13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- description of microstructural features of brittle fracture of irradiated RPV steels. 2. Local brittle fracture criteria Currently two local cleavage fracture criteria are mainly used in brittle fracture modelling. Traditional formulation is written as [4] σeq ≥ σY (1a) σ1 ≥ SC, (1b) where σeq is the equivalent stress, σY, the yield stress, σ1, the maximum principal stress and SC, the critical brittle fracture stress, which is independent of temperature, strain rate and stress triaxiality. Another local criterion of cleavage fracture was formulated and verified in the papers [6-10]. This formulation is written in the form d eff T 1 nuc m σ ≡σ + ⋅σ ≥σ ε , (2a) S (æ) 1 C σ ≥ , (2b) where the effective stress is σeff = σeq-σY, æ ∫ = ε p eq d is the accumulated plastic strain, p eq dε is the equivalent plastic strain increment, σd is the critical stress for microcrack nucleation and mTε is the concentration coefficient for the local stress near the microcrack-nucleating particles. This coefficient depends on temperature T and plastic strain and may be written as mTε=mT(T)⋅mε(æ). From the physical viewpoint the parameter σd is the strength of carbides or carbide-matrix interfaces or other particles on which cleavage microcracks are nucleated. The functions SC(æ), mT(T) and mε(æ) are calculated as [6-10] [ ] 1/2 d 2 1 CS(æ) C C exp(Aæ) − − = + , (3) mε(æ)= S0/SC(æ), (4) mT(Т) = m0⋅σYs(Т), (5) where C1, C2, Ad are material constants, S0≡SC(æ=0) is the stress of start for the nucleus microcrack, m0 is a constant which may be experimentally determined and σYs is the temperature-dependent component of the yield stress. From the physical viewpoint, Eqs. (1a) and (2a) are the conditions for cleavage microcracks nucleation, and Eqs. (1b) and (2b) are the conditions of their propagation. In criterion (1) the condition (1a) is the simplest requirement to reach a minimum plastic strain corresponding to yield stress that is usually equal to 0.2%. As distinct from condition (1a), cleavage microcrack nucleation according to condition (2a) depends on the maximum principal stress, plastic strain and temperature and is characterized by the critical stress σd. It is important that the plastic strain when microcrack is nucleated may exceed 0.2% and increases with the temperature growth [8, 10]. It may be noted that the connection of cleavage microcrack nucleation with plastic deformation seems to be quite clear from the physical point of view. Nevertheless, when formulating the local cleavage fracture criterion this connection was explicitly used only near twenty years ago [6, 11]. Now this consideration is widely used in other models, for example, [12]). The important consequences follow from this difference between (1a) and (2a). In criterion (2) two critical parameters - SC and σd may control cleavage fracture and this depends on material properties and loading conditions, mainly, on the ratio SC/σY, stress triaxiality and temperature. For example, the brittle fracture of smooth specimens is controlled by (2b) and, by contrast, the brittle fracture of
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