ultimte lod), d u (ultimte displcement), nd stiffness for ech fom specimen. Sttic tests were performed in compression, indenttion, sher, nd flexurl mode for the foms of different densities, where specimens were loded t constnt rte of 2 mm/min. The verge vlue of stiffness F u nd d u for ech series of the specimens ws determined Compression test The mechnicl response of foms under compression must e well chrcterised in order to nlyse the extent of dmge in sndwich structures. Qusi-sttic compression tests were crried out on foms to study the mechnicl ehviour under compression for different densities. Four foms of different densities were considered for the sttic tests. The series of specimens, hving nominl dimensions of (mm) were cut from fom pltes with 60, 80, 100 nd 200 kg/m 3 densities. The size of the specimens ws chosen to otin resonle compromise etween the mximum mount of reltive compression of entire group of specimens. Ech test ws repeted five times under the sme nominl condition to determine the significnce of response vriility. Test dt showed tht the repetility of the tests ws excellent for qusi-sttic loding. These specimens were compressed t constnt rte of 2 mm/min etween two steel pltes Results of compression test Polymeric foms often exhiit very high strin rte dependence compred to tht of solid metllic mterils. This dependence is due to the solid mteril properties nd due to the presence of fluid, generlly ir, inside the fom. The ir is compressed or forced outside, depending on the fom cellulr structure when the foms deform. The compressive response is lso relted to its reltive density, which is the rtio of fom density nd the density of the solid from which it is mde. Typicl lod displcement curves for rigid closed cell PVC fom for ll densities, otined y compressing specimen qusi-stticlly long one direction re shown in Fig.1. Fig. 1. Stress versus normlised displcement for four core densities in monotonic compression tests. As the reltive density increses the cell size decreses. It mens tht more of the solid is contriuted to the fom mechnicl properties rther thn the gs within the cells. For this reson s the density of PVC foms increses, the increse of compressive strength ws oserved, s cn e seen in Fig.1 for higher density foms (e.g. 200 kg/m 3 ). These curves exhiit three definite regions: liner elstic, long plteu nd densifiction. The ehviour is liner elstic with slope equl to Young s modulus of the fom t smll displcements, usully less 5 %. The fom cells egin to collpse y plstic collpse, plstic yielding 35

3 or rittle crushing, depending on the mechnicl properties of the cell wlls s the lod increses. The collpse progresses t roughly constnt lod, giving the stress plteu, until the opposing wlls in the cell meet nd touch ech other, when densifiction cuses the stress to increse steeply. For the chrcteristion of foms nd the evlution of their potentil pplictions, the determintion of their compression ehviour, or, more specificlly, their compression strength nd the length nd slope of the plteu re importnt. It is oserved from Fig. 1 tht the vlue of stress for strting the plteu region is incresed from 1 MP for lower density (60 kg/m 3 ) up to 5 MP to higher density (200 kg/m 3 ), when the density of foms increses. Higher stress is needed for the strt of plteu region for high density foms. Vlues of mechnicl constnts otined from the results re presented in Tle 1. Tle 1. Chrcteristic of foms Properties Density (kg/m 3 ) Young s modulus E (MP) Poisson rtio Chrcteristic t the strt of plteu: - stress σ1 (MP) - strin ε1 (%) Chrcteristic t the end of plteu: - stress σ1 (MP) - strin ε1 (%) All these prmeters in this tle re found to increse with the increse of density of foms. The work done per unit volume in deforming the fom to given strin or displcement is simply the re under the stress-strin curve. Very little energy is sored in the liner elstic region; it is long plteu of the stress-strin curve tht llows the energy sorption t nerly little increse in the lod. It is pprent tht with the increse in core density, the energy soring rte incresed. Cell size decreses with the increse of the core density, which offer higher resistnce in the deformtion of the fom; thus, more energy is needed in compression. Incresing the density of the foms, increses the slope of the liner elstic prt, rises the plteu nd reduces the strin t which densifiction strts. Also s the density of PVC fom incresed, the rpid densifiction occurs t smller strins, which led to the shortening of stress plteu tht cn e oserved for the fom of 200 kg/m 3 density Sttic indenttion test Sndwich ems cn fil in severl different wys. Indenttion is one of the filure modes strongly ffecting the ehviour of sndwich composites. Indenttion of the core generlly occurs due to the concentrted lodings tht cn pper in the corners or in the joints. Indenttion cn lso e cused y ccidentl drops of lods on the cores. Therefore it is lso necessry to tke into ccount the effect of core indenttion in the overll ehviour of sndwich composites. A systemtic discussion of filure modes for sndwich ems with fom cores nd some tests exhiiting filure y core indenttion cn e found [7 14]. Since the min interest in this study is the mechnicl ehviour in foms, we will consider, mong the possile filure modes tht which is governed y deformtion of the fom core, tht is, the filure y core indenttion. Procedure of experiments for indenttion tests ws the sme s used in the compression tests. In these tests, indenttion ws performed with roller of 35 mm dimeter. The rte of the indenttion ws 2 mm/min. The length, width, nd thickness of the specimens were 100, 40 nd 15 mm respectively Results from sttic indenttion test During the test, the lod increses pproximtely linerly until the displcement of 1 mm. After this foms continuously ehve in non-liner mnner. Typicl lod displcement curve for the sttic indenttion tests is shown in Fig. 2. Three distinct regions cn e identified from the lod displcement curves. Ech core density displyed the initil elstic region up to initil dmge, followed y stress plteu, then region of densifiction. Ech of three phses correltes to the specific filure mechnisms tht the fom undergoes during indenttion. The liner region is controlled y the stretching of cell fces with the ppliction of lod for closed cell PVC foms. The cell wll touches the other one nd further strins the solid itself, giving the finl region of rpidly incresing lod until the cells hve completely collpsed. However, efore the filure the core deforms lrgely in proximity of the point of ppliction of the lod nd fom deformtion ffects significntly the overll mechnicl response up to filure. Fig. 2. Evolution of lod versus displcement for foms of four densities in sttic indenttion The indenttion response of the foms is relted to its reltive density. For this reson s the density of the PVC foms increses, the increse in filure lod is oserved. Also, s density of PVC fom incresed, the sudden densifiction occurs t lower strin, which leds to the shortening of plteu, nd cn e pprently seen in Fig. 2 in the cse of fom 200 kg /m 3 of density. 36

4 3.4. Sher test Sher tests were performed for fom specimens of 60, 80 nd 200 kg/m 3 densities in order to study the sher ehviour of foms. For ech type of the core, the series of specimens, hving nominl dimensions of (mm) were cut from fom pltes. The size of the specimens ws chosen to otin resonle compromise etween the mximum mount of reltive compression of entire group of the specimens. These tests were crried out using Instron Mchine with lod cell of 100 kn Results from sttic sher test The lod increse linerly with incresing of the strin t the eginning of the test. Fig. 3 shows stress-strin curves for the foms of three different densities. It is oserved tht t the strin of 3.5 %, the first slip occurs during loding of the fom 60 kg/m 3 of density. This slip is cused y the crcks in the corners of the core mteril due to the stress concentrtion, where the corners of the core mteril re glued to the loding pltes. The men sher stress is out 0.7 MP t the first slip. The relile vlue of sher modulus is ssumed to e verge slope in the stress intervl of 0.1 nd 0.5 MP. After these slips, the curves rech the mximum stress of out 0.78 MP nd numerous crcks occur cusing totl frcture of the specimen t the strin of 15 %. Similr trend ws oserved for other fom of 80 kg/m 3. The sttic chrcteristic increses with the increse of the density s seen in the Tle Flexurl sttic test Flexurl tests were performed for the PVC fom specimens of four densities. Fom specimens of (mm) dimensions were cut from the fom pnels for qusi-sttic tests using sw fitted with dimond coted stel lde. The specimens were tested in three point ending t spn length of 160 mm. Tests were conducted on the Instron mchine in displcement control t cross hed speed of 5 mm/min Results from sttic flexurl test Numer of sttic tests were performed in order to otin strength nd stiffness dt, which would e used for ftigue tests. Fig. 4 shows the lod displcement curves in sttic tests for PVC foms of four densities. All the curves revel the non-liner elstic ehviour up to the frcture. The filure process nd lod displcement responses were indictive of ductile mteril ehviour. The properties of the core mteril in prticulr re lso found to control the sttic strength of the foms. The higher core density, the etter sttic strength. Lower density core mteril tends to fil in sher, while higher density core mteril tends to crush under the centrl roller in three point ending. All the foms show tht there is no increse in the lod fter yield point, while the displcement increses until filure. Fig. 4. Lod displcement curves for cores of four densities in three point ending sttic tests Fig. 3. Stress-strin curves of 60, 80 nd 200 kg/m 3 densities foms in sttic sher tests Tle 2. Sher chrcteristics of PVC foms for different densities Proprieties Density (kg/m 3 ) Sher modulus G (MP) Elstic limit: stress τ (MP) strin γ Chrcteristic t filure: stress τ (MP) strin γ Not determined The sttic strength increses with the increse of density nd filure is chrcterised y ductile mnner. The initil portion of the lod displcement curve is liner elstic with some decresing slope. After the pek point, the lod displcement response follows the plteu where cellulr structure collpses y the uckling cell wlls nd edges. The wlls of loded cells strt to come in to the contct with ech other t higher strins (displcements) nd further strin compresses the solid leding to filure. The filure should initite on the compression side. However, the filure ws chrcterised y lmost verticl crck tht initited on the tension side of the em. The filure process nd the lod deformtion response re indictive of ductile to rittle filure. Vlues of filure lods, filure displcements nd stiffness re presented in Tle 3. All the prmeters re found to increse with the increse of density. 37

5 Tle 3. Sttic chrcteristics of PVC foms for different densities Vlues Density (kg/m 3 ) Filure lod (N) Filure displcement (mm) Stiffnes (N/mm) Sttic filure nlysis Foms re generlly composed of lrge numer of interconnected cells, forming cellulr network. The cell is the sic unit of the fom; its morphology nd the chemicl composition of the struts oth define the finl properties of the fom. The study of the cell is, therefore, key pproch to develop nd design new foms ccording to the given specifictions nd requirements. The deformtion of the foms is dependent on the mechnicl properties of the mteril, of which the cells re mde, nd on the dimensions nd rrngement of the struts nd pltes, of which cells re composed. In the closed celled PVC foms under the compressive loding the struts nd pltes re oriented in loding directions under pure compression, while the olique struts nd pltes, tht is the struts nd pltes in ny other direction, will hve ending stresses induced in them. Since most of the memers in the foms re olique struts nd pltes, the ending is elieved to e the most importnt deformtion mechnisms in the foms. Becuse of the usully lrge slenderness rtio of struts for the low-density foms, the strut in the loding direction will rther fil y elstic uckling thn y compressive lod. The uckling of the struts will e resisted y the cell wlls. Actully, ny ending or uckling of the strut will produce tension nd compression in the ttched cell wlls. It hs lso een reported tht strut lwys will move to the stte where one wll is in the tension nd other in the compression. When the strut is in tension ll the ttched cells re stretched in tensile direction. In ddition, the wlls re stretched in the tensile direction sujected to the tensile strins due to the ending of olique struts. For this reson the strins in the wlls re lrger thn those in the struts nd the cell wlls re likely to e ruptured first. Cell rrngements of the foms under compressive loding chnge continuously nd these chnges were oserved in the lod displcement curves continuously. Smll sections of the foms from the selected filed specimens were cut trnsversely to the em xes 5 mm wy from the filed centre region. Similrly, specimens were cut from the foms of the four densities (60, 80, 100 nd 200 kg/m 3 ). Also, virgin specimens were cut from the fom plnks. Fig. 5 8 show representtive SEM microgrphs tken t suitle mgnifiction for the virgin nd the filed specimens of four densities. From the SEM pictures of dmged specimens it cn e clerly seen tht for the low-density foms, some cells re deformed y uckling, while in other cells the wlls touch ech other. In low density foms, fter deformtion cells shpe ecome irregulr due to the uckling, ending, nd stretching of the cell wlls. Fces of the closed cells cn e seen opened due to the compression of the djcent cells. Compressed ir inside the cells ruptures the fces of the cell nd opens it. The core cells of the PVC fom re unique mong rigid plstic foms since it cn e compressed up to 50 percent of its thickness without rupture or crumling of the cell wlls. It cn e seen in the dmged specimens of ll the densities. Fig. 5. SEM photos of 60 kg/m 3 density fom of undmged () nd dmged () specimens in indenttion tests Fig. 6. SEM photos of 80 kg/m 3 density fom of undmged () nd dmged () specimens in indenttion tests Fig. 7. SEM photos of 100 kg/m 3 density fom of undmged () nd dmged () specimens in indenttion tests Fig. 8. SEM photos of 200 kg/m 3 density fom of undmged () nd dmged () specimens in indenttion tests The oservtion of the frcture topogrphies of the vrious test specimens shows tht the totl dmge 38

6 depends on the mode of the rupture of unit cells of constituting mteril. In the cse of the compression test or indenttion the totl dmge of the test specimens is otined y the progressive collpse of the unit cells. The initition of this collpse is oserved in the zones of the contct of the test specimen with the rigid fces of supports or indentors. In the cse of the sher test the totl rupture of the test specimen is cused y the progressive rupture in sher of the unit cells. The zone of dmge is situted in the middle prt of the test specimen ccording to the sher line. In the cse of the flexurl test the totl rupture of the test specimen hs the rittle nture. This rupture is cused y the instntniously rupture of the unit cells in tense fce. In the compressed fce under centrl support the sme unit cells collpse zone is oserved. 4. CONCLUSIONS The sttic strength of the fom (core mteril) directly depends on their density. The sttic tests highlighted the performnces of the higher density fom. The foms show some elsticity until the filure of the cells. Lower density core mteril tends to fil in sher, while higher density core mteril tends to crck in the stress concentrtion zone. The oservtion of the frcture surfces of the filed specimens in flexurl tests permitted to revel tht the rittle rupture of the foms is otined y rupture-of the tensile fce cused y rupture of the cells nd itercellulr rupture in tension. However, the dmge in the compression nd indenttion tests is less rittle thn tht the flexion cused y consequence collpse of the cells wls. The dmge of the foms in the sher test is oserved y the internl longitudinl crcking of the specimen induced y the rupture of the cells nd the itercellulr rupture in the sher. It is to e noted tht the frcture nd rupture of specimen or cruching of the cells of the fom is the result of dmge processes induced rndomly in the test specimens. These processes re the min cuse of chrcteristic dispersion even t the sme loding conditions with the sme type of specimens. These dispersions re due to the heterogeneous nture of the fom relted to their friction (volume frction, density, dimension of the specimens, etc.). REFERENCES 1. Berthelot, J.-M. Composite Mterils. Mechnicl Behviour nd Structurl Anlysis. New York, Springer Edition. 1999: 645 p. 2. Ferreir, J. A. M., Cost, J. D. M. Sttic Behviour of PVC Fom Composite Sndwich Pnel J. Cellulr Polymers 17 (3) 1998: pp Zenkert, D. An Introduction to Sndwich Construction. EMAS Pulishing, London, 1995: 277 p. 4. Zenkert. D. Hndook of Sndwich Construction, London, EMAS Pulishing, Chmeleon Press Limited, 1997: 442 p. 5. Allen, H. G. Anlysis nd Design of Structurl Sndwich Pnels. London, Pergmon Press, 1969: 283 p. 6. Cprino, G., Teti, T., Mess, M. Long Term Behviour of PVC Foms Cores for Structurl Sndwich Constructions. Sndwich Construction 3 Proceeding of Third Int. Conf. on Sndwich Construction, University of Southmpton, U.K. Septemer 12 15, London, EMAS Pulishing, 465 p. 7. Gison, L. J., Ashy, M. F. Celulr Solids: Structure nd Properties. Oxford, New York, Beijing, Frnkfurt, So Pulo, Sydney, Tokyo, Toronto, Pergmon Press, 1988: 357 p. 8. Trintfillou, T. C., Gison, L. J. Filure Mode Mps for Fom Core Sndwich Bems J. Mteril Science Engineering : pp Trintfillou, T. C., Gison, L. J. Deonding in Fom- Core Sndwich Pnels J. Mterils nd Structures (Mtériux et construction) : pp Dniel, M. I., Aot, J. L. Friction, Testing nd Anlysis of Composite Sndwich Bems J. Composite Science nd Technology : pp Uemur, M., Iwi, I. Flexurl Testing nd Evlution Methods of Advnced Composite Mterils Séminire Frnco-Jponis sur les les mtériux composite. Pris, le Bourget 1990 : pp Fournier, P., Pelissou, O., Chteuminois, A., Lrge- Toumi, B. Post-Buckling Method Applied to Sttic nd Ftigue Chrcteristion of Composites Mterils. EACM Amsterdm, Edit. BCS, EACM-CT, 1992: pp Bienind, W. K., Roerts, G. D., Ppdopoulos, D.-S. Effect of Contct Stress in Four Point Bend Testing of Grphite/Epoxy nd Grphite/PMR-15. Composite Bems SAMPE. USA, : pp Reifsnider, K. L., Scott, C, Jermy, D. The Mechnics of Composite Strength Evlution Composites Science nd Technology : pp