1. Effect of residual stresses on the fatigue stress range of a pre-deformed stainless steel AISI 316L exposed to combined loadingDarko Jagarinec, Nenad Gubeljak, 2024, original scientific article Abstract: AISI 316L austenitic stainless steel is utilized in various processing industries, due to its abrasion resistance, corrosion resistance, and excellent properties over a wide temperature range. The physical and mechanical properties of a material change during the manufacturing process and plastic deformation, e.g., bending. During the combined tensile and bending loading of a structural component, the stress state changes due to the residual stresses and the loading range. To characterize the component’s stress state, the billet was bent to induce residual stress, but a phase transformation to martensite also occurred. The bent billet was subjected to combined tensile–bending and fatigue loading. The experimentally measured the load vs. displacement of the bent billet was compared with the numerical simulations. The results showed that during fatigue loading of the bent billet, both the initial stress state at the critical point and the stress state during the dynamic loading itself must be considered. Analysis was demonstrated only for one single critical point on the surface of the bent billet. The residual stresses due to the phase transformation of austenite to martensite affected the range and ratio of stress. The model for the stress–strain behaviour of the material was established by comparing the experimentally and numerically obtained load vs. displacement curves. Based on the description of the stress–strain behaviour of the pre-deformed material, guidelines have been provided for reducing residual tensile stresses in pre-deformed structural components.
Keywords: metastable austenitic stainless steel, fatigue, residual stress, phase transformation
Published in DKUM: 28.11.2024; Views: 0; Downloads: 8
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2. Experimental characterization and phase-field damage modeling of ductile fracture in AISI 316lVladimir Dunić, Nenad Gubeljak, Miroslav Živković, Vladimir Milovanović, Darko Jagarinec, Nenad Djordjevic, 2024, original scientific article Abstract: ) Modeling and characterization of ductile fracture in metals is still a challenging task in
the field of computational mechanics. Experimental testing offers specific responses in the form of
crack-mouth (CMOD) and crack-tip (CTOD) opening displacement related to applied force or crack
growth. The main aim of this paper is to develop a phase-field-based Finite Element Method (FEM)
implementation for modeling of ductile fracture in stainless steel. (2) A Phase-Field Damage Model
(PFDM) was coupled with von Mises plasticity and a work-densities-based criterion was employed,
with a threshold to propose a new relationship between critical fracture energy and critical total
strain value. In addition, the threshold value of potential internal energy—which controls damage
evolution—is defined from the critical fracture energy. (3) The material properties of AISI 316L steel
are determined by a uniaxial tensile test and the Compact Tension (CT) specimen crack growth test.
The PFDM model is validated against the experimental results obtained in the fracture toughness
characterization test, with the simulation results being within 8% of the experimental measurements.
Keywords: phase-field damage modeling, ductile fracture, crack-tip opening displacement, crack growth, resistance curve, finite element method, simulations
Published in DKUM: 19.09.2024; Views: 0; Downloads: 12
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3. Vpliv zaostalih napetosti na rast utrujenostne razpoke v upognjenem delu podnice tlačne posodeDarko Jagarinec, 2017, doctoral dissertation Abstract: Postopek izdelave podnice ustvarja kompleksno deformacijsko-napetostno stanje, ki ga ni možno enostavno prenesti v laboratorijske pogoje testiranja. V nalogi smo pokazali, da s predhodnim upogibanjem in nato nateznim obremenjevanjem ustvarimo sestavljeno napetostno stanje, ki ga je možno analizirati le v eni kritični točki na notranji strani upognjenca. Z numeričnim modeliranjem napetostnega stanja smo se približali eksperimentalno določenim razmeram med utrujanjem s kombiniranim obremenjevanjem pred-deformiranih vzorcev. Na osnovi opravljenih numeričnih analiz, ki so bile podprte z eksperimentalnim testiranjem, kako v procesu upogibanja s tremi različnimi stopnjami pred deformacije tako tudi v procesu cikličnega natezno-upogibnega obremenjevanja z različnimi amplitudami sil in ob upoštevanju spremembe amplitude in napetostnega razmerja, smo dobili vrednosti dinamične trdnosti skladne s S-N Wohlerjevim diagramom. S tem smo potrdili, da je v doktorski disertaciji predstavljen postopek in model pravilen za določitev dejanskih napetostnih razmer v pred-deformiranih upognjenih vzorcih. V doktorski nalogi je pokazano, da zaostale napetosti v metastabilnem avstenitu AISI 316L, ki so posledice deformacije in fazne transformacije v martenzitno mikrostrukturo, vplivajo na iniciacijo in širjenje utrujenostnih razpok, pri čemer se napetostno razmerje v materialu razlikuje od razmerja sil R utrujanja zaradi teh zaostalih napetosti Rs. To smo pokazali na osnovi določitve napetostnega stanja z numeričnim modeliranjem in z eksperimentalno dobljenimi vrednostmi na S-N Wöhlerjevi krivulji, ki se ujemajo z rezultati upogibnega utrujanja ploščatih preizkušancev s polkrožnim utorom.
Keywords: metastabilni avstenit, podnica, S-N krivulja, utrujanje, zaostale napetosti, upogibanje, lomna žilavost
Published in DKUM: 29.11.2017; Views: 1959; Downloads: 259
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4. Fracture behaviour of thin sheet stainless steelNenad Gubeljak, Darko Jagarinec, Jožef Predan, John Landes, 2012, published scientific conference contribution Abstract: The differences in fracture behavior between the compact tension C(T) and the middle tensile M(T) specimens make structure integrity assessment uncertain. Two different types of specimens C(T) and M(T) specimens made from stainless steel have been used for fracture toughness testing at the room temperature by the principles of the ASTM 1820-05 standard procedure. Stable crack initiation and crack propagation occurred for the C(T) specimens at lower values of crack driving force than for the M(T) specimens. Crack tip opening displacement-CTOD has been directly measured on the surface of specimens by using a stereo-optical grading method. The critical crack tip opening displacement at crack initiation CTODi has been measured as a plastic Stretch Zone Width (SZW) during a post test fractographic inspection. Comparison between the CTOD-R curves of both types of specimens shows some difference between the C(T) and the M(T) specimens, but a more significant difference appeared in the crack driving force, as consequence of different constraint (triaxiality) of the C(T) versus the M(T) specimens. Therefore, the result obtained by test on laboratory C(T) specimens cannot be directly used as fracture toughness material properties in a structure integrity assessment, except as a conservative lower bound estimate.
Keywords: stainless steel, compact tension specimen, middle tension specimen, CTOD-R curves
Published in DKUM: 10.07.2015; Views: 1351; Downloads: 96
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6. PROJEKTIRANJE TLAČNEGA REAKTORJA 1000 LDarko Jagarinec, 2010, undergraduate thesis Abstract: Diplomska naloga opisuje projektiranje tlačnega reaktorja. Projektiranje tlačnega reaktorja je izvedeno za slovensko podjetje, ki se ukvarja s proizvodnjo tlačnih posod. Tlačni reaktor je v bistvu tlačna posodo v kateri se dogajajo kemične reakcije.
Prvi del diplomskega dela je sestavljen iz teorije o tlačnih posod, omenjena je zakonodaja in predpisi. Trdnostni preračun je izveden po AD 2000 Regelwerk, kjer so računati vsi deli tlačnega reaktorja (dna, plašč, noge, varnostni ventil, prirobnice, tesnila, vijaki). Opisana so nerjavna jekla katera uporabljamo kot osnovni material in izveden je natezni preizkus za material kateri je uporabljen kot osnovni material za tlačni reaktor. V drugem delu je izvedena numerična analiza dna z vsemi priključki in obravnavani so kritični deli. Numerična analiza je izvedena z programom ABAQUS CAE 6.7.1, modeliranje in dokumentacija je izvedena z programoma Autodesk Inventor 2010 in Autodesk AutoCAD Mechanical 2010. Na koncu so elaborati za tlačni reaktor in nasveti za projektiranje.
Keywords: tlačni reaktor, tlačna posoda, projektiranje, AD – 2000 Regelwerk, nerjavna jekla, varnostni ventil, trdnostna analiza, metoda končnih elementov, ABAQUS CAE 6.7.1, Autodesk Inventor 2010, Autodesk AutoCAD Mechanical 2010
Published in DKUM: 05.08.2010; Views: 4261; Downloads: 586
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