|
|
The specimen size effect on R-curve and on crack tip blunting
Mrňa, Tomáš ; Konopík,, Pavel (referee) ; Stratil, Luděk (advisor)
The thesis deals with the determination of fracture toughness using compact tension (CT) test specimens at elevated and high temperatures. The experimental material steel P91 designated for application at temperatures 550650°C was used. The fracture toughness in the ductile fracture region of the steel was characterized by the R curve, which characterises the resistance against crack propagation depending on the crack length. The effect of temperature on the R curve at range 23600°C was evaluated. Next the specimen size effect using three sizes of CT specimens at 23°C and the effect of loading rate (2, 0,2 a 0,02 mm/min) at 600°C was examined. The results showed that the temperature has distinct effect on the R-curve, which yields minimal values at 400°C. Only the smallest test specimen size with thickness 6.25mm showed the specimen size effect giving about 10% lower values of toughness comparing to larger specimens. The effect of loading rate was clearly distinguishable. The values of toughness varied about 20% of the toughness value comparing individual loading rate.
|
|
|
Determination of Fracture Mechanical Characteristics From Sub-Size Specimens
Stratil, Luděk ; Džugan, Jan (referee) ; Haušild, Petr (referee) ; Dlouhý, Ivo (advisor)
The standards of fracture toughness determination prescribe size requirements for size of test specimens. In cases of limited amount of test material miniature test specimens offer one from the possibilities of fracture toughness evaluation. Because of small loaded volumes in these specimens at the crack tip the loss of constraint occur affecting measured values of fracture toughness. In such cases the size requirements for valid fracture toughness characteristics determination are not fulfilled. These specimens can be even on limits of load range of test devices and handle manipulation by their small dimensions. The important task related to these specimens is, apart from methodology of their preparation and measurement of deformations, the interpretation of measured values of fracture toughness and their possible correction to standard test specimens. Moreover, in the upper shelf region of fracture toughness quantification and interpretation of size effects is still not resolved sufficiently. This thesis is by its aims experimentally computational study focused on evaluation of size effect on fracture toughness in the upper shelf region. The size effect was quantified by testing of miniature and large specimens’ sizes in order to determine J R curves. Two geometries of miniature test specimens, there point bend specimen and CT specimen, were used. The experimental materials were advanced steels developed for applications in nuclear and power industry, Eurofer97 steel and ODS steel MA956. Finite elements analyses of realized tests together with application of micromechanical model of ductile fracture were carried out in order to evaluate stress strain fields at the crack tip in tested specimens from Eurofer97 steel. By comparison of experimental results and numerical simulations of J R curves the mutual dependencies between geometry of specimens and element sizes at the crack tip were derived. On the basis of acquired relationships, the methodology of J R curve prediction for standard specimen size from limited amount of test material was proposed. Main contribution of thesis is description of effect of material’s fracture toughness level on resistance against ductile crack propagation in miniature specimens. For material where significant crack growth occurs after exceeding the limit values of J integral (Eurofer97), the loss of constraint is considerable and highly decreases resistance against tearing. Miniature specimens then show significantly lower J R curves in comparison with standard size specimens. This effect is the opposite to the behaviour of miniature specimens in transition region. In case of material with low toughness, in which significant crack growth occurs in the region of J integral validity (ODS MA956), the effect of constraint loss is small without large impact on resistance against tearing. In such case miniature specimens demonstrate comparable J R curves as specimens of larger sizes. Next important contribution is proposed methodology for prediction of J R curve from small amount of test material using micromechanical modeling.
|
|
|
Fracture toughness testing at high temperature range using miniaturized CT specimens
Lokvenc, Martin ; Chlup, Zdeněk (referee) ; Stratil, Luděk (advisor)
This thesis deals with a high temperature testing of fracture toughness and studies the size effect on measured values using miniature size CT specimen. Two types of specimen geometry were manufactured from P91 steel, the standard size and the quarter size specimen. J-R curves were obtained in the temperature range from 23°C to 600°C. No specimen size effect was observed at room temperature tests. The realized experiments together with fractography analysis demonstrated the drop of toughness at 400°C caused by the effect of dynamic strain aging.
|
|
|
High temperature service embrittlement of EUROFER´97 steel
Stratil, Luděk ; Jan, Vít (referee) ; Hadraba, Hynek (advisor)
The thesis describes effect of long-time ageing on the microstructure and properties of the Eurofer´97 steel. The ferritic-martensitic reduced activation steel Eurofer´97 is candidate structural material for in-core components of proposed fusion reactors. Thesis is focused on examination and description of brittle-fracture behaviour of the steel. Properties of the steel were investigated in as-received state and state after long-time ageing. Detailed microstructure studies were carried out by means of optical and electron microscopy and also by means of quantitative electron microscopy. Mechanical properties were evaluated also in both states by means of hardness tetsing, tensile testing and Charpy impact testing. Fractography analysis of fracture surfaces was carried out on samples after Charpy impact testing.
|
|
|
The Gurson-Tvergaard-Needleman model calibration from sub-size test specimens by size corresponding to the half of Charpy specimen
Stratil, Luděk ; Šiška, Filip ; Hadraba, Hynek ; Dlouhý, Ivo
The paper deals with a utilization of miniaturized tensile specimens for purposes of local approach application from limited amount of test material. The motivation for application of those miniaturized specimens is the utilization of volume of test material corresponding to the half of Charpy specimen. The tensile testing of miniaturized specimens was conducted at room temperature in ductile regime of the Eurofer97 steel followed by detailed fractography and quantitative analysis of micromechanism of damage. The parameters of Gurson-Tvergaard-Needleman micromechanical model of ductile fracture were calibrated. Except for the slight differences in strength and deformation characteristics no other differences between two sizes of tensile specimens were found. The calibrated sets of parameters of GTN model using the same element size were very similar. The application of different calibrated sets of GTN model parameters for simulation of damage in cracked specimen (R-curve) is sensitive to the element size and needs its calibration.
|
|
| |
|
|
A calibration of Gurson-Tvergaard- Needleman micromechanical model of ductile fracture
Stratil, Luděk ; Hadraba, Hynek ; Šiška, Filip ; Dlouhý, Ivo
Fracture mechanics characteristics of ductile fracture often depend on geometry and size of specimens. A micromechanical modelling of material damage proposes a tool for treatment of geometry and size effects in fracture mechanics. Application of micromechanical model for certain materiál requires a calibration of model‘s parameters. This contribution presents the calibration procedure of Gurson-Tvergaard-Needleman model of ductile fracture for Eurofer97 steel. The identification of ductile damage of the steel and its tensile properties at various level of stress triaxiality by testing smooth and notched bars were determined in the previous study. The calibration procedure was performed by hybrid method as a combination of FEA simulations of tensile tests and fractography analyses. The Gurson-Tvergaard-Needleman model performance is greatly influenced by true stress-true strain curve of the material.
|
|
|
Study of ductile damage mechanism for Eurofer97 steel
Stratil, Luděk ; Hadraba, Hynek ; Dlouhý, Ivo
The contribution deals with the investigation of ductile damage mechanisms acting in the EUROFER97 steel. For evaluation of material response to various levels of the stress triaxiality the tests on smooth and notched tensile bars were performed. The broken tensile specimens were fractographically and metallographically analyzed and also a quantitative study of mirco-void initiation and nucleation was performed. The more serve conditions of the stress triaxiality in the notched specimens increase the strength characteristics and lower their fracture strain. The nucleation of the voids by decohesion mechanism and gradual void’s growth form substantial part of damage process of the material, finally rapid void coalescence takes place. The higher stress triaxiality promotes growth of voids and on restricts void’s nucleation.
|
|
|
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.
|
|
|
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.
|
guest ::
