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Modelling of ductile fracture for sub-sized three-point-bend geometry
Stratil, Luděk ; Hadraba, Hynek ; Kozák, Vladislav ; Dlouhý, Ivo
The contribution deals with the simulation of R-curve using complete Gurson model of ductile fracture. The R-curve was experimentally determined for a Eurofer97 steel on sub-sized three-point-bend geometry in previous study. To apply complete Gurson model the parameters describing the voids´ behaviour and characteristic length parameter need to be determined. The nucleation parameters were identified by single specimen method of smooth tensile test specimen and from metallographic examination of fracture micro-mechanism. The characteristic length parameter was derived by fitting load versus deflection curves of sub-sized specimens. The simulations of the tests were carried out by software ABAQUS in Standard and Explicit modules. The identification was supported by parametric studies. Comparing experimental and simulated R-curve the ductile tearing was not successfully achieved. Insufficient calibrated parameters as a result non-uniqueness problem of single specimen method were found.
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A simulation of R-curve by complete Gurson model
Stratil, Luděk ; Hadraba, Hynek ; Kozák, Vladislav ; Dlouhý, Ivo
The contribution deals with the simulation of R-curve using completed Gruson model of ductile fracture. The R-curve was experimentally determined for a Eurofer97 steel in previous study. The simulations of the tests were carried out by FEM software ABAQUS 6.11 in Standard and Explicit modules. The Gurson parameters were identified by single specimen method of smooth tensile test specimen and from metallographic examination of fracture micro-mechanism. The identification was supported by parametric studies. Comparing experimental and simulated R-curve the ductile tearing was not successfully achieved. Insufficient calibrated parameters as a result non-uniqueness problem of single specimen method were found.
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Embrittlement of ferritic 14%Cr ODS steel in air and in Pb/Pb-Bi coolants at elevated temperatures
Hadraba, Hynek ; Dlouhý, Ivo ; Hojná, A. ; Di Gabriele, F.
The work was focused to describing influence of long-time high-temperature exposition of the ferritic 14%Cr ODS steel to the liquid Pb and Pb-Bi metals environments. The steel was during exposition in the liquid Pb and Pb-Bi at 600°C and 550°C respectively depleted from Cr and Ti formed with Pb and Bi the surface oxide scale. In addition the isothermal ageing of the steel at 650°C caused precipitates coarsening and porosity development by dislocation annealing in the bulk of the steel. Tensile and impact bend tests reveals outstanding embrittlement of the isothermally aged ODS steel and the steel after exposition to the liquid Pb and Pb-Bi compared with the steel in as-received state.
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The Role of Microstructure in Fracture Resistence Control of TiAl Based Intermetallics
Dlouhý, Ivo ; Haga, H. ; Chlup, Zdeněk ; Hadraba, Hynek ; Hasegawa, M. ; Fukutomi, H.
The analysis of the role of microstructure in fracture performance of TiAl intermetalics at room and elevated temperatures has been carried out. For Ti-46Al-0.7Cr-0.1Si-7Nb-0.2Ni alloy different regimes of thermal and thermal-mechanical treatment were applied enabling to control substantially microstructural parameters. Attention was paid to grain (colony) size effect, effect of lamellas thickness and morphology and, in addition, to β-phase effect. For separate treatment regimes the flexural strength and fracture toughness were evaluated in addition to microstructural parameters. Fractographic analysis enabled to evaluate fracture micromechanisms; development of shear ligament toughening was observed in almost all cases. This is the key mechanism controlling the fracture behaviour. Based on findings obtained key aspects of the effect of separate microstructural components have been discussed.
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Stress state analysis of sub-sized pre-cracked three-point-bend specimen
Stratil, Luděk ; Kozák, Vladislav ; Hadraba, Hynek ; Dlouhý, Ivo
The contribution deals with finite element analysis of stress state in sub-sized pre-cracked three-point bend specimen (KLST). A studied material was Eurofer´97 steel. The true stress-true strain curve was obtained from tensile test. The three point bend tests were carried out on plain and side-grooved specimens at ambient temperature and were simulated using ABAQUS 6.10. The stress state in KLST specimen was compared with the stress state of standard specimen. The analysis confirmed the stress state in KLST specimens at the crack tip does not correspond to conditions of standard fracture toughness determination because loss of constraint.
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Determination of R-curves from sub-sized three-point-bend geometry
Stratil, Luděk ; Hadraba, Hynek ; Dlouhý, Ivo
Submitted article deals with determination of R-curves from sub-sized three-point-bend geometry. R-curves were measured by single and multi-specimen method on KLST samples fabricated from 25 mm sheet of Eurofer´97 steel. The applicability of single specimen method for given tested geometry and steel was verified. The curves determined by this are well comparable. However results obtained by single specimen method just from one specimen could be biased by material variability. Measured values are in good agreement with published results. The side-grooved specimens show lower values of J-integral and weaker crack-length effect.
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Influence of surface hardening on fracture behaviour of steels
Dlouhý, Ivo ; Chlup, Zdeněk ; Hadraba, Hynek
The paper is focused on the phenomena associated with fracture in surfaře layer produced by three different procedures. Standard cementing, nitriding and surface hardening were applied onto surfaces of six low alloy structural steels. For fracture behaviour evaluation Charpy type specimen with U notch were applied the surface of the notch radius was hardened too. The samples were loaded by means of an instrumented impact pendulum. The evaluation and classification of different fracture behaviour modes have been based on evaluation of load . flexure tracen supported by fractographic analysis. Knowledge obtained were summarised into general schematics of failure modes. As the most detrimental for the fracture resistance the surface cementing was found. The surface hardening showed the minimal negative effect on crack initiation and relatively small fracture resistence decrease.
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Mechanisms of the embrittelement and the effect of ageing on the brittle-fracture properties of the Eurofer ´97 steel
Stratil, Luděk ; Hadraba, Hynek ; Dlouhý, Ivo
The embrittlement of metal alloys is known phenomenon described as a loss of toughness. This effect is usually caused by segregation of the undesirable traps elements on the grain boundaries or unsuitable precipitation. The iron-based alloys, especially alloyed by chromium, are very susceptible to the embrittlement. The most common mechanisms of the embrittlement of these steels are briefly described in this contribution. The Eurofer´97 is a advanced steel developed for application in nuclear industry mainly alloyed by chromium and tungsten. The susceptibility to the embrittlement of the steel was studied by long-term isothermal ageing. Detailed microstructure studies were carried out in as received and aged state, the fracture behaviour was assessed by instrumented impact tester. The applied ageing caused slight microstructure changes including carbide and subgrain coarsening and slight shift of the impact transition curve.
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Recent Approaches to Brittle Fracture Characteristics of Steels
Dlouhý, Ivo ; Hadraba, Hynek ; Kozák, Vladislav ; Chlup, Zdeněk ; Holzmann, Miloslav
A brief overview of the current approaches for evaluating selected brittle fracture properties of steels relating to fracture resistance of welded joints. A critical analysis of the existing solutions to a singleparameter description of a crack tip stress field, linear-elastic and elastic-plastic fracture mechanics. Two-parameter fracture mechanics as the next development stage of fracture mechanics and impacts of its knowledge on fracture toughness measurement. Master curve for fracture toughness description of steels in the transition region. Local approach as the next fracture mechanics concept enabling prediction of global brittle fracture properties. Application of micromechanical crack tip fracture models (toughness scaling models) for correction of fracture mechanics data in their transfer from a specimen of one geometry to a specimen of another geometry and size (transferability). Evaluation of fracture properties of heterogeneous materials according to the SINTAP procedure.
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