The Definition Method and Optimization of Atomic Strain Tensors for Nuclear Power Engineering Materials

A common measure of deformation between atomic scale simulations and the continuum framework is provided and the strain tensors for multiscale simulations are defined in this paper. In order to compute the deformation gradient of any atom m, the weight function is proposed to eliminate the different contributions within the neighbor atoms which have different distances to atom m, and the weighted least squares error optimization model is established to seek the optimal coefficients of the weight function and the optimal local deformation gradient of each atom. The optimization model involves more than 9 parameters. To guarantee the reliability of subsequent parameters identification result and lighten the calculation workload of parameters identification, an overall analysis method of parameter sensitivity and an advanced genetic algorithm are also developed

Unified Constitutive Model and Numerical Implementation of NiTi Alloy Involving Phase Transformation and Plasticity

In order to actually describe the mechanical behavior of phase transformation and plasticity for NiTi shape memory alloy (SMA), the master equations which based on irreversible thermodynamics theory was derived by assuming two internal variables to characterize phase transformation evolution and plastic evolution. Thus a unified constitutive model was developed by summarizing master equations of phase transformation and plasticity in the loading process of NiTi alloy. Adopting semi-implicit stress integration algorithm to update inelastic strain increment, the phenomenological-based constitutive model was numerically implemented with FORTRAN code. The numerical simulation results were in good agreement with experimental data so that the proposed model validation was conducted. The results show that the proposed model not only can describe well the different deformation stages of NiTi alloy, but also the constitutive behaviors subjected to different strain rates. And it provides the basis for the practical application of NiTi alloy in the condition of impact and high speed cutting

A Unified Constitutive Model for Creep and Cyclic Viscoplasticity Behavior Simulation of Steels Based on the Absolute Reaction Rate Theory

In this work, the viscoplasticity and creep behavior for modified 9Cr-1Mo and 316 stainless steels were investigated. Based on the absolute reaction rate theory, a unified constitutive model incorporating internal state variables was proposed to characterize the evolution of the back stress. Also, the model was implemented by the ABAQUS system with the semi-implicit stress integration. Compared to the experimental data, the results demonstrated that the proposed approach could effectively simulate the cyclic softening and hardening behavior for such structural steels

Effect of the Preexisting Fissure with Different Fillings in PMMA on Blast-Induced Crack Propagation

In order to study the dynamic crack propagation law in fissured rock under the different fillings, a borehole with 7 mm diameter was processed in the center of a polymethyl methacrylate (PMMA) specimen. The preexisting fissure with different angles (θ = 0°, 45°, and 90°) and different distances (L = 20, 30, 40, 50, and 60 mm) was prefabricated around the borehole. Air, soil, and water were employed as fillings in the fissure, respectively. The experiment of explosive loading was carried out by a single detonator, and the dynamic crack propagation process of the experimental specimens was simulated by nonlinear dynamics software AUTODYN. The results show that the blast-induced cracks are the most favorable and unfavorable to propagate when θ = 0° and θ = 45°, respectively. The length of the far-end wing crack decreases with the increase of the distance L, and the length of the far-end wing crack in the air-filled specimens is larger than those in soil-filled and water-filled specimens. The damage-pressure curve of the far-end wing crack initiation point shows “S”-type change, and the damage-pressure curve shows two obvious damage evolution processes of initial nonlinear and later linear stages. With the increase of the angle, the distance from the borehole to the crack initiation point decreases and the compressive stress wave peak value should increase, but the tensile force peak value decreases. Meanwhile, the relationships between pressure and average velocity of the initiation point and L, θ, and fillings are established, respectively. The numerical simulation agrees with the experimental results well. It can be seen that the fillings types, angle, and distance have a mutual restraint relationship with the reflected and absorbed stress wave energy. The phenomenon of crack propagation under different fillings can be explained well from the viewpoint of discontinuity degree and stress wave energy, which reveals the general law of blast-induced crack propagation

First-Principles Study of the Effect of Titanium Doping on Carbon Monoxide Poisoning Properties of Zirconium-Cobalt Alloys

It is very important to study impurity gas poisoning in ZrCo alloy because it is associated directly with the performance of ZrCo alloy as a hydrogen storage material. In this work, the effects of atomic replacement on the mechanism and properties of CO impurity gas poisoning in doped (Ti) ZrCo hydrogen storage alloys were investigated using the first principles method, based on the pseudopotential plane wave method. The adsorption energy, lattice constant, density of states, and charge density difference of the compounds before and after doping were calculated. Then, surface adsorption models of the ZrCo and Zr0.8Ti0.2Co alloys were established with the assistance of a conventional model. The resulting adsorption energy values of the clean surface and the surface adsorption energy values in the presence of CO impurity gases manifested that the Ti element-doped Zr0.8Ti0.2Co alloy was more susceptible to CO gas poisoning compared to ZrCo, which was consistent with the existing experimental results. In addition, by analyzing the conventional model, the electrons from the doped atoms overlapped with the surrounding electrons of C atoms, the phenomenon of orbital hybridization occurred, and the interactions increased. Consequently, Ti doping was not conducive to ZrCo to improve the ability to resist CO poisoning. The research results of this paper have laid a good foundation for the study of the effect of Ti doping on the antitoxicity performance

Investigation of the Propagation of Stress Wave in Nickel-Titanium Shape Memory Alloys

Based on irreversible thermodynamic theory, a new constitutive model incorporating two internal variables was proposed to investigate the phase transformation and plasticity behavior in nickel-titanium (NiTi) shape memory alloys (SMAs), by taking into account four deformation stages, namely austenite elastic phase, phase transition, martensitic elastic phase, and plastic phase. The model using the material point method (MPM) was implemented by the FORTRAN code to investigate the stress wave and its propagation in a NiTi rod. The results showed that its wave propagation exhibited martensitic and austenitic elastic wave, phase transition wave, and plastic wave. However, a double-wave structure including the martensitic and austenitic elastic wave and plastic wave occurred when the martensitic elastic wave reached the phase transformation wave. Thus, the reflection wave at a fixed boundary exhibited a different behavior compared with the elastic one, which was attributed to the phase transition during the process of reflection. It was found that the stress increment was proportional to the velocity of phase transition wave after the stress wave reflection. In addition, the influences of loading direction and strain rate on the wave propagation were examined as well. It was found that the phase transition wave velocity increased as the strain rate increased. The elastic wave velocity of martensite under compressive conditions was larger than that under tensile loading. In contrast, the plastic wave velocity under compression was less than that subjected to the tensile load

Numerical modeling of heat transfer during hydrogen absorption in thin double-layered annular ZrCo beds

In this work a three-dimensional (3D) hydrogen absorption model was proposed to study the heat transfer behavior in thin double-layered annular ZrCo beds. Numerical simulations were performed to investigate the effects of conversion layer thickness, thermal conductivity, cooling medium and its flow velocity on the efficiency of heat transfer. Results reveal that decreasing the layer thickness and improving the thermal conductivity enhance the ability of heat transfer. Compared with nitrogen and helium, water appears to be a better medium for cooling. In order to achieve the best efficiency of heat transfer, the flow velocity needs to be maximized. Keywords: Hydrogen storage, ZrCo metal hydride, Heat transfer, Three-dimensional simulatio

Hugoniot States and Mie–Grüneisen Equation of State of Iron Estimated Using Molecular Dynamics

The objective of this study was to develop a micromechanical approach for determining the Mie–Grüneisen EOS parameters of iron under the Hugoniot states. The multiscale shock technique (MSST) coupled with molecular dynamics (MD) simulations was employed to describe the shocked Hugoniot relation of single-crystal (SC) and nanocrystalline (NC) iron under high pressures. The Mie–Grüneisen equation of state (EOS) parameters, the cold pressure (Pc), the cold energy (Ec), the Grüneisen coefficient (γ), and the melting temperature (Tm) are discussed. The error between SC and NC iron results was found to be less than 1.5%. Interestingly, the differences in Hugoniot state (PH) and the internal energy between SC and NC iron were insignificant, which shows that the effect of grain size (GS) under high pressures was not significant. The Pc and Ec of SC and NC iron calculated based on the Morse potential were almost the same with those calculated based on the Born–Mayer potential; however, those calculated based on the Born–Mayer potential were a little larger at high pressures. In addition, several empirical and theoretical models were compared for the calculation of γ and Tm. The Mie–Grüneisen EOSs were shown on the 3D contour space; the pressure obtained with the Hugoniot curves as the reference was larger than that obtained with the cold curves as the reference