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It is proposed that the fatigue strength of structures in nuclear power plants with a quite long service life be analyzed in two stages: first, the type and location of the process and the effect of basic factors are determined by method of direct analysis of stable cycles and then a more accurate stepped analysis of the deformation kinetics is performed. The real properties of a material are systematized, taking account of creep, in ways that depend on the different types of processes. The methods for calculating the conditions and mechanisms of the progressing shape change and alternating-sign flow are refined. An example of the instability of the processes of cyclic deformation, which limits the possibility of operating structures beyond the limits of elastic adaptability, is examined. The possibilities and conditions for algorithmitization of the choice of material models and ensuring computational accuracy in calculations of deformation kinetics are discussed.Modern computer methods and program packages have made possible, in principle, computational analysis of the kinetics of deformation and fracture during the entire life time of a structure taking account of almost any geometric shapes, loading schedules, and physical and geometric nonlinearities. The growth of such possibilities has also increased requirements for the conventional problems of understanding the mechanisms of processes in order to control them in a rational manner; safety assessments which impose stringent specific requirements on the information content and computational accuracy have been added to strength evaluations [1,2]. The reliability and accuracy of the results as well as the laboriousness of the calculations began to depend, first and foremost, on the completeness and accuracy of the initial data, the choice of model for the material and methods for predicting fracture rather than on simplifications of the computational geometric scheme, as done in the past.The initial data on the conditions of loading and the initial post-fabrication state of a structure are always incomplete and inaccurate; the requirements for them depend not only on the objective of the calculations but also on the expected characteristics of the process, which are unknown before a calculation is completed. Most of the many material models presented in the well-known packages of the finite-element method are phenomenological, i.e., they describe definite phenomena in a limited range of conditions. Universal models describing quite diverse effects cannot be constructed differently, such as on the basis of physical ideas about the micromechanisms of deformation and fracture. Such models have not been ade-

The plastic properties of materials must be used more fully as working temperatures and temperature gradients increase. The results of tests performed on refractory chromium-nickel alloys with combined low-cycle strain with prescribed total amplitudes and strain increments per cycle are presented. The results are compared with those obtained with rigid, cyclic, sign-alternating, and monotonic strain. A method of constructing mathematical models of a material that take account of short-and long-time characteristics of a metal in combination with the possibility of their direct use in readily available packages implementing the fi nite-elements method is described.The serviceability, strength, safe life, stability, and safety of the high-temperature reactors now being designed can be secured only by making full use of the plastic and strength properties of the structural materials. Thus far, plastic deformation under the conditions of normal operation has been permitted only in local strictly size-bounded zones of stress concentrations [1,2]. The urgency of going outside these frames is dictated by the prospects for advancing nuclear power by building power-generating units with thermal and fast reactors with coolant temperatures above 500-600°C as well as new-generation reactor facilities, including high-temperature gas and space facilities with temperature 900-1500°C. In addition, it is proposed that conventional heat-resistant and austenitic corrosion-resistant steel as well as new refractory and fi re-and radiation-resistant alloys with the problems of deformation and fracture mechanics formulated in a new way be used [3][4][5][6][7].Low-cycle inelastic deformation includes three types of processes, differing by the consequences, the material properties, and the effect of operational factors: sign-alternating deformation, which is characteristic for local zones of concentration, and progressing shape change -the accumulation of residual displacements and their combinations [8]. The properties of a material in the presence of sign-alternating fl ow are determined during tests in a rigid loading cycle set by the deformation of the working part of the sample. In the presence of progressing shape change, they are close to the diagram of monotonic single deformation with elastic relief of loading. There are many works devoted to the investigation of processes of the fi rst two types [9][10][11][12][13], but the general case of combined deformation, which is characteristic for the widest range of the parameters of the external actions, has not been adequately studied. The tests of materials for such conditions are limited [14][15][16][17]; there are no standards for performing them. Mathematical models of the kinetics of the deformation properties of materials for combined long-time loading processes, which are necessary for calculating the strength and longevity of structures, are also weakly developed.

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