Title Model probabilističke procjene trajnosti zavarenih pomorskih konstrukcija Title (english) Probabilistic model for durability assessment of welded marine structures Author Darko Pastorčić Mentor Goran Vukelić (mentor)Committee member Radoslav Radonja (predsjednik povjerenstva)Committee member Joško Parunov (član povjerenstva)Committee member Špiro Ivošević Granter University of Rijeka Defense date and country 2022-03-03, Croatia Scientific / art field, TECHNICAL SCIENCES Universal decimal classificationUDC ) 51 - Mathematics 621 - Mechanical engineering. Nuclear technology. Machinery 629.5 - Watercraft engineering Abstract U ovom radu provedene su eksperimentalna i numerička analiza degradacije mehaničkih svojstava čelno zavarenih uzoraka brodograđevnog čelika AH36, dugotrajno izloženih korozivnom okolišu sjevernog Jadrana na 6, 12, 24 i 36 mjeseci. Na osnovu dobivenih rezultata razvijen je probabilistički model zamornog vijeka trajanja pomorskih konstrukcija. Uzorci su podvrgnutim standardnim razarajućim testovima, dok je površina uzoraka ispitana optičkim i pretražnim mikroskopima. Na temelju eksperimentalnih
... More rezultata i primjenom stohastičkih procesa izrađen je model korodiranih površina uzoraka u zoni taljenja, zoni utjecaja topline i osnovnom materijalu. Za simulaciju statičkog vlačnog pokusa napravljen je konačnoelementni model svih uzoraka koji uključuje SMCS (engl. stress modified critical strain) kriterij duktilnog loma s različitim α parametrima žilavosti materijala i modelima očvršćivanja za sve zone zavarenih uzoraka. Rezultati pokazuju da porastom trijaksijalnosti, koja ovisi o broju i veličini jamica kod jamičaste korozije, vrijednost kritične plastične deformacije pada što uzrokuje raniju pojavu loma. Najveća je degradacija mehaničkih svojstava kod uzoraka iz zone plime i oseke, kod kojih su vlačna čvrstoća i naprezanje tečenja smanjeni za četvrtinu nakon tri godine izlaganja. Osnova probabilističke procjene zamornog vijeka koncept je granične funkcije stanja uz primjenu Monte Carlo metode. Kalibracija parametara teorije mehanike loma sa S-N (engl. stress vs. number of cycles curve) analizom izvršena je posebno razvijenim računalnim rutinama i to za slučajeve kada početna veličina pukotine nije poznata i kada je zadana kao trogodišnja jamica jamičaste korozije. Vjerojatnost zamornog loma i parametar materijala C iz Paris-Erdoganove jednadžbe, a time i propagacije pukotine osjetljivi su na izbor početne veličine pukotine. Kod ažuriranja vjerojatnosti loma nakon nerazarajućeg ispitivanja iza četvrte i pete godine eksploatacije postoje razlike u probabilističkim procjenama loma. Less Abstract (english) In this study, an experimental and numerical analysis of degradation of mechanical properties of butt welded samples of shipbuilding AH36 steel, after long term exposure to a corrosive environment in the north Adriatic lasting 6,12, 24 and 36 months, were conducted. Based on the obtained results a probabilistic model to estimate the fatigue life of free corroded marine structure was established. The samples were subjected to the standard destructive testing while the surface of the samples
... More was tested using optical and scanning microscopy. Using the experimental results as a basis and applying stochastic processes, a model of the sample corroded surface in the weld metal, heat affected zone and the base metal was developed. In order to simulate static tensile testing a FE model was applied, that included the SMCS (stress modified critical strain ) criteria for ductile fracture with different 𝛼 parameters and hardening models for all zones of the welded samples. Results show that by the increase of the triaxiality, which depends on the number and size of the pits in pitting corrosion, the value of critical plastic deformation decreases which causes an earlier fracture. The biggest degradation of mechanical properties was found in the sea tide zone samples, where the tensile strength and the yield strength are reduced by a fourth of their original value after three years of exposure. The basis of the fatigue life probability estimate is the concept of the boundary state function while applying the Monte Carlo method. The calibration of parameters of the fracture mechanics theory using S-N analysis was performed with specially devised computer routines for the case when the initial size of the crack is unknown and for the case when it is set as a three year old corrosion pit. The probability of fatigue fracture and material parameter C from the Paris-Erdogan equation, consequently the crack propagation, are sensitive to the choice of the initial crack size. When updating failure probability after nondestructive testing, after the fourth and fifth year of service life, there are differences in probability estimates. Less Keywords
zavarene konstrukcije
degradacija materijala
korozivni okoliš
trajnost brodograđevnog čelika
stohastičko modeliranje
vjerojatnost zamornog loma
Keywords (english)
welded structures
material degradation
corrosive environment
shipbuilding steel durability
stochastic modelling
probability fatigue failure
Language croatian URN:NBN urn:nbn:hr:187:860866 Promotion 2023-05-19 Study programme Title: Postgraduate (doctoral) university study programme - Maritime Studies Type of resource Text Extent IX, 127 listova : ilustr., graf. prikazi File origin Born digital Access conditions Open access Terms of use Created on 2023-04-25 09:50:04
