The paper presents a physico-mechanical model for predicting creep rupture in neutron-irradiated materials. The model is based on the approach whereby damage is described as voids on grain boundaries. The equations for void nucleation and growth which were proposed by the authors earlier are augmented to include neutron irradiation of material. Constitutive equations are derived to describe viscoplastic deformation of material including void propagation. A criterion for plastic stability of a unit cell is employed as a fracture criterion.
We present some results of prediction of creep rupture strength and plasticity for austenitic materials prior to and after irradiation with variable neutron flux rates, based on physicomechanical model as outlined in Part 1. The calculated results are compared with the available experimental data.Keywords: creep rupture strength, plasticity, void, neutron flux rate.
Creep Rupture Strength and Plasticity of 1Kh18N10T Steel in Its Initial State.Based on the physico-mechanical model for intercrystalline fracture [1], calculations were performed to predict creep rupture strength and plasticity in 1Kh18N10T austenitic steel under uniaxial loading. For calculating the creep rupture strength and plasticity curves we simulated steady-load tests and therefore creep rupture strength is expressed in terms of stresses active at the initial time of testing, i.e., conventional stresses. It is known that under the creep tension conditions the cross section of a specimen is reduced [2]. The true stress F true (i.e., the stress corrected for the cross-section reduction) was calculated bywhere F c is the conventional stress in the specimen. The calculation involves temperature-dependent and temperature-independent parameters. The last-mentioned ones include Ω, R 0 , k η , ρ max , and d g .* According to [3], the initial radius of a void arisen R 0 is taken 5 10 4 ⋅ − mm, Ω = ⋅ − 1 21 10 29 . m 3 [4]. As per [5] the grain size d g was set equal to 0.1 mm. The k d g η ratio was determined from the data on stress rupture plasticity in a unirradiated material at T =°650 C. We used ρ max and c α as adjustable parameters and found them by the criterion of the best agreement between the experimental and calculated data on creep rupture strength at various temperatures in the life time range up to 10 3 h. Also, it was established that c α parameter can be constant, i.e., independent of temperature. Hence, we obtained c α = 9 and ρ max =1000 m −2 .The temperature-dependent parameters are as follows: D b b δ governs the grain-boundary diffusion, a c , n c , and m c determine creep {Eq. (32) in [1]}, a p controls the strain hardening, and σ Y is the yield stress. To describe the temperature dependence of D b b δ we used (25) [1], where δ b b D 0 13 2 10 = ⋅ − mm 3 /s and Q b =167 kJ/mole [4].* Hereinafter we will use the notation as in Part 1 [1] unless otherwise defined.
We propose an engineering method which permits predicting the creep crack growth rate under neutron irradiation conditions. Theoretical analysis of the creep crack-tip stress-strain state is carried out. Calculations are performed to determine the effect of neutron flux intensity (flux) and pre-irradiation dose (fluence) on the crack growth rate.
We analyze the available methods for prediction of fatigue fracture resistance, which allow for material creep in a deformation cycle and neutron irradiation. Benefits and drawbacks of the available methods are discussed. We present a new method for the fatigue fracture strength prediction, which suffers no disadvantages inherent to the well-known methods. The proposed method has been verified on austenitic steels tested at elevated temperatures.Keywords: fatigue fracture resistance, creep, neutron flux level.Introduction. Various parts of a fast neutron reactor plant operate in a wide temperature range -from 220 to 580°C. Moreover, some of them are subjected to intensive neutron irradiation. During the reactor transient modes, such as reactor going-up, shutdown cooling, fast emergency protection conditions, the temperature distribution in structural elements is highly nonuniform, resulting in thermal stresses and strains. The transient processes, when repeated many times, may lead to fatigue damage. Since the structural elements operate at temperatures above the temperature threshold of creep, damage should be considered as a case of interplay between fatigue and creep. The problem becomes even more complicated as this process is accompanied by an intensive neutron irradiation of the structural elements, which contributes to a considerable deterioration of ductility and creep rupture strength of material. The static strength and ductility characteristics of a material correlate with its resistance to fatigue fracture; therefore, neutron irradiation has a significant effect on the cyclic strength of the material.The objective of the present work has been to review the currently available methods and to further develop them to enable prediction of fracture resistance in cyclic loading under viscoelastoplastic deformation and neutron irradiation conditions. 1. Current Status. Let us discuss the currently available methods for predicting fatigue damage in a material under creep and neutron irradiation conditions.In [1][2][3][4], the researchers used a so called strain range partitioning method which essentially consists in dividing the damage into components induced by the time-dependent and time-independent strains. Generally, alternating inelastic deformation is split into the four components: a) plastic tensile strain followed by plastic compression strain Δε pp ;
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