A simple model for the oxidation of carbon-containing composites

The isothermal oxidation mechanism of the carbon-containing composites has been investigated based on the experimental data reported in the literature. The results showed that the oxidation kinetics was affected not only by temperature and time but also by carbon content and the sample shape. For the oxidation kinetics, a series of quantitative kinetic models have been developed based on the controlling step. In this model, the effects of carbon content, sample size and temperature on the reaction fraction have been especially discussed. Incorporation of the experimental data into the new model indicates that a good agreement has been reached.     

Ti—Mn基储氢合金电化学容量的一个简易模型

针对Ti-Mn基储氢合金循环稳定性差这一问题。提出了一关于Ti-Mn基储氢合金循环稳定性的简易模型,并引入两个表示储氢合金本征属性的因子;活化特征因子β和容量衰减因子A。采用该模型可以描合金活化与容量衰减的全过程。实验测量了Ti0.26Zr0.07V0.21Mn0.1Ni0.3Cr0.03和Ti0.07V0.19Mn0.1Ni0.3Cr0.05合金的活化与充放电循环过程,理论与实验有较好的吻合关系。     

A simple model of throughput and pressure development for single screw

To be able to predict the throughput of a single-screw extruder or the metering time of an injection moulding machine for a given screw geometry, set of processing conditions and polymeric material is important both for practical and designing purposes. The model is based on viewing the entire screw simply as a pump, conveying a solid and a molten fraction. The evolution of the solid fraction is the essence of the plastication process, but under a particular hypothesis on solid bed acceleration, its influence on the throughput is nil. This allows getting a good estimate on the throughput and pressure development along the screw. Calculations are compared to a large set of experiments available from the literature. Consistent agreement with these published results is obtained, both for throughput and pressure along the screw. The effect of the plasticating process on the throughput is non-existent if the plastication length is short, and more visible if the plastication length takes a good part of the screw length (for instance, at higher screw rotation frequency). This diminishes the throughput value and widens the pressure peak. The model also shows that the screw geometry is the most important parameter, followed by polymer rheology and processing conditions. Melting properties and length seem to intervene to a lesser extent. Finally, the model is used for screw design, highlighting the influence of the compression zone on throughput.     

A simple analytical model for burr type prediction in drilling of ductile materials

This paper proposes a new analytical model to predict the type of burr at drilling exit. The model is based on the theory of slip-planes and is specially developed to predict burr type formation in drilling of ductile materials. First the analytical model is set up, based on mechanical and geometrical considerations. Then it has been validated through experimental drilling tests on aeronautical aluminum by predicting burr type and thickness. The experimental results show that the model is suitable in the drilling of ductile materials and its validity domain has been established.     

A half-space Peierls–Nabarro model and the mobility of screw dislocations in a thin film

In this paper, a half-space Peierls–Nabarro (HSPN) model is proposed to re-examine the mobility of a screw dislocation along a thin film/substrate (half-space) interface. In this configuration, the screw dislocation is subjected to an image force due to the free surface, and we are concerned with the interaction between the dislocation and the free surface. Unlike the original Peierls–Nabarro (P–N) model, the HSPN model takes into account the effect of the image force, which leads to modifications on analytical expression of the Peierls barrier stress. The modified Peierls stress is a function of the thin film thickness, which allows us to accurately predict the mobility of a dislocation in the interface between the thin film and the substrate. Based on the proposed HSPN model, we have found that the Peierls stress of a surface screw dislocation may be about 5–15% less than that in bulk materials.     

A simple and efficient model for mesoscale solidification simulation of globular grain structures

A simple model for the solidification of globular grains in metallic alloys is presented. Based on the Voronoi diagram of the nuclei centers, it accounts for the curvature of the grains near triple junctions. The predictions of this model are close to those of more refined approaches such as the phase field method, but with a computation cost decreased by several orders of magnitude. Therefore, this model is ideally suited for granular simulations linking the behavior of individual grains to macroscopic properties of the material.     

A stochastic model for austenite phase formation during arc welding of a low alloy steel

Modeling the microstructure of heat-affected zone (HAZ) in weld area can be useful in predicting mechanical behavior of the weldment. A multi-scale model is proposed to calculate the temperature distribution and to predict the microstructure evolution within the HAZ. Finite difference method was used to develop a computer model for studying the cooling curves during welding, and a stochastic method to analyze the austenite formation and its grain growth in HAZ. The droplet of liquid metal detached from electrode in manual arc metal welding is an important concern in studying the temperature distribution and the austenite grains growth. Both heat content of liquid metal detached from electrode and heat generated by electrical arc were used in calculation of temperature distribution. The stochastic model simulates the austenite phase formation and its growth during welding based on the kinetics of these processes. With this model, it is possible to visualize the topology of austenite phase. This multi-scale model was applied to the welding of low alloy steel. The observed morphology was in good agreement with that predicted by the model.     

A stochastic model to simulate the formation of a thermal spray coating

We present a three-dimensional, stochastic model of thermal spray coating. It is capable of predicting coating porosity, thickness, roughness, and the variation of these properties with spray parameters. The model assigns impact properties to molten droplets landing on the substrate by generating random values of process parameters, assuming that these properties follow normal distributions with user-specified means and standard deviations. We prescribed rules to calculate splat sizes after droplet impact and their interaction with each other. Porosity is assumed to be solely due to the curl-up of the splats as a result of thermal stresses. We use a Cartesian grid to define the computational domain and to track the shape and position of the deposited coating. The surface of the coating and the location of pores within it are specified using a variable known as the “volume fraction,” defined as the fraction of the volume of a computational cell occupied by coating material. Results are given for the variation of coating porosity, thickness and roughness with varying particle speed, size, and spraying gun speed. Predicted trends agree with experimental observation.     

嵌入随机概率模型的模具项目模板及其应用

针对模具生产中存在的不确定性因素,在考虑企业关键资源有限性的前提下,采用嵌入随机概率模型的模具项目模板,实现多项目调度,能够为企业制定较为准确且符合实际生产情况的项目计划。     

New model for prediction of ferrite number of stainless steel welds

Abstract

A new semi-empirical model for predicting the ferrite content of stainless steel welds has been developed. This model predicts the ferrite number of stainless steel welds as a function of composition. The model is based on an equation representing the free energy change between ferrite and austenite. This model has been derived from published data of experimental weld metal compositions and their corresponding ferrite numbers. The predictive capability of this model was found to be good and describes the effect of alloying elements on the ferrite number. This model is comparable in accuracy to currently available constitution diagrams but is applicable to a wider range of alloy compositions.     

A concurrent scheme for passing dislocations from atomistic to continuum domains

This paper presents a concurrent atomistic-continuum (CAC) methodology for three-dimensional dynamic simulation of dislocation nucleation, migration and interaction. The method is based on a new continuum field formulation of balance laws with relevant atomistic information (the arrangements and interactions of atoms) considered. In this work, we show that the new CAC method allows the smooth passage of dislocations through sharp interfaces between the atomistic and the coarse-grained finite element domains without unphysical reflection of dislocations or the need for heuristic rules; meanwhile, complex dislocation phenomena such as dislocation nucleation, dynamic strain bursts associated with nucleation and migration avalanches, formations of Lomer-Cottrell locks, dislocation-rigid boundary interactions, formation of intrinsic and extrinsic stacking faults, deformation twinning, and curved dislocation loops can be reproduced by the CAC method. All of the CAC simulations are directly compared with the corresponding atomic-level molecular dynamics (MD) simulations. The efficiency, accuracy and potential applications of the method are discussed along with necessary additional development of criteria for coarse graining.     

A three-dimensional cellular automaton model of dendrite growth with stochastic orientation during the solidification in the molten pool of binary alloy

A three-dimensional (3D) cellular automaton model is developed for the prediction of dendrite growth with stochastic orientation during solidification process in the molten pool of binary alloy. An angle-information transfer method is proposed for improving cellular automaton technique to simulate the growth of the dendrites whose preferred growth direction owns stochastic misorientation with respect to the direction of the coordinate system. Dendrite morphologies and solute distributions of single dendrite growth and multi-dendrite growth are able to be obtained by the simulation using present model. The model is also employed to study the difference between two-dimensional simulation and 3D simulation on solute segregation and dendritic growth. Using the established model, 3D multi-columnar dendrites with stochastic crystallographic orientations can be obtained efficiently, and the competitive growth and impinging of dendrites can be reproduced in practice. The simulation results agree well with the experimental results.     

A transfer-matrix method for analysis of multicomponent diffusion with any number of components

A transfer-matrix method (TMM) is presented for the development of concentration and flux profiles in multicomponent diffusion involving any numbern of components. From interdiffusion fluxes or concentration gradients available at an initial positionx _s, the authors derive expressions for the transfer matrix and its integral so that the concentrations or interdiffusion fluxes of the components can be obtained at any coordinatex. The TMM requires data for interdiffusion coefficients, which are obtained as average values over selected regions by the method of moments developed by Dayananda. Expressions for the concentrations are also obtained from initial conditions on the fluxes or the concentration gradients. The method is also applicable to the case when all the concentrations are known at two ends of a region over which the diffusion coefficients are considered constant. The integration of the fluxes over time, or over the coordinatex, can be evaluated using the transfer-matrix approach, provided the value of the interdiffusion flux is given at a given coordinate. The TMM is applicable to any number of components and can be regarded as a compact generalization of the solutions available for ternary diffusion couples with constant interdiffusion coefficients. An application of the method is illustrated with the experimental data for a ternary Cu-Ni-Zn diffusion couple, and the results are compared with those based on the Fujita-Gosting solution. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

Evaluation of the Peierls stress for boundary dislocations

The Peierls stress for 1/6〈111〉 twinning dislocations and 1/2〈111〉 perfect dislocations in a bcc structure has been evaluated. The calculations have been performed using the Peierls-Nabarro formalism. The Peierls stresses have been determined from the migration energy of a twin boundary γ_tbm and the energy of an unstable stacking fault γ_us. The values of γ_tbm and γ_us have been calculated using a set of generalized many-body interatomic potentials. The potentials were defined so as to ensure different stability of the bcc structure relative to other structures. It has been shown that this approach provides realistic values of the Peierls stress. The values of the Peierls stress for 1/6〈111〉 twinning dislocations are very sensitive to the model parameters, unlike those for 1/2〈111〉 perfect dislocations.     

一种简易刀具补偿编程方法

介绍了国内传统数控铣加工中存在的计算繁琐问题,提出了基于CAD的数控手工编程,在CAD上可找到手工编程所需的坐标点,去掉繁琐的计算和自动编程中冗长的程序及增加的通信问题,并在生产中验证了这种方法的正确性和有效性。实践证明,该手工编程方法具有快、精、准和提高生产效率等优点。