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Vysokoteplotní tahové vlastnosti kvazikrystalické Al slitiny
Chlup, Zdeněk ; Dlouhý, Ivo
Low density, sufficient strength and high temperature resistance are the basic requirements for materials used for aerospace and automotive applications. The nanostructured aluminium alloy reinforced by quasi-crystals prepared by powder metallurgy supposes to be a good alternative to currently used alloys. The aim of this paper is to describe high temperature tensile properties of nanostructured quasi-crystalline aluminium alloy including the changes in fracture mechanism at different strain rates and temperatures. High temperature behaviour was assessed by tensile tests of cylindrical testpieces at temperatures ranging from 20C to 550C. Two strain rates 2.7x10 -4 and 1.1s -1 were applied over the whole temperature range of interest. Fractographic analysis was used to describe fracture mechanism and its changes with increasing temperature and loading rate. The material under investigation proved the potential for high temperature applications
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Fracture toughness and fracture micromechanisms in fibre reinforced brittle matrix composites
Dlouhý, Ivo ; Chlup, Zdeněk ; Boccaccini, A. R.
Fracture toughness and fracture micromechanisms in fibre reinforced brittle matrix composites. For monolithic ceramics, the fracture toughness determination using a specimen with straight and/or chevron notch in the three (four) point bend test for monolithic ceramic materials can be taken as standard techniques. In case of composite materials, mainly of brittle matrix reinforced by fibres the crack growth resistance increases as the crack propagates. In addition, the long fibres stimulate delaminations along their interfaces with the matrix perpendicularly to the major crack. Fracture toughness determination in cases when delaminations take place is difficult. The chevron notch technique appears to be the only technique that enable to obtain exact fracture toughness characteristics by its natural ability to "keep" the mode I major crack propagation. Based on fractographic analyses and comparisons of data from chevron and straight notch technique the corresponding evidences are supplied.
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Influence of sample geometry (size) effect on fracture toughness characteristics of brittle materials
Chlup, Zdeněk ; Dlouhý, Ivo
Influence of sample geometry (size) on fracture toughness characteristics of brittle materials. The fracture toughness is an important characteristic for structures design even more when inherently brittle material is used. Ceramics materials are those ones where knowledge of exact fracture resistance is crucial and frequently lack of this knowledge can disqualify such material for application in a given component. Over last few years a big effort was paid in investigation in this field. Recently a standardized methodology for fracture toughness evaluation was introduced. However there are many restrictions, mostly in geometrical requirements, and often it is difficult to reach them especially when new material is developed. Data obtained from different geometries of specimen and/or loading conditions time to time it is necessary to compare. The paper attempts to reveal questionable points when data from various sources are compared.
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Lomové chování kompozitů s křehkou matricí vyztuženou vlákny
Chlup, Zdeněk ; Dlouhý, Ivo ; Boccaccini, A. R. ; Chawla, K. K.
A number of examples exist that indicate the potential for increasing the fracture resistance of brittle matrix by dispersing brittle fibres. For applications development the real composite constituents response during mechanical loading under presence of flaws appears to be important. Theoretical and experimental knowledge sampled on glass matrix composites reinforced by fibres is shortly summarised in the paper. Borosilicate glass with continuous Nicalon fibres and the same matrix containing the chopped carbon fibres as well as the ZrO2 particles (hybrid composite) have been in focus of interests. Chevron notch technique was mainly used for the fracture toughness evaluation. High fracture toughness level of both composites has been proved when testing the as received state. Damage introduced by thermal ageing and/or thermal cycling has been discussed based on fracture toughness values and fracture behaviour analyses
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Přenositelnost lomové houževnatosti z malých těles na tělesa standardních rozměrů
Dlouhý, Ivo ; Chlup, Zdeněk ; Kozák, Vladislav
The capability of pre-cracked Charpy type specimens for prediction of fracture toughness characteristics for standard (1T) bend specimens has been analysed. The toughness scaling model and the master curve methodology have been combined for these purposes. Cast low-carbon manganese steel cut from midthickness of thick plate was used as an experimental material being tested as a candidate material for container casks of spent nuclear fuel. The Dodds-Anderson toughness scaling model was applied for toughness data correction from small scale pre-cracked Charpy specimens. This approach was combined with master curve methodology for the fracture toughness - temperature diagram and loading rate effects predictions supplying good correlation of predicted data with real behaviour
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Lomové chování kompozitů s křehkou (skelnou) matricí
Dlouhý, Ivo ; Chlup, Zdeněk ; Boccaccini, A. R.
A number of examples exist that indicate the potential for increasing the toughness of brittle matrix by dispersing different reinforcements. For further development of these advanced materials the real material response during mechanical loading under presence of flaws appears to be important. Theoretical and experimental knowledge sampled on different kinds of brittle matrix composites is summarised in the paper. These include glass matrix with metallic particles, alumina platelets, continuous Nicalon fibres, both the chopped fibres and ZrO2 particles (hybrid composite). Almost all of these composites were tested in as received state but also after thermal exposition and/or thermal shocking applied according to supposed composite application. Chevron notch technique was mainly used for the fracture toughness evaluation. Damage will be explained on base of fracture micromechanism
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