橡胶粘结颗粒材料粘弹性性能的试验研究与离散元模拟

制备了颗粒规则四方排列和六方排列的橡胶粘接颗粒材料试样,实验测试了所制备试样在单向拉伸载荷下的应力松弛曲线和不同应变率时的应力应变曲线。基于所测试的应力松弛曲线,采用曲线拟合方法得到了所测试材料的宏观Burger’s粘弹性本构模型参数。采用离散元模型中单元间连结模型代表颗粒间橡胶粘接剂的作用,并基于试样的宏观Burger’s模型参数与离散元模型中细观Burger’s连结模型参数间的关系,建立了橡胶粘接颗粒材料的无厚度胶结离散元分析模型。最后采用所建立的离散元模型计算了所测试试样的松弛和拉伸力学性能。离散元预测结果与实验结果的对比表明,采用无厚度胶结离散元模型能较好的计算颗粒规则排列的橡胶粘接颗粒材料松弛和拉伸力学性能,但基于应力松弛实验拟合而来参数不能准确反应橡胶粘接剂在高应变率条件下其力学性能的应变率相关性。     

6061-T6铝合金动态拉伸本构关系及失效行为

采用HMH-206高速材料试验机开展了6061-T6铝合金在0.001～100 s?1应变率范围内的静、动态拉伸力学性能实验,分析了其应力-应变响应特征和应变率敏感性,讨论了应变率对6061-T6铝合金流动应力和应变率敏感性指数的影响,并基于实验结果对Johnson-Cook本构模型进行了修正。结合缺口试件的实验结果和模拟数据,得到了材料的Johnson-Cook失效模型参数,并对模型的准确性和适用性进行了验证。结果表明,在拉伸载荷作用下,6061-T6铝合金表现出明显的应变硬化特征和应变率敏感性,其流动应力随应变率的升高而提高,修正的Johnson-Cook本构模型可以描述材料的动态塑性流动行为,建立的Johnson-Cook失效模型能够表征材料的断裂失效行为。     

纤维增强橡胶复合材料拉伸与破坏的试验分析

鉴于橡胶制品的广泛应用,本文利用Zwick020材料试验机和Zwick010双向拉伸材料试验机分析了一类纤维增强橡胶复合材料在不同温度条件下的单向拉伸及双向拉伸的力学行为.得到了不同温度条件下材料单向拉伸及双向拉伸的应力-应变关系曲线和材料的破坏条件,由此分析了温度条件和加载条件对纤维增强橡胶复合材料拉伸变形和破坏等力学性能的影响.同时利用实验结果拟合出材料的一类应变能函数及材料常数.     

有机玻璃非线性应力松弛行为表征与本构模型

通过实验测定了有机玻璃(PMMA)在不同初始应变率和初始应变下的应力松弛曲线。分析了三种应变率(0.001s~(-1)、0.01s~(-1)和0.1s~(-1))下不同初始应变(5%、7%、10%和15%)的实验结果。结果表明:初始加载应变率会影响有机玻璃高分子链段的运动速度,进而对应力松弛行为产生影响;不同初始应变对应力松弛的影响,则体现为不同变形程度对PMMA高分子链运动速度的影响。将PMMA的应力松弛视为两个阶段即应力快速松弛段和应力缓慢松弛段,基于对这两个阶段的分析,提出了一个非线性应力松弛本构模型。对实验得到的应力松弛曲线采用上面提出的本构模型进行数值拟合,吻合结果良好。此外,还对模型中参数的影响因素进行了分析讨论。结果表明该本构模型可以用于描述PMMA的非线性应力松弛行为。     

帘线/橡胶复合材料各向异性黏-超弹性本构模型

帘线/橡胶复合材料广泛应用于轮胎等重要工程领域,为了描述其在服役条件下的大变形、非线性、各向异性和高应变率等材料力学行为,基于纤维增强复合材料连续介质力学理论,提出了一种考虑应变率效应的帘线/橡胶复合材料各向异性黏-超弹性本构模型. 该模型中单位体积的应变能被解耦为便于参数识别的基体等容变形能、帘线拉伸变形能、剪切应变能和黏性应变能四部分. 给出了模型参数的确定方法,并通过拟合文献中单轴拉伸、偏轴拉伸实验数据,得到了模型参数. 利用该模型预测了不同加载和变形条件下的力学行为,并将预测结果与实验结果对比分析. 结果表明, 考虑黏性模型和不考虑黏性模型对不同应变率变形条件下的预测结果相差很大,且考虑黏性模型的预测结果与实验结果吻合很好. 因此,与不考虑黏性模型相比,所提出的各向异性黏-超弹性本构模型能更好地表征帘线/橡胶复合材料在大变形、高应变率条件下的力学特性.      

温度对炭黑填充橡胶复合材料力学性能影响的实验研究

采用岛津万能试验机对天然与丁苯共混橡胶(NSBR)试件进行不同温度下多步松弛循环加卸载实验和单向拉伸加卸载循环实验，对比两者的宏观力学响应，并研究温度对炭黑填充复合材料力学性能的影响．结果表明：(1)随着温度的升高，未填充橡胶逐渐变“硬”，而炭黑填充橡胶是“先变软后变硬”．(2)随着温度的升高，橡胶的迟滞损耗呈线性减小．(3)比较单向拉伸和应力松弛平衡点的应力应变曲线，填充橡胶“先变软后变硬”的温度转折点不同，这是由于应力松弛过程基本消除了粘弹性效应．     

含热冲击预损伤的陶瓷基复合材料损伤本构模型

陶瓷基复合材料结构在服役过程中不可避免地经受热冲击(较高的热应力梯度)而产生热机械损伤, 因此, 建立含循环热冲击预损伤材料的损伤本构模型, 以描述材料在热机械载荷作用下的力学行为, 对材料结构损伤容限设计与结构完整性评估非常重要. 本文首先对经历了循环热冲击的材料进行单调拉伸损伤实验, 发现对于含循环热冲击预损伤的材料, 其弹性模量的下降与所施加的应变直接相关. 然后在连续介质损伤力学的框架下, 基于平面应力假设, 建立了含循环热冲击预损伤材料的损伤演化模型, 该模型所涉及的参数可通过一个偏轴(45$^\circ$)以及两个正轴(平行于两个主方向)的单调拉伸试验获得. 最后, 采用经典塑性理论对由基体损伤引起的非弹性应变进行了描述. 本文所提出的应变损伤宏观模型可以描述陶瓷基复合材料在热机械载荷作用下的损伤演化, 同时弥补了含预损伤的陶瓷基复合材料在机械载荷下损伤本构模型在理论及实验研究方面的不足.      

一种高强度钢的动态力学本构参数识别和数值模拟

分析了N-1钢在不同应变率条件下(10-3~103s-1)的应力应变关系实验结果,得到了N-1钢的应 变率强化特性。修正了平板试件断裂面中心的真实应力,并通过对真实应变与最小截面的厚度缩减率关系的 研究,得到了断裂面中心的真实应变,从而识别出了N-1钢的修正Johnson-Cook本构模型的参数。利用所确 定的本构关系模型,对N-1钢平板试件的杆-杆型冲击拉伸过程进行了数值模拟,模拟结果与实验结果较吻 合,验证了采用的修正的Johnson-Cook本构模型参数的实验识别方法以及数值模拟方法的正确性。     

含热冲击预损伤的陶瓷基复合材料损伤本构模型

陶瓷基复合材料结构在服役过程中不可避免地经受热冲击(较高的热应力梯度)而产生热机械损伤,因此,建立含循环热冲击预损伤材料的损伤本构模型,以描述材料在热机械载荷作用下的力学行为,对材料结构损伤容限设计与结构完整性评估非常重要.本文首先对经历了循环热冲击的材料进行单调拉伸损伤实验,发现对于含循环热冲击预损伤的材料,其弹性模量的下降与所施加的应变直接相关.然后在连续介质损伤力学的框架下,基于平面应力假设,建立了含循环热冲击预损伤材料的损伤演化模型,该模型所涉及的参数可通过一个偏轴(45°)以及两个正轴(平行于两个主方向)的单调拉伸试验获得.最后,采用经典塑性理论对由基体损伤引起的非弹性应变进行了描述.本文所提出的应变损伤宏观模型可以描述陶瓷基复合材料在热机械载荷作用下的损伤演化,同时弥补了含预损伤的陶瓷基复合材料在机械载荷下损伤本构模型在理论及实验研究方面的不足.     

室温下镁合金板循环塑性随动强化本构模型研究

本文在具有各向异性屈服强度和拉压不对称的CPB06屈服准则的基础上,建立了基于随动强化的循环塑性本构模型．通过引入滑移、孪晶以及去孪等不同变形模式下的背应力演化方程,对室温下镁合金板材异常循环硬化行为进行了模拟．选取了AZ31B-O和AZ31B两种镁合金板材,通过拉伸-压缩-拉伸(T-C-T)和压缩-拉伸(C-T)等不同加载路径下的部分实验曲线确定模型的参数,采用三次插值多项式建立了背应力参数与上一变形模式中累积的等效塑性应变（即预应变）之间的函数关系．使用本模型对剩下的实验曲线进行了预测,发现预测结果与实验结果有良好的一致性,说明了当前模型的正确性．     

Experimental identification and mathematical modeling of viscoplastic material behavior

Uniaxial torsion and biaxial torsion-tension experiments on thin-walled tubes were carried out to investigate the viscoplastic behavior of stainless steel XCrNi18.9. A series of monotonic tests under strain and stress control shows nonlinear rate dependence and suggests the existence of equilibrium states, which are asymptotically approached during relaxation and creep processes. Strain controlled cyclic experiments display various hardening and softening phenomena that depend on strain amplitude and mean strain. All experiments indicate that the equilibrium states within the material depend on the history of the input process, whereas the history-dependence of the relaxation and creep behavior appears less significant. From the experiments the design of a constitutive model of viscoplasticity is motivated: The basic assumption is a decomposition of the total stress into an equilibrium stress and a non-equilibrium overstress: At constant strain, the overstress relaxes to zero, where the relaxation time depends on the overstress in order to account for the nonlinear rate-dependence. The equilibrium stress is assumed to be a rate independent functional of the total strain history. Classical plasticity is utilized with a kinematic hardening rule of the Armstrong-Frederick type. In order to incorporate the amplitude-dependent hardening and softening behavior, a generalized arc length representation is applied [14]. The introduction of an additional kinematic hardening variable facilitates consideration of additional hardening effects resulting from the non-radiality of the input process. Apart from the common yield and loading criterion of classical plasticity, the proposed constitutive model does not contain any further distinction of different cases.The experimental data are sufficient to identify the material parameters of the constitutive model. The results of the identification procedure demonstrate the ability of the model to represent the observed phenomena with satisfactory approximation.     

Comparison of the rheology of polymer melts in shear,and biaxial and uniaxial extensions

The experimental properties of different polymer melts, polystyrene, high density polyethylene and low density polyethylene are compared for the first time in three different deformations: step shear, step biaxial extension and steady uniaxial extension. Properties of three other melts are also studied in step biaxial and shear experiments. For our comparative purposes some data of Laun and Winter from the literature are used, as well as new data reported here. In all the step strain experiments, the stresses can be factored into a time dependent relaxation modulus and a strain dependent damping function. The data are interpreted using a differential constitutive equation of Larson which satisfies this time-strain separability and has a single parameter that describes the strain softening character of the material. Results show that differences in the properties of the melts are most pronounced in uniaxial extension and least in biaxial extension. All melts follow the Doi-Edwards prediction relatively closely in biaxial extension. In uniaxial extension, the branched material shows a strong strain hardening effect although its shear and biaxial properties are similar to the other melts. The constitutive model gives a reasonably good fit to the data in all three deformations for unbranched materials for the same value of the adjustable parameter; the model, however, fails for the branched low density polyethylene.     

Thermo-mechanical response of nylon 101 under uniaxial and multi-axial loadings: Part I,Experimental results over wide ranges of temperatures and strain rates

A comprehensive study of the thermo-mechanical response of a thermoplastic polymer, nylon 101 is presented. Quasi-static and dynamic compression uniaxial and multi-axial experiments (stress states) were performed at a wide range of strain rates (10⁻⁵ to 5000 s⁻¹) and temperatures (−60 to 177 °C or −76 to 350 °F). The material is found to be non-linearly dependent on strain rate and temperature. The change in volume after plastic deformation is investigated and is found to be negligibly small. The relaxation and creep responses at room temperature are found to be dependent on strain rate and the stress–strain level at which these phenomena are initiated. Total deformation is decomposed into visco-elastic and visco-plastic components; these components have been determined at different levels of deformation. Results from non-proportional uniaxial to biaxial compression, and torsion experiments, are also reported for three different strain rates at room temperature. It is shown that nylon 101 has a response dependent on the hydrostatic pressure.     

Viscoelasticity of brain corpus callosum in biaxial tension

Computational models of the brain rely on accurate constitutive relationships to model the viscoelastic behavior of brain tissue. Current viscoelastic models have been derived from experiments conducted in a single direction at a time and therefore lack information on the effects of multiaxial loading. It is also unclear if the time-dependent behavior of brain tissue is dependent on either strain magnitude or the direction of loading when subjected to tensile stresses. Therefore, biaxial stress relaxation and cyclic experiments were conducted on corpus callosum tissue isolated from fresh ovine brains. Results demonstrated the relaxation behavior to be independent of strain magnitude, and a quasi-linear viscoelastic (QLV) model was able to accurately fit the experimental data. Also, an isotropic reduced relaxation tensor was sufficient to model the stress-relaxation in both the axonal and transverse directions. The QLV model was fitted to the averaged stress relaxation tests at five strain magnitudes while using the measured strain history from the experiments. The resulting model was able to accurately predict the stresses from cyclic tests at two strain magnitudes. In addition to deriving a constitutive model from the averaged experimental data, each specimen was fitted separately and the resulting distributions of the model parameters were reported and used in a probabilistic analysis to determine the probability distribution of model predictions and the sensitivity of the model to the variance of the parameters. These results can be used to improve the viscoelastic constitutive models used in computational studies of the brain.     

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.     

Experimental analysis and modeling of biaxial mechanical behavior of woven composite reinforcements

This paper presents experimental studies on the mechanical behavior of fiber fabrics using a biaxial tensile device based on two deformable parallelograms. The cross-shaped specimens are well adapted to fabrics because of their lack of shear stiffness. Tension versus deformation curves, for different strain ratios, are obtained in the case of composite woven reinforcements used in aeronautic applications. It is shown that the tensile behavior of the fabric is strongly nonlinear due to the weaving undulations and the yarn contraction, and that the phenomenon is clearly biaxial. A constitutive model is described and identified from the experimental data. The essential role played by the yarn crushing will be pointed out.     

采用瓣叶复合材料的生物心脏瓣膜的力学分析

采用微观组织结构分析及宏观复合材料分析结合的方法,分析了猪主动脉瓣的非线性复合材料性质,提出了一种适用于猪主动脉瓣的非线性复合材料本构模型,用提出的非线性复合材料本构模型,对闭合承载状态下的等厚度与变厚度几何模型的猪主动脉瓣的应力分布及变形进行了有限元数值模拟,发现：与各向同性瓣叶相比,单向增强复合材料的瓣叶不但具有较强的承载能力,而且具有较大的柔软性。     

柔性硅泡沫薄片的短时松弛行为研究

根据多孔硅泡沫材料的单轴压缩和应力松弛实验,利用最小二乘法的LM法拟合得到硅泡沫材料的本构关系.基于上述模型开展了组合结构中多孔泡沫薄片应力松弛行为的数值模拟,得到了短时松弛过程中硅泡沫结构件的应力变化规律.     

Relaxation and retardation functions of the Maxwell model with fractional derivatives

A four-parameter Maxwell model is formulated with fractional derivatives of different orders of the stress and strain using the Riemann-Liouville definition. This model is used to determine the relaxation and retardation functions. The relaxation function was found in the time domain with the help of a power law series; a direct solution was used in the Laplace domain. The solution can be presented as a product of a power law term and the Mittag-Leffler function. The retardation function is determined via Laplace transformation and is solely a power law type.The investigation of the relaxation function shows that it is strongly monotonic. This explains why the model with fractional derivatives is consistent with thermodynamic principles.This type of rheological constitutive equation shows fluid behavior only in the case of a fractional derivative of the stress and a first order derivative of the strain. In all other cases the viscosity does not reach a stationary value.In a comparison with other relaxation functions like the exponential function or the Kohlrausch-Williams-Watts function, the investigated model has no terminal relaxation time. The time parameter of the fractional Maxwell model is determined by the intersection point of the short- and long-rime asymptotes of the relaxation function.