mechanical properties of the fragment material and, in particular, by impact toughness of material in initial state (Fig. 4 c). a b c Figure 4. Cumulative number-mass distributions of shell fragments from brittle (a) and ductile (b) carbon steels, and dependence of impact toughness of the brittle (I) and ductile (II) steels on the exponents in the exponential relations describing distributions of fragments (c) 2.5. Concentration criterion of defects At different stages of tension and fatigue, the average length of the microcracks Lav, their density n, fraction of the damages ω, and the values of concentration criterion k suggested in [10] and estimated by the relation k = n -1/2/ Lav (4) were evaluated. It allowed to find that the transition from the exponential to the power - law relation with development of damage accumulation process is accompanied by reduction of the concentration criterion related to the coalescence of microcracks in the plastic deformation zone. Fig. 5. Dependences of the average length (Lav), density of microcracks (n) and the concentration criterion (k) on the relative fraction of damages (ω) at the different loading stages of damage development (II, III, IV) (at the stage I plastic zone forms) Four stages of the damage accumulation were found. At the first (I) stage, in the notch tip of specimen, slip bands appear and plastic zone is formed (before approaching the yield strength); at the second stage (II), the formation and the accumulation of the microcracks occur. Interaction of microcracks at the third stage (III) leads to their coalescence and initiation of macrocrack. At the fourth stage (IV), the main crack appears in a secondary plastic zone in its tip; the area of the
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