PSI - Issue 20

Victor Petrov et al. / Procedia Structural Integrity 20 (2019) 87–92 Victor Petrov et al. / Structural Integrity Procedia 00 (2019) 000–000

90 4

branches. In the third area (feather zone) there is a certain “plateau”, in view of the constancy of the average velocity of the crack. There is also a fourth area, not clearly reflected, but which is essentially the “break through” zone. Thus, due to viscoelastic character of deformation, at negative temperatures the crack dynamics corresponds to the mechanism of brittle fracture across the width of the sample, and the area of the “break through” zone is significantly small. Further, based on the provisions thermofluctuational theory, when the destruction is not seen as the ultimate state of the material occurring when the critical values of stress or strain, and as a kinetic process of damage accumulation that develops in the body from the moment of load application, it is assumed that mechanical stress reduces the activation barrier, thus facilitating the rupture of cohesive bonds in the polymer. The immediate destruction of the polymer due to the formation of cracks in areas of stress concentration, and the origin and development of cracks are considered as a consequence of the kinetic process of the thermalfluctuation break ties by Bartenev(1984) according to the activation dependencies (longevity law) by Zhurkov: where τ is approximately equal to 10 -13 - 10 -14 sec for classical solids and 10 -13 sec for the solid polymers. The value of U o in contrast to the γ coefficient does not depend on the physical state and supramolecular structure of the polymer, and is determined by its type and structure of polymer chains. In the framework of the thermalfluctuation theory preexponent τ о in (1) is interpreted as the period of thermal vibrations of the atoms in the polymer chain. The main conclusion about the nature of the process of destruction of solids, which follows from equation (1), is that the cause of destruction of solids at temperatures above 0 K is thermal motion, and stress only increases the probability of breaking bonds and reduces the probability of their recombination the Value of Uo in contrast to the coefficient γ does not depend on the physical state and supramolecular structure of the polymer, and is determined by its type and structure of polymer chains. In the framework of the thermalfluctuation theory preexponent τо in (1) is interpreted as the period of thermal vibrations of the atoms in the polymer chain. It is also necessary to take into account the interpretation of structural size in fracture mechanics, proposed by G. Neiber and V. V. Novozhilov in the eponymous criterion of the following type:        U 0  kT    0 exp (1)

d  0

d 1

dr



 

(2)

c

The main feature is the introduction of some structural size d . According to Goldstein and Osipenko (1978, 1993) consider it as a linear size characterizing an elementary fracture cell at a given scale level. The generalized structural approach treats the change in the structural size d in the process of damage accumulation as the difference between the mechanisms of destruction at different scales by Lepov at al (2015). At the macroscopic level d determined by quasi-static tests of samples with cracks and in accordance with the Neiber– Novozhilov criterion (2) can be expressed in terms of static fracture toughness and strength by a simple formula:

2 2 2 c IC d K  

(3)

Thus, the following model of brittle crack growth under combined static-dynamic loading is proposed:

 kT B W R 

  

  

 d X A

exp

(4)