60Si2Mn螺旋弹簧的压缩应力松弛行为与贮存寿命预测

为评价60Si2Mn螺旋压缩弹簧的室温松弛特性,利用InstronE3000K8953型小吨位电子动静态疲劳试验机,对其在不同温度和初始应力水平条件下进行了高温压缩加速应力松弛试验,研究了环境温度、初始应力水平对松弛行为的影响.基于粘弹性体模型,揭示了应力松弛过程中弹性应变向塑性应变的转化特性与塑性应变随松弛时间的变化规律.在对应力松弛前后弹簧丝材金相和TEM微结构进行对比分析的基础上,探讨了应力松弛的微观机制.结果表明,环境温度与初始应力水平对松弛速率具有显著影响.基于应力松弛过程的热激活特性,建立了60Si2Mn螺旋压缩弹簧的贮存寿命预测方程,并对不同应力水平下弹簧的室温和高温贮存寿命进行了合理预测.     

耦合蠕变损伤的Chaboche粘塑性模型的研究

Chaboche粘塑性统一本构模型由于没有包含损伤变量,在应力恒定时只能得到不变的塑性应变速率,因此不能描述蠕变第三阶段的加速过程.该文按照Lemaitre有效应力的概念,采用Kachanov损伤演化方程,推导了耦合损伤的Chaboche粘塑性流动方程和硬化方程,并链接到有限元软件ANSYS中,将之用于高温合金钢P91的蠕变寿命计算,结果表明耦合损伤模型的模拟值与实验数据基本吻合;为该模型描述蠕变损伤和疲劳交互作用奠定了基础.     

汽车玻璃高温蠕变特性的实验与理论研究

通过对汽车用挡风玻璃进行不同温度、不同应力水平下的单轴拉伸蠕变实验,揭示了其非线性蠕变变形特性.实验发现,不同温度和载荷下的蠕变曲线形状类似,高温(550℃～590℃)下该材料的减速蠕变变形不很明显,主要表现为稳态和加速蠕变变形,且其断裂数据符合Monkman-Grant关系,应力和温度对其影响不显著.基于实验结果,本文建立了非线性高温蠕变本构方程.理论预测与实验数据进行了比较,结果表明所提出的本构方程能较好地描述汽车玻璃高温蠕变变形的全过程.     

硅泡沫的超弹压缩和应力松弛的不确定性表征

硅泡沫垫层材料由于制备过程中的发泡随机性,在实验测试中往往表现出单轴压缩和应力松弛性能的分散性。本文基于二阶Hyperfoam超弹性本构模型和二阶Prony应力松弛级数模型,由实验数据的规律性,从不确定性量化角度引入各材料参数的相关性,将多参数不确定性转化为单个参数不确定性,分别建立了形式简单且能够表征不确定性因素的超弹压缩和应力松弛本构模型。在此基础上,分别联合实验统计的单值压缩数据和单值松弛数据,反演出了完整的超弹性本构曲线和应力松弛曲线,与实验结果吻合较好,误差不超过2%,合理地预测了材料的压缩和松弛性能演化规律,并为数值模拟提供了直接的材料参数输入条件。     

旋转壳非线性蠕变分析

基于现有的蠕变理论,本文推导出复杂应力状态下的蠕变本构方程,以内力与转角为基本未知量,得到旋转壳在轴对称载荷作用下的非线性蠕变控制方程.对基本未知量在空间上采用样条配点法离散,在时域上采用初应力法离散,求出壳体在不同时域的蠕变应变和应力,而最终可求出蠕变后的稳态应力重新分布.计算结果表明同,本文分析方法是有效的.     

粘塑性金属材料蠕变实验及流变模型研究

对固体金属材料Lc4铝合金、TC11钛合金及优质碳素结构钢(20#、35#)在高温下进行了多组蠕变实验,对电沉积镍薄膜在常温下进行了多组加、卸载和蠕变实验,根据实验曲线,采用修正的Burgers流变模型来描述这些材料的蠕变行为,建立了积分型蠕变本构方程和微分型应力应变本构关系,确定了蠕变积分核函数,并对由蠕变方程推导的...     

缝合式C/SiC复合材料非线性本构关系及断裂行为研究

C/SiC复合材料具有高比强度、高比模量和优良的热稳定性能等一系列优点, 广泛应用于航空航天领域中. 裂纹扩展进而引起的脆性断裂是其主要失效形式之一, 因而材料的断裂性能分析对材料的结构设计和应用有重要的指导意义. 本文开展了缝合式C/SiC复合材料简单力学试验和断裂试验, 研究了材料在不同载荷下的力学响应及断裂特征. 基于缝合式C/SiC复合材料简单力学试验, 建立了材料宏观非线性损伤本构方程, 并模拟了缝合式C/SiC复合材料单边切口梁和双悬臂梁的断裂行为. 本构方程采用简单函数描述了材料在复杂应力状态下的非线性应力-应变曲线, 并考虑了反向加载过程中造成的裂纹闭合. 基于商业有限元软件ABAQUS, 通过编写UMAT子程序实现非线性损伤本构方程, 采用单个单元验证了建立的本构方程的有效性. 在此基础上, 采用线弹性损伤本构和非线性损伤本构分别模拟了缝合式C/SiC复合材料单边切口梁和双悬臂梁的断裂行为. 采用非线性损伤本构方程模拟的力-位移曲线结果与试验结果更为吻合, 非线性损伤本构预测的失效载荷与试验失效载荷更为接近, 验证了所建立的非线性损伤本构方程的准确性, 为C/SiC复合材料断裂行为的研究提供了借鉴, 为缝合式C/SiC复合材料结构的设计和应用提供了理论基础.      

深部灰岩三轴蠕变特性实验研究

采用MTS815岩石材料力学试验机,在温度28℃条件下,对重庆市万盛区某煤矿深部掘进巷道灰岩进行了三轴蠕变试验,对不同偏应力水平下该巷道灰岩的蠕变特性进行了分析。结果表明,蠕变试验的曲线显示该灰岩蠕变具有非线性特征,其蠕变特性随着围压的增加而更加明显。对深部灰岩的蠕变模型进行初步探讨,回归得出深部灰岩在不同应力水平下蠕变应变随时间变化的Burgers蠕变方程。分析表明,Burgers模型能较好地反映该深部灰岩的蠕变规律。该模型所确定的本构关系可以作为该深部灰岩蠕变本构模型来预测其蠕变发展。     

STUDY ON NONLINEAR CONSTITUTIVE RELATIONSHIP AND FRACTURE BEHAVIOR OF STITCHED C/SiC COMPOSITES$^{\bf 1)}$

C/SiC复合材料具有高比强度、高比模量和优良的热稳定性能等一系列优点, 广泛应用于航空航天领域中. 裂纹扩展进而引起的脆性断裂是其主要失效形式之一, 因而材料的断裂性能分析对材料的结构设计和应用有重要的指导意义. 本文开展了缝合式C/SiC复合材料简单力学试验和断裂试验, 研究了材料在不同载荷下的力学响应及断裂特征. 基于缝合式C/SiC复合材料简单力学试验, 建立了材料宏观非线性损伤本构方程, 并模拟了缝合式C/SiC复合材料单边切口梁和双悬臂梁的断裂行为. 本构方程采用简单函数描述了材料在复杂应力状态下的非线性应力-应变曲线, 并考虑了反向加载过程中造成的裂纹闭合. 基于商业有限元软件ABAQUS, 通过编写UMAT子程序实现非线性损伤本构方程, 采用单个单元验证了建立的本构方程的有效性. 在此基础上, 采用线弹性损伤本构和非线性损伤本构分别模拟了缝合式C/SiC复合材料单边切口梁和双悬臂梁的断裂行为. 采用非线性损伤本构方程模拟的力-位移曲线结果与试验结果更为吻合, 非线性损伤本构预测的失效载荷与试验失效载荷更为接近, 验证了所建立的非线性损伤本构方程的准确性, 为C/SiC复合材料断裂行为的研究提供了借鉴, 为缝合式C/SiC复合材料结构的设计和应用提供了理论基础.     

金属橡胶材料基于微弹簧组合变形的细观本构模型

研究了金属橡胶材料细观本构模型。依据金属橡胶材料变形的主要特征,分析了微弹簧沿轴向和径向变形的规律,分别推导了应力应变关系的解析表达式,由两种微弹簧的组和变形构造了代表性体积单元-微元体。为了反映因铺层引起的工艺各向异性,本构方程中引入了一个新的材料参数铺层比例系数iβ,该参数可以较好地表征细观结构中微弹簧取向的分布状况。在大量试验的基础上,提出了确定铺层系数的实验方法,进而建立了包含金属橡胶材料细观结构信息的本构方程。理论与实验结果比较表明,本文建立的本构模型能较好的反映材料的力学行为。     

Viscoelasticity and viscoplasticity of polypropylene/polyethylene blends

Observations are reported on polypropylene/polyethylene blends with various concentrations of components in uniaxial tensile tests with constant strain rates, relaxation tests, and creep tests at room temperature. A model is developed for the viscoelastic and viscoplastic responses of polymer blends at arbitrary three-dimensional deformation with small strains. Material constants in the constitutive equations are determined by fitting the experimental data. It is found that all adjustable parameters evolve with blend composition following an analog of the rule of mixture. Lifetime of blends under condition of creep rupture is evaluated by numerical simulation.     

A MICROMECHANICAL FRAMEWORK WITH AGGREGATE-MASTIC INTERFACE EFFECT FOR PREDICTING UNIAXIAL COMPRESSION CREEP OF ASPHALT MIXTURE

According to the elastic-viscoelastic correspondence principle, an elastic microme- chanical framework taking the inclusion-matrix interface effect into account is extended for predicting viscoelastic properties of asphalt mixture, which is simply treated as elastic coarse aggregate inclusions periodically and isotropically embedded in a viscoelastic asphalt mastic matrix. The Burgers model is adopted for characterizing the matrix mechanical behavior, so that the homogenized relaxation modulus of asphalt mixture in compression creep is derived. After a series of uniaxial compression creep tests are performed on asphalt mastic in different temperature and stress conditions in order to determine the matrix constitutive parameters, the framework presented is validated by comparison with the experiment, and then some predictions of uniaxial compression creep behavior of asphalt mixture in different temperature and stress conditions are given.     

纳米颗粒增强镍基MEMS器件材料的蠕变性能研究

利用同步辐射LIGA微铸复合工艺,将纳米氧化物增强颗粒复合到微电子机械系统(MEMS)结构材料中。制作了专用夹具,采用微力材料试验机测量了纳米Al2O3颗粒增强镍基复合材料的强度为1GPa;将恒加载速率/载荷法和恒载荷法相结合,利用纳米压痕仪测量了该材料的室温蠕变速率敏感指数m。结果表明,LIGA复合技术得到的纳米颗粒增强镍基复合材料具有较高的强度;MEMS器件材料在室温下会发生蠕变;材料在相同压深下最大载荷不随加载速率而改变,加载段粘弹性和粘塑性变形极少;主要由局部高应力导致压痕蠕变;材料的蠕变速率敏感指数m值为0.004,说明纳米Al2O3颗粒可有效增强基体材料的抗蠕变能力;且不同恒.P/P下获得的m值基本相同,表示此种材料对加载速率不敏感。     

An experimental study of uniaxial creep,cyclic creep and relaxation of aisi type 304 stainless steel at room temperature

Following previous work (Krempl, 1979), a servocontrolled testing machine and strain measurement at the gage length were used to study the uniaxial rate(time)-dependent behavior of AISI Type 304 stainless steel at room temperature. The test results show that the creep strain accumulated in a given period of time depends strongly on the stress-rate preceding the creep test. In constant stress-rate zero-to-tension loading the creep strain accumulated in a fixed time-period at a given stress level is always higher during loading than during unloading. Continued cycling causes an exhaustion of creep ratchetting which depends on the stress-rate. Periods of creep and relaxation introduced during completely reversed plastic cycling show that the curved portions of the hysteresis loop exhibit most of the inelasticity. In the straight portions, creep and relaxation are small and there exists a region commencing after unloading where the behavior is similar to that at the origin for virgin materials. This region does not extend to zero stress.The results are at variance with creep theory and with viscoplasticity theories which assume that the yield surface expands with the stress. They support the theory of viscoplasticity based on total strain and overstress.     

Prediction of nonlinear viscoelastic behavior of polymeric composites using an artificial neural network

Creep tests at constant stress are performed for the carbon-fiber reinforced epoxy composite at various temperatures and initial stresses. A nonlinear viscoelastic constitutive model is developed, and its material parameters are determined by fitting it to creep test data. Model results are found to agree very well with the experimental data at low temperature and low stress conditions. However, the agreement deteriorates at high temperatures, particularly in the vicinity of the glass transition temperature.An alternative model based on an artificial neural network (ANN) is developed to predict the stress relaxation of the polymer matrix composite. The ANN model is trained and validated with 9000 experimental data sets obtained from stress relaxation tests performed at various constant strain (initial stress) and constant temperature conditions. Training of the ANN employs a scaled conjugate gradient method. The optimal brain surgeon algorithm is employed to optimize the topology. The optimal ANN configuration has 88 processing elements (3 in the input layer, 45 in the first hidden layer, 39 in the second hidden layer, and 1 in the output layer) and 410 links. The predictions of the ANN model are found to be more accurate over a wider range of stress and temperature conditions than those of the explicit nonlinear viscoelastic model, in particular near the glass transition temperature.     

A viscoplastic constitutive model incorporating dynamic strain aging effect during cyclic deformation conditions

In this paper, a viscoplastic constitutive model previously proposed by the authors was extended to apply to the cyclic deformation analysis of the modified 9Cr-1Mo steel. A series of cyclic deformation tests were conducted on modified 9Cr-1Mo steel at various temperatures, including those under anisothermal conditions. Furthermore, cyclic evolution of state variables used in the authors' constitutive model was experimentally measured. Based on the test results, cyclic softening behavior depending on the temperature and its history was introduced into the constitutive model. The extended model was applied to simulations of inelastic deformation behavior under monotonic tension, stress relaxation, creep, isothermal cyclic deformations including stress relaxation and anisothermal cyclic deformations. It was found that the present constitutive model has a capability of describing the inelastic deformation behavior of modified 9Cr-1Mo steel adequately at various loading conditions.     

Nonlinear dependence of viscosity in modeling the rate-dependent response of natural and high damping rubbers in compression and shear: Experimental identification and numerical verification

The rate-dependent behavior of filled natural rubber (NR) and high damping rubber (HDR) is investigated in compression and shear regimes. In order to describe the viscosity-induced rate-dependent effects, a constitutive model of finite strain viscoelasticity founded on the basis of the multiplicative decomposition of the deformation gradient tensor into elastic and inelastic parts is proposed. The total stress is decomposed into an equilibrium stress and a viscosity-induced overstress by following the concept of the Zener model. To identify the constitutive equation for the viscosity from direct experimental observations, an analytical scheme that ascertains the fundamental relation between the inelastic strain rate and the overstress tensor of the Mandel type by evaluating simple relaxation test results is proposed. Evaluation of the experimental results using the proposed analytical scheme confirms the necessity of considering both the current overstress and the current deformation as variables to describe the evolution of the rate-dependent phenomena. Based on this experimentally based motivation, an evolution equation using power laws is proposed to represent the effects of internal variables on viscosity phenomena. The proposed evolution equation has been incorporated in the finite strain viscoelasticity model in a thermodynamically consistent way. Simulation results for simple relaxation tests, multi-step relaxation tests and monotonic tests at different strain rates using the developed model show an encouraging correlation with the experiments conducted on HDR and NR in both compression and shear regimes. Finally, an approach to extend the proposed evolution equation for rate-dependent cyclic processes is proposed. The simulation results are critically compared with the experiments.