Národní úložiště šedé literatury Nalezeno 3 záznamů.  Hledání trvalo 0.01 vteřin. 
Short Crack Growth in Materials for High Temperature Applications
Mazánová, Veronika ; Obrtlík, Karel (oponent) ; Krupp, habil Ulrich (oponent) ; Polák, Jaroslav (vedoucí práce)
: Advanced highly-alloyed austenitic stainless steel Sanicro 25 based on Fe-Ni-Cr matrix was studied under the conditions of low cycle fatigue at room temperature and at elevated temperature of 700 °C. Several state-of-the-art experimental techniques were used to investigate the mutual effects of alloy composition, microstructural changes and deformation mechanisms, which determine the overall damage resistance of the material. The main effort has been focused on the fatigue crack initiation and the short crack growth, two stages which play an essential role in the resulting fatigue life of the material in the service. • The internal deformation mechanisms were correlated with the evolution of the surface relief in the form of persistent slip markings. It was found that the high planar character of the dislocation slip causes the high cyclic plastic strain localization into the persistent slip bands and thus the early Stage I crack nucleation associated with the presence of the persistent slip markings in all studied samples. The twin boundary cracking was found to be related to the presence of the persistent slip marking along the surface trace of twin boundary as well. • The intergranular crack initiation was observed only rarely at high strain amplitude loading. It was found to be related to the presence of the persistent slip markings at the grain boundary. Owing to high number of local incompatibilities at the grain boundary caused by the shape of the persistent slip markings the grain boundary cracks under the external tensile loading. • The crack growth mechanisms of natural short cracks were studied on the samples subjected both to low and high strain amplitude cycling. The role of microstructure was analysed using experimental techniques and discussed. • The crack growth rates of the longest cracks were measured on the samples with a shallow notch. The results were analysed using the fracture mechanics approaches based both on amplitude of KI and of J-integral as well as on the plastic strain amplitude. All approaches were discussed in the relation to the Manson-Coffin fatigue life curve. The simple power law of the short crack growth based on the plastic strain amplitude shows very good correlation to the fatigue life law. • In the case of cycling at elevated temperature the role of oxidation was studied. The brittle cracking of the oxidized grain boundaries was found to play a major role in the early stages of crack nucleation. Later the crack growth path changes to be preferentially transgranular. The crack growth path is very similar to that studied in room temperature cycling.
Fatigue crack initiation and growth in 316L steel in torsional cyclic loading
Karol, Michal ; Chlupová, Alice ; Mazánová, Veronika ; Kruml, Tomáš
Fatigue crack initiation and growth study in 316L austenitic stainless steel was made in cyclic\ntorsion. The experiments on hollow cylindrical specimens were performed at room temperature using fully reversed shear strain controlled cycles. The specimens used were polished mechanically and electrolytically to enable surface damage and crack propagation observation using optical light microscope, SEM. It was found that high density of extrusions and intrusions are formed on the specimen surface due to cyclic loading. TEM observations revealed that dislocation arrangement in well-known ladder-like structure is responsible for the localization of cyclic plastic deformation and for the origin of surface roughness in which the fatigue crack nucleate. The path of fatigue cracks leading to failure was observed, too. The crack path\nwas found to be dependent upon the applied shear strain amplitude.
Fatigue crack growth in 316L under uniaxial and torsional loading
Karol, Michal ; Chlupová, Alice ; Mazánová, Veronika ; Kruml, Tomáš
A study of fatigue crack initiation and growth in 316L austenitic stainless steel is reported. Fatigue experiments were performed at room temperature on full cylindrical specimens cycled axially (tension-compression) and on hollow cylindrical specimens tested in reversed torsion. Microstructure and damage evolution (crack initiation and growth) on the surface of mechanically and electrolytically polished specimens were observed using light and electron microscopy. An analysis of the orientation of microcracks and macrocracks which led to failure was made. Axially loaded specimens exhibited presence of several microcracks which resulted in macrocrack propagating perpendicularly to specimen axis. In the case of torsional loading, orientation of macrocrack propagation was dependent on applied load and presence of notches. High amount of short cracks initiated parallel to specimen axis. Long cracks exhibited a tendency to bifurcate with crack branches oriented at approximately 45° to the specimen axis.

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1 Mazánová, Vladislava
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