A damage function used for prediction of low cyclic fatigue life

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Linear stability analysis for a thermoviscoplastic material under cyclic axial loading

Some mechanical properties exhibit a very strong dependence upon temperature; these evolutions can be properly analyzed by the steady state response in cyclic loading. To relate experimental conditions to thermomechanical characteristics, the existence and the stability of steady state solutions are studied for cylinders submitted to cyclic compression. The material, considered as rigid viscoplastic, is modeled by a non-Newtonian temperature dependent viscous law. Closed form solutions are obtained in the framework of a large deformation theory by neglecting thermal expansion and inertia effects. Steady state regime is analyzed. The stress versus strain rate response and the temperature distribution are established as functions of the geometry of the cylinder, the loading characteristics and the material parameters. The stability of steady state solutions is analyzed with use of a linear perturbation scheme.Received: 4 July 2002, Accepted: 5 August 2004, Published online: 24 February 2005PACS: 46.15.Ff, 83.60.St Correspondence to: F. Dinzart     

Measurement of uniform and localized heat dissipation induced by cyclic loading

An experimental technique is presented for measuring the heat dissipation and localization during cyclic loading of materials. The temperature field is measured by a number of thermistors and an infrared camera, which scans the specimen surface continuously. The specimen is mounted inside an isothermal chamber. The measured whole-field temperature can be used for detection of damage propagation and localization. The resolution of the technique under various boundary conditions is discussed using a onedimensional model for the heat loss under steady-state conditions. Applications of the technique are demonstrated for specimens made of fiber-reinforced ceramic and polymer matrix composites (PMCs). A methodology is proposed for measuring changes in damping and stiffness properties of viscoelastic polymer matrix composites using the temperature rise of a cyclic loaded specimen. It is demonstrated that for a ceramic matrix composite, where interfacial frictional sliding gives rise to heat dissipation, the temperature resolution can be used for detection of stress-strain hysteresis with an accuracy better than that of the stress-strain data.     

A constitutive model and elastoplastic analysis of structures under cyclic loading

A constitutive model for cyclic plasticity is briefly outlined. Then the model is implemented in a finite element code to predict the response of cyclic loaded structural components such as a double-edge-notched plate, a grove bar and a nozzle in spherical shell. Comparision with results from other theories and experiments shows that the results obtained by using the present model are very satisfactory.The Project Supported by National Natural Science Foundation of China.     

Interfacial debonding of coated-fiber-reinforced composites under tension-tension cyclic loading

A new degradation function of the friction coefficient is used. Based on the double shear-lag model and Paris formula, the interfacial damage of coated-fiber-reinforced composites under tension-tension cyclic loading is studied. The effects of strength and thickness of the coating materials on the debond stress, debond rate as well as debond length are simulated. The subject supported by the National Natural Science Foundation of China (No.59778034), Teaching and Research Award Program for Outstanding Young Teachers in Higher Education Institutions of MOE, China and The Hong Kong Polytechnic University (G-S737)     

Thermodynamic based model for the evolution equation of the backstress in cyclic plasticity

A nonlinear kinematic hardening rule is developed here within the framework of thermodynamic principles. The derived kinematic hardening evolution equation has three distinct terms: two strain hardening terms and a dynamic recovery term that operates at all times. The proposed hardening rule, which is referred in this paper as the FAPC (Fredrick and Armstrong–Phillips–Chaboche) kinematic hardening rule, shows a combined form of the Frederick and Armstrong backstress evolution equation, Phillips evolution equation, and Chaboche series rule. A new term is incorporated into the Frederick and Armstrong evolution equation that appears to have agreement with the experimental observations that show the motion of the center of the yield surface in the stress space is directed between the gradient to the surface at the stress point and the stress rate direction at that point. The model is further modified in order to simulate nonproportional cyclic hardening by proposing a measure representing the degree of nonproportionality of loading. This measure represents the topology of the incremental stress path. Numerically, it represents the angle between the current stress increment and the previous stress increment, which is interpreted through the material constants of the kinematic hardening evolution equation. This new kinematic hardening rule is incorporated in a material constitutive model based on the von Mises plasticity type and the Chaboche isotropic hardening type. Numerical integration of the incremental elasto-plastic constitutive equations is based on a simple semi-implicit return-mapping algorithm and the full Newton–Raphson iterative method is used to solve the resulting nonlinear equations. Experimental simulations are conducted for proportional and non-proportional cyclic loadings. The model shows good correlation with the experimental results.     

循环加卸载损伤大理岩的动力学特性

利用MTS 815电液伺服岩石实验系统进行上限应力为80%、85%、90%、95%单轴抗压强度的大理岩单轴压缩循环加卸载实验,每种上限应力条件分别设置20、40、60、80次循环。再利用分离式Hopkinson压杆对损伤岩样进行动力学实验。分析了循环加卸载上限应力及循环次数对大理岩塑性应变的影响,揭示了大理岩动态力学参数和破碎吸收能随损伤变量的演化规律。实验结果表明:塑性应变与循环次数呈正相关,且上限应力越大,塑性应变趋于稳定所需的循环次数也会增大;动态单轴抗压强度、动态弹性模量随损伤变量增加呈指数衰减;破碎吸能占比以损伤变量D=0.343为临界点分为两个阶段,D<0.343时,破碎吸能占比稳定在10%左右,数值约为13 J,当D>0.343时破碎吸能占比随损伤变量增加不断增大。研究结果可为岩体工程的设计、施工及支护参数的选取提供参考。     

不同循环加载应力下花岗岩Kaiser效应特征的试验研究

为了更准确地获取花岗岩的Kaiser效应点对应的应力值σAE,利用RMT-100B岩石力学伺服试验机和Micro-Ⅱ Express型声发射监测系统,研究不同循环加载应力σci下岩石Kaiser效应特征.结果表明:相比单次加载,在循环加载作用下,试样的应力-应变曲线在加载初期下凸减弱,压密阶段变短;由于试样的加卸载曲线...     

周期荷载作用下煤岩声发射特征的颗粒流模拟

为研究周期荷载应力水平对煤样声发射特征的影响,采用PFC数值软件开展了 3种不同应力水平的等幅周期荷载数值模拟试验,分析了周期荷载应力水平对煤样破坏循环次数、声发射计数及损伤演化特征的影响.研究结果表明:周期荷载作用下,煤样破坏过程中的声发射活动呈现初始、相对平静和活跃三阶段演化规律,且在煤样破坏前的周期荷载卸载阶段及...     

球壳塑性变形下的应变增长现象

应变增长现象会对容器安全形成威胁。以往研究涉及的应变增长现象大多在壳体弹性变形范围内,本文中实验观察到球壳塑性变形时的应变增长现象,应变增长系数(最大应变值与第一个应变峰的比值)最大值达到1.16。实验还获得了容器内壁压力-时间曲线,并利用球壳响应理论分析出应变增长现象是由容器内壁的周期性多脉冲载荷引起的,该载荷存在3个较明显的脉冲,前两个脉冲对应变增长现象起主要作用。     

CONSTITUTIVE RELATION OF UNSATURATED SOIL BY USE OF THE MIXTURE THEORY(Ⅰ)-NONLINEAR CONSTITUTIVE EQUATIONS AND FIELD EQUATIONS

The nonlinear constitutive equations and field equations of unsaturated soils were constructed on the basis of mixture theory. The soils were treated as the mixture composed of three constituents. First, from the researches of soil mechanics, some basic assumptions about the unsaturated soil mixture were made, and the entropy inequality of unsaturated soil mixture was derived. Then, with the common method usually used to deal with the constitutive problems in mixture theory, the nonlinear constitutive equations were obtained. Finally, putting the constitutive equations of constituents into the balance equations of momentum, the nonlinear field equations of constituents were set up. The balance equation of energy of unsaturated soil was also given, and thus the complete equations for solving the thermodynamic process of unsaturated soil was formed. Foundation items: the National Natural Science Foundation of China (59678003); Special Research Plan of the Education Department of Shaanxi Province (01JK178) Biographies: HUANG Yi (1936-) ZHANG Yin-ke (1964-)     

Two notes on nonlocal field theory

In this paper, two fundamental problems completely unsolved in nonlocal field theory are studied. The first is the dependence of nonlocal residuals. By studying this problem, a theorem concerning the relationship between the residuals of nonlocal body force and nonlocal moment of momentum is given and proven. The other problem is how to give the stress boundary conditions in the linear theory of nonlocal elasticity. The stress boundary conditions obtained in this paper can not only answer why the nonlocal stress solution satisfying the boundary conditionst _ji ^(s) n _j ¦O ₂ =p _i (p _i is a constant) on the surface of crack does not exist but also give a model of the molecular cohesive stress on the crack tip.     

拉压循环载荷下正交异性钢桥面板肋-面板焊缝裂纹扩展分析

采用四步法计算了考虑循环载荷中压应力影响的正交异性钢桥面板的肋-面板焊缝表面裂纹扩展。第一步是基于正交异性钢桥面板的疲劳分析模型,计算肋-面板焊缝处的应力,第二步是通过肋-面板焊缝的三维局部模型,用Schwartz-Neumann交替法计算焊缝表面裂纹的应力强度因子分布,第三步是用二维断裂力学模型和增量塑性损伤模型,计算循环载荷中的压应力对裂纹扩展的影响,第四步是用第二步中的三维裂纹分析结果和第三步中的二维断裂力学模型得到的裂纹扩展公式,计算钢桥面板的肋-面板焊缝表面裂纹扩展。计算结果表明,对应于正交异性钢桥面板肋-面板焊缝处的循环应力,本文所用模型的裂纹尖端反向塑性区导致裂纹扩展率增加50%以上。研究结果为正交异性钢桥面板肋-面板焊缝裂纹的疲劳寿命分析提供了研究基础。     

A crystallographic model for the orientation dependence of low cyclic fatigue property of a nickel-base single crystal superalloy

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基于遗传进化的混凝土湿度场差分反演算法

建立混凝土湿度场差分反演模型,并用遗传算法对其进行了优化求解。根据混凝土湿度扩散方程,确定其内部与边界的差分格式,并对具体实例进行了求解;针对经验参数计算结果与实测结果吻合较差的问题,提出了基于实测数据的差分反演模型,利用遗传算法对模型进行优化求解,所得结果与实测数据误差较小。该算法根据实测结果,可获得湿度场的实际分布情况,对类似问题有一定的参考作用。     

CONSTITUTIVE RELATION OF UNSATURATED SOIL BY USE OF THE MIXTURE THEORY (Ⅱ)-LINEAR CONSTITUTIVE EQUATIONS AND FIELD EQUATIONS

The linear constitutive equations and field equations of unsaturated soils were obtained through linearizing the nonlinear equations given in the first part of this work. The linear equations were expressed in the forms similar to Biot’s equations for saturated porous media. The Darcy’s laws of unsaturated soil were proved. It is shown that Biot’s equations of saturated porous media are the simplification of the theory. All these illustrate that constructing constitutive relation of unsaturated soil on the base of mixture theory is rational.     

INVESTIGATION ON KANE DYNAMIC EQUATIONS BASED ON SCREW THEORY FOR OPEN-CHAIN MANIPULATORS

IntroductionDynamicsofroboticmanipulatorsisthebasisofdynamiccontrolandmechanicaldesignofroboticmanipulators.Dynamicanalysisofrobotscanemploytheapproachesoftheoreticalmechanicsanddynamicequationsofanalyticalmechanics,suchasNewton_Eulerdynamicequations,Lagrange_Eulerdynamicequations ,Routhequation ,Appellequation ,andKaneequations.Theultimateresultsarethesamewhenweinvestigateononekindofmotionsaboutoneobjectivebydifferentdynamicequations,butthespeedandquantitiesofcomputationoftheseequationsare…     

基于能量平衡原理的有限单元耦合方法

结构非线性数值计算分析应真实反映局部损伤破坏细节,以作为损伤演化全过程分析的依据。对同类构件,有限单元耦合方法可以解决破坏细节与整体模拟的空间尺度差异问题。基于能量平衡原理,建立了梁与实体单元、梁与壳单元以及壳与实体单元的耦合方程,适用于结构的损伤数值计算。对某RC框架结构原位推覆试验的损伤数值分析表明,有限单元耦合模型能正确反映整体结构的承载力和变形性能,并且能准确反映局部损伤破坏细节。     

基于流固耦合理论的长寿命陀螺仪浮子内部流场及温度场分析

为精确计算出陀螺仪内部温度场,有效改善影响陀螺仪精度的温度梯度分布状况,采用了精度更高的流固耦合计算方法,结合FLUENT软件,对卫星等上使用的某型长寿命陀螺仪浮子组件进行了内部流场及温度场计算。结果表明,浮子内部结构形式对高速旋转的电机产生的介质流场有较大影响,合理的结构设计有利于降低电机功耗;陀螺仪浮子表面存在最大温差为0.52℃的温度梯度,其所产生的液流力矩会对陀螺仪精度产生一定影响;浮子表面温度分布表现出一定的对称性和规律性。计算结果为下一步旨在减小温度梯度、提高陀螺仪精度的精密分区温控提供了数据基础。     

Elasto-plastic constitutive relations of unidirectional fiber composites for cyclic loadings

The clasto-plastic constitutive behaviors of continuous fiber reinforced composites under cyclic loadings are studied by the micromechanics method in which the equal-strain model is used in the fiber direction, the equal-stress model in the other directions. It is supposed that fiber is linearly elastic and matrix is clastic-viscoplastic. The constitutive equations of the matrix are described by Bodner-Partom's unified constitutive theory. Boron/Aluminum composite, as an example, is investigated in detail for an understanding of the stress-strain relations and initial yield behaviors of metal matrix composites. Present results are compared with the experimental data.The project was supported by the Chinese National Natural Science Foundation.