THE EFFECT OF LOADING RATE AND DIRECTION OF FORMATION ON FRACTURE TOUGHNESS OF RIGID POLYURETHANE FOAMS

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1 Journl of Engineering Studies nd Reserch Volume 18 (01) No THE EFFECT OF LOADING RATE AND DIRECTION OF FORMATION ON FRACTURE TOUGHNESS OF RIGID POLYURETHANE FOAMS LINUL EMANOIL 1*, MARSAVINA LIVIU 1 1 Politehnic University of Timisor, Deprtment of Strength of Mterils, Blvd. M. Vitezu, No. 1, 300, Timisor, Romni, Abstrct: This pper presents the effect of loding rte, (ELR), nd direction of formtion, (DF), of rigid polyurethne foms, (PUR 40 nd PUR 140), on frcture toughness. Nominl densities of used foms in the experimentl progrm were 140 kg/m 3, (for ELS) nd 40 kg/m 3, (for DF), which is closed-cell rigid foms widely used for sndwich cores. Determintion of frcture toughness for Mode I frcture of studied mterils hs mde by three-point bending tests, (3PB), on specimens with notches, t room temperture (0 ± ºC). All the specimens were cut from one nd the sme plte. The specimens were subjected to 3PB t loding speed of mm/min, except smples for determining the ELR where, 0, 00 nd 400 mm/min loding speeds were used, nd were tken into ccount the fct tht the lod must ct exctly on the notch direction. All the specimens present brittle filure without plstic deformtion. Keywords: polyurethne fom, closed cells, frcture toughness, three point bend tests 1. INTRODUCTION The min use of rigid polyurethne (PUR) fom is for sndwich pnels becuse it presents high stiffness, they re used s core, nd tht fces cn use different types of mterils (crbon fiber, luminum), depending on pnel destintion. Also, these types of cellulr mteril re used for pcking nd cushioning becuse of their structure shows gret cpcity to bsorb impct energy [1, ]. The min chrcteristics of foms re high porosity nd workbility, good energy bsorption cpcity, nd weight reltively low compred with other mterils with the sme mechnicl chrcteristics [3]. Mny efforts hve been mde in recent yers to determine the frcture toughness of such foms under sttic nd dynmic loding conditions, [4]. McIntyre nd Anderson, [5], using single edge notch specimen, mde of rigid closed-cell PU foms mesured the K IC for different densities. They found tht the frcture toughness is independent of crck length nd liner correltion of K IC with density for foms with densities less thn 00 kg/m 3. At higher densities the correltion becomes non-liner. Liner reltionship between K IC nd density (90-35 kg/m 3 ), ws lso obtined by Dnielsson, [6] for PVC Divinicel HD using three point bend specimens. Burmn, [7] presented frcture toughness results for two commercil foms Rohcell F51 (density 5 kg/m 3 ) nd Dyvinicell H100 (density 100 kg/m 3 ) using SENB specimens. Vinn nd Crlsson, [8] presented results of frcture toughness for PVC foms of different densities, (36, 80, 100, 00 AND 400 kg/m 3 ). Kbir nd Ssh, [9] using 3PB tests hve determined frcture toughness for polyvinyl chloride (PVC) nd polyurethne (PU) * Corresponding uthor, emil: linul_emnoil@yhoo.com 01 Alm Mter Publishing House

2 Journl of Engineering Studies nd Reserch Volume 18 (01) No foms. Also, frcture toughness ws investigted by Mrsvin nd Linul, [3], nd Linul et l., [10] for three different densities of rigid polyurethne (PUR) foms, (40, 140 nd 00 kg/m 3 ). In order to estblish tht vlid K IC hs been determined, it is first necessry to clculte conditionl result, K Q, which involves construction on the test record, nd to then determine whether this result is consistent with the size of the specimen. Figure 1 presents the force-displcement curve for determining the criticl lod P Q. Fig. 1. Force-displcement curve for determining the criticl lod P Q. Determining the criticl lod is s follows: Drw best stright line (AB) to determine the initil complince, C (C=tnθ; 1,05C=tnθ ). C is given by the reciprocl of the slope of line (AB). Drw second line (AB ) with complince 5 % greter thn tht of line (AB). If the mximum lod tht the specimen ws ble to sustin, P mx, flls within lines (AB) nd (AB ), use P mx to clculte K Q. If P mx flls outside line (AB) nd line (AB ), then use the intersection of line (AB ) nd the lod curve s P Q. Furthermore, if P mx /P Q < 1.1, use P Q in the clcultion of K Q. However, if P mx /P Q > 1.1, the test is invlid [11]. For specimen tht meet the condition, L/=4, K Q is determined by the following the reltion: K PQ = B Q 1 f, with: 0 < < 1 (1) with f(x) non-dimensionl function given by: f = () where: P Q - force cting on the specimen; b - specimen thickness; - specimen height; - crck length; S - spn length. In order for result to be considered vlid ccording to these test methods, the following size criteri must be stisfied:, B,( ).5 σ K Q ys (3)

3 Journl of Engineering Studies nd Reserch Volume 18 (01) No. 1 1 where σ ys is the yield stress of the mteril for the temperture nd loding rte of the test. In cse tht the condition (3), the criticl stress intensity fctor, K IC, is considered to be equl to the clculted stress intensity fctor, K Q, so: K IC = K Q (4). EXPERIMENTAL PROCEDURE Experimentl tests for determining the sttic frcture toughness were mde on the Strength of Mterils Lbortory from the Fculty of Mechnicl Engineering from Timisor on tension-compression Zwick Roell 005 testing mchine of 5 kn, (Figure ). Fig.. Zwick Roell 005 testing mchine used for 3PB tests. Tests were performed t room temperture, (0 ± ºC), using specimens with the shpe nd dimensions shown in Figure 3. For determining the frcture toughness of studied mterils we used notched specimens loded in three point bending. Fig. 3. Shpe nd dimensions of the used specimens for 3PB tests. In the experimentl progrm ws used rigid polyurethne fom with 40 nd 140 kg/m 3. Figure 4 presents the shpe of the specimens used for experimentl tests. Both specimens nd notches were cut from one nd the sme rectngulr plte with blde thickness of 0.6 mm.

4 Journl of Engineering Studies nd Reserch Volume 18 (01) No ) b) Fig. 4. Shpe of the specimens used for 3PB tests: ) ELR; b) DF. The specimens were subjected to 3PB. The loding speed ws mm/min for determining the influence of direction of formtion, nd, 0, 00 nd 400 mm/min for determining the ELR. For ech type of test 5 specimens were used, nd the tests were performed ccording with ASTM D (Stndrd Test Methods for Plne-Strin Frcture Toughness nd Strin Energy Relese Rte of Plstic Mterils), nd were tken into ccount the fct tht the lod must ct exctly on the notch direction..1. The effect of loding rte (ELR) The men vlues of the frcture toughness obtined from the experimentl tests for rigid polyurethne fom with 140 kg/m 3 density, ccording to loding rte, re listed in Tble 1. Density ρ, [Kg/m 3 ] 140 Tble 1. Men vlue of the mechnicl chrcteristics for nlyzed foms fter 3PB tests. Smples dimensions Frcture Loding Criticl High Spn Crck toughness, idt rte, lod, length length, K B [mm] [mm/min] P [mm] S [mm] [mm] Q [N] IC [MPm 0.5 ].5 σ [mm] Figure 5 presents the lod-displcement curves for studied fom nd Figure 6 showed the vrition of frcture toughness versus loding rte... The effect of loding direction (DF) The frcture toughness of nisotropic fom depends on the direction in which the crck propgtes. This is the best defined with two subscripts, the first indicting the norml to the crck plne, the second the direction of crck propgtion, [1]. In Figure 7 is shown the smpling of the 3PB specimens from rectngulr SF plte, nd Figure 8 shown the influence of this trining pln nd direction of the lod on the mechnicl chrcteristics t 3PB. Loding direction emphsizes nisotropic behvior of the fom. Men vlue of frcture toughness obtined re listed in Tble. K Q ys Density ρ, [Kg/m 3 ] 40 idth B [mm] Tble. Men vlues of frcture toughness versus loding direction. Smples dimensions Criticl Spn Crck Loding High lod, length length, direction [mm] P [N] S [mm] [mm] Frcture toughness, K IC [MPm 0.5 ].5 σ () (3) K Q ys

5 Journl of Engineering Studies nd Reserch Volume 18 (01) No Fig. 5. Lod-displcement curves for ELR. Fig. 6. Vrition of frcture toughness versus loding rte. Fig. 7. The smpling of the 3PB specimens from rectngulr SF plte.

6 Journl of Engineering Studies nd Reserch Volume 18 (01) No Fig. 8. Lod-displcement curves for 3PB tests. Effect of loding direction. For ll tested specimens plne strin condition (3) ws fulfilled. From the Tble it cn be observed tht the vrition of frcture toughness with lod direction is insignificnt, (for this type of fom). Brittle frcture ws observed for ll tested specimens. The liner elstic behvior ws confirmed during the tests when no cushioning occurs nd no plstic deformtions remin fter the test [3]. 3. MICROSTRUCTURAL ANALYSIS OF INITIAL AND BROKEN SURFACES For nlysed foms hs mde micro structurl nlysis. The nlysis ws done for both before, (initil surfce), nd fter, (broken surfce), 3PB tests on the Lbortory from the Fculty of Building nd Architecture t Lublin University of Technology, Lublin, Polnd. Initil nd broken surfces of rigid polyurethne fom used in 3 PB experimentl progrms re presented in Figure 9. Also, in sme figure is shown the cellulr structure of fom hving closed cell with ρ = 40 kg/m 3 respectively 140 kg/m 3 density. All tested specimens show qusi-brittle frcture without plstic deformtions nd cushioning. 4. CONCLUSIONS This pper presents two prmeters which influence the frcture toughness of rigid polyurethne foms: effect of loding rte (ELR) nd effect of loding direction (DF). The vlues of frcture toughness for PUR foms re in the rnge MP m 0.5. Frcture toughness decrese with incresing of loding speed (Figure 6 nd Tble 1). Loding direction emphsizes nisotropic behvior of the fom. For this type of polyurethne fom with 40 kg/m 3 density, ws obtined pproximtely the sme frcture toughness (Tble ). All the specimens present brittle filure without plstic deformtion.

7 Journl of Engineering Studies nd Reserch Volume 18 (01) No Initil surfce Broken surfce 50.0 ) 40 kg/m 3 density; Initil surfce Broken surfce 5.10 b) 140 kg/m 3 density; Fig. 9. The microstructure of polyurethne foms used for compression tests.

8 Journl of Engineering Studies nd Reserch Volume 18 (01) No ACKNOLEDGMENT This work ws prtilly supported by the strtegic grnt POSDRU 6/1.5/S/13, (008) of the Ministry of Lbour, Fmily nd Socil Protection, Romni, co-finnced by the Europen Socil Fund Investing in People. REFERENCES [1] Gibson, L. J., Ashby, M. F., Cellulr solids, Structure nd properties, Second edition. Published by the Press Syndicte of the University of Cmbridge, [] Linul, E., Mrsvin, L., Cernescu, A.V., Effect of loding speed, the direction of formtion nd density of rigid polyurethne foms subjected to compression, Act Tehnic Npocensis, Series: Mechnicl Engineering. Mteril Science, vol. 53, 010, p [3] Mrsvin, L., Linul, E., Frcture toughness of polyurethne foms. Experimentl versus micromechnicl models, Frcture of Mterils nd Structures from Micro to Mcro Scle. 18 th Europen Conference on Frcture, Dresden, Germny, August - September, vol. 03, 010. [4] Altenbch, H., Ochsner, A., Cellulr nd Porous Mterils in Structures nd Proceses, CISM Courses nd Lectures, vol. 51. Ed. Springer, ien, New York, Udine, 010. [5] McIntyre A., Anderson, G.E., Frcture properties of rigid PU fom over rnge of densities, Polymer, vol. 0, 1979, p [6] Dnielsson, M., Toughened rigid fom core mteril for use in sndwich construction, Cellulr Polymers, vol. 15, 1996, p [7] Burmn, M., Ftigue crck initition nd propgtion in sndwich structures, Report No.98-9, Stockholm, 1998 [8] Vin, G.M., Crlsson, L.A., Mechnicl Properties nd Frcture Chrcteriztion of Cross-Linked PVC Foms, J. Sndw Struct Mter, vol. 4, 00, p [9] Kbir Md.E., Sh M.C., Jeelni S., Tensile nd frcture behvior of polymer foms, Mterils Science nd Engineering A, vol. 49, 006, p [10] Linul, E., Mrsvin, L., Cernescu, A., Determinre tencittii l rupere pentru mterile celulre de tipul spumelor poliuretnice, Mecnic Ruperii. Lucrrile celui de-l XV-le Simpozion Ntionl de Mecnic Ruperii, Editur Universittii Petrol-Gze din Ploiesti si Editur Universittii Lucin Blg din Sibiu, 6-7 Noiembrie, 009, p [11] ASTM D Stndrd Test Methods for Plne-Strin Frcture Toughness nd Strin Energy Relese Rte of Plstic Mterils.