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Fracture behaviour of ceramic cellular solids under tensile loading
Řehořek, Lukáš ; Dlouhý, Ivo ; Chlup, Zdeněk
Cellular structures are promising type of architecture for a lot of materials whether made of metals, plastic or ceramic aimed at applications where lightweight, damping and and/or stiff properties are required. For trouble-free use of this materials all types of mechanical behaviour have to be known. The tensile tests of open cell ceramic foam material based on alumina are presented in this paper and complemented of acoustic emission data and digital image correlation for measuring deformations. From the results follows that ceramic foams under tensile loading behave significantly different from widely used compressive or bending tests.
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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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Tensile strength of alumina based ceramics foam
Řehořek, Lukáš ; Chlup, Zdeněk
In last years cellular materials are getting to focus of an interest in a number of industrial applications and science fields. These materials are often used for filtration of different mediums such as melts or in aerospace industry and even in tissue engineering. Hence, there is evolved effort to understand and describe mechanical and physical properties of these interesting materials. While behaviour of these materials in compression is well understand and published, tensile tests are not so frequent and concerning ceramic cellular materials they are highly rare. The methodology of evaluation of tensile properties of model ceramic foam material VUKAPOR((R))A used like filters in metallurgy was developed. This methodology is based on fixation of ceramic foam sample by specialised resin to a holder. Homogenous transfer of forces is ensured by this technique.
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Quantification of increasing fracture toughness of glass matrix reinforced by alumina platelets composite
Kotoul, M. ; Pokluda, J. ; Šandera, P. ; Dlouhý, Ivo ; Chlup, Zdeněk ; Boccaccini, A. R.
A borosilicate glass matrix composite containing alumina platelets was considered to investigate toughening mechanisms and crack tip behaviour in dispersion reinforced brittle matrix composites. Fracture toughness was determined applying the chevron notched specimen technique and fractographic analysis was employed to reveal the active toughening mechanisms with increasing content of reinforcement. A roughness-induced shielding effect has been quantified to prove the relation between fracture toughness and fracture surface roughness. Theoretical calculations of the fracture toughness enhancement based on a modified crack deflection model developed by Faber and Evans, combined with the influence of the increase in Young’s modulus, were found to be in good agreement with experimental data. The crack deflection model was further extended to capture a synergy between crack deflection and the contribution of residual stresses to toughening in the investigated composites.
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Dynamic fracture toughness determination for inherently brittle materials
Flašar, Petr ; Chlup, Zdeněk ; Dlouhý, Ivo
Dynamic fracture toughness measurement of brittle materials using Charpy impact test. A method of dynamic fracture toughness evaluation of brittle materials using Charpy pendulum machine is presented. For low-speed loading rates a specimen damping technique was developed, in high-speed loading rates regime an eccentric onepoint bending test for DKC evaluation was used. A new technique of time to fracture tf measurement was designed using gold-layer deposition on specimen surface. Analytical and numerical analysis of studied problematic was carried out and compared to experimental data.
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Influence of loading conditions on fracture toughness values determined on ceramics
Chlup, Zdeněk ; Dlouhý, Ivo ; Hadraba, Hynek ; Flašar, Petr ; Řehořek, Lukáš
Fracture toughness is generally an important material characteristic. In structural inherently brittle materials as ceramics and ceramics based composites it plays all the more important role. Therefore exact determination of reliable values of fracture toughness is an important matter. Aim of this paper is to analyse influence of selected parameters (applied technique, specimen dimensions, span length, temperature and loading rate) on fracture toughness values. To fulfil this aim, among others, a statistical analysis based on the normal distribution was undertaken. Some of the effects are connected to the material and some of them to the applied methodology. Those linked to the material are hardly describable by a mathematical formula however gives an outlook on possible general trends. On the other hand the span of supporting rollers is an example of parameter whose influence has been described by mathematical formula.
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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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Determination of characteristics of inherently brittle materials using impact tester
Flašar, Petr ; Chlup, Zdeněk ; Dlouhý, Ivo
In this work, the mothodology of dynamic fracture toughness evaluation of brittle materials using Charpy pendulum machine is presented. For low-speed loading rates specimen damping technique was developed, in high-speed loading regime a modified DKC method was used. A new technique of time to fracture tf measurement was designed using gold-layer deposition on specimen surface. Analytical and numerical analysis of the loading tup – specimen interaction during the test was carried out and compared to experimental data.
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