Inverse Stress Relaxation and Viscoelastic Recovery of Multifilament Textile Yarns in Different Test Cycles

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1 ISSN 9 MATERIALS SCIENCE (MEDŽIAGOTYRA). Vol., No.. Inverse Stress Relxtion nd Viscoelstic Recovery of Multifilment Textile Yrns in Different Test Cycles Rit POCIENĖ, Arvyds VITKAUSKAS Deprtment of Textile Technology, Kuns University of Technology, Studentų, LT- Kuns, Lithuni Received August ; ccepted Jnury The rticle presents the results of the experimentl reserch of inverse stress relxtion (IR) nd viscoelstic recovery (VR) tht tke plce in cette nd polyester multifilment yrns in dependnce on the mechnicl pre-history. The ove-mentioned time-effects re investigted in two different stress relxtion (R-) nd creep (C-) testing cycles. The fct tht inverse stress relxtion process tkes plce in C- test cycle, i.e. fter previous sustining the specimen t constnt lod is experimentlly confirmed. It is shown tht viscoelstic recovery is the slower process thn the inverse stress relxtion. At identicl elongtions of the yrns t the end of loding period the inverse relxtion nd viscoelstic recovery processes go on similrly regrdless of the chrcter of testing cycle. Keywords: relxtion, inverse relxtion, creep, recovery, yrn, viscoelsticity. INTRODUCTION Strength nd deformility of textile yrns re importnt only mechnicl properties determining the ehviour of yrns in woven or knitted frics mking-up s well s the ehviour of fric in end-use. The ehviour of textile mterils s of ny polymeric odies is mostly viscoelstic. The response of mteril to the specific mechnicl ction depends not only on the ction itself ut lso on the former ctions undergone, i. e., it depends on mechnicl pre-history of mteril [, ]. This implies tht time-dependence of the response of ny textile mteril opposing the pplied forces, should e tken into ccount [ ]. Viscoelstic properties cn serve s n index of gretly vrious purposes, e. g. for comprtive evlution of mterils or s criterion t the control of the specific process. Experimentl investigtion of viscoelstic properties in textile fires nd yrns is commonly sed on the results of tensile tests tht cn e relted to two min groups of testing cycles, tking into considertion the conditions of mechnicl ction on the mteril nd the prmeters mesured. The tests t constnt elongtion ε t (Fig., ) re relted to the first group. In these tests stress relxtion (R) s well s inverse stress relxtion (IR) (smooth lines) or viscoelstic recovery (VR) in length of specimen (dotted lines) cn e oserved. The tests t constnt force or lod F c (Fig., ) re relted to the second group. In this cse creep (C) s well s viscoelstic creep recovery (CR) (dotted lines) or inverse stress relxtion (IR) (smooth lines) cn e oserved in the tests. Numerous works re done reveling the fetures of creep, recovery nd stress relxtion in textiles: the pulictions [, 7 ] cn serve s the proper exmples. The phenomenon of inverse stress relxtion ws first mentioned in 9 y Stein, Hlsey nd Eyring [], i. e. Corresponding uthor. Tel.: +7-7-; fx.: E-mil ddress: rit.pociene@ktu.lt (R. Pocienė) sustntilly lter thn the ove mentioned phenomen. Inverse relxtion is often met with under prcticl conditions [ 7], so its study is mtter of gret interest. Despite of the fct tht the studies of the inverse relxtion ecme more intensive during lst decdes [ - ], knowledge on the regulrities of the phenomenon re oviously insufficient. Except for theoreticl model proposed in [], up to now there re no experimentl dt on the inverse relxtion mnifesting in the testing cycle of the second group (Fig., ). Strin (ε ), stress (F ) Strin (ε ), stress (F ) R F t ε t θ t F θ F ε VR IR t t t θ t Time (t ) ε c C θ t F c F ε θ ε CR IR t t t θ t Time (t ) Fig.. The testing cycles: test t constnt elongtion; test t constnt force

2 In some pulictions the ttempts hve een mde to predict the development of one viscoelstic ftereffect y the dt on the nother one [, 7, ]. Presumptively positive results could e otined predicting the inverse relxtion y viscoelstic recovery, for the oth phenomen strt to develop t the solutely identicl mechnicl pre-history (points F in Fig., nd ). Such prediction would e especilly useful ecuse the inverse relxtion test implictes much more prolems thn the comprtively simple recovery test [7]. The im of this study is to investigte experimentlly the chrcter of oth inverse relxtion nd viscoelstic recovery in different textile yrns t identicl mechnicl pre-history. A specil considertion is pid to the inverse relxtion development following the creep in the yrns ecuse there re no ny experimentl dt on the effect in the yrn eing undergone y constnt force. EXPERIMENTAL Two different types of multifilment yrns were tken for the experimentl investigtion: cette (CA). tex nd polyester (PES). tex. All specimens were preconditioned, then conditioned nd tested in the tmospheres ccording to ISO 9. The experiments were provided on Zwick/Z universl testing mchine. To ensure the setting of testing cycle prmeters with s possile higher ccurcy the guge length ws tken 7 mm, i. e., lrger thn it is customry used in yrn testing. Ech specimen ws pretensioned to. mn/tex efore the testing. The testing cycle t constnt elongtion R- (for symols used elow in Fig., ) consisting of four phses ws provided in two different regimes (R-IR nd R-VR). The first three phses were identicl for oth regimes: ) constnt rte (v t ) extension up to the strin ε t, ) sustining t ε t up to the time t* = (t t t ) = θ t, while stress relxtion (R) ws going on nd mesured, ) constnt rte (v ) retrction down to pretension (F ). The fourth phse ws different for the prticulr regime. For the regime R-IR the specimen ws sustined t strin ε corresponding to F, nd the inverse relxtion (IR) ws going on nd mesured in time t** = (t t ). For the regime R-VR, in the lst fourth phse viscoelstic recovery VR, i.e the decrese in specimen strin ε ws going on nd mesured in time t** = (t t ) t constnt pretension F. The testing cycle t constnt force (for symols used elow look t Fig., ) ws lso provided in two regimes (C-IR nd C-CR). The first three phses were identicl for oth regimes: ) constnt rte (v t ) extension up to the force F c corresponding to the strin ε c, ) sustining t force F c up to the time t* = (t t t ) = θ t, while creep (C) ws going on nd mesured, c) constnt rte (v ) retrction down to pretension (F ). For the regime C-IR the fourth phse ws identicl to tht of the regime R-IR, while for the regime C-CR the fourth phse ws identicl to tht of the regime R-VR. In ll tests the rte of extension (v t ) ws mm/min (. % / s) nd it ws equl to the rte of retrction (v ). The limits of extension (ε t, F c ) during the first phses of the regimes, nd the sustining times (θ t ) during the second phses were vried. The inverse relxtion (IR) nd the viscoelstic recovery (VR, CR) were minly mesured during the time t** = s. In some cses the mesuring time ws prolonged up to t** = s. To exmine the chrcters of oth IR nd CR processes running in the C- testing cycle s distinct from the corresponding IR nd VR processes running in the R- testing cycle the tests were provided in two different modes: Mode I: The level of force F c in C- testing cycle is equl to the corresponding level of force F θ in R- testing cycle. Mode II: The level of strin ε t in R- testing cycle is equl to the corresponding level of strin ε θ in C- testing cycle. RESULTS AND DISCUSSIONS Stress relxtion curves of CA nd PES yrns re shown in Fig.. Stress relxtion process is very distinctive in CA yrn, where the ovious yield point is oserved in its stress-strin curve. With increse of the set extension level (ε t ) the vlues of stress increse s well. However, when the extension level of CA yrn is in the yield zone (ε t = 9. %), from time t* s the vlues of stress re lower thn t elongtion ε t = %. This nomlous ehviour of cette yrn ws formerly noticed y Meredith [], who explined it s kind of ditic sudden stretch. In our cse, the extension rte is not so high to rise the temperture of the specimen. It is possile tht the ove-mentioned nomly of the ehviour is relted to complex entropy chnges in stretched yrn. Extending the CA yrn over the yield point, the relxtion curves ecome lmost prllel. The yield zone is not chrcteristic for PES yrn so s extension level increses, relxtion is going on t higher stresses throughout the whole oservtion time θ t. Creep curves of the yrns t constnt forces in the C- test cycle re presented in Fig.. With increse of the force (F c ), elongtions of the yrns during creep process increse s well. Creep of CA yrns is especilly intense t yield zone nd over it, while in PES yrn t the zone where the slope of its stress-strin curve mrkedly grows up through the most structure chnge, i.e. when ε c > %. Due to chrcteristic yield zone in stress-strin curve of CA yrn the dt otined in R- nd C- cycles when testing in mode I re unlikely comprle. It is seen in Fig., tht in R- test cycle t t* ( ) s stresses of CA yrn during relxtion process nrrowly differ etween themselves while the relxtion is going on t completely different elongtions ε t. Stress development (IR) curves of the yrns in oth test cycles of mode I (the vlues of F c, mn/tex for CA/PES yrn re: 9.7/.9 t θ t = s,./7. t θ t = s,./. t θ t = s,./99.7 t θ t = s) re shown in Figure, nd,. In R- test cycle of CA yrn the mount of stress increse is slightly dependent on the sustining time efore the retrction. Due to superposition of inverse relxtion nd ordinry stress relxtion processes, the distinct mximum in the curves is oserved. With increse of the sustining time θ t, the inverse 9

3 F, mn/tex 9 7. t*, s F, mn/tex. t*, s Fig.. Stress relxtion curves of the yrns. CA yrn; ε t : %, %, 9. %,. % (mode II); PES yrn; ε t : %, %, %,. %,. % (mode II) ε, % ε, %. t*, s. t *, s Fig.. Creep curves of the yrns. CA yrn; F c, mn/tex :., 7.,., 7.; PES yrn ; F c, mn/tex : 7., 99., 99.7,., 9. relxtion process ecomes slower nd less ffected y the stress relxtion process. Therefore, the mximum in the curves moves towrds longer time of oservtion (t**). After θ t = s the mximum is moved so considerly tht it could not e reched even till oservtion time t** = s. The IR process is more slow s the sustining time is incresed. In C- test cycle the mximum in IR curves moves towrds time t** xis in dependence on the sustining time θ t in the similr wy s in the R- cycle. Nevertheless, the increse in stress (IR) is pproximtely four times lower thn in R- test cycle. In the tests y mode I the vlues of strin ε t in R- cycle is 9. %, i.e. ove the yield point, while ll vlues of strin ε θ in C- cycle t constnt force re elow the yield point (ε θ =. % t θ t = s, ε θ =. % t θ t = s, ε θ =. % t θ t = s, ε θ =. % t θ t = s). So, the increse in stress depends more on the sustining time thn on the yrn elongtion: the increse in stress t time θ t = s is the highest despite the mount of strin (ε θ ) efore the retrction is the lowest (ε θ =. %). It is supposed tht this fct is ssocited with more pronounced orienttion of polymer chins during creep if compred to tht during relxtion t constnt elongtion. For the PES yrn the mximum in the stress development (IR) curves is eyond the oservtion time, ut the tendency of its movement towrds longer time of oservtion (t**) cn e distinguished. The chrcter of stress increse dependence on test regime is nlogous to tht of CA yrn. However, for PES yrn the difference in the vlues of strins when testing in the different cycles is not so distinct s for CA yrn: in R- cycle ε t =. %, while in C- cycle t θ t = s ε θ =. %, t θ t = s ε θ =. %, t θ t = s ε θ =. %, nd t θ t = s ε θ =. %. Therefore, inverse relxtion is quite comprle in its mount when testing in the different cycles. The results oviously showed tht the presupposition of the inverse stress relxtion process s tking plce in C- test cycle, i.e. fter previous sustining the specimen t constnt lod is proven out. The effect must e lso credily chrcteristic for ny for ny viscoelstic polymeric mteril. The viscoelstic recovery (VR, CR) of the yrns in mode I depends on the test regime similrly to the IR process (Fig., nd Fig., ). Due to the resons discussed ove creep recovery vlues of the yrns in the C- testing cycle re lower thn in R- testing cycle: pproximtely ten times lower for CA yrn nd pproximtely. times lower for PES yrn. Much more comprle results re otined in R- nd C- test cycles, when testing in mode II, i.e. when identicl elongtions vlues t time instnt t θ re mintined in oth test cycles (Fig., nd Fig. ). The vlues of strin ε t = ε θ, % for CA/PES yrn re the following: 9./. t 7

4 F F, mn/tex. 7 ε ε, % Fig.. Inverse stress relxtion () nd viscoelstic recovery () curves of CA yrn in R- (, left scle) nd C- ( , right scle) test cycles, mode I (ε t = 9. %); θ t : s, s, s, s F F, mn/tex 9 7. ε ε, % F F, mn/tex. ε ε, % Fig.. Inverse stress relxtion (IR, left scle) nd viscoelstic recovery (VR (CR), right scle) curves of PES yrn in R- ( ) nd C- ( ) test cycles. mode I, (ε t =. %); mode II, (F c = 9. mn/tex); IR curves s θ t : s, s, s, s; VR nd CR curves s θ t : s, s F F, mn/tex. ε ε, % Fig.. Inverse stress relxtion () nd viscoelstic recovery () curves of CA yrn in R- (, left scle) nd C- ( , right scle) test cycles, mode II (F c = 7. mn/tex); θ t : s, s, s, s 7

5 θ t = s,./7. t θ t = s, 9.7/.7 t θ t = s, nd./. t θ t = s. It is seen tht when the sme strin vlue is held on efore the retrction the mount of stress increse nd of viscoelstic contrction in R- nd C- cycles re similr nd tht with increse of sustining time efore the retrction the processes go on slower similrly to mode I. As result, it is oviously seen tht t identicl elongtions of the yrns t the end of loding period the inverse relxtion nd viscoelstic recovery processes go on similrly regrdless of the chrcter of testing cycle. CONCLUSIONS The inverse stress relxtion process is proven out s tking plce in the yrns in C- test cycle, i.e. fter previous sustining the specimen t constnt lod. The effect must e lso credily chrcteristic for textile frics nd for ny polymeric mteril. Viscoelstic recovery is the slower process thn the inverse stress relxtion. The time during which the yrns re undergone y specified constnt elongtion or constnt lod is the most effective fctor influencing the mount nd chrcter of oth inverse relxtion nd viscoelstic recovery in the yrns. At identicl elongtions of the yrns t the end of loding period the inverse relxtion nd viscoelstic recovery processes go on similrly regrdless of the chrcter of testing cycle. REFERENCES. Ledermn, H. Elstic nd Creep Properties of Filmentous Mterils nd other High Polymers. nd print. Wshington: The Textile Foundtion, 9.. Meredith, R. The Mechnicl Properties of Textile Fires. Amsterdm: North-Hollnd Pul. Co, 99.. Nchne, R. P., Sundrm, V. Anlysis or Relxtion Phenomen in Textile Fires Prt I: Stress Relxtion The Journl of The Textile Institute () 99: pp. 9.. Mnich, A. M., Ussmn, M. H., Brell, A. Viscoelstic Behvior of Polypropylene Fiers Textile Reserch Journl 9 () 999: pp... Menrd, K. P. Dynmic Mechnicl Anlysis: A Prcticl Introduction. CRC Press LLC, Wu, X., Wng, F., Wng, S. Properties of Wool/PET Composite Yrns Textile Reserch Journl 7 () : pp Aott, N. J. Extension nd Relxtion of Nylon Filments Textile Reserch Journl () 9: pp. 7.. Guthrie, J. C., Wierley, J. The Effect of Time on the Elstic Recovery of Fires Journl of the Textile Institute () 9: pp Gupt, V. B., Gopl Krishnn, Y. Creep nd Recovery Behviour of Oriented Nylon Filments Indin Journl of Textile Reserch () 97: pp Meredith, R. Relxtion of Stress in Stretched Cellulose Fires Journl of the Textile Institute () 9: pp. T T.. Stlevich, A. M., Tirnov, V. G., Meshchninov, Yu. N., Vol f, L. A. Anlyticl Description of the Process of Stress Relxtion in Synthetic Yrn Technology of the Textile Industry U.S.S.R Mnchester, The Textile Institute 97: pp... Vng, X., Yu, L. Y. The Stress Relxtion of Wool t High Strining Rte Journl of the Textile Institute () 99: pp. 9.. Inoue, M., Niw, M. Tensile nd Tensile Stress Relxtion Properties of Wool/Cotton Plied Yrns Textile Reserch Journl 7 () 997: pp. 7.. Stein, R., Hlsey, G., Eyring, H. Mechnicl Properties of Textile. IV. Textile Reserch Journl () 9: pp... Vitkusks, A., Mtukonis, A. Phenomenologicl Presenttion of the Inverse Stress Relxtion of Yrn Under Tension Lod Technology of the Textile Industry U.S.S.R Mnchester, The Textile Institute 9: pp. 9.. Vngheluwe, L., Kiekens, P. Simultion of Procedures to Avoid Set Mrks in Weving Cused y Relxtion Textile Reserch Journl 7 () 997: pp Vitkusks, A. Viscoelstic Properties of Textile Yrns. Reserch Prolems Fires & Textiles in Estern Europe () 99: pp... Mnich, A. M., de Cstellr, M. D. Elstic Recovery nd Inverse Relxtion of Polyester Stple Fier Rotor Spun Yrns Textile Reserch Journl () 99: pp Vngheluwe, L. Relxtion nd Inverse Relxtion of Yrns After Dynmic Loding. Textile Reserch Journl, vol., No. 9, 99: pp... Nchne, R. P., Sundrm, V. Anlysis or Relxtion Phenomen in Textile Fires Prt II: Inverse Relxtion The Journl of the Textile Institute () 99: pp... Vitkusks, A. Simultion of Inverse Stress Relxtion Following Creep in Textile Mterils Mterils Science (Medžigotyr) () 99: pp. 7.. Kothri, V. K., Rjkhow, R., Gupt, V. B. Stress Relxtion nd Inverse Stress Relxtion in Silk Fiers Journl of Applied Polymer Science : pp. 7.. Milšius, V. Investigtion of the Stress Relxtion nd the Inverse Stress Relxtion of the Chemicl Yrns nd the Woven Fric when the Relxtion Processes re Not Liner Behviour Tekhnologij tekstil noi promyshlennosti (Technology of the Textile Industry U.S.S.R.) 97: pp. 9 (in Russin).. Ymguchi, T., Kitgw, T., Yngw, T., Kimur, H. Reltionship Between Stress Relxtion nd Tensile Recovery of Filment Yrns Journl of the Textile Mchinery Society of Jpn 7 () 9: pp. 9.. Vitkusks, A. Inverse Relxtion nd Delyed Elstic Recovery of Textile Yrns: Resemlnt or Disprte? Textiles nd The Informtion Society Ppers Presented t The 7 th World Conference of The Textile Institute (My, 997, Thessloniki, Greece), vol. III, pp