Heat Conduction in a Phase Field Model for Martensitic Transformation

We study the martensitic transformation with a phase field model, where we consider the Bain transformation path in a small strain setting. For the order parameter, interpolating between an austenitic parent phase and martensitic phases, we use a Ginzburg-Landau evolution equation, assuming a constant mobility. In [1], a temperature dependent separation potential is introduced. We use this potential to extend the model in [2], by considering a transient temperature field, where the temperature is introduced as an additional degree of freedom. This leads to a coupling of both the evolution equation of the order parameter and the mechanical field equations (in terms of thermal expansion) with the heat equation. The model is implemented in FEAP as a 4-node element with bi-linear shape functions. Numerical examples are given to illustrate the influence of the temperature on the evolution of the martensitic phase. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A comparison of numerical models for one-dimensional Stefan problems

In this paper, we present a critical comparison of the suitability of several numerical methods, level set, moving grid and phase field model, to address two well-known Stefan problems in phase transformation studies: melting of a pure phase and diffusional solid-state phase transformations in a binary system. Similarity solutions are applied to verify the numerical results. The comparison shows that the type of phase transformation considered determines the convenience of the numerical techniques. Finally, it is shown both numerically and analytically that the solid-solid phase transformation is a limiting case of the solid–liquid transformation.     

Phase‐field systems for multi‐dimensional Prandtl–Ishlinskii operators with non‐polyhedral characteristics

Hysteresis operators have recently proved to be a powerful tool in modelling phase transition phenomena which are accompanied by the occurrence of hysteresis effects. In a series of papers, the present authors have proposed phase‐field models in which hysteresis non‐linearities occur at several places. A very important class of hysteresis operators studied in this connection is formed by the so‐called Prandtl–Ishlinskii operators. For these operators, the corresponding phase‐field systems are in the multi‐dimensional case only known to admit unique solutions if the characteristic convex sets defining the operators are polyhedrons. In this paper, we use approximation techniques to extend the known results to multi‐dimensional Prandtl–Ishlinskii operators having non‐polyhedral convex characteristicsets. Copyright © 2002 John Wiley & Sons, Ltd.     

Micromechanical Approach of Martensitic Transformation Problem in Steels

To determine the mechanical behavior of material involving the martensitic phase transformation (for example, steels like 100Cr6), a representative volume element (RVE) model including phase transformation criterion is desireable at micromechanical approach. A framework combining the Eshelby's inclusion theory as well as continuum mechanics with phase-transformation (PT) critical condition at RVE model is presented briefly. And application of this model to estimate the critical aspect ratio of martensitic plate or lath inside homogeneneous stress field is also included, where the RVE can be under uniaxial tension/compression or pure shear loading case. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling of stress relaxation associated with an electrically-induced phase transformation

Stress relaxation due to an electrically-induced phase transformation in a ferroelectric crystal bar stretched by a hard-loading device is studied in the one-dimensional setting of electromechanics. According to the proposed model, the stress relaxation rate is governed by a nonlinear ordinary differential equation which resulted from the kinetic relation that controls the evolution of the phase transformation. A numerical analysis basing on simplifying approximation indicates that the stress starts to decrease when the intensity of the applied electric field reaches a critical value and that a stronger electric field results in a quicker stress reduction.     

带交易费用的证券组合投资选择的优化模型

本文利用在约束条件中加入证券多样化选择约束的办法来抵减非系统风险 ,就证券组合投资的选择问题 ,建立了带交易费用的综合考虑收益和风险的多目标规划模型 ,然后通过变换将不可微的多目标规划问题转化为一个多目标线性规划问题 ,最后给出了问题的一个算法和算例     

Macroscopic and mesoscopic modeling based on the concept of generalized stresses for cutting simulation

Based on the concept of generalized stresses proposed by GURTIN [2] and FOREST et al. [1] macro- and meso-scopic modelling are presented. For the macroscopic modelling we develop a multi-mechanism model for strain rate and temperature dependent asymmetric plastic material behavior accompanied by phase transformation with consideration of the trip-strain. Furthermore, we extend the multi-mechanism model with the gradient of phase fraction, which is considered as an extra degree of freedom. For mesoscopic modelling a phase field model is implemented for describing phase transformations. For the scenario of a cutting process we have a martensite-austenite-martensite transformation. A generalized principle of virtual power is postulated involving generalized stresses and used to derive the constitutive equations for both approaches. Furthermore, parameters of the multi-mechanism model related to visco-plasticity with SD-effect and the trip-strain are identified for the material DIN 100Cr6. In the examples a cutting simulation for testing the multi-mechanism model and a phase-transformation simulation are shown. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

激光熔凝加工中瞬时温度场数值模拟

以激光熔凝表面强韧化处理为背景,应用空间弹塑性有限单元和高精度数值算法同时考虑材料组织性能的变化模拟工件的温度场,主要研究激光熔凝加工中瞬时温度场数值模拟,同时考虑相变潜热的影响,为第二步热应力场及残余应力的数值模拟做准备．用算例验证了模型的正确性,最后给出了激光熔凝加工不同时刻温度场分布．     

Phase field modeling of complex crack patterns in multi-physics problems

Recently developed continuum phase field models for brittle fracture show excellent modeling capability in situations with complex crack topologies including branching in the small and large strain applications. This work presents a generalization towards fully coupled multi-physics problems at large strains. A modular concept is outlined for the linking of the diffusive crack modeling with complex multi field material response, where the focus is put on the model problem of finite thermo-elasticity. This concerns a generalization of crack driving forces from the energetic definitions towards stress-based criteria, the constitutive modeling of degradation of non-mechanical fluxes on generated crack faces. Particular assumptions are made on the generation of convective heat exchanges approximating surface load integrals of the sharp crack approach by distinct volume integrals. The coupling effect is also shown in generation of cracks due to thermally induced stress states. We finally demonstrate the performance of the phase field formulation of fracture at large strains by means of representative numerical examples. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Indirect interaction between nuclear spins in easy-axis antiferromagnets

We consider two atoms with nuclear spins I = 1/2 belonging to a regular chain (spin register) of isotopes substituting basic nuclear-spin-free atoms in a plate of an antiferromagnet with the easy anisotropy axis. A constant external magnetic field is assumed to be directed along the easy axis perpendicular to the plate plane and to have a constant gradient along the register direction. For a simple model, we diagonalize the spin Hamiltonian in a spin-wave approximation using the Bogoliubov-Tyablikov transformation. We show that in the presence of a nonuniformity of the external magnetic field, the indirect interaction between nuclear spins caused by the hyperfine interaction of nuclear spins with virtual spin waves in the antiferromagnet can increase and even oscillate depending on the distance between the considered spins if the local field at the midpoint between them is close to the field of the orientational phase transition to the spin-flop phase in a homogeneous antiferromagnet.     

A Phase Field Model for Martensitic Transformations

The martensitic transformation is described using a phase field model which is in mathematical terms the regularization of a sharp interface approach. In this work, up to two martensitic orientation variants are considered. The evolution of microstructure is assumed to follow a time dependent Ginzburg-Landau equation. The coupled problem of the mechanical balance equation and the evolution equations is solved using finite elements and an implicit time integration scheme. In this work, the global energy evolution during the martensitic transformation and the influence of external loads on the formation of the different martensitic phases are studied in 2d. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low Dimensional Born-Infeld Equations Coupled with a Collisionless Matter Model

We consider the Born-Infeld nonlinear electromagnetic field equations and study its Cauchy problem in the case that the Vlasov equation is considered as a matter model. In the present paper, the Vlasov equation is considered on the so-called one and one-half dimensional phase space, and in consequence the Born-Infeld equations are reduced to a quasilinear hyperbolic system with two unknowns. A transformation is introduced in order to make the field equations easy to handle, and suitable assumptions are made on initial data so that the nonlinearity of the field is controlled.     

油田高含水期稳产措施配置多层目标规划

措施规划对于延长油田稳产年限 ,提高采油速度及提高最终采收率是十分必要的 .有些学者建立了油田稳产措施规划的整体或区块规划模型 ,但没有考虑实际油田生产各生产层系的地质特性和所采取措施的差别 .本文针对油田开发实际中存在多层现象 ,以区块的各个生产层为基础 ,建立了油田措施的多层目标规划模型 ,并采用合理的算法进行求解 .应用结果表明 ,多层目标规划使措施配置更精细 ,更能反映生产实际 ,是解决油田措施配置问题的一项有力工具     

Transformation strains for bainitic variant evolution in steel

In our work, we supplement a thermodynamically consistent multi-scale model for the bainitic phase transformation in a low alloy steel, which takes into account the mechanisms of elasto-viscoplasticity, phase transformations and heat conduction as well as the poly-crystalline structure of steel. In order to obtain realistic simulation results, the microscopic conversion procedures for the austenite-to-bainite transformation have to be described in an appropriate way. To this end, the transformation strains for the crystallographic variants have been adjusted. For calculation of transformation strains a theory for the formation of dislocated martensite, more precisely a hierarchical packet-block structure, is used. The calculated transformation strains are used in a simulation of bainite phase transformation in a polycrystalline RVE. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analysis of a thermomechanical phase transition model

Subject of this work is a macroscopic thermomechanical model of phase transitions in steel. Effects like transformation strain and transformation plasticity induced by the phase transitions are considered and used to formulate a consistent thermomechanical model. The resulting system of state equations consists of a quasistatic momentum balance coupled with a nonlinear stress-strain relation, a nonlinear energy balance equation and a system of ODEs for the phase volume fractions. We prove the existence of a unique weak solution using fixed-point arguments. A key issue is a regularity analysis for the mechanical subsystem to obtain continuity of the stress tensor. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A constitutive model for a high alloyed cast CrMnNi TRIP-steel

The macroscopic material model, proposed by Papatriantafillou [1], is adopted to describe the rate dependent flow behaviour and the temperature and stress state dependent γ-α′ phase transformation of a newly developed cast TRIP-steel. Simple test of the implemented model revealed that the evolution of the martensite volume fraction is not predicted correctly in case of inelastic deformation and subsequent unloading. Therefore, we present an improved model for the γ-α′ phase transformation and show that these predictions are due to the choice of the corresponding thermodynamical driving force. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Micromechanical modeling of bainitic phase transformation

We develop a micromechanical material model for phase transformation from austenite to bainite for a polycrystalline low alloys steel. In this material (e.g. 51CrV4) the phase changes from austenite to perlite-ferrite, bainite or martensite, respectively. This work is concerned with phase transformation between austenite and n-bainite variants in differently orientated grains. The characteristic features of bainite formation are the combination of time-dependent transformation kinetics and lattice shearing in the microstructure. These effects are considered on the microscale and transferred to the polycrystalline macroscale by means of homogenisation of stochastically orientated grains. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)