numero25

A. Shanyavskiy, Frattura ed Integrità Strutturale, 25 (2013) 36-43; DOI: 10.3221/IGF-ESIS.25.06 39 The discussed process of particles formation directed to crack growth retardation [16]. That is why cracking of metals during FGA formation cannot be strongly expressed in term of crack growth rates now because it is not conventional process. It can be only say that the estimated crack growth in area out of FGA is less than all period of subsurface metals cracking from the moment of start to form FGA. Based on the performed consideration, it can be summarized process of FGA formation during metals subsurface fatigue cracking in the next manner: (1) In steels under cyclic loads there in the area of UHP takes place diffusion carbon with thin layers formation by the occurred free surface. The main role in fracture surface formation played matrix rotation instability that directed to for spherical particles and spherical in shape surface patterns. Carbon is good seal for preventing crack edges interaction effect that is why its moves in area of the crack. Hydrogen influenced this process being in matrix and trapped inclusions; (2) In titanium alloys there takes place diffusion of rest gases inside of the crack area and, also, some of the matrix chemical components can go in [11]. Based on the considered sequence of fatigue crack growth events we can conclude that in all discussed cases of steels fatigue cracking there were four areas of different processes of fracture surface formation: (1) Crack origination because of formation FGA, FSF, or Supergrain cracking; (2) Smooth surface formation with or without BP that depended on the stress concentration around inclusions and cracked Supergrain orientation in space according to acting external loading; (3) Drastically change to increasing of fracture surface roughness because of metals cracking by grain boundary of by slip bands; (4) Drastically change to fast fracture with dimpled surface formation. From considered sequence of formed fracture surface areas one can be conclude that it is not only one way for metals deformation and destruction processes to realize fatigue crack propagation mechanisms on the all discussed stages. There can be intensive cracking by the grain before fast fracture without influence of this mechanism on the crack origination in one matrix. In another metal crack origination has been performed with supergrain but FSF took place in the area of origin. Consequently it can be conclude that in the case of subsurface crack origination there takes place some specific self- organized process of matrix destruction that has to be investigated further more precisely to discover regularities in this process. Q UANTUM - MECHANICAL NATURE OF PROPAGATED CRACKS n the case of subsurface fatigue cracking it was introduced Paris-Hertzberg equation for metals in the next form [1]: 1/2 3 / ( / [ ] ) eff da dN b K E b   (1) In Eq.(1), E is Young’s modulus, b – modulus of Burger’s vector. Earlier it was theoretically considered crack growth process in metals based on non-continuous approach [6]. It was introduced knowledge about minimum value of crack increment such as quant of metals cracking, a q , and this value has been calculated for steels [18] and Al-based alloys [19, 20]. Cascade of crack increments being more than a q –value was demonstrated and strongly expressed evidence of crack growth rate hierarchy was introduce in the next form: 1/ 1 ( / ) / ( / ) ( / ) q i i T T da dN da dN     (2) In Eq. (2), T  and T  are theoretical shear and tension stress, respectively, q=1,2,4,8… . Many experiments were done to confirm the relation (2) for Al-based alloys [19, 21], Fig.3. It was discovered constant values of crack growth rates in the crack growth direction with their hierarchy in accordance with Eq. (2). Following by physical approach to metal failure it is very easy to shown that for all metals cannot be realized crack increment in one cycle less than two lattice. For, example, one dislocation has in homogeneity in two lattices at its tip, Fig.4. If sliding process is dominant ahead of a crack tip there will be interaction effect between two dislocations that directed to form inhomogeneities in the distance 4b (see Fig.12b). Values of a q were calculated and shown that for steels it is 0.5nm [18], but for Al-based alloys it is 2nm [19], [21]. Quantum-mechanical nature of subsurface crack growth can be seen based on test results under tension of wide range of metals in area of infinitesimal deformations. It was discovered quantum-mechanical nature for quantization of Yang Modulus [22]. In the range of stress levels for metals subsurface cracking realizes so small deformations that they ought to be considered as metals extension in area of infinitesimal deformation. In this case it was shown that knowledge about elastic modulus disappeared because in this area of deformation exists quantization of modulus by the order [22]: ( /2) ( /4) 2.06(1 )(2 / 3) [1 /(2 )] s p m E A T T      (3) I