13th International Conference on Fracture June 16–21, 2013, Beijing, China (c) The rate of change of void volume concentration nuc def v dσ dρ is presented in the form d def v max v nuc def v σ ρ ρ dσ dρ , (1) where def vρ is the concentration of deformation voids in a unit of volume of a material matrix, max vρ is the maximum volume concentration of void nucleation sites. In (Eq. 1) σnuc is stress which controls nucleation of discontinuity near some barriers [4, 5]. In case of void nucleation the following barriers are considered: inclusions of second phase or coarse carbides and others. According to papers [4, 5] σnuc can written in the form σnuc = σ1+ mTε·σeff , (2) where the parameter mTε is concentration coefficient of local stress near dislocation pile-up; σ1 is maximum principal stresses; σeff = σeq – σY is effective stress; σeq is equivalent stress; σd is a local strength of matrix-inclusion interface. In general case σd depends on neutron dose and does not depend on test temperature [4, 5]. It is necessary to note that equation for σnuc is similar to equation proposed in paper [6]. From (Eq. 2) it follows that as a neutron dose D increases, σnuc will increase at the expense of an increase of σY and, correspondingly, σ1. According to papers [4, 5], as a dose D increases, σd decreases. Then from (Eq. 1) it follows that irradiation results in an increase of void concentration. This conclusion following from the considered equations is confirmed by the experimental data. It is shown in paper [7] that dimple concentration on a specimen surface in an irradiated condition is higher than that in the initial one. (d) When analysing the growth of vacancy and deformation voids the Huang’s equation [8] is used. To describe a void growth under conditions of their interaction an additional factor was introduced into Huang’s equation in the form f 1 1 dæ, 1 α 3 V dV void void f (3) where eq m eq m σ σ 2 3 exp σ σ 0.427 α k ; 1 σ σ 0, for 1 σ σ for 0,25, k eq m eq m ; (4) p eq dε æ is the Odquist's parameter; p eq dε is equivalent of a plastic strain increment, f is material void volume fraction Σ Σ V V V f , (5) In (Eq. 5) VΣ is the total volume of vacancy and deformation voids in a material matrix of the volume V. (e) The criterion of a unit cell plastic collapse or, in other words, the criterion of plastic instability is used as fracture criterion [3, 9] -2-
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