Creep simulation of asymmetric effects by use of stress mode dependent weighting functions

The paper presents a framework for creep modeling of materials exhibiting different behaviors in different loading scenarios, such as tension, compression and shear, respectively. To this end an additive decomposition of the flow rule is assumed into a sum of weighted stress mode related quantities. The characterization of the stress modes is obtained in the octahedral plane of the deviatoric stress space in terms of a single scalar variable, such that stress mode dependent scalar weighting functions can be constructed. Furthermore the numerical implementation into a finite element program of the resulting set of constitutive equations and aspects of the sensitivity analysis for parameter identification are addressed. Verification of the constitutive equations is succeeded for an aluminum alloy AK4-1T and a superalloy René 95, respectively. In two finite element examples the proposed model is applied to investigate the relaxation behavior of a square plate with circular hole and the evolution of creep damage in a gasturbine blade subjected to centrifugal and thermal loads.     

弹体贯穿钢筋混凝土数值模拟

详细描述了依据损伤原理建立的连续损伤模型,并对该模型进行了改进。在LS DYNA程序用户自定义材料模型中加入改进的连续损伤模型,并对弹体侵彻钢筋混凝土的穿孔过程进行了数值模拟,其结果与实验结果相吻合,模型可以用于钢筋混凝土的动态破坏预报。     

A thermo-viscoelastic–viscoplastic–viscodamage constitutive model for asphaltic materials

A temperature-dependent viscodamage model is proposed and coupled to the temperature-dependent Schapery’s nonlinear viscoelasticity and the temperature-dependent Perzyna’s viscoplasticity constitutive model presented in Abu Al-Rub et al., 2009, Huang et al., in press in order to model the nonlinear constitutive behavior of asphalt mixes. The thermo-viscodamage model is formulated to be a function of temperature, total effective strain, and the damage driving force which is expressed in terms of the stress invariants of the effective stress in the undamaged configuration. This expression for the damage force allows for the distinction between the influence of compression and extension loading conditions on damage nucleation and growth. A systematic procedure for obtaining the thermo-viscodamage model parameters using creep test data at different stress levels and different temperatures is presented. The recursive-iterative and radial return algorithms are used for the numerical implementation of the nonlinear viscoelasticity and viscoplasticity models, respectively, whereas the viscodamage model is implemented using the effective (undamaged) configuration concept. Numerical algorithms are implemented in the well-known finite element code Abaqus via the user material subroutine UMAT. The model is then calibrated and verified by comparing the model predictions with experimental data that include creep-recovery, creep, and uniaxial constant strain rate tests over a range of temperatures, stress levels, and strain rates. It is shown that the presented constitutive model is capable of predicting the nonlinear behavior of asphaltic mixes under different loading conditions.     

钢筋混凝土剪力墙受火性能非线性分析

为了研究钢筋混凝土剪力墙的抗火性能,本文基于纤维元模型,提出了一种非线性分析方法。该方法无需计算截面的弯矩-曲率曲线族,计算过程中采用了高温下耦合的混凝土本构关系,其中考虑了混凝土高温热膨胀应变、瞬态热应变和徐变。编制了相应的非线性计算分析程序,程序考虑了轴力二阶效应的影响,计算得到了剪力墙跨中平面外挠度以及火灾下其跨...     

无粘结预应力混凝土板火灾行为的非线性分析

火灾试验研究表明,火灾下无粘结预应力混凝土受弯构件的变形非常大,其中转动也很大,尤其是单面受火的板,是典型的材料非线性、几何非线性问题。如何对其进行精确、高效地求解是值得关注的问题。而基于S-R分解原理的更新拖带坐标有限元法有诸多优点:有利于跟踪变形物体中各点的变形;保证单元的质量守恒;在有限元增量法求解时,还可以避免对坐标的修正;而且将转动作为一个独立的自由度,提高了求解效率。在升温过程中,虽然预应力钢筋的应力处处相等,但其各点的温度不同,导致各点的温度应变、蠕变、塑性应变均不同。预应力筋必然产生滑移。本文采用该方法对火灾下无粘结预应力钢筋混凝土板进行编程分析。通过实际算例验证该算法的可靠性,该方法求解效率高,精度也比较好。     

Multiaxial creep and cyclic plasticity in nickel-base superalloy C263

Physically-based constitutive equations for uniaxial creep deformation in nickel alloy C263 [Acta Mater. 50 (2002) 2917] have been generalised for multiaxial stress states using conventional von Mises type assumptions. A range of biaxial creep tests have been carried out on nickel alloy C263 in order to investigate the stress state sensitivity of creep damage evolution. The sensitivity has been quantified in C263 and embodied within the creep constitutive equations for this material. The equations have been implemented into finite element code. The resulting computed creep behaviour for a range of stress state compares well with experimental results. Creep tests have been carried out on double notched bar specimens over a range of nominal stress. The effect of the notches is to introduce multiaxial stress states local to the notches which influences creep damage evolution. Finite element models of the double notch bar specimens have been developed and used to test the ability of the model to predict correctly, or otherwise, the creep rupture lifetimes of components in which multiaxial stress states exist. Reasonable comparisons with experimental results are achieved. The γ^′ solvus temperature of C263 is about 925 °C, so that thermo-mechanical fatigue (TMF) loading in which the temperature exceeds the solvus leads to the dissolution of the γ^′ precipitate, and a resulting solution treated material. The cyclic plasticity and creep behaviour of the solution treated material is quite different to that of the material with standard heat treatment. A time-independent cyclic plasticity model with kinematic and isotropic hardening has been developed for solution treated and standard heat treated nickel-base superalloy C263. It has been combined with the physically-based creep model to provide constitutive equations for TMF in C263 over the temperature range 20–950 °C, capable of predicting deformation and life in creep cavitation-dominated TMF failure.     

An application of a damage constitutive model to concrete at high temperature and prediction of spalling

A characteristic feature of concrete under uniaxial compression is the development of cracks parallel to the loading direction. A damage constitutive model proposed by Ortiz [Ortiz, M., 1985. A constitutive theory for the inelastic behaviour of concrete. Mech. Mater. 4, 67–93] can predict the transverse tensile stress responsible for these cracks by considering the interaction between the aggregate and the mortar and the development of damage in the latter. When concrete is exposed to high temperature, as is the case during fire, the failure mode is thermal spalling. In order to improve the prediction of the stresses involved in this failure Ortiz’s model is extended to account for the effects of high temperature. Published experimental results for uniaxial and biaxial compression at high temperatures are used to calibrate the temperature dependence of some of the material properties. The transient creep strain is accounted for by modifying the constrained thermal strain. The stress analysis is coupled with hygro-thermal analysis of heat, mass transfer and pore pressure build-up. The effect of pore pressure on the damage evolution is modeled by applying a body force in the stress analysis module proportional to the pressure gradient. A numerical example of concrete under fire is solved and the computed results are discussed. Spalling is predicted when the damage variable reaches its maximum value of unity. The predicted depth and time of spalling for a range of variation of permeability and initial liquid water content are presented. They are in good agreement with published experimental results.     

尼龙6/蒙脱土复合材料蠕变行为研究

针对经典黏弹性模型不能很好分析黏弹性材料的蠕变行为问题,运用分数阶导数的类标准线性体模型与Prony级数模型研究了黏弹性材料尼龙6/蒙脱土复合材料的蠕变行为.采用原位聚合法制备了尼龙6/蒙脱土复合材料,在室温环境下对其进行蠕变实验.然后运用分数阶导数的类标准线性体模型和Prony级数模型对复合材料的蠕变实验数据进行分析...     

On a finite-strain viscoplastic law coupled with anisotropic damage: theoretical formulations and numerical applications

Based on a dissipation inequality at finite strains and the effective stress concept, a Chaboche-type infinitesimal viscoplastic theory is extended to finite-strain cases coupled with anisotropic damage. The anisotropic damage is described by a rank-two symmetric tensor. The constitutive law is formulated in the corotational material coordinate system. Thus, the evolution equations of all internal variables can be expressed in terms of their material time derivatives. The numerical algorithm for implementing the material model in a finite element programme is also formulated, and several numerical examples are shown. Comparing the numerical simulations with experimental observations indicates that the present material model can describe well the primary, secondary and tertiary creep. It can also predict the anisotropic damage modes observed in experiments correctly.     

A multi-fiber approach for modeling corroded reinforced concrete structures

Taking into account the specific behavior of the steel/concrete interface is of primary importance to predict properly the structural response of RC structures. Several constitutive models have been proposed in the literature within the framework of nonlinear finite element method (2D and 3D). Such approaches usually lead to high computational costs due to the large number of degrees of freedom. In the present paper, a multifiber-based model including the steel/concrete interface behavior is proposed. Despite the fact that the kinematics of the multi-fiber approach is based on the theory of beams, this simplified strategy accounts for local phenomena such as the relative sliding between concrete and steel. Furthermore, this steel/concrete interface constitutive model can be extended to model the loss of bond properties due to corrosion. The numerical implementation aspects are described and local responses at the Gauss point level are exposed in the cases of monotonic loadings with and without corrosion. The efficiency and the reliability of the proposed approach are tested on structural case studies which highlight a good agreement between numerical and experimental results. This multifiber-based model provides a pertinent tool for the engineers concerns with the structural assessment of degraded reinforced concrete structures.     

损伤因素对单层平面索网静力性能影响研究：有限元分析与试验验证

运用通用有限元软件ANSYS建立三维有限元模型,对横索与竖索连接节点失效、索预应力损失和锚固端失效这三种损伤因素的不同损伤工况下,单层平面索网结构的受力性能进行了非线性有限元分析,并与相应的试验结果进行了全面对比分析。结果表明：本文的有限元模型能够准确地分析计算上述三种损伤因素对单层平面索网结构受力性能的影响,包括结构...     

A phenomenological model for anisotropic creep in a multipass weld metal

Creep strength of welded joints can be estimated by continuum damage mechanics. In this case constitutive equations are required for different constituents of the welded joint: the weld metal, the heat-affected zone, and the parent material. The objective of this paper is to model the anisotropic creep behavior in a weld metal produced by multipass welding. To explain the origins of anisotropic creep, a mechanical model for a binary structure composed of fine-grained and coarse-grained constituents with different creep properties is introduced. The results illustrate the basic features of the stress redistribution and damage growth in the constituents of the weld metal and agree qualitatively with experimental observations. The structural analysis of a welded joint requires a model of creep for the weld metal under multiaxial stress states. For this purpose the engineering creep theory based on the creep potential hypothesis, the flow rule, and assumption of transverse isotropy is applied. The outcome is a coordinate-free equation for secondary creep formulated in terms of the Norton–Bailey–Odqvist creep potential and three invariants of the stress tensor. The material constants are identified according to the experimental data presented in the literature.     

基于Burgers模型考虑损伤的岩石蠕变全过程研究

岩石蠕变是一个显著的非线性问题，而经典的Burgers 模型是一种线性蠕变模型，必然不能全面地描述岩石蠕变的全过程．当岩石处于高应力水平下的加速蠕变阶段，岩石内部损伤会不断产生和累积，蠕变呈现出明显的非线性特征．为了更好地描述考虑损伤的岩石蠕变全过程，本文假定加速蠕变阶段的损伤演化方程，将损伤引入到Burgers 模型中，提出了一种非线性蠕变损伤模型．然后以乔后盐矿试件蠕变实测数据为分析案例，验证模型在应用于实际工程时的准确性和适用性．最后根据Origin 软件数值拟合的结果，表明本文所提出的非线性蠕变损伤模型能很好地描述岩石蠕变全过程，模型参数取值也在合理范围之内，模型拟合曲线与实验所得数据具有较好的一致性．     

耦合蠕变损伤的粘塑性模型用于疲劳-蠕变交互作用的研究

该文对550℃下30CrMoNiV5-11转子钢进行了轴向应力控制的带保持时间的疲劳实验.按照Le-maitre有效应力的概念,采用Kachanov损伤演化方程,推导了耦合损伤的粘塑性流动方程和随动硬化方程,使用返回映射和向后欧拉方法实现应力更新,并在有限元软件ANSYS的接口程序UMAT中嵌入用户程序.对试样疲劳-蠕变交互行为进行模拟,结果表明,耦合蠕变损伤变量后,模拟结果较不含损伤变量模型的模拟结果精度大为提高.     

SIMULATION OF THE IN-PILE BEHAVIORS EVOLUTION IN NUCLEAR FUEL RODS WITH THE IRRADIATION DAMAGE EFFECTS

Based on the commercial computational software, a three-dimensional finite ele- ment model to simulate the thermo-mechanical behaviors in a nuclear fuel rod is established; By taking into consideration irradiation-swelling of the pellet and the irradiation damage effects in the cladding together with the coupling effects between the temperature field and the mechanical field, the user subroutines to define the special material performance and boundary conditions have been developed independently and validated. Three-dimensional numerical simulation of the thermo-mechanical coupling behaviors in a nuclear fuel rod is carried out, and the evolution rules of the important thermal and mechanical variables are obtained and analyzed. The research re- sults indicate that： （i） the fuel pellets will be in contact with the cladding at high burnup, which will induce a strong mechanical interaction between them; （2） the irradiation creep effect plays an important role in the mechanical behavior evolution in the nuclear fuel rod.     

界面特性对短纤维金属基复合材料蠕变行为的影响

基于短纤维增强金属基复合材料(MMC)的单纤维三维模型(三相)，利用粘弹性有限元分析方法对影响金属基复合材料的蠕变行为的因素进行了较为系统的分析。研究中主要讨论了界面特性和纤维取向角对金属基复合材料的蠕变性能的影响。研究结果发现，界面特性诸如厚度、模量和应力指数都对纤维最大轴应力和稳定蠕变率产生影响：稳态蠕变率随界面模量的增大而逐渐减小，当高于基体模量时基本保持不变；纤维轴应力的变化与蠕变率正好相反。稳态蠕变率随界面厚度、应力指数的增加而增大；而轴应力则随之减小。同时不同的纤维取向也影响金属基复合材料蠕变时的轴应力分布和稳态蠕变率。     

水平荷载下型钢再生混凝土柱-钢梁组合框架受力性能非线性数值分析

在型钢再生混凝土柱-钢梁组合框架拟静力试验的基础上,采用Abaqus软件建立该组合框架有限元模型并进行非线性数值分析,获取组合框架的变形图、应力云图及荷载-位移骨架曲线,分析组合框架的受力破坏特征,验证了有限元模型的合理性,并对组合框架进行了参数分析。结果表明,数值模拟计算值和试验值对比误差较小,数值计算模型能够较好地模拟该组合框架的受力性能;组合框架符合强柱弱梁的破坏机制;另外,提高型钢强度或再生混凝土强度对组合框架承载力和刚度有利,但对其变形能力不利;框架承载力和刚度随着梁柱线刚度的增加而提高;增大轴压比对于组合框架的抗震性能和延性不利。研究结论可为该类绿色组合框架的工程应用提供参考。     

Continuum damage models with non-conventional finite element formulations

In recent years, some research effort has been devoted to the development of non-conventional finite element models for the analysis of concrete structures. These models use continuum damage mechanics to represent the physically non-linear behavior of this quasi-brittle material. Two alternative approaches proved to be robust and computationally competitive when compared with the classical displacement finite element implementations. The first corresponds to the hybrid-mixed stress model where both the effective stress and the displacement fields are independently modeled in the domain of each finite element and the displacements are approximated along the static boundary, which is considered to include the inter-element edges. The second approach corresponds to a hybrid-displacement model. In this case, the displacements in the domain of each element and the tractions along the kinematic boundary are independently approximated. Since it is a displacement model, the inter-element boundaries are now included in the kinematic boundary. In both models, complete sets of orthonormal Legendre polynomials are used to define all approximations required, so very effective p-refinement procedures can be implemented. This paper illustrates the numerical performance of these two alternative approaches and compares their efficiency and accuracy with the classical finite element models. For this purpose, a set of numerical tests is presented and discussed.     

关节软骨两相多孔介质非线性模型的有限元方法

基于混合物理论的两相多孔介质模型可以准确描述关节软骨的力学行为，关节软骨的渗透率与固体相体积应变相关。本文研究这一模型的非线性有限元法。具体采用伽辽金加权残值法得到有限元平衡方程，编制了有限元程序，进而对关节软骨围限压缩蠕变和应力松弛行为进行了数值模拟。与视渗透率为常数的线性模型的计算结果比较表明，在变形较大时，渗透率随固体相体积应变变化这一非线性效应不容忽视。