7075铝合金高温流变行为的研究

采用圆柱试样在Gleeble-1500热模拟机上进行高温压缩变形实验，研究了7075铝合金在高温塑性变形过程中流变应力的变化规律。实验在温度为250－500℃、应变速率为0．05－50s^-1的条件下进行。结果表明：应变速率和变形温度的变化强烈影响着合金流变应力的大小，流变应力随变形温度升高而降低，随就变速率提高而增大，可用ZenerHollomon参数的双曲正弦形式来描述7075铝合金高温压缩变形时的流变应力行为。     

基于CuZnAl形状记忆合金的框架结构抗震减振控制

利用形状记忆合金的本构关系,分析了在自由应力状态下的残余应变与温度的关系.使用动态数据采集分析仪,测试了安装CuZnAl形状记忆合金耗能器的框架结构的减振性能;应用电子万能拉伸试验机测试了CuZnAl形状记忆合金的应力-应变滞回曲线,借助透射电子显微镜,分析了CuZnAl形状记忆合金在热机械循环过程中显微组织的变化;比较了不同热处理工艺对CuZnAl形状记忆合金热机械循环后的滞回曲线面积及减振性能的影响.结果表明:CuZnAl形状记忆合金用于框架结构具有较好的减振效果,可明显降低框架结构的振动频率,其中马氏体CuZnAl形状记忆合金耗能器具有较大的耗能能力,经室温水淬时效7 h处理后减振效果最好.     

MODELLING OF P/M FORMING PROCESSES BY FEM

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单榫头叶片精锻成形规律三维热力耦合有限元模拟研究

单榫头叶片是航空发动机中的一类重要叶片.本文以单榫头叶片精锻过程为研究对象,采用热力耦合方法,利用自行开发的面向叶片精锻过程的三维刚粘塑性热力耦合有限元数值模拟系统3D-CTM,模拟分析了不同工艺参数(变形速度、摩擦因子以及坯料锻造温度)下的单榫头叶片精锻过程,研究了不同工艺参数对叶片精锻成形规律的影响.该研究对制定叶片精锻工艺以及发展叶片精锻技术具有重要的理论意义和实用价值.     

Cold forming processes: some examples of predictions and design optimization using numerical simulations

Predicting the behaviour of steel during a deformation process, and then under service conditions, is one of the main challenges in cold forming. The design of optimized forging schedules, by means of classical trial+errors procedures, has become increasingly heavy in terms of time and cost in a competitive environment. Simultaneously, the improvement of steel qualities requires the microstructure, constitutive behaviour and deformability to be known a priori regarding a targeted application. During the last few years, numerical simulations have become a very efficient tool to reach these goals.

In this paper, we give examples of innovating forging sequences developed by numerical simulations, including the investigation of damage in tools and forged parts. In case of specific processes with very determined geometry — such as wire drawing — we show how systematic numerical studies may lead to predictive models of force, local strains and residual stress…

However, reliable predictions from numerical simulations require reliable input data, including constitutive laws, friction conditions and propensity to ductile damage. These data must be characterized under realistic sollicitations. Typical cold forging loadings are indeed very severe: local strains up to 600%, strain rates locally greater than 1000 s⁻¹, and subsequently, plastic heating over 500°C.

To characterize the constitutive behaviour, the standard upset test between grooved dies is used along with an original methodology to derive the strain hardening curve from the experimental force-displacement recording. Tool elastic deformations, specimen strain heterogeneity… are taken into account. This enables a precise determination of the strain hardening curve up to about 100% of strain. The extrapolation of the flow stress to greater deformations is then very easy and reliable. Such a test can be performed under quasi-static and isothermal conditions (0.1 s⁻¹ but also adiabatic and rapid conditions (up to 10 s⁻¹). This procedure was adapted to a Pellini hammer, which enables very simple characterization at 800 s⁻¹. The comparison of all these flow curves lead to the formulation of an original constitutive model, which accounts for the effects of plastic heating, strain rate, dynamic aging…

In order to predict ductile fracture during the forging process, the most classical criteria were tested over a wide range of experimental loading conditions. None of them were general enough to solve all the cold forming problems. On the other hand, mesoscopic models describing the deformation of the metal matrix around inclusions or second phases have proved to be in good agreement with the various experimental observations. An original plasticity criterium, based on the recent works in porous plasticity theory, has been developed and already displays promising capabilities. Simple experimental procedures enable a reliable classification of steel qualities, heat treatments… in term of forgeability.

Finally, the friction problem is treated using different methodologies based on the forming process considered. For forging operations, a fine analysis of the force-displacement curves in direct extrusion stages may lead to a precise measurement of the friction coefficient under pressures from about 200 MPa up to 1000 MPa and for sliding rates between 1 and 100 mm/s. For wire drawing process, a model relying on an analytical approach using a significantly improved slice method has been developed: the comparison of the experimental drawing force and the predicted one gives the friction coefficient in industrial processing conditions (speed up to 5 m/s).     

PRIMARY RECRYSTALLIZATION TEXTURES OF 70-30 BRASS ROLLED BY CROSS SHEAR ROLLING IN SINGLE DIRECTION

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Constitutive model of rock under static-dynamic coupling loading and experimental investigation

1 Introduction Rock fragmentation, boring, explosive, rockburst etc in engineering domain of mineral are almost concerned with cracking of rock experiencing dynamic loading, and a plenty of results have been obtained[1]. But the related studies are almos…     

Nonclassical constitutive model involving void evolution of casting magnesium alloy

The void evolution equation and the elastoplastic constitutive model of casting magnesium alloy were investigated. The void evolution equation consists of the void growth and the void nucleation equations. The void growth equation was obtained based on the continuous supposition of the material matrix, and the void nucleation equation was derived by assuming that the void nucleation follows a normal distribution. A softening function related to the void evolution was given. After the softening function was embedded to a nonclassical elastoplastic constitutive equation, a constitutive model involving void evolution was obtained. The numerical algorithm and the finite element procedure related to the constitutive model were developed and applied to the analysis of the distributions of the stress and the porosity of the notched cylindrical specimens of casting magnesium alloy ZL305. The computed results show satisfactory agreement with the experimental data.     

软黏土不排水循环应力应变关系的数值模拟

对程星磊等提出的总应力形式增量弹塑性本构模型进行二次开发,以模拟复杂应力状态下软黏土的响应。通过Newton-Raphson算法,对材料非线性问题进行迭代求解;针对本构模型中应力反向等一系列关键性问题,应用欧拉切线算法编写了有限元程序,并结合子增量方法提高了计算精度。预测了软黏土在轴对称应力状态下的响应,得到了应力应变关系曲线,将其与单元预测结果进行比较,二者趋于一致,从而验证了有限元程序编写的合理性。利用该程序模拟三轴不固结不排水试验,模拟结果与试验吻合良好,表明该本构模型有限元程序可以反映轴对称应力状态下软黏土的不排水应力应变特性,可应用于更加复杂边值问题的模拟计算。     

Improving the Control Energy in Model Reference Adaptive Controllers Using Fractional Adaptive Laws

This paper presents the analysis of the control energy consumed in model reference adaptive control (MRAC) schemes using fractional adaptive laws, through simulation studies. The analysis is focused on the energy spent in the control signal represented by means of the integral of the squared control input (ISI). Also, the behavior of the integral of the squared control error (ISE) is included in the analysis. The orders of the adaptive laws were selected by particle swarm optimization (PSO), using an objective function including the ISI and the ISE, with different weighting factors. The results show that, when ISI index is taken into account in the optimization process to determine the orders of adaptive laws, the resulting values are fractional, indicating that control energy of the scheme might be better managed if fractional adaptive laws are used.