超高分子薄膜双向拉伸的数值模拟

针对一类超高分子聚合物薄膜材料(UHMWPE),通过单向拉伸实验获得了载荷-大变形曲线,并由弹塑性线性强化本构关系,拟合得到了相应的弹性模量、屈服应力、切线模量(硬化模量)等材料参数。将较大尺寸试件的有限元模拟结果和相应的实验结果进行对比,指出弹塑性线性强化本构关系可适用于UHMWPE薄膜大变形分析。应用弹塑性大变形有限元数值模拟了直线、抛物线、三次曲线、双圆弧等4种常见曲线导轨双向拉伸变幅区内的应力分布。为了减弱拐点处的应力集中,提出了圆弧-直线-圆弧的导轨曲线,计算分析表明,优化后的导轨曲线可以使变幅区薄膜的应力分布更加均匀,并且处于较低水平,达到了导轨优化的效果。     

Plasticity Induced by Twin Lamellar Structure in Magnesium Alloy

Effect of {10-12} twins on the mechanical properties of magnesium alloy has received considerable research interest. A hot-rolled AZ31 Mg alloy sheet was subjected to dynamic plastic deformation with the aim of introducing {10-12} twin lamellar structure. It has been found that higher strength and better ductility are obtained when tensile loading is perpendicular to the c axis of twin region of the twin lamellar structured sample, indicating that the plasticity improvement caused by twins depends on the special strain path. The fracture morphology of the twin lamellar structured sample shows a dimple fracture mode under tensile loading perpendicular to the c axis, while the cleavage fracture with river pattern has been observed in other fractured samples. Above experimental results indicate that the interaction of dislocations and twin lamellae may play an important role in improving mechanical properties of Mg alloy.     

等双轴预拉伸下介电弹性体驱动器力电稳定性分析

针对等双轴预拉伸条件下的介电弹性体驱动器,开展了驱动器力电特性有限元仿真计算,建立了拉伸率与名义电场强度之间的关系,将仿真结果与已有的理论计算结果进行了对比,二者完全重合;同时根据等双轴预拉伸情况下系统的状态方程,分析不同预拉伸水平下极限电致应变、电场强度、电位移等参数关系,获取该介电弹性体驱动器稳态工作区间;有限元建模及分析方法可以为等双轴预拉伸驱动器的设计提供参考。     

Orientation dependence of mechanically induced grain boundary migration in nano-grained copper

Tensile tests were carried out on gradient nanograined copper samples to investigate the grain orientation dependence of mechanically induced grain boundary migration(GBM) process. The relationship between GBM and the orientations of nanograins relative to loading direction was established by using electron backscatter diffraction. GBM is found to be more pronounced in the grains with higher Schmid factors where dislocations are easier to slip. As a result, the fraction of high angle grain boundaries decreases and that of low angle grain boundaries increases after GBM.     

基于解析法的非均匀张力作用下运动薄膜的稳定性

目的针对高速印刷运动薄膜在传输制备过程中由于相邻导纸辊等支撑副的平行度误差,料膜沿宽度方向张力分布易出现不均匀性问题,研究提高印刷设备的工作稳定性的方法。方法根据D’Alembert原理建立运动薄膜的横向振动微分方程,引入无量纲量,将微分方程转化为无量纲形式,利用解析法求解运动薄膜的复频率方程。结果得到不同参数下无量纲复频率与无量纲速度的变化曲线,当增大张力系数、张力比,或者减小长宽比时,薄膜系统达到发散失稳的临界无量纲速度增大,运动薄膜系统越稳定。结论获得了在不同张力比、张力系数、长宽比等参数下薄膜系统达到发散失稳的临界无量纲速度。     

Microstructure, Textures and Deformation Behaviors of Fine-grained Magnesium Alloy AZ31

Channel die compression and initial textures are used to activate different deformation mechanisms in a fine-grained magnesium alloy AZ31. The σ-εcurves, microstructures and, particularly, textures are analyzed to reveal different deformation mechanisms and to compare with those of coarse grained samples. Dominant double-prismatic slip, {1012} twinning and basal slip are detected in three types of samples, respectively, which is similar to those of coarse grained samples. The detrimental effect of shear band formation or {1011} twinning is limited in fine grained microstructure. In addition to the higher flow stress at low temperature an early decrease in flow stress at higher temperature is also found in fine-grained samples in comparison with their coarse-grained counterparts. This softening is ascribed to the early dynamic recrystallization or grain boundary glide.     

Twin Growth and Texture Evolution in an Extruded AM30 Magnesium Alloy During Compression

The aim of this study was to examine and quantify the growth of extension twins and the associated texture change in an extruded AM30 magnesium alloy during compression along the extrusion direction. Three stages of twin growth with increasing strain were observed due to twin-dislocation interactions, together with increasing texture volume fraction of {1210}〈0001〉 and {01- 10}〈0001〉 components. Stage I was characterized by a relatively slow and gradually accelerating growth. A steady-state twin growth was reached in stage II, where the twin width increased linearly with increasing strain. Stage III twin growth became decelerated, exhibiting a plateau-like character.     

镁合金织构演化晶体塑性力学模型的研究进展

晶体塑性力学建立于位错理论,能够将材料的塑性变形行为和微观组织变化联系起来,被广泛应用于形变织构的研究中。简述了镁合金的塑性变形机制,介绍了晶体塑性力学在镁合金织构演化模拟中的应用发展过程,重点对泰勒模型、弹塑性自洽模型、粘塑性自洽模型、弹粘塑性自洽模型的研究过程及其应用现状进行了综述。最后指出了晶体塑性力学模型在镁合金织构演化应用中存在的问题,并展望了其在镁合金变形织构模拟中的应用前景。     

Orientation dependence of dislocation structure evolution during cold rolling of aluminum

A well-organized dislocation structure forms in many polycrystalline metals during plastic deformation. This structure is described qualitatively with no explanation of the quantitative characterization. In this work, the evolution of dislocation structure in commercial purity aluminum is described by means of the excess dislocation density and by quantitative characterization of the cell structure as seen on a plane surface. The measurements were performed on a pseudo-internal surface of a split specimen deformed by channel die deformation. The results show a clear dependence of cell structure formation on orientation of the crystallite with respect to the imposed deformation gradient with the largest excess dislocation density occurring in grains of {0 1 1}[1 2 2] orientation for plane strain deformation. Neighboring grain and non-local effects are shown to be of importance in the type of dislocation structures that evolve.     

Plasticity induced crack closure in Middle‐Crack Tension specimen: numerical versus experimental

Numerical studies played a major role in the current understanding of plasticity induced crack closure (PICC), however it is well known that the numerical models simplify the reality by considering discrete crack propagations, relatively high fatigue crack growth rates, sharp cracks and that propagation occurs at a well‐defined load. The objective of this paper is to find if despite these limitations, the numerical predictions of PICC are close to the experimental results. Experimental work was developed to obtain crack opening values in 1‐mm‐thick middle‐tension (M(T)) specimens of AA6016‐T4 aluminium alloy. The opening level was determined from remote compliance data captured by a pin microgauge placed at the centre of the specimen. A numerical model was developed and optimized replicating the experimental test in terms of sample geometry, loading parameters, material behaviour and crack opening measurement technique. The adoption, in the numerical analysis, of a remote measurement technique for determining PICC was found to be robust and adequate for mesh refinement studies. A good agreement was found between experimental and numerical results.     

Fracture Analysis of Composites in High-speed Tension

In this paper the results of a high-speedtension experiment of the SiC_w/Al compositehave been reported and a simplified theoreticalmodel has been developed to study the fracturemechanism of composites in high-speed tension.This theoretical model provides a new explanationfor the increase of dynamic fracture strengthof composites in high-speed tension.     

SiC颗粒增强镁基复合材料界面稳定性的第一性原理计算

针对SiC颗粒增强镁基复合材料界面的理论研究较少,大多研究仅停留在表征层面等问题。本文采用第一性原理方法,计算了四种不同SiC(0001)/Mg(0001)界面模型的电荷密度、布局分析和界面分离功。结果表明:对于同种终端的SiC(0001)/Mg(0001)界面模型中,顶位型结构比心位型结构的稳定性好;不同终端的SiC(0001)/Mg(0001)的界面模型中,Si终端结构比C终端结构更加稳定,其中Si终端顶位型结构稳定性最好,其分离功为W_sep=3.297 J/m²,界面间距为d₀=2.651nm。界面的键合方式主要为C-Mg共价键和Mg-Si离子键。     

高稳晶振的秒级频率稳定度的测量

介绍了在不同的条件下，对高稳晶振的秒级频率稳定度进行测量的方法。     

开卷系统速度和张力的鲁棒控制

开卷过程中的速度和张力系统具有时变、强耦合的特点,为了提高系统的控制性能,采用了多变量H∞鲁棒控制策略。建立了带结构化扰动的系统模型,用结构奇异值方法对控制系统的鲁棒稳定性和鲁棒性能进行了分析。仿真结果表明,所设计的控制器能满足闭环系统稳定性和抗干扰性能要求。     

国产软铝材料失稳时的应力应变分析

本文利用金属薄膜试验求出的软铝塑性应力应变关系．分析了国产软铝材料在简单拉伸、柱形薄壳和球形薄壳失稳时的应力和应变．     

带横向内筋张紧轮旋压成形分析

目的解决某型号带横向内筋张紧轮旋压成形困难的问题。方法运用有限元模拟软件,建立了张紧轮的有限元模型,选取了不同的坯料壁厚和旋轮圆角半径,对张紧轮旋压成形过程进行了数值模拟。结果获得了旋压成形过程中坯料壁厚大小和旋轮圆角半径大小对旋压力和成形质量的影响,并对结果进行了总结。结论通过分析壁厚和预成形轮形状对张紧轮旋压成形质量的影响,得出了最优的工艺参数值,对实际生产具有指导意义。     

Improvement of formability and mechanical properties of magnesium alloys via pre-twinning: A review

Twinning can generate the change of texture and a large of twin boundaries, which can greatly influence the mechanical properties of magnesium alloys. Thus, pre-twinning can be considered to be a simple and feasible method to improve the mechanical properties of magnesium alloys. Recently, some studies have confirmed that pre-twinning can be an effective way to enhance the strength, formability and mechanical anisotropy of magnesium alloys. Based on these results, some aspects of the present research on the improvement of mechanical properties via pre-twinning are reviewed. The relevant mechanisms have been summarized. Finally, for this research field, a few critical scientific problems are also proposed.