A three-dimensional model describing stress-temperature induced solid phase transformations: thermomechanical coupling and hybrid composite applications

Between composite materials, shape memory alloy (SMA) composites are having a more and more relevant role. Typically, SMA wires are embedded in a metallic or a polymeric matrix to obtain materials with native multi-functionality and adaptive properties. This work approaches the computational study of the mechanical response of a composite in which SMA wires, previously deformed, are activated by electrical current heating, and accordingly try to recover the original shape inducing a shape change or a prestress in the structure. In particular, since the SMA behaviour is strongly affected by the thermo-mechanical coupling, in the first part of this work we present a 3D phenomenological model able to take into account this aspect. The model time-discrete counterpart is used to develop a 3D solid finite element able to describe the thermo-electro-mechanical coupled problem due to shape memory alloy response and to Joule effect. Finally, in the second part of the paper, we employ the developed computational tool to simulate different feasible SMA composite applications. Copyright © 2004 John Wiley & Sons, Ltd.     

钢管混凝土的等效本构关系研究

在总结现有钢管混凝土本构关系分析的基础上,将钢管混凝土简化为同一尺度的横观各向同性体来研究,并在弹性条件下利用应变能相等原理得到了宏观弹性系数,同时根据试验结果假设进入弹塑性阶段后各模量为二次抛物线变化,建立了钢管混凝土这种组合结构新的等效本构关系.并结合具体算例研究了弹性条件下材料强度变化和含钢率对各模量的影响,同时与前人的研究结果进行了比较,结果表明,该本构关系具有理论依据明确,参数易确定,能很好地反映组合构件材料物理性能的优点,为钢管混凝土结构进一步进行数值分析奠定了基础.     

岩土本构关系的特殊性和统一性

在阐明岩土塑性变形的基本特殊性,特别是应力应变关系与应力路径相关性,产生机理的基础上,明确指出了导致这些特性的内在的共同的根源是,岩土塑性变形中存在两个相对独立的变形,即体积应变和剪切应变,两者之间存在着强烈的非线性的相互作用.这种耦合作用造成岩土的力学响应不仅具有多样性和复杂性,而且也具有内在的统一性.     

Thermomechanical design in the automotive industry

In this paper thermomechanical fatigue assessment in the automotive industry is discussed. The design strategy is based upon a consistent approach of the thermomechanical loading, the mechanical constitutive law of the material, the damage parameters and the fatigue strength criteria. The good understanding of these different steps allows one to perform predictive calculations of automotive parts subjected to thermomechanical loading. The main hypotheses and modelling choices are presented and results are illustrated by a series of computations on real 3D structures. Cracked area and lifetime prediction are described in the case of aluminium alloy cylinder heads subjected to transient thermal loadings.     

Vibration Analysis of Tb-Dy-Fe Magnetostriction Actuator and Transducer

In this paper, theoretical analysis of longitudinal free vibration was carried out for Tb-Dy-Fe series magnetostrictive actuator and transducer. The formulations considered two constitutive laws; in one we employ the standard square nonlinear constitutive equation of magnetostriction and in the other we employ the linear piezomagnetic equation. The results obtained from the nonlinear equation can be reduced to the linear piezomagnetic equation when the amplitude of the excitation magnetic field provided by the coil is very small compared to the bias magnetic field and its frequency does not induce resonance of the system. For the case of a relatively large excitation magnetic field, which usually exists for an actuator, the nonlinear constitutive equation should be adopted in order to provide an accurate prediction for the design and analysis of actuator and transducer. Another important aspect is the resonance of the Tb-Dy-Fe series transducers that was revealed in the analysis using the nonlinear constitutive equation. The resonance not only appears at the natural frequencies of the system, but also arises when the frequency of excitation current in the coil happens to be half of one of the natural frequencies of the system. This conclusion cannot be reached using the linear piezomagnetic formulation.     

锈蚀钢筋与混凝土间黏结性能试验

为研究锈蚀对钢筋与混凝土间黏结性能的影响,制作20个中心拉拔试件.通过电化学加速锈蚀得到理论锈蚀率为0%、0.5%、1%、2%、5%的拉拔试件,完成了不同锈蚀率下钢筋与混凝土的拉拔试验.从钢筋几何外形特征与混凝土强度在锈蚀过程中变化的角度分析了锈蚀率对极限黏结强度的影响,以割线刚度的方法定量研究了锈蚀率对黏结刚度的影响.阐述了锈胀裂纹、黏结刚度及试件破坏模式随锈蚀率变化的规律.试验结果表明:随着锈蚀率增大,钢筋与混凝土间的黏结强度、黏结刚度呈现先增大后减小趋势.黏结刚度的退化与锈胀裂纹的出现具有一定相关性.通过整理国内外学者黏结强度退化数据,得到了两段式黏结强度退化试验模型及一定程度上可供工程参考的黏结强度退化保守模型.最后,基于两段式黏结-滑移模型得到了不同锈蚀率下钢筋与混凝土间黏结-滑移本构模型.     

15CrMoG耐高温合金钢Johnson-Cook本构模型建立与精度分析

目的 掌握不同条件下15CrMoG的高温流变规律。方法 在变形温度为1 173.15、1 273.15、1 373.15、1 473.15 K,应变速率为5、10、15、20 s-1条件下,对15CrMoG进行等温压缩实验。根据实验数据构建15CrMoG的Johnson-Cook本构模型,并对应变速率强化参数C和温度敏感系数D进行分别修正和同时修正。将修正后的3种模型预测值与实验值进行线性拟合,分别得到各模型的相关系数。引入相对误差ARE函数,计算不同条件下各模型的平均误差,通过对比得到精度较高的模型。结果 15CrMoG具有温度和应变速率敏感性,高温流变曲线整体呈现先上升后平稳的趋势。拟合取值的局限性导致未修正的本构模型的精度较差。在较低变形温度下,修正后的模型均具有良好的预测精度,在高变形温度下,修正温度敏感系数的模型具有更好的适应性。3种修正模型预测值与实验值的线性相关系数均在0.9以上,具有良好的相关性。在低变形温度下,3种模型的相对误差均较小;在高变形温度下,修正温度敏感系数的Johnson-Cook模型具有较高的预测精度,最大误差为4.75%。结论 对Johnson-Coo...     

低活化马氏体钢热变形行为及机理型本构建模研究

利用Gleeble-3500热模拟试验机研究了低活化马氏体钢在变形温度为850～950 ℃、应变速率为0.001～1 s^-1条件下的热变形行为。建立了流变应力本构方程,并评估了该方程的预测能力。绘制了低活化马氏体钢在不同应变下的热加工图。结果表明：在较高的应变速率条件下,该材料主要发生动态回复,在较高变形温度和较低应变速率下具有明显的动态再结晶特征;本构方程的预测结果与实验结果符合良好;变形温度870～930 ℃、应变速率0.001～0.01 s^-1和变形温度920～950 ℃、应变速率0.3～1 s^-1分别是真应变为0.4和0.6下最优的热加工区域。     

基于神经网络的BT20钛合金本构关系模型

在Thermecmastor-Z型热模拟试验机上对BT20钛合金进行了变形温度800～1 100℃及应变速率0.001～70 s-1的热模拟压缩实验。以实验数据为基础,运用BP神经网络算法原理,建立了BT20钛合金在高温变形条件下的应力与应变、应变速率和变形温度关系的预测模型,并对模型的泛化能力进行了误差评价。结果表明:通过BP神经网络建立的合金本构关系模型具有较高的预测精度,预测结果的相对误差均在3%以内,能很好地满足实际应用的需求。此外,该模型能够客观、真实地描述BT20钛合金的高温动态变形行为,为材料高温本构关系模型的建立提供了快捷、有效的工具。