S. Ya. Yarema

Plastic deformation in the vicinity of a crack and the criteria of fracture a review

ConclusionsThe existing solutions of problems concerning the determination of stresses and strains in bodies with cracks on the basis of the classical theory of plasticity are approximate and, in most cases, numerical. Solutions obtained by different authors for the same problem commonly differ among themselves and do not agree entirely with observations. In this connection it is necessary to work out optimal methods (programs) for solving such problems with more complete consideration of the actual conditions, in particular the three-dimensional character of the stress-strain state. It is also necessary to develop further the theoretical basis for setting up elastic-plastic problems, using current advancements in the physics of deformation and fracture of solids, because classical theories are not capable of describing the discrete mechanism of the phenomena. In this connection we may point to a method of solving elastic-plastic problems by modeling thin slip bands by surfaces of displacement discontinuity.Experimental investigations are very important, but at the present time they are conducted in sporadic and unsystematic fashion. There are as yet no full explanations of the causes leading to localized or diffuse plastic zones. Data on small plastic zones and their configuration in the thickness of the material are inadequate. We need direct investigations of plastic zones at cracks and of the formation of prefracture zones within them.The final and most important purpose of such investigations is the development of criteria of fracture in fairly plastic bodies with cracks. In this respect criteria of critical crack opening are distinguished. The actual ways of using these in practice, however, are not clearly defined, particularly in constructional design. An exception is found in those cases where the δk model is suitable, although the limits of its application have not yet been established. In this connection, further investigation is necessary, both of the δk model and of the creation of new models that take into account the variety of actual cases. Special attention should be given to precritical crack growth in prolonged loading (static or cyclic) and to the effect of surface-active media.

General features of fatigue fracture diagrams of metals

In this paper the results of investigations of fatigue fracture diagrams (v-K diagrams) of materials or products (depicting the relationship between the fatigue crack growth rate, v, and stress intensity factor range in a cycle, ΔK) performed in the Physico-mechanical Institute are summarized. Typical diagrams are described and their main features in normalized coordinates are clearly demonstrated. On this basis the defining parameters of crack growth rate curves (v-K curves), ΔKth, Kfc, m, and ΔK*, characterizing the crack extension resistance of material are validated. Convenient expressions for analytical approximating v-K curves by splines are proposed. The main observed deviations of v-K curves from typical ones are shown.

The effect of low temperatures on the rate and microfractographic features of the development of a fatigue crack in low-carbon steel

Several investigations of the development of a fatigue crack at low temperatures have indicated that with a decrease in temperature the rate of the growth of the crack decreases [1-3]. However, as a result of tests of mediumand low-carbon steels at temperatures of +20 to --160~ it has been established that tnheeffect of reduced temperature on the growth rate of a crack depends substantially upon the value of the amplitude of stress [1-4] and that the growth rate of a crack decreases at low amplitudes and increases at higher ones. In addition, in [5] it was shown that in 15G2AFDps steel at reduced temperature the growth rate of a crack does not always change steadily. Until a certain temperature (--95~ it decreases at all amplitudes and then depending upon the level of stresses either continues to decrease or increases and then again decreases. Regrettably the complex configuration of a crack in the cylindrical samples used in these investigations very much complicates a quantitative analysis of the stresses in the vicinity of its contour. A similar inconsistent temperature relationship of the propagation rate of a crack was observed in testing high-temperature tempered 65G steel [6]. Until now the reasons for this behavior of materials have not been studied.

Cracking time during repeated-impact loading as a function of test temperature

Conclusions1.The temperature range over which there is a sharp decrease in durability becomes narrower and is displaced toward positive temperatures as the cycle amplitude is increased during repeated-impact loading of specimens with annular cracks.2.The values of the parameter K1c determined during static tests are higher than those obtained in repeated-impact tests. This difference is slight at T=20°C and increases as the temperature is reduced, reading a maximum at T=−45°C.3.The cracking time under repeated-impact loading is governed by the ratio of the stress-intensity constant corresponding to the initial crack length to the value of K1c at the temperature in question and is expressed by the exponential relationship

Crack development in a sintered carbide in combined deformation of types I and II

N = Ae^{B\frac{{k_ \bullet }}{{K_{1c} }}}

Development of fatigue failure of low-carbon sheet steel at room and low temperatures

over the range 0.2