numero25

A. Shanyavskiy, Frattura ed Integrità Strutturale, 25 (2013) 36-43; DOI: 10.3221/IGF-ESIS.25.06 37 It should be corrected knowledge about meaning of the word that used to such indication environment state as “vacuum”. Usually considered: (1) low vacuum with gas pressure less than 100 Pa; (2) middle pressure with 0.1<p <100Pa; (3) high vacuum with 10-5 Pa<p<0.1Pa; (4) ultra-high-vacuum with p<10-5 Pa. Tests in vacuum performed under continual pumping with vacuum pumps to support needed level of low pressure in specimen chamber but not less that 10 -5 Pa. At the same time it should be clear that always in specimen chamber exist pressure of gases. This situation attests itself as dynamical vacuum. It means that inside of camera when tests perform exists low pressure always. That is why it should be corrected meaning of word “vacuum’: it is environment with low pressure of gases or other elements. The problem of subsurface crack initiation and propagation has been briefly discussed in the case of titanium alloy VT3-1 [11]. Following by this consideration was clear that subsurface fatigue cracking performs not in vacuum but in low level pressure of rest cases existing in metals which are enough for environment influence consideration on this process. Moreover, investigating of crack initiation processes, occurring and developing in chemically clean metals because of different types of defects formation, such as, for instance, pore’s, microcracks and etc., the theory of strength suggested that there inside of the defect volume has to be vacuum state. But it contradicted with physics of phase transformation [12]. From the diagram of phase transformation for each chemical element followed that solid phase cannot be in equilibrium state at the contact with vacuum. The solid body equilibrium state can be possible, if exist interaction effect between its own vapor or liquid in the triple point, T3, where simultaneously exist three phases. For example, if there exists in solid body spherical pore, it needs to be fill up of vapor or liquid metal. If volume of pores will be increased under external stressing, quantity of liquid and gas phases also will have increase. Nevertheless, but it is clear that subsurface fatigue cracking in metals takes place not in vacuum but in specific environment having low value of rest gases or other elements pressure. At the surface layer atomic stress-state is not the same that subsurface. There is biaxial stress-state at the matrix by the surface and hydrostatic case takes place subsurface. That is why energy dissipation at the surface is more effective than subsurface. Moreover, at the surface takes place disordering in atomic state. That is why there is more effective energy dissipation process under low level of deformation than subsurface. Subsurface hydrostatic state influenced not only residual state because of non-elastic deformation but there metals can have transition in Ultra-High-Plasticity (UHP) state that has been discussed in the first part of our paper. Also, there can be developed deformations not only during subsurface crack origination because of material volume rotations [11] but for crack propagation too. So, it should be considered metals critical state for crack origination and propagation by the three criteria: (1) non-elastic local deformation; (2) whirls formation under hydrostatic stress-state; (3) self-organized introducing in area of cracked metals own vapor or liquid because free surface can be creates only in environment conditions with some pressure. Below will be discussed process of subsurface fatigue crack propagation in steels in gigacycle or Ultra-High-Cycle-Fatigue (UHCF) regime based on the introduced above criteria. M ECHANISM OF FGA FORMING e will discuss process of FGA formation during crack propagation based on discovered fracture surface patterns for investigated steels [2, 13]. Typical fracture surface patterns for all specimens are shown in Fig.1. There are spherical and elliptically shaped particles and the same shape has roughed fracture surface in FGA. It is not granules. It is nano-structure of metals that has been performed during cyclic loading. In some FGA places there very clear seen “dimples” because several particles have been moved from their position during deformation process during crack propagation. Size of particles is not more than 50nm. It is typical nano-structure for investigated metals. Investigations subsurface layers in the depth direction for one and another part of fracture surfaces in FGA discovered there the same ensemble of particles which structure has difference from matrix structure [2]. Chemical composition investigations of the FGA in steels performed by the fracture surface have shown that there is more percent of carbon than exists in metals composition [14]. Following consideration of non-elastic diffusion in metals this effect can be clear explained. During metals plastic deformation in the case of hydrostatic pressure at the crack tip there takes place local matrix heating for propagating crack, carbides destruction, and, in another way, carbon from matrix has possibility to move inside of free area for occurred crack, and carbon moves there during crack propagation. This effect excluded ultra-high-vacuum state inside of the growing crack. Because of hydrostatic stress-state the diffusion process can be considered based on the model that has been introduced for explanation subsurface crack occurring in the W