During the 20th century leading industrial countries successively passed through several "epochs" - first that of steel, then aluminum and other light alloys, then plastics, structural ceramics, etc. Quite recently, analyzing the achievements of the respective schools of research, the international industrial journal Metals of Eurasia stressed that out of the aforesaid list of materials the most important now are the new ones which are associated with improvements and "structural changes" in world economics and consequently, with further progress of our industrial civilization. A detailed report on the subject has been published by Prof. Nikolai Sklyarov, Dr. Sc. (Tech.) of the Ail-Union (now All-Russia) Institute of Aviation Materials (VIAM). It was there that some 40 years ago the term "structural materials" was born which has since received broad recognition by specialists.
Incidentally, these materials had long been regarded as substances with permanent composition and balanced structure and a set of relatively stable properties, operating in a prescribed regime. Later on the situation turned out to be even more complicated. There were growing requirements concerning the mechanical, thermal, corrosion and radiation parameters of these materials. As a result they began to incorporate multicomponent phases of different nature and in what specialists call metastable state. As was established later-instead of samples characterized by an unchanging crystal lattice, in actual fact they are full of micro defects-dislocations produced by strains and stresses. Formed along their borders are local "breaks" of spatial continuity, there appears what we call cold creep (when residual deformation grows even at room temperature), microstructure keeps growing leading to the ultimate failure of the whole structure.
Naturally enough, one did not have to wait long for the scientists' response to this changing situation. VIAM specialists, for example, put forward a fundamentally new approach to the safety and dependability of aircraft component structures. And they suggested something really paradoxical: while in the past everything was done to prevent cracks of any kind developing in aircraft structures, today cracks and fissures are "tolerated" not only in microstructures but in places visible by the naked eye. And that is despite the fact that maximum flight safety requirements remain as stringent as ever...
As was established in the subsequent studies what they call the micro-flaws dislocations in the crystal lattice are an objective natural factor which can never be "ruled out" by specialists. This being so, it would be much more productive to try and control this process-regulate its growth and not let it reach the critical limit. With that aim in view it is important to study the kinetics of fissure formation. And, as is often the case, the idea was suggested by an incident recalled by the author of the article.
During a pre-flight tests of an airliner which was to carry the Soviet leader Nikita Khrushchev on his visit to the United States engineers discovered cracks in the aluminum body of the motor gondola. Tests and checks were started under the direction of VIAM Deputy Director, Acad. Sergei Kishkin, and head of one of the Institute labs, Dr. Josef Friedlander (later-academician). The obvious question was: could they guarantee one hundred percent safety for the coming mission? Having checked the parameters of the appropriate structures, the experts came to the conclusion that the strains in the aircraft body could have no direct impact on the coming mission. And the unexpected flaws were the results of numerous engine starts on the ground-they were heated and cooled down time after time, thus provoking the development of cracks in the gondola.
This and several other examples of this kind proved that the safety of materials depends on the two main factors. Above all on the rate of growth of cracks, and simultaneously on the organization of control of these processes. And it is also important to produce structures of separate sections whose "boundaries" prevent the growth of cracks up to critical dimensions. Specialists established what they call "basic parameters" for both these factors. These include the high breaking point (for steel - 250 kgs/mm 3/2 , titanium - over 300, aluminum alloys - over 120 kgs/mm 3/2 ) and low rate of creep (about 0.1 - 0.2 mm/kilocycle).
Prof. N. Sklyarov points out that problems of strength of materials today are pushed into the background by the factors of dependability and safety. And while in the past engineers "gave priori-
ty" to metals-the strength of atoms bonds in the crystal lattice-today they focus on the activation energy of chemical bonds. For example, while at the start of the 20th century specific strength of aluminum alloy in a vertically suspended wire varied from 14 to 16 km, the latest composite materials developed at VIAM have strength of some 150 km. Or take matrixes and alloying components. For a long time they consisted of metals like aluminum, nickel, titanium, etc. Today attention is focused on intermetallide compounds, like nickel with aluminum or titanium, of titanium with aluminum, etc., and these are used as strain hardening components and also matrixes. There is, however, one "but" in this process: until the middle of the last century the share of elastic strains in mechanical engineering amounted to 0.2 - 0.3 percent and, as a rule, specialists easily take this into account. Today deformations of this kind can reach 12 percent and more. As a result, metal becomes "rubberized" which causes colossal strains concentrations at joint points and connections, etc. This has to be taken into account and "special promises" are offered so far only by materials on the basis of chemical compounds of carbon - carbofibers orborofibers.
Summing it up, Prof. N. Sklyarov says one cannot ignore the achievements of science of the late 20th century. These include the heat treatment of the surface of materials using as the heat source concentrated energy fluxes in plasma, ionic, induction and laser processes. One also has to take into account nanotechnologies as natural development of directional crystallization and monocrystalline casting, the transition of microstructure formation of materials onto atomic level. And there is no end to this list. The progress of science continues.
N. M. Sklyarov , "Konstruktsiya Splava " (Structure of Alloy), Journal METALLY YEVRAZII (Metals of Eurasia), No. 4, 2003
Prepared by Vladimir GOLDMAN
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