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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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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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Transparent composites with ceramic and glass matrix
Polena, J. ; Dlouhý, Ivo
Nowadays transparent composites with ceramic or glass matrix are one of the most promising opto-mechanical materials. They combine low surface density (for aplications as security windows) with high strength. In addition they restrain the unfavourable brittle nature of ceramics. These facts make them the leading candidates for construction aplications, where also optical transparency is needed. The main mechanisms of stiffening are based on the disipaton of fracture energy on the fibre/matrix interface (interphase respectively). Therefore the focus of research is in optimalisation of the interface (interphase respectively) as well as in endeavours to keep good optical transparency.
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Instrumented indentation test used for mechanical properties determination
Brumek, J. ; Strnadel, B. ; Dlouhý, Ivo
Focusing on the materials with non-linear stress-strain behavior an instrumented indentation study is presented. Indentation tests were applied on steel R7T. The observed data were analyzed and mechanical properties, such as yield stress, strain hardening exponent and elastic modulus were assessed. Experimental results of load-depth (P-h) test were compared with FEM solution. FEM analysis was applied on two-dimensional model described by elastic-plastic elements. Presented method of prediction in mechanical characteristics is useful to judge deformations of steel and other materials under contact stressing. Experimental results of P-h relation and additional FEM analysis of indentation test provide very helpful information of mechanical behaviors in contact of two bodies.
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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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Evaluation of KQ toughness and Kefc fracture toughness of hypoeutectoid steel R7T
Holzmann, Miloslav ; Jurášek, L.
Interpretation of the characteristic KQ using linear-elastic fracture mechanics coupled with Irwin´s concept of the effective crack length. The effective yield stress Ref as a basic parameter controlling the level of KQ and its scatter. The use of Irwin´s concept of effective crack length for determining fracture toughness Kefc. Validity of Irwin´s concept up to certain ratio of Fmax/FGY. The scatter of the fracture toughness Kefc, i. e. probability distribution of fracture toughness Kefc diagram. This diagram can be used in the case of application of probabilistic fracture mechanics for determining cumulative fracture probability of the given crack.
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Reference temperature prediction from tensile test parameters
Šmida, T. ; Babjak, J. ; Dlouhý, Ivo
Reference temperature of fracture toughness temperature diagram has been predicted based on tensile test data. Neuron analysis was applied to solve these properties correlation. For fracture toughness determination three-point bend specimens have been applied, the reference temperature being calculated applying multi-temperature method. Different strength and deformation properties have been evaluated from standard tensile specimens focusing on data from localised deformation during specimen necking. Totally 29 data sets from low alloy steels have been generated for the analysis. Good correlation of predicted and experimentally determined values has been found. Further analysis with higher number of data sets is needed to increase the accuracy of predicted values and decrease the number of input data.
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