Procjena radnog vijeka kućišta ventila

Galić, Ivica (2012) Procjena radnog vijeka kućišta ventila. = Life assessment of valve body. Doctoral thesis , Sveučilište u Zagrebu, Fakultet strojarstva i brodogradnje, UNSPECIFIED. Mentor: Opalić, Milan.

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01_10_2012_Doktorat.pdf Jezik dokumenta:Croatian

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Abstract (Croatian)

U radu je provedena trodimenzijska analiza nastanka i rasta pukotine kao i rast hipotetskih pukotina na prolaznim i troputim kućištima ventila s različitim debljinama stijenki. Linearno elastičnom analizom kućišta ventila određeno je stanje naprezanja na kućištima opterećenih unutarnjim tlakom. Numerički model je verificiran eksperimentalnim ispitivanjem, pomoću metode tenzometrije, mjerenjem deformacija troputog kućišta ventila D\N100 i prolaznog kućišta D\N50, opterećenih unutarnjim tlakom. Poznavanjem stanja naprezanja te pomoću ovisnosti amplitude deformacije o broju ciklusa do nastanka pukotine određeno je potencijalno mjesto i broj ciklusa do nastanka pukotine na homogenim kućištima ventila. \Nakon toga je provedena analiza rasta nastale pukotine do kritične duljine inkrementalnim povećavanjem duljine pukotine i određivanjem koeficijenta intenzivnosti naprezanja za svaki inkrement. Na osnovu određenih koeficijenata i poznavanjem karakteristika materijala određeni su brojevi ciklusa rasta pukotine korištenjem numeričke integracije Parisovog zakona. Korištenjem dijagrama procijene loma za vrijednosti kritičnih duljina pukotine uočeno je da neće doći do sloma kućišta ventila, nego da će se pojaviti slučaj propuštanja prije loma. Kako je u praksi rijetkost da se procesom lijevanja dobije homogena struktura, proveden je postupak određivanja najveće dopustive greške u stijenci kućišta ventila. Pukotine koje su smještene u području na kojima su najveća glavna naprezanja imaju najbrži rast a time i kućišta s tim pukotinama imaju najkraći radni vijek. U cilju definiranja najveće dopustive greške s kojom će kućište imati traženi radni vijek, određeni su brojevi ciklusa potrebni za rast hipotetskih pukotina do kritične duljine i na kućišta na kojima nije došlo do nastanka pukotine. Na osnovu brojeva ciklusa rasta pukotine, grafičkim putem određene su najveća duljina pukotina čiji daljnji rast do kritične duljine neće biti kraći od radnog vijeka kućišta ventila. Na greškama koje imaju duljine najvećih dopustivih pukotina, proveden je postupak određivanja nastanka oštre pukotine, na osnovu čega je uočeno da je broj ciklusa do nastanka oštre pukotine kod ovakvih grešaka jako kratak i da se u odnosu na traženi radni vijek može zanemariti.

Abstract

Three dimensional numerical analysis of fatigue crack initiation, initiated cracks and hypothetic cracks growth were performed on two and three-way valve bodies with different wall thicknesses. Linear elastic finite elements method was used to determine stress states on valve bodies which were loaded by inner pressure. The numerical model was validated experimentally using strain gages and measuring strains on three-way valve body DN100 and two-way valve body DN50, loaded by inner pressure. The time to fatigue crack initiation and potential crack location on homogenous valve body were determined using stress state and local strain-life. After that, analyses of crack growth from initial to critical size were conducted by incremental increased crack length and calculated stress intensity factor for every increment. The time for crack growth was determined using numerical integration of Paris law and material characteristics. Using failure assessment diagram, it was noticed that would happened leak before break. Since a homogenous structure is very rarely obtained by sand casting, the biggest allowable failure size in the valve body wall was determined. The cracks, which were located in the area of maximum principal stress, had fastest growth and valve bodies with those cracks had shorter life. To define the biggest allowable failure for which valve body would have defined life, the number of stress cycles needed for hypothetic crack growth to critical size was determined. According to this number of cycles, the biggest allowable crack size for which the growth wouldn’t be shorter then valve body life was graphically determined. A procedure was conducted determining number of stress cycle for crack initiation on the valve bodies with failure which had size of biggest allowable crack and it was noticed that this time was very short according to valve body life.

Item Type: Thesis (Doctoral thesis)
Uncontrolled Keywords: Kućište ventila; nastanak pukotine; rast pukotine; zamor materijala; metoda kritičnih ravnina; MKE; X-FEM
Keywords (Croatian): Valve Body; crack initiation; crack growth; fatigue; critical plane method; FEM; X-FEM
Date Deposited: 22 Sep 2014 18:00
Last Modified: 16 Oct 2015 12:57
URI: http://repozitorij.fsb.hr/id/eprint/1995

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