G. S. Pisarenko over many years was engaged in intensive investigations in the new and promising area, which he qualified as the strength of materials and structural elements under extreme conditions. Those operating conditions were characteristic of many structural elements of novel equipment in engineering industries, which were progressing rapidly during the second half of the 20th century.All this called for creation of new materials such as high-temperature steels and alloys, high-strength titanium and aluminum alloys, composite ceramic and cermet materials, including heat-shielding ones, refractory metals and alloys, carbon-graphite materials, etc.Designing of structures from the above materials is impossible without knowledge of their strength and deformability characteristics and the regularities in the in-service behavior of structures. Moreover, it is important not only to determine the characteristics of some particular properties of materials under certain conditions but also to develop the criteria of their ultimate state that would make it possible to calculate the strength and life of real structures using the results obtained in testing laboratory specimens and taking into account the structure geometry, stress state, size, and other factors.The development of the ultimate state criteria of materials is a challenging scientific problem. In attacking it one should consider the great number of different mechanisms of deformation and fracture of materials (for instance, plastic deformation, creep, cyclic, low-and high-cycle fatigue, thermal fatigue, corrosion cracking, wear, frettingcorrosion, erosion, etc.) occurring in operation and resulting in various conditions of the ultimate state (failure).Furthermore, it is necessary to take into account the large variety of materials used in engineering and significant difference in their behavior under mechanical, thermal, and themomechanical loading, as well as some peculiar features of real materials such as scatter in properties, the presence of manufacturing and in-service defects, instability of properties in the course of long-term operation, particularly under conditions of radiation embrittlement, specific state of the material surface layer, etc.330

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We present the results of investigation of the kinetics of damage to structural elements made of refractory alloys (including the elements with protective coatings) under cyclic thermal loading. The influence of the stress-strain state on crack propagation under conditions of thermal fatigue loading is analyzed.The development of scientifically substantiated approaches and creation of methods capable of taking into account the actual kinetics of damage of the materials in the process of operation is one of the most urgent tasks in the solution of the problems of reliability of structural elements and evaluation of their residual service life and survivability.The main aim of the present work is to study the processes accompanying the initiation and development of thermal fatigue cracks in structural elements made of refractory alloys (with and without protective coatings) under conditions of cyclic thermal loading in high-temperature gas flows. In the proposed formulation, the present work is of primary importance for understanding the mechanisms of crack propagation under nonisothermal loads and substantiation of the possibility of application of new and well-known models of contemporary fracture mechanics to the phenomenon of thermal fatigue of structural materials of the indicated classes.In view of the fact that the properties of the protected material differ from the properties of the coating and the inevitable presence of a specific transition zone on the boundary of the coating and the protected material, the base-coating system should be studied as a structure.The formation of the fields of residual stresses (which can hardly be controlled) and low reliability of the available data on the character of changes in the properties of the base material in the transition zone significantly complicate the problem of correct determination of the actual stress-strain state (SSS) of the material and the proper choice of the criteria of limiting state and durability of the coatings.In the solution of the posed problems, we follow the following major directions: -the analysis and generalization of the experimental data on the mechanisms of thermal fatigue damage to different materials in the stage following the initiation of a thermal fatigue crack depending on the intensity of thermomechanical load and corrosion influence of the gas flow;-the investigation of the dynamics of thermal and stress-strain states of the material under cyclic loading in the presence of a crack by the experimental methods and methods of numerical simulation;-the analysis of the regularities of propagation of thermal fatigue cracks in alloys with protective refractory coatings in directionally crystallized materials; 386 0039-2316/06/3804-0386