EXPONENTIAL MAP ALGORITHM FOR STRESS UPDATES IN ANISOTROPIC MULTIPLICATIVE ELASTOPLASTICITY FOR SINGLE CRYSTALS

This paper presents a new stress update algorithm for large-strain rate-independent single-crystal plasticity. The theoretical frame is the well-established continuum slip theory based on the multiplicative decomposition of the deformation gradient into elastic and plastic parts. A distinct feature of the present formulation is the introduction and computational exploitation of a particularly simple hyperelastic stress response function based on a further multiplicative decomposition of the elastic deformation gradient into spherical and unimodular parts, resulting in a very convenient representation of the Schmid resolved shear stresses on the crystallographic slip systems in terms of a simple inner product of Eulerian vectors. The key contribution of this paper is an algorithmic formulation of the exponential map exp: sl(3) → SL(3) for updating the special linear group SL(3) of unimodular plastic deformation maps. This update preserves exactly the plastic incompressibility condition of the anisotropic plasticity model under consideration. The resulting fully implicit stress update algorithm treats the possibly redundant constraints of single-crystal plasticity by means of an active set search. It exploits intrinsically the simple representation of the Schmid stresses by formulating the return algorithm and the associated consistent elastoplastic moduli in terms of Eulerian vectors updates. The performance of the proposed algorithm is demonstrated by means of a representative numerical example.     

CONSISTENT LINEARIZATION FOR THE EXACT STRESS UPDATE OF PRANDTL–REUSS NON-HARDENING ELASTOPLASTIC MODELS

This paper presents a consistent algorithm, which combines the advantages of the exact time integration of Prandtl–Reuss elastoplastic models and the quadratic asymptotic convergence of Newton–Raphson iteration strategies. The consistent modulus is evaluated by a full linearization of the exact stress update procedure. Numerical tests for a thin wall tube subjected to combined loads of tension and torsion are performed to illustrate the accuracy and efficiency of the consistently linearized exact stress update algorithm described in the paper. For comparison purpose numerical results of the radial return method are also given.     

GEOMETRICALLY NON-LINEAR AND ELASTOPLASTIC THREE-DIMENSIONAL SHEAR FLEXIBLE BEAM ELEMENT OF VON-MISES-TYPE HARDENING MATERIAL

A three-dimensional elastoplastic beam element being capable of incorporating large displacement and large rotation is developed and examined. Elastoplastic constitutive equations are applied to the beam element based upon the assumption of small deformational strain leading to a material formulation which is completely objective for the application of stress update procedures. The continuum-type equations of plastic model of J₂ mixed hardening are transformed into the beam equations by satisfying beam hypotheses. An effective stress update algorithm is proposed to integrate elastoplastic rate equations by means of the so-called multistep method which is a method of successive control of residuals on yield surfaces. It avoids severe divergence when the displacement increments become large which is usual for the continuation methods. Material tangent stiffness matrix is derived by using consistent elastoplastic modulus resulting from the integration algorithm and is combined with geometric tangent stiffness matrix. Different from other elements, the present element is shear flexible and can satisfy the plasticity condition in a pointwise fashion. A great number of numerical examples are analysed and compared with the literature. The proposed beam element is verified to be not only quite accurate but also very effective for the analyses of pre-buckling and large deflection collapse of spatial framed structures.     

热辅助超声滚压温度场参数影响激光熔覆涂层表面粗糙度研究

目的 降低激光熔覆涂层的表面粗糙度。方法 采用激光熔覆技术制备铁基激光熔覆涂层,采用超声温滚压耦合热处理工艺对熔覆层进行熔覆后强化加工,重点研究温度场参数对成形表面粗糙度的影响,通过方差分析(ANOVA)确立参数显著性,同时利用响应曲面法(RSM)构建温度场参数影响铁基涂层表面粗糙度的预测模型,并进行参数优化。结果 加热温度和保温时间对成形试样表面粗糙度的影响显著。在实验参数范围内,试样的表面粗糙度与加热温度呈正相关,与保温时间呈负相关。实验结果表明,在相同保温时间下,在加热温度100、250、400 ℃条件下试样的表面粗糙度Ra分别为0.237、0.158、0.096 μm;在相同加热温度下,在保温时间为0.5、1、2 h条件下试样的表面粗糙度Ra分别为0.156、0.164、0.170 μm。可见与保温时间相比,加热温度对涂层表面粗糙度的影响更显著。参数优化分析结果表明,在实验参数范围内,在400 ℃加热温度和0.5 h保温时间条件下,试样具有最小的表面粗糙度Ra(0.089 μm)。结论 相较于车削及常温滚压工艺,采用超声温滚压耦合热处理工艺可进一步降低激光熔覆涂层的表面粗糙度,在实验参数范围内,加热温度400 ℃和保温时间0.5 h是最优的温度场参数组合。     

高性能可塑料的研制与应用

以特级高铝矾土为主要原料,加入适量复合微粉、膨胀剂、保湿剂,采用复合结合剂,能够生产出可塑性能优良、高温强度高的可塑料.     

塑性流动法则的相关性

介绍、分析和比较由最大耗散法则与热力学原理两种方法推导得到的率无关塑性变形流动法则的形式,得出了塑性流动法则总是与屈服函数有关,可由屈服函数和耗散函数确定材料进入塑性状态后不可恢复应变的结论.由此得到的流动法则,避免了不相关流动法则中塑性势函数意义不清的缺点.     

1Cr18Ni9Ti不锈钢低周疲劳随动硬化实验研究

为了研究单调加载对1Cr18Ni9Ti不锈钢材料后继循环塑性硬化和流动特性的影响,对该材料进行了带平均应变的低周疲劳实验研究.对不同应变幅值的对称应变及非对称的应变循环进行了屈服面半径和背应力演化分析,揭示了材料的随动硬化和各向同性硬化演化对应变幅值和平均应变的依赖性.实验结果为建立循环加载和单调加载耦合的复杂加载条件下的本构模型提供了实验基础.     

膨胀性二灰土的施工工艺探讨

主要介绍用微膨胀的高塑性粘土做二灰土的施工工艺,侧重于高塑性粘土的砂化改良,详细阐述了试验路的施工过程,并简单介绍了二灰土施工过程中表面起皮和松散的处理方法。     

医用镁丝多道次大塑性拉拔变形工艺及微观组织研究

本文以医疗器械行业常用材料纯镁丝为研究对象,采用一种新型大塑性多道次转角拉拔(M-PAD)变形工艺,建立了M-PAD变形模型,并模拟了拉拔过程中纯镁材的等效应力、等效应变变化图;通过设计便于进行拆卸与实验操作的M-PAD模具,对纯镁M-PAD变形工艺参数进行了试验研究.结果表明:M-PAD拉拔变形有利于晶粒的均匀细化,...     

DM1 Transgenic Mice Exhibit Abnormal Neurotransmitter Homeostasis and Synaptic Plasticity in Association with RNA Foci and Mis-Splicing in the Hippocampus

Myotonic dystrophy type 1 (DM1) is a severe neuromuscular disease mediated by a toxic gain of function of mutant RNAs. The neuropsychological manifestations affect multiple domains of cognition and behavior, but their etiology remains elusive. Transgenic DMSXL mice carry the DM1 mutation, show behavioral abnormalities, and express low levels of GLT1, a critical regulator of glutamate concentration in the synaptic cleft. However, the impact of glutamate homeostasis on neurotransmission in DM1 remains unknown. We confirmed reduced glutamate uptake in the DMSXL hippocampus. Patch clamp recordings in hippocampal slices revealed increased amplitude of tonic glutamate currents in DMSXL CA1 pyramidal neurons and DG granule cells, likely mediated by higher levels of ambient glutamate. Unexpectedly, extracellular GABA levels and tonic current were also elevated in DMSXL mice. Finally, we found evidence of synaptic dysfunction in DMSXL mice, suggestive of abnormal short-term plasticity, illustrated by an altered LTP time course in DG and in CA1. Synaptic dysfunction was accompanied by RNA foci accumulation in localized areas of the hippocampus and by the mis-splicing of candidate genes with relevant functions in neurotransmission. Molecular and functional changes triggered by toxic RNA may induce synaptic abnormalities in restricted brain areas that favor neuronal dysfunction.