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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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Prediction of Fracture Toughness Transition from Tensile Test Parameters Applying Artificial Neural Networks
Dlouhý, Ivo ; Hadraba, Hynek ; Chlup, Zdeněk ; Kozák, Vladislav ; Šmida, T.
Reference temperature localizing the fracture toughness temperature diagram on temperature axis was predicted based on tensile test data. Regularization artificial neural network (ANN) was adjusted to solve the interrelation of these properties. For analyses, 29 data sets from low-alloy steels were applied. The fracture toughness transition dependence was quantified by means of master curve concept enabling to represent it using one parameter - reference temperature. Different strength and deformation characteristics from standard tensile specimens and notched specimens, instrumented ball indentation test etc. have been applied. A very promising correlation of predicted and experimentally determined values of reference temperature was found.
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Lomová houževnatost a mechanismy porušování intermetalika na bázi TiAl
Dlouhý, Ivo ; Chlup, Zdeněk ; Hadraba, Hynek ; Kozák, Vladislav
The paper is focused on the analysis of the role of microstructure in fracture performance of TiAl intermetalics at room and elevated temperatures. Tensile properties, flexural strength and fracture toughness have been evaluated for Ti-40Al-2Cr-2Nb-1B and Ti-46Al-0,7Cr-0,1Si-7Nb-0,2Ni alloys. Positive Nb effect on fracture resistance was found; fracture toughness of the high Nb alloy was increased at contemporary increase of tensile and flexural strength. Fracture surfaces were evaluated using scanning electron microscopy showing the key role of mechanical twinning in deformation of both alloys. As a governing mechanism of the crack initiation a microcrack nucleation at boundaries between -TiAl grains was confirmed. Based on findings obtained key aspects of the effect of separate microstructural components have been discussed. Summary of toughening effects acting in this group of intermetalics has been provided.
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Rozvoj kuželových trhlin při "Ball on the Three Balls Test"
Chlup, Zdeněk ; Harrer, W. ; Kozák, Vladislav ; Řehořek, Lukáš
Proposed ball on three balls test takes advantage of biaxial loading and very simple experimental setup allowing usage of various test-piece geometries. The loading configuration is insensitive to small inaccuracies of disk shape. The only relevant parameter playing significant role is the thickness of test-piece. The three supporting and one loading balls eliminate disadvantage of ring of ring test mentioned above. From the experiences of numerical analyses as well as experimental results is still unclear influence of contact stresses between test-piece and loading ball on resulting stress field respectively the influence of cone crack developed due to such stresses. The main aim of this paper is to describe possibility of cone crack development due to contact stresses.
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Application of centrally loaded disk on the three supports for strength evaluation of brittle materials
Chlup, Zdeněk ; Kozák, Vladislav ; Supancic, P.
Development in advanced ceramics and brittle matrix composites enables use of these materials for industrial application irrespective of their brittleness. Obtaining relevant mechanical properties is extremely essential for components design. However, mechanical properties corresponding to the real loading conditions are unknown due to absence of testing procedure providing such results. The proposed "ball on three balls" test is a promising method for biaxial loaded components. Aim of this contribution is to describe advantages and limitations when B3B test is used from both configuration and material point of view. Numerical analysis of three-dimensional model at various loading, geometrical and material conditions was used. Optimal testing conditions were determined and possible errors were identified from error analysis.
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Fracture toughness and damage mechanisms of intermetalic based on TiAl
Dlouhý, Ivo ; Chlup, Zdeněk ; Hadraba, Hynek ; Kozák, Vladislav
The paper concentrates on the analysis of the role of microstructure in fracture behaviour of TiAl intermetalics at room and elevated temperatures. Tensile properties, flexural strength and fracture toughness have been evaluated for alloys Ti-40Al-2Cr-2Nb-1B and Ti-46Al-0,7Cr-0,1Si-7Nb-0,2Ni. Positive effect of Nb on fracture resistance has been found; fracture toughness of the high Nb alloy increased at contemporary increase of tensile and flexural strength. Fracture surfaces have been evaluated using scanning electron microscopy showing the key role of mechanical twinning in deformation of both alloys followed. As a governing mechanism of the crack nucleation a microcrack formation at boundaries between mechanical twins and gama-TiAl grains has been confirmed. Extensive development of shear ligament toughening was observed supplying quite good values of fracture toughness in high niobium alloy.
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