Damage Constitutive Relationship of Coupled Plasticity-creep for Magnesium Alloy

Based on a simple mechanical model and an appropriate definition of generalized time,a constitutive equation is obtained for coupled plasticity and creep behavior of materials.A damage evolution based on a spherical void model for mixed hardening materials is obtained by means of Gurson's model.The damage evolution law is embedded in the adopted constitutive equation,and a damage constitutive equation is obtained for coupled plasticity and creep behavior of die-casting magnesium alloy.The response of magnesium alloy subjected to cyclic loads is analyzed.The validity of the proposed model is demonstrated by the satisfactory agreement between the experimental and calculated results.     

Cyclic Creep Constitutive Model of Magnesium Alloy at High Temperature

With high strength-to-weight ratio and excellent technological properties, Magnesium Alloy plays a more and more important role in automotive industry. But the inferior high temperature creep resistance limited its application to power components such as engine and transmission cases. In order to investigate the cyclic creep behavior of Magnesium Alloy at high temperature, creep tests of plate specimens AM50 were conducted. Through the analysis about the microstructure and defects of AM50, an isotropic and scalar damage parameter was introduced into an existing creep constitutive model. Furthermore, a Fortran code based on numerical algorithm was developed to simulate plasticity, cyclic creep, and recovery phenomenon observed. Comparisons between calculated results and experimental data show good agreement.     

Comprison Research of Creep Expriments and the Analysis of the Theorial Model of Soft Rocks

Uniaxial and triaxial creep experiments of soft rocks aredone by Chen Applying Method and the Instron-1346 eletronic-fluid serving compression machine. The authors use the five-substance model and discuss the one and three-dimentional constitutive equations and creep ones. It is found that the new model well describes the full creep procedure of soft rocks. The rheological parameters of the creep equations are calculated by the Marquart Method and the forecast curves are drawn. In comprison with the experimental curves,the theorial results are proved.     

Deformation Analysis of Thermo Elastic Plastic on ReinforcedConcrete Structure Subjected to Fire

To investigate the mechanical behavior of reinforced concrete structure subjected to fire, the incremental thermo elastic plastic creep constitutive equations of steel and concrete in consideration of temperature and creep deformation were obtained based on different yield rules and the elastic plastic theories. The nonlinear analysis for reinforced concrete simply supported slab was carried out with consideration the variation of the mechanical properties of steel and concrete with temperature. The effectiveness and applicability of the constitutive equations were verified with the test results from relevant literature. The nonlinear analysis was employed for one bay one storey reinforced concrete frame under fire and the variation of displacement at partial nodes was analysed. It was found that the reinforced concrete structure at a high temperature would have a significant deformation. And the displacement change curves of beam column joints of reinforced concrete frame were not monotonous trend and there were inflexion points. The vertical displacement values of beam column joints were less than those at midspan of cross beam.     

NONPROPORTIONAL CYCLIC PLASTICITY FOR METALLIC MATERIALS

In order to describe nonproportional cyclic plasticity of polycrystalline metallic materials, the hardening function is separated into three factors, in which,f1 is related to the hardening parameter determined by plastic strain range, and f2 and f3 are related to the change of plastic deformation path characterized by a measure of nonproportionality. The main features of this work are: (a)cyclic hardening is described by f1, and additional hardening is separated into instantaneous and hereditary one which is described by f2 and f3 respectively; (b)a general form of the evolution of f1(i= 1,2, 3) is proposed; (c)parameters a1 and a3 are introduced to keep the memory to the largest cyclic hardening and additional hardening respectively; (d)various hardening mechanisms are embedded into a thermomechanically consistent constitutive equation. The capability of the developed model is verified with the analysis of biaxially nonproportional experiments by Benallal et al, Tanaka et al and Ohashi et al.     

Triaxial creep model and its stability analysis of gas containing coal 

Investigation on the creep property of coal containing gas is an effective means for understanding the mechanism of delay outburst of coal and gas in a coal mine. This paper conducts triaxial creep tests on the specimens of gas containing coal using a self made creep experimental apparatus to determine the creep law for coal containing gas. An improved triaxial creep model is developed for containing gas by coupling a Nishihara model with another visco elasto plastic body which can describe the deformation property of non Newtonian fluids. The parameter fittings are conducted based on the experimental dada.The present model is examined in terms to the results of triaxial creep tests in various stress level cases. The result shows that the proposed model for gas containing coal can properly represent the creep deformation at different creep stages, especially at the accelerating stage. The instability condition of the model for coal containing gas is also discussed according to stability theories of differential equation solution.     

基于能量守恒定律对西原模型的改进与验证

在油气钻井工程中,由岩石蠕变引起的井眼缩径或井壁坍塌时有发生,本文基于能量守恒定律分析岩石蠕变的3个阶段,阐述传统的蠕变模型难以研究岩石加速蠕变阶段的原因。为此,在西原模型的基础上串联了一个绕组元件,建立改进西原模型及相应的本构方程。根据油气开发过程中所钻遇地层部分岩心的蠕变实验数据,对建立的改进西原模型进行验证,表明该模型能够很好地反映出岩石的蠕变特性,尤其是在岩石所受荷载大于长期强度的情况下,对凸显岩石加速蠕变阶段效果明显。该改进西原模型的提出可为进一步揭示岩石蠕变的客观规律提供理论依据。     

NONLINEAR ELASTIC-DAMAGE CONSTITUTIVE EQUATION OF BRITTLE ROCK AND EXPERIMENAAL VERIFICATION

on the basis of the experimental studies of mechanical properties in brittle rock,the generalized elastic modulus,generalized elastic Poisson ratio,irreversible deformation Poisson ratio and damage deformation characteristic arediscussed.And then,a new damage variable is defined.A new nonlinear elastic-damage constitutive equation is built and the damage evolution equation is proposed.The nonlinear elastic-damage constitutive equation is verified in the uniaxialcompression test of quartz sandstone in Chongqing.The results show that reasonableagreement exists between the theoretical analysis and the experimental data.     

Plastic upper bound theory reliability analysis of coal gangue hills

We established the plasticity limit analysis reliability limit state equation and the reliability calculating formula for gangue hills using the JC method. This work was based on the plasticity mechanics limit analysis upper bound theory and traditional slice methods. A reliability index of gangue hill stability was derived using this equation. It was found that the safety coefficient of gangue hills derived by the limit state equation risks failure if the variability of cohesion and internal friction angle are not considered. When the variability of the strength parameters rises, the reliability theory more consistently projects practicality than the limit state equation for gangue hill stability assessment.     

Application of Meshless Method in Dynamic Mechanics

As a new type of meshless method which has appeared in recent years, the Reproducing Kernel Particle Method has such meshless features as the need for nodes only without classified units, and is advantageous in the process of calculation. The present study introduces the Reproducing Kernel Particle Method and applies it to the research of nonlinear dynamic mechanics. The dynamic process involves different kinds of nonlinearity. The study assumes that deformation of dynamic analysis belongs to small one and that the material nonlinearity has been taken into account. When under small strain, the increment constitutive law and the total Lagrangian model of calculation are adopted to deduce the dynamic control equation by the Reproducing Kernel Particle Method. The instances of calculation demonstrate that this method is effective in the analysis of dynamic problems.     

INCREMENTAL FORM OF ENDOCHRONIC CONSTITUTIVE EQUATION AND ITS APPLICATIONS

An incremental formula is derived from integral form of endochronic plastic constitutive equation, which greatly reduces the error caused by the one which was directly obtained from differential form of the constitutive equation. An elastoplastic matrix is then proposed, based on which a stiffness finite element approach is developed. The analysis of the residual stress at the inner skin of an autofrettaged thick-walled cylinder agrees well with the experimental result. The calculated stress-strain fields of a double - edge - notched plate subjected to cyclic zero-to-tension loading are also quite reasonable. The numerical process is steady and quickly convergent, and the developed approach can easily be applied to practical engineering analysis.     

Numerical Simulation of Eelastoplastic Multi-scales Model for Casting Magnesium Alloy

To realize the transforms of stress and strain and obtain the multi-scale constitutive equation across micro/meso/macro scales.strain-stress curves for magnesium alloy(AM60) and magnesium matrix are carried out with the machine MTS.By means of mixture law,mechanical property of particle is obtained.Based on the character of magnesium alloy structure,the finite element model of unit-cell included complex micro-structures is carried out.Via finite element numerical simulation of magnesium alloy unit-cell,the methodology overcome the limitation of present analytical method.Finally the multi-scale constitutive equation has been used for the analysis of the tensile stress vs. strain curve for magnesium alloy.Results show satisfactory agreement between the stress vs strain curve by the present methodology and the experimental data for AM60.     

早龄混凝土的压缩与拉伸徐变及其研究现状

早龄混凝土的拉伸、压缩徐变规律及其结构徐变应力计算方法是对早期裂缝进行有效预测并控制的关键。既有的徐变研究主要侧重于成熟混凝土,而早龄混凝土徐变相关的科学研究还有待进一步深入。对早龄混凝土的压缩和拉伸徐变研究成果、测试方法及其徐变应力计算方法进行了详细综述。研究表明：目前混凝土早龄期拉伸、压缩徐变试验测试尚无规范可循,相关试验数据较为缺乏;混凝土早龄期徐变预测模型基本未考虑其在低应力水平下的非线性性质;早龄混凝土结构非线性徐变应力理论分析方法亦不尽完善。基于系统试验研究和固化徐变理论建立混凝土非线性徐变理论模型,对早龄混凝土结构采用同时考虑受拉和受压不同应力松弛特性的非线性徐变应力理论计算方法,应可提高早龄结构的有限元仿真精度。     

限制性剪切蠕变的渐进式破坏发育模式与本构方程分析

以标准红砂岩方形试样为研究对象,运用自行研制开发的软弱煤岩剪切蠕变试验设备和煤岩细观力学特性测控软件、超声波检测设备,对红砂岩限制性剪切、限制性剪切蠕变过程的破坏过程、破坏模式、渐变特性进行了分析。结果表明：红砂岩试样在限制性剪切、限制性剪切蠕变条件下表现出明显的渐变破坏特性;大部分试验过程中试样内部的超声波速减小幅度较小,仅在临近破坏处有明显减小;红砂岩限制性剪切蠕变破坏发育过程表现为先形成主控破裂面,又在其附近区域形成丰富微裂隙,并发育、相互贯通,最后克服剪切面摩擦力并发生突发式破坏,且伴有较大断裂声;其破坏模式发育过程呈渐进式,破坏模式呈突发式;建立了一种可以表征红砂岩剪切蠕变渐进式破坏发育过程的本构模型。     

结构消(耗)能元件芯材SN490B本构关系数值模拟

采用消(耗)能元件的结构在遭受地震作用时,元件芯材首先屈服进入塑性阶段,利用其滞回变形消耗地震输入能量,保护主体结构,元件芯材本构关系的数值模拟是对采用消(耗)能元件结构进行抗震分析与设计的基础。为更真实地模拟结构消(耗)能元件芯材在单调和循环荷载下的本构响应,更准确地对采用消(耗)能元件结构进行结构弹塑性地震响应分析,对常用作消(耗)能元件芯材的日本高延性钢材SN490B的单调、循环加载本构及循环骨架曲线进行了数值模拟,包括：采用Esmaeily-Xiao二次流塑性模型模拟材料在单调荷载作用下弹性段、屈服段、强化段和二次流塑段4个阶段;采用混合强化模型模拟材料循环荷载作用下的本构响应,运用大型通用有限元软件ABAQUS结合数值模拟参数对16种不同循环加载制度下的循环加载试验进行模拟,并与试验结果进行对比;采用Ramberg-Osgood模型、无量纲化的Ramberg-Osgood模型及两段式模型模拟循环骨架曲线。研究结果表明：所采用数学模型可以较好地模拟SN490B钢材单调、循环加载本构响应及循环骨架曲线,数值模拟与试验结果拟合较好。     

A Thermomechanically Consistent Constitutive Model for Finite Elastoplastic Deformation

Based on a thermomechanically consistent mechanical model consisting of springs and plastic dashpots, a plastic constitutive equation for large deformation is derived. Then the incremental form of elastoplastic constitutive equation is developed, which can easily be applied to the finite element analysis and other numerical approaches. The method for the determination of the involved material constants is suggested, which is based on the concept of nonstress configuration proposed by Lee. The necking process of a circular bar subjected to large elastoplastic deformation is simulated and the comparison between the analytical and experimental results is quite satisfactory. The developed model does not use the eoncept of a yield surface, which effectively improves the convergence and computation efficiency.     

蠕变模型对约束钢梁抗火性能分析的影响

钢材在高温和荷载作用下产生明显蠕变变形,影响火灾中结构的变形和受力性能。现有的蠕变模型较多,但没有一个广泛适用的蠕变模型。不同的蠕变模型对钢结构抗火分析结果有很大影响。为了量化蠕变模型对约束钢梁抗火性能分析的影响,对5种常用的蠕变模型进行了对比分析。采用编写的约束钢梁计算程序,分别计算5种蠕变模型下约束钢梁的抗火性能并与试验数据进行对比。结果表明,采用Norton蠕变模型的计算结果与试验数据吻合最好。最后对影响约束钢梁抗火性能参数进行了研究,研究发现,Harmathy蠕变模型对约束梁抗火性能分析结果影响最大;不同蠕变模型对不同荷载比、约束刚度下的约束钢梁抗火性能影响程度均不同。