Finite element analysis of fracture resistance parameters for stationary semi-elliptical surface cracks in high strength steel

This study deals with numerical simulations based on elastic-plastic finite element method for nonlinear stress analyses on pre-cracked tensile specimens with high strength properties. The influence of crack geometry on fracture resistance properties is considered with numerical analysis on models with two different cracks. Results on fracture resistance parameters are obtained for both simulated tensile panels with different crack lengths and width, referred as short and long crack. The obtained numerical results for crack resistance behaviour of tensile specimens are compared with data results from existing experimental investigations on specimens with same shape and material properties. Also, comparison is made with results obtained from similar standard experimental procedure. It is shown, that a proper combination of numerical and experimental procedures can lead research to significantly accurate results when investigating the fracture response of materials

Finite element analysis of fracture resistance parameters for stationary semi-elliptical surface cracks in high strength steel

This study deals with numerical simulations based on elastic-plastic finite element method for nonlinear stress analyses on pre-cracked tensile specimens with high strength properties. The influence of crack geometry on fracture resistance properties is considered with numerical analysis on models with two different cracks. Results on fracture resistance parameters are obtained for both simulated tensile panels with different crack lengths and width, referred as short and long crack. The obtained numerical results for crack resistance behaviour of tensile specimens are compared with data results from existing experimental investigations on specimens with same shape and material properties. Also, comparison is made with results obtained from similar standard experimental procedure. It is shown, that a proper combination of numerical and experimental procedures can lead research to significantly accurate results when investigating the fracture response of materials

Local approach to ductile fracture of low steel welded joints

Predmet disertacije je analiza nastanka žilavog loma i stabilnog rasta prsline mehanizmom žilavog loma u niskolegiranom čeliku i zavarenim spojevima primenom lokalnog pristupa. Razmatrani su spojevi koji sadrže jedan ili dva metala šava, od kojih jedan ima manju (undermatch - UM), a drugi veću (overmatch - UM) čvrstoću od osnovnog materijala. Ispitivane su epruvete za savijanje u tri tačke (SENB) sa početnom prslinom u metalu šava kod svih spojeva. Početna prslina kod spojeva sa dva metala šava imala je tri različita položaja: samo u overmatch delu, samo u undermatch delu, ili kroz oba metala šava - po celoj debljini spoja. Takođe, analiziran je razvoj oštećenja u šavnim cevima bez početne prsline, sa koncentratorom napona tipa zareza. Ovakva geometrija je aproksimacija cevovoda oštećenih lokalnom korozijom, čiji integritet se u literaturi po pravilu procenjuje u odnosu na plastični kolaps uz zanemarujivanje loma kao mehanizma otkaza. U ovom radu je razmotren nastanak prsline na dnu žleba kao kriterijum gubitka nosivosti oštećenog dela cevi. Proračuni metodom konačnih elemenata (MKE), koji uključuju primenu lokalnog pristupa (kompletnog Gursonovog modela - CGM) u analizi zavarenih spojeva i cevi sa simuliranim korozionim oštećenjima, su urađeni u programskom paketu Abaqus. Analiziran je uticaj heterogenosti materijala, izazvane različitim zateznim osobinama osnovnog materijala i dva metala šava, na ponašanje spojeva pri lomu. Zbog odgovarajućeg opisivanja oštećenja, određene su mikrostrukturni parametri koji utiču na mehanizam žilavog loma čelika i zavarenih spojeva čelika - sadržaj nemetalnih uključaka i srednji slobodni put između njih. Ispitan je uticaj širine spoja i dužine početne prsline na ponašanje pri lomu. Utvrđeno je da kod OM spojeva povećanje širine spoja dovodi do smanjenja, a kod UM spojeva do povećanja otpornosti prema lomu. Takođe, uočeno je da se razlika u rezultatima dobijena ispitivanjem UM spojeva širine 12 i 18 mm jako mala, i pokazano je da za širine preko 12 mm heterogenost materijala nema većeg uticaja na rast prsline. Imajući u vidu lokalni karakter oštećenja mehanizmom žilavog loma, naročita pažnja je posvećena uticaju mreže konačnih elemenata na otpornost prema lomu. Ispitivanjem spojeva sa jednim metalom šava (OM ili UM) zaključeno je da postoji razlika u veličini elementa koja najbolje opisuje početak rasta i rast prsline. Naime, za OM spojeve ova veličina odgovara srednjem slobodnom putu između nemetalnih uključaka, dok je za UM spojeve ona tri puta veća...The subject of this dissertation is analysis of ductile fracture initiation and stable crack growth by ductile fracture mechanism in low alloyed steel and welded joints. The analysis is conducted using local approach to fracture. Welded joints with one or two weld metals are examined; one of them has lower strength (undermatch - UM) while the other has higher strength in comparison with the base metal (overmatch - OM). Precracked three point bending (SENB) specimens are analysed. In the joints with two weld metals, the pre-crack has three different positions: only in overmatch part, only in udermatch part or through both of them - through thickness dirrection. The damage development is also analysed in the seam pipes, without a pre-crack but with a notchtype stress concentrator. This geometry is aproximation of the pipeline damaged by local corosion; in the literature, integrity of such structures is often assessed based on the plastic collapse, neglecting the fracture as failure mechanism. In this work, fracture initiation at the bottom of the notch is applied as the failure criterion for damaged portion of the pipeline. Finite element calculations, which include the application of local approach (complete Gurson model - CGM) in analysis of welded joints and pipes with simulated corrosion damages, are conducted using the software package Abaqus. The inflence of the material heterogeneity, caused by different tensile properties of the base metal and two weld metals, on the fracture behavior of joints is analysed. For appropriate damage description, microstructural parameters affecting the ductile fracture mechanism in steel and its welded joints are determined - volume fraction of non-metallic inclusions and mean free path between them. The influence of joint width and initial crack length on the fracture behavior is assessed. For OM joints, the increase of the joint width decreases the fracture resistance, while fracture resistance of UM joints grows. It is also shown that the difference in results for UM joints with 12 and 18 mm width is rather small, and the increase of width over 12 mm does not have a significant influence on the crack growth. Bearing in mind the local character of the damage by ductile fracture mechanism, special attention is devoted to the influence of the finite element mesh to the fracture resistance. Testing the joints with one weld metal (OM or UM) revealed a difference in element sizes that result in the most adequate description of the crack growth initiation and stable growth. For OM joints, this size approximates the mean free path between the non-metallic inclusions, while in the case of UM joints element size is three times larger..

Srednji i trenutni maseni protok goriva - automatizacija diskontinualnog merenja u realnom vremenu

U radu su prikazani rezultati razvoja prototipa automatizovanog procesorski upravljanog laboratorijskog sistema za merenje srednjeg i trenutnog masenog protoka goriva u realnom vremenu na principu promene hidrostatičkog pritiska u mernoj posudi. Sa stanovišta obrade signala, metodom virtuelnog eksperimenta, testirane su tri statističke metode pokretnih sredina i metode najmanjih kvadrata i aproksimacionog kubnog splajna. Eksperimentalna verifikacija sprovedena je u vanmotorskim uslovima, korišćenjem piezo-rezistivnog senzora IHTM Tpr 110 i 12-bit akvizicionog sistema ED428

Srednji i trenutni maseni protok goriva - automatizacija diskontinualnog merenja u realnom vremenu

U radu su prikazani rezultati razvoja prototipa automatizovanog procesorski upravljanog laboratorijskog sistema za merenje srednjeg i trenutnog masenog protoka goriva u realnom vremenu na principu promene hidrostatičkog pritiska u mernoj posudi. Sa stanovišta obrade signala, metodom virtuelnog eksperimenta, testirane su tri statističke metode pokretnih sredina i metode najmanjih kvadrata i aproksimacionog kubnog splajna. Eksperimentalna verifikacija sprovedena je u vanmotorskim uslovima, korišćenjem piezo-rezistivnog senzora IHTM Tpr 110 i 12-bit akvizicionog sistema ED428

An overview of application of micromechanical models in ductile fracture analysis of welded joints

Fracture of welded joints has been an important research and industrial topic for a long time, having in mind the key role of welded joints in ensuring the safe operation and integrity of welded structures. This work contains an overview of application of micromechanical models to ductile fracture of welded joints. The main benefit of these models, in comparison with the classical fracture mechanics approach, is consideration of the local quantities (stress and strain) in prediction of damage development. The damage is quantified through the value of the damage parameter, which is typically related to the void nucleation, growth and coalescence for ductile fracture of metallic materials, i.e. the description of the material can be related to the actual material behaviour during fracture. Most of the presented studies, including those published by the present authors, are performed on steel as the base material, and the rest deal with aluminium alloys

Micromechanical modelling of ductile fracture - local approach

Micromechanical modelling of ductile fracture by using the local approach has been presented as the review of previously obtained results for welded joints made of low alloyed high strength steel. Experimental work was performed on 3PB specimens and tensile panels, which were then modelled by finite element method, using two- and three-dimensional meshes, respectively. The local approach was used to simulate both for crack initiation and growth during ductile fracture process

Numerical simulation for studying constraint effect on ductile fracture initiation using complete Gurson model

Nastanak žilavog loma u zavarenim spojevima niskolegiranog čelika povišene čvrstoće je predviđen primenom mikromehaničkog kompletnog Gursonovog modela (CGM). Ograničeno deformisanje oko vrha prsline (eng. 'constraint') i promena troosnosti napona u ligamentu su razmatrani na epruvetama za savijanje u tri tačke SE(B) i na kompaktnim epruvetama za zatezanje C(T), uzimajući u obzir uticaj razlike u mehaničkim osobinama (eng. 'mismatch') i širine metala šava. Analizirana je prenosivost parametra koji odgovara nastanku prsline, u zavisnosti od troosnosti napona ispred njenog vrha i zaključeno je da se dobijene vrednosti korišćenog parametra mogu koristiti za obe geometrije razmatrane u radu.Ductile fracture initiation in high-strength low alloyed welded steel joints is predicted using micromechanical complete Gurson model (CGM). The crack tip constraint and variation of stress triaxiality in ligament are considered on single-edge notch bend SE(B) and compact tension C(T) specimens, including the effect of strength mismatching and different weld metal width. According to the analysis of stress triaxiality in front of the crack tip, transferability of fracture initiation parameter was studied. As a result of the analyses on specimens, fracture initiation parameter determined by using CGM can be transferred from one geometry to another if their triaxial conditions are found to be similar

Assessment of the integrity of welded pipes

Predmet rada je analiza integriteta zavarenih cevi izrađenih od čelika API J55 visokofrekventnim kontaktnim zavarivanjem (VF). Eksperimentalna ispitivanja mehaničkih osobina osnovnog materijala su izvršena na cevima povučenim iz ekspolatacije posle 70 000 sati rada. Uticaj oštećenja tipa površinske prsline na integritet cevi ispitan je hidrostatičkim pritiskom cevi sa aksijalnom površinskom prslinom u osnovnom materijalu. Ponašanje pri lomu je ispitivano korištenjem modifikovanih kompaktnih epruveta za zatezanje (CT), s početnom prslinom u osnovnom materijalu, zavarenom spoju i zoni uticaja toplote (ZUT). Kritična vrednost faktora intenziteta napona KIc određena je na osnovu kritične vrednosti J integrala JIc. Osim eksperimentalnog istraživanja, na osnovu dobijenih vrednosti za Kr i Sr primenom dijagrama analize loma (FAD) izvršena je procena integriteta zavarenih cevi sa aksijalnom površinskom prslinom na spoljašnjoj površini.The subject of the paper is analysis of the integrity of welded pipes made of API J55 steel by high frequency contact welding (HF). Experimental research on the mechanical properties of the base material was conducted on pipes withdrawn from exploatation after 70 000 hours at service. Defect influence of the surface crack on the integrity of pipes was tested using hydrostatic pressure of pipes with axial surface crack in the base material. Fracture behaviour was tested using modified compact specimen (CT), with the initial crack in the base material, welded joint and heat affected zone (HAZ). Critical value of the tensile strength factors KIc was determined based on the critical value J of the integral JIc. Apart from the experimental research, based on the derived values of Kr and Sr and by applying fracture analysis diagram (FAD) an assessment of the integrity of welded pipes with axial surface crack on the outer surface area was conducted

Numerical prediction of ductile fracture resistance of welded joint zones

This study deals with the numerical prediction of ductile fracture initiation and development in welded joints of a high strength low alloyed steel. Having in mind the material heterogeneity in the joint zone, a combined experimental-numerical procedure is applied for determination of properties of the weld metal and heat affected zone - HAZ (both coarse-grained and fine-grained portion). Single smooth tensile specimen is tested, and the surface strains are determined during this test using stereometric measurement. Combined with numerical analysis, this enabled determination of stress-strain curves, which are subsequently used in numerical analysis of fracture of pre-cracked specimens. Two different geometries are considered: standard single-edge notched bend (SENB) specimens and surface-cracked tensile specimens. In each of them, the crack is positioned either in weld metal or between the coarse-grained and fine-grained HAZ. Micromechanical model (complete Gurson model, by Z.L. Zhang) is applied in numerical analysis. Higher resistance to ductile fracture initiation and crack growth in HAZ is successfully predicted, as well as constraint effect caused by different crack shapes