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Reliability Problems in Methods of Residual Creep Life Assessment
Kloc, Luboš ; Sklenička, Václav
The service life of components designed to operate for long periods under loading at elevated temperatures is limited by the creep deformation and fracture phenomena. Possibility of a reliable prediction of the residual creep life is of major practical and economical relevance. A number of approaches has been used for the estimation of the creep damage and residual life; these approaches could be divided into two principal branches. The microscopic methods are based on the microstructural observations of accumulated damage in the samples of material after service, while macroscopic meth- ods use mechanical testing of the samples under some reference conditions. However, both approaches still do not provide enough reliable results for wide practical use, be- cause of result scatter and missing generally applicable models for the creep damage evolution. The most important factors which make the results unreliable are analyzed in the present paper. These are mainly: i) production scatter in the material's properties, ii) inhomogeneity of the material, iii) sampling problem in microscopic methods, iv) extrapolations involved in macroscopic methods. Some suggestions to overcome these problems are made and the need for a new approach and new experimental techniques is presented.
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Problems in Theories of Low Stress Creep Mechanisms
Kloc, Luboš ; Sklenička, Václav ; Fiala, J.
Viscous creep behaviour of many materials including creep resistant steels was observed at temperatures close to one half of absolute melting point and at very low stresses. The experimental results were interpreted as Coble diffusional creep and/or Harper-Dorn dislocation creep. Some of the results are in a very good agreement with the Coble theory of diffusional creep and support an idea that the diffusional creep is an important deformation mechanism under certain loading conditions. Nevertheless, another results are not fully compatible with the theoretical predictions. Some authors dispute the role or even the very existence of the diffusional creep and offer the Harper-Dorn creep mechanisms instead. In fact, there are many theories trying to deescribe Harper-Dorn creep mechanism, but none of them is capable to explain all observed properties. Transition (or primary) stage observed at low stress creep regime exhibits also some properties which contradict all existing theories. The observed effects which cannot be explained by the current theories are: i) a large scatter of creep rates observed for coarse grain materials, ii) creep rates much higher than those predicted by the diffusional creep theory while the creep rate dependence on grain size is maintained, iii) transition stage duration is independent of stress, iv) transition strain is dependent on temperature. Despite the problems in theoretical description, the experiments show that the viscous creep regime in structural materials must be considered as an important behaviour of the structural materials.
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Creep of intermetallic alloy TiAl at very low strain rates
Mareček, P. ; Kloc, Luboš ; Fiala, J.
Intermetallic alloys are very advanced materials for high-temperature applications. There are series of attractive properties exceed coventional up to now used creep resistance steels. The creep behaviour of TiAl alloy was investigated under an applied stress lower than 100 MPa at temperatures 700-850 °C. The strain rate was lower than 10-9 s-1 under testing conditions, which have importance for technical applications. In relation to results, the creep behavior of intermetallic alloy TiAl shows a transition to a mechanism with low stress exponent in the range of minimum strain rates, similar to creep resistance steels with evidence change of deformation mechanism under strain rate 10-10 s-1 . However, this transition in TiAl alloys is not as clear as in creep resistant steels.
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Vliv zatěžovací historie na creep 9% chromové oceli
Kloc, Luboš ; Sklenička, Václav
The creep life assessment is currently based on the phenomenological models mostly derived from the constant stress or constant load creep experiments. Since the loading history of the parts in industry used to be complicated and frequently unknown, the model data can be invalid. Stress change creep experiments were done on the P-91 type creep resistant steel at loading conditions corresponding to the low stress creep regime. The experiment was interrupted, that is cooled down and heated up again several times. The interrupts were done during the ``quasi-steady state'' when the creep rate is stabilized. The effect of both stress changes and temperature cycles on creep properties was analyzed. While the temperature cycle introduce some additional creep strain, the stress changes reduce the creep rate considerably. Taking into account the proportionality between the creep strain and creep damage nucleation, it is possible to conclude that small stress changes during primary stage can improve the creep strength of the steel, while the temperature changes have opposite effect.
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