Title Numeričko modeliranje pukotina u metalnim i polimernim materijalima Title (english) Numerical modelling of cracks in metal and polymer materials Author Ivica Skozrit Mentor Zdenko Tonković (mentor)Committee member Jurica Sorić (predsjednik povjerenstva)Committee member Zdenko Tonković (član povjerenstva)Committee member Mladen Šercer (član povjerenstva)Committee member Mirko Husnjak (član povjerenstva)Committee member Zoran Ren (član povjerenstva) Granter University of Zagreb Defense date and country 2011-03-23, Croatia Scientific / art field, TECHNICAL SCIENCES Universal decimal classificationUDC ) 51 - Mathematics 53 - Physics Abstract U radu je provedeno numeričko modeliranje pukotina u metalima te eksperimentalno i numeričko modeliranje zareznog djelovanja u polimernim materijalima. Na temelju opsežnih numeričkih analiza debelostjenih metalnih cijevi opterećenih unutarnjim tlakom s polueliptičkim pukotinama različitih dimenzija, predloženi su izrazi za točniju procjenu Jintegrala i graničnog tlaka u odnosu na do sada predložena rješenja. Pritom je u izrazima za referentno naprezanje i J-integral umjesto graničnog tlaka predložena nova funkcija opterećenja. To omogućava da rezultati za J-integral dobiveni novom metodom referentnog naprezanja neznatno odstupaju od rješenja dobivenih inkrementalnom teorijom plastičnosti, što je istraženo na primjerima debelostjenih cijevi generatora pare tipa VVER 1000. Usporedbom rješenja dobivenih primjenom predloženog izraza za granični tlak za debelu cijev s pukotinom s dostupnim rezultatima iz literature, dan je kritički osvrt na postojeća rješenja. Na temelju analiza plastičnog kolapsa (sloma) cijevi generatora pare pokazano je da rezultati dobiveni primjenom izraza iz literature za tlak plastičnog kolapsa mogu biti vrlo nepouzdani. Pritom je detaljno istražen utjecaj izbora naprezanja tečenja na procjenu plastičnog kolapsa cijevi. Umjesto faktora naprezanja tečenja koji ovisi samo o vrsti materijala, u izrazu za tlak plastičnog kolapsa cijevi predložen je novi faktor, koji osim o materijalu ovisi i o dimenzijama pukotine. To omogućuje točnije modeliranje kako plitkih, tako i dubokih pukotina u cijevima u odnosu na postojeća rješenja. Osim toga, vrlo detaljno je istražena primjena tehnike podmodeliranja za izračunavanje parametara mehanike loma u debelostjenim cijevima. Predložene su optimalne veličine podmodela u ovisnosti o duljini pukotine za izračunavanje parametara linearno elastične i elastoplastične mehanike loma. U području istraživanja polimera, na temelju eksperimentalnih rezultata iz literature i s pomoću formulacija sličnih onima izvedenim za metalne materijale, izveden je numerički algoritam za modeliranje ponašanja polietilena srednje gustoće za slučaj puzanja, sprezanjem efekata viskoelastičnosti i viskoplastičnosti. \Nova predložena formulacija namijenjena je analizi mehanizama primarnog i sekundarnog puzanja te loma pri sporom rastu pukotine u polietilenu. Modeliranje realnog ponašanja materijala omogućava točnije izračunavanje parametra mehanike loma C*-integrala u vrhu pukotine u odnosu na do sada predložena rješenja. Rezultati numeričkih testova uspoređeni su s dostupnim eksperimentalnim i numeričkim rješenjima. Osim toga, u radu su provedena eksperimentalna istraživanja lomnog i vremenski ovisnog ponašanja polietilena PE100 pri različitim temperaturama i brzinama deformacije, što u literaturi nije istraženo. Određeni su parametri materijala za primarnosekundarni zakon puzanja čime se je numeričko modeliranje približilo realnom ponašanju materijala. Rezultati numeričkih analiza zareznog djelovanja u cijevima od polietilena PE100 uspoređeni su s eksperimentalnim rješenjima iz literature.
Abstract (english) Numerical modelling of cracks in metals and experimental and numerical modelling of notches in polymer materials are presented in this thesis. Thick-walled metal pipes with semielliptical cracks of various dimensions were subjected to internal pressure. On the basis of extensive numerical analysis of the case, expressions for a more accurate assessment of the Jintegral and limit pressure in comparison to the previously recommended solutions are proposed. A new loading function is recommended instead of limit pressure in the expressions for reference stress and the J-integral. As a result, the results for the J-integral obtained by using the new reference stress method deviate insignificantly from the results obtained from the incremental theory of plasticity. This is illustrated by the examples of the thick-walled metal pipes of the VVER 1000 steam generators. The results obtained by using the proposed expressions for limit pressure in thick-walled metal pipes with cracks were compared with the results available in literature. An analysis of the plastic collapse of the pipes in steam generators has shown that the results obtained by using expressions for plastic collapse pressure from literature may indeed be very unreliable. The impact of the selection of the flow stress factor on the estimation of the plastic collapse pressure in pipes was investigated. Instead of the flow stress factor which depends only on the type of material, a new factor, which depends both on the material and on the crack dimensions, is proposed for the plastic collapse pressure expression. This provides more exact modelling not only for shallow cracks but also for deep cracks in pipes when compared to the existing solutions. Furthermore, a very detailed investigation was conducted into the use of the sub-modelling technique when calculating fracture mechanics parameters in thick-walled metal pipes. Optimal sizes of the sub-model related to the length of the crack are proposed for calculating the parameters of the linear elastic and the elastic-plastic fracture mechanics. Regarding the research into polymers, a numerical algorithm for modelling the behaviour of medium-density polyethylene for the case of creeping, including viscoelastic and viscoplastic effects, was derived on the basis of experimental results available in literature and of formulations similar to those proposed for metal materials. The new numerical algorithm proposed is intended for analysing the mechanisms in primary and secondary creep and fracture during the slow crack growth in polyethylene. Modelling the real behaviour of materials allows a more accurate calculation of parameters of fracture mechanics. The results of numerical tests were compared to the available experimental and numerical solutions. In addition, the paper includes the conducted experimental research into the fracture and the time-dependent behaviour of PE100 polyethylene at various temperatures and deformation rates, which has not been reported in literature. Particular material parameters for a primarysecondary creep law were determined. This brought the numerical modelling close to the real behaviour of materials. The results of the numerical analysis for notched action in pipes made of PE100 polyethylene were compared to experimental results in literature.
Keywords
metoda konačnih elemenata
mehanika loma
koeficijent intenzivnosti naprezanja
J-integral
granično opterećenje
opterećenje plastičnog kolapsa
metoda referentnog naprezanja
polietilen
puzanje
C-integral
Keywords (english)
finite element method
fracture mechanics
stress intensity factor
J-integral
limit load
plastic collapse load
reference stress method
polyethylene
creep and C-integral
Language croatian URN:NBN urn:nbn:hr:235:006579 Study programme Title: Mechanical Engineering and Naval Architecture Type of resource Text File origin Born digital Access conditions Open access Terms of use Created on 2020-05-15 17:54:36
