Cyclic loading effect on the mechanical behaviour and morphology ...

442.2. Fatigue testsFatigue tests have been conducted using a Universaltesting machine (type ZWICK ROELLE Z005), with a<strong>loading</strong> capacity of 5 kN. This machine is equipped with acomputer controlled system for the data recording and dataprocessing (testxpert).<strong>Cyclic</strong>-test series at constant rate (speed) of 10mm/min have been carried following a prescribed displacementfor a number of cycles varying between 1 and10000, corresponding to two levels of <strong>loading</strong>s, ie. 50%and 70% of ΔL se . Each tests series is followed by a pullingout<strong>loading</strong> until failure occurs. The cycling frequency ismaintained constant at the value of 2 Hz. It should be mentionedthat a simple pulling-out test has been carried outprior to cycling, in order to investigate the <strong>effect</strong> of the<strong>loading</strong> rate. Based on the results the maintained a testingrate is 10 mm/min. To investigate the <strong>effect</strong> of the cyclingon the material morphology, observations using opticalmicroscope have been carried out.3. Results and discussionThe mechanical behaviour of PEHD 80 when subjectedto static pulling-out stresses is presented by meansFig. 3 Load-displacements curve (for v = 10 mm/min)of load-displacements curve in Fig. 3. At constant rate of<strong>loading</strong>; the elastic modulus was calculated for the deformationrange within of 0.05 to 0.25 %, according to theISO 527–1: 1993 (F) norms. The experimental values ofmechanical properties are presented in Table 2.Table 2Mechanical properties of the PEHD 80E,Δ LN/mm 2 F S e , N S e ,Δ LFmmMax , NFmax ,mm163 585 11.8 750.07 317.02Figs. 4 and 5 show the mechanical behaviour ofPEHD 80 under cyclic pulling-out corresponding to twolevels of <strong>loading</strong>: 50% and 70% of Δ L S e . These figuresillustrate the fact that high density polyethylene exhibits anelastic behaviour as of the most polymers during cyclic<strong>loading</strong>. A large hysterics loop is observed for the firstcycle, then, it reduces according to the increase of thenumber of cycles. The present observations were found tobe in close agreements to many results reported in the literature,e.g. [6].Fig. 6 illustrates the progression of <strong>loading</strong>against the number of cycles. This figure shows a typicalcyclic softening of the material from the early stage of<strong>loading</strong>, which reduces progressively for the followingcycles. Beyond 1000 cycles an apparent stability of thematerial is observed, this, was valid for both levels of <strong>loading</strong>.In Fig. 7 the elastic modulus is plotted against thenumber of cycles. From this figure it could be argued thatthe linear Young modulus increases three times from thefirst cycle. This could be attributed to the alignment oflamelas and to the morphological evolution of material [6].This phenomenon could also be explained by the wideningof the “covalent” links proportions, parallel to the directionof stresses, which causes increases of the elasticitymodulus [7].N = 1 cycleN = 10 cycleN = 100 cyclesN = 1000 cyclesN = 10000 cyclesFig. 4 Load-displacements curves (for a <strong>loading</strong> level of 50% Δ L S e )

45N = 1 cycleN = 10 cycleN = 100 cyclesN = 1000 cyclesN = 10000 cyclesFig. 5 Load-displacements curves, (for a <strong>loading</strong> level of 70% Δ L S e )Load (N)60050040030070%50%2000 1 2 3 4 5 6 7 8 9 10Number of cyclesLoad (N )g60050040030070%50%2000 10 20 30 40 50 60 70 80 90 100Number of cyclesLoad (N )60050040030070%50%Load (N)60050040030070%50%2000 100 200 300 400 500 600 700 800 900 1000Number of cycles2000 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000Number of cyclesFig. 6 <strong>Cyclic</strong> <strong>loading</strong>s against the number of cycles, (for level of <strong>loading</strong> 50% and 70% Δ L S e ))y modulus (MPaElasticitNumber of cyclesFig. 7 Elasticity modulus against number cycles (for levelof <strong>loading</strong> 50% and 70% Δ L S e )Observations using the optical microscope of thePEHD 80 for noncycled and cycled structures, illustratedin Figs. 8 and 9, for different level of <strong>loading</strong>s, show clearorientation of lamellas towards the direction of <strong>loading</strong>s.The increasing level of alignment is more pronounced forthe samples of the material cycled 100 times. The increaseof the level of <strong>loading</strong> leads to an increase of the lamellasalignment, mainly for the level 70% (Fig. 9, b). Figs. 8, dand 9, d show that at 1000 cycles, the structure follows alamellas pack rearrangement with lower density and betterorientation, compared to the noncycled structure.

46a120Xb120Xc120Xd120XFig. 8 Structures of the PEHD 80: (a) noncycled, (b) 1 cycle, (c) 100 cycles and (d) 1000 cycles (for <strong>loading</strong> level of50%ΔL Se )120Xa120Xb120Xcd120XFig. 9 Structures of the PEHD 80 (a) noncycled, (b) 1 cycle, (c) 100 cycles and (d) 1000 cycles (for <strong>loading</strong> level of70%ΔL Se )

474. ConclusionThe present research work is a part of the comprehensiveexperimental investigation on mechanical behaviourof high density polyethylene PEHD 80 under cyclic<strong>loading</strong>.The cycling of the PEHD80 leads to a considerableincrease of the elasticity modulus from the first cycles.<strong>Cyclic</strong> softening of the material for the first 1000 cycleswas observed, followed by cyclic stability beyond thisnumber of cycles.Optical microscopic observations illustrate a crystallinelamellas orientation parallel to the axis of appliedload, the alignment degree is a function of the level of<strong>loading</strong> mainly for the 1000 cycles.References1. Hubert, L. et all. Physical and mechanical propertiesof polyethylene for pipes in relation to molecular architecture.I. Microstructure and crystallization kinetics.-Polymer, 2001, 42, p.8425-8434.2. Hamouda, H.B.H. et all. Creep damage mechanismsin polyethylene gas pipes.-Polymer, 2001, 42, p.5425-5437.3. Baer <strong>effect</strong> of strain rate JMSc, 2000, 35, p.1857-1866.4. Byoung-Ho et all. Fracture initiation associated withchemical degradation: observation and modelling.-Int.J. of Solids and Structures, 2005, 42. p.681-695.5. Zhou, Y., Lu, X., Zhou, Z. and Brown, N. The relativeinfluence of molecular structure on brittle fractureby fatigue and under constant loads in polyethylene.-Polymer Eng. and Science, 1996, v.36. No16, p.2101-2107.6. Meyer, R.W., Pruitt, L.A. The <strong>effect</strong> of cyclic truestrain on the morphology, structure, and relaxation behaviourof ultra high molecular weight polyethylene.-Polymer, 2001, 42, p.5293-5306.7. Ashby, M.F., Jones, D.RH. Matériaux 2. Microstructureet Mise en Œuvre.-Paris: Dunod, 1991.L. Fatmi, A. Rouili, N. HamlaouiCIKLINĖS APKROVOS POVEIKIS DIDELIO TANKIOPOLIETILENO MECHANINĖMS SAVYBĖMS IRMORFOLOGIJAIR e z i u m ėDidelio tankio polietileniniai vamzdžiai(PEHD 80) iki šiol plačiai naudojami gamtinėms dujomstransportuoti ir paskirstyti. Eksploatacijos metu juos veikiavidinis bei atmosferos slėgiai, grunto poslinkiai, jais tekančioskysčio dinaminės jėgos. Šie įtempiai sukelia didelesvamzdžių medžiagos deformacijas ir keičia jos mechaninessavybes. Šiame darbe eksperimentiškai tyrinėta ciklinėsapkrovos įtaka PEHD 80 markės vamzdžių funkcionavimui.Bandymų metu bandiniai buvo tempiami iki suirimo.Tempimo kreivės, apibūdinančios PEHD 80 vamzdžiųmechanines savybes, rodo, kad pirmųjų bandymų ciklųmetu formuojamos didelės histerezės kilpos, kurios, didėjantciklų skaičiui, mažėja. Nustatyta, kad ciklinė apkrovadidina medžiagos linijinį standumą ir ciklinį sipnėjimą.Stebint mikroskopu matyti, kad makromolekulių grandinėsišsidėsčiusios tempimo kryptimi.L. Fatmi, A. Rouili, N. HamlaouiCYCLIC LOADING EFFECT ON THE MECHANICALBEHAVIOUR AND MORPHOLOGY OF THE HIGHDENSITY POLYETHYLENES u m m a r yHigh density polyethylene pipes (PEHD 80) arestill widely used in the distribution and transportation ofnatural gas. While in use, these materials are exposed tointernal pressures, to earth pressures, to soil movementsand to the dynamic <strong>effect</strong> of the liquid displacements.These stresses induce inevitably, considerable deformationsof the tube’s material and affect its mechanical properties.In the present work, the <strong>effect</strong> of cyclic <strong>loading</strong> onthe behaviour of PEHD 80 tubes is experimentally investigated.Samples are tested to cyclic <strong>loading</strong>s followed bythe pulling (tension) until failure of the material. Thecurves expressing the mechanical behaviour obtained indicateclearly that the PEHD 80 exhibits large hystereticloops for the first <strong>loading</strong> cycles, and then the loops decreaseaccording to the increase of the cycle number. Itwas concluded that cyclic <strong>loading</strong> causes a considerableincrease in the linear stiffness and the cyclic softening ofthe material. At microscopic levels, the observations haveshowed parallel orientation of the macromolecular chainsto the tension axis.L. Фamми, A. Рoуили, Н. ГaмлoуиВЛИЯНИЕ ЦИКЛИЧЕСКОЙ НАГРУЗКИ НАМЕХАНИЧЕСКИЕ СВОЙСТВА И МАРФОЛОГИЮПОЛИЭТИЛЕНА БОЛЬШОЙ ПЛОТНОСТИР е з ю м еПолиэтиленовые трубы (PЕHD 80) изготовленныеиз полиэтилена большой плотности, до сих поршироко используются в газопроводах. Во время эксплуатацииони подвергаются воздействиям внутреннегои атмосферного давлении, сдвигам грунта, динамическимсилам протекающей жидкости. Возникшиенапряжения при этом вызывают большие деформацииматериала трубы, которые меняют ее механическиесвойства. В работе экспериментально исследуется влияниециклической нагрузки на функциональность трубгазопровода марки PЕHD 80. Во время экспериментовобразцы нагружались до разрушения. Диаграмы деформирования,характеризующие механические свойстватруб марки RЕHD 80, показывают, что во времяпервых циклов формируются большие петли гистерезиса,которые при увеличении числа циклов уменьшаются.Установлено, что циклическая нагрузка увеличиваетлинейную жесткость и циклическое разупрочнениеобразца. На уровне микроскопических исследованийцепи макромолекул распределяются по направлениюрастяжения.Received September 24, 2007