Modelling of the deformation process under thermo-mechanical fatigue conditions
Introduction
In the technology which uses appliances subject to a high and variable in time temperature, thermo-mechanical fatigue is one of common causes of cracking of machine parts and equipment components. Currently, an evaluation of the degree to which such parts are damaged is most often conducted based on creep characteristics; data concerning fatigue properties determined at constant temperatures are used as well. However, when trying to enhance the accuracy of forecasting the behaviour of materials, fatigue induced by the action of a cyclically variable temperature, due to its significance and a different nature compared to fatigue at a constant temperature, should be treated in a different way [1], [2], [3], [4], [5], [6]. It is impossible to reflect accurately the interrelationships between damages induced by simultaneous mechanical and thermal loads through isothermal tests. In this connection, a rapid development of fatigue investigations has been observed recently.
The investigations take into account the influence of a simultaneous action of a variable in time temperature and mechanical loads. The number of teams involved in such research is growing and the results of the research are more and more frequently applied to evaluate and forecast the residual life of industrial appliances’ components. In this scope research is also being conducted at the Department of Mechanics of Materials, Silesian University of Technology, and first of all, refers to materials applied in the power engineering industry. This study is focused on the problem of modelling the deformation of a specimen subjected to thermo-mechanical fatigue. The approach presented in the paper addresses one of the problems the solution of which is necessary to describe the behaviour and to evaluate the durability of components subjected to the action of a cyclically variable temperature which induces a variable, non-uniform thermal stress field in them.
Section snippets
Technical example and selection of material for the research
In a majority of power engineering and chemical industry installations, fatigue of a thermal and mechanical nature works parallel to creep processes, whereas the creep and fatigue phenomena cannot be separated. The only problem to be still solved is which of the above-mentioned processes should be considered as predominant and which research methodology reflects the best operating conditions of an appliance. When trying to select a representative object, where thermo-mechanical fatigue
Conditions of material investigations
When the material behaviour is observed in the conditions of a cyclically variable temperature, it is necessary to isolate and define the quantities related to deformations. A deformation measured in laboratory conditions during the specimens examination is relative elongation called total deformation. This strain is a sum of thermal strain of measuring length of a specimen and its relative elongation induced by the action of load.
The investigation results presented in the paper were carried
Method of investigation results processing
Within the thermo-mechanical fatigue process characteristics, one can enumerate characteristics in the form of dependencies between strains, stresses, temperature and time, fatigue process parameters and the number of cycles required until sample failure [1], [2], [3], [4], [5].
During fatigue tests, hysteresis loops are recorded in the force/total strain system. On the basis of the total strain values recorded, diagrams describing the changes in mechanical and thermal strain with relation to
Model presentation of the deformation process
One of the methods commonly applied in the strain process analysis of the low-cycle fatigue conditions is an approach referring to the steady state which should be characterised by stability of the characteristics in the form of a hysteresis loop for the strain range selected at random. Such assumption may be fulfilled in certain cases only. Most frequently, characteristics of the state known as steady depend on the load history and the problem connected with the accuracy evaluation of their
Verification and application examples
Function f[ε′,T] is assumed for a given material on the basis of the low-cycle fatigue investigation results at constant temperatures.
The simple form of this function has been assumed:where A, C, D, n are material constants determined from mathematical description of isothermal low-cycle fatigue hysteresis loops course.
The results of the presented method application describe the course of the strain process in the conditions of thermo-mechanical fatigue of the P91
Acknowledgements
The studies have been performed as a part of Research Project No. 3 T08A 02027 financed by KBN (Polish Research Scientific Committee). The authors kindly acknowledge Polish Research Scientific Committee for the support in accomplishment of the research programme.
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