Phase Transformation Accommodated Plasticity In Nanocrystalline Nickel

Based on detailed x-ray diffraction and transmission electron microscopy we have found body-centered-cubic (bcc) Ni upon room-temperature rolling of nanocrystalline (nc) face-centered-cubic (fcc) Ni. The bcc phase forms via the Kurdjumov-Sachs (KS) martensitic transformation mechanism when the von Mises equivalent strain exceeds similar to 0.3, much higher than accessible in tensile testing. The fcc and bcc phases keep either the KS or the Nishiyama-Wasserman orientation relationship. Our results provide insights into the deformation physics in nc Ni, namely, the fcc-to-bcc phase transformation can also accommodate plasticity at large plastic strains. (C) 2008 American Institute of Physics.

弹塑性有限元的一些解法比较

<正> 1.弹塑性有限元分析的基本公式根据von Mises 屈服准则和Prandtl-Reuss塑性流动律,可以导出弹塑性阶段的应力增量-全应变增量之间的本构关系:{dσ}=[D_(eP)]{dε} (1)其中{dσ}为应力增量列阵,{dε}为应变增量列阵,[D_(eP)]为弹塑性系数矩阵,它的表达式为:其中(?)为有效应力,[D_e]为弹性系数矩阵,H=(?)/((?)~p)为有效应力和有效塑性应变曲线的斜率.增量形式的平衡方程为:[K]{△u}={△P} (3)其中[K]为总体刚度矩阵,{△u}为位移增量列阵,{△P}为外载荷增量列阵.2.几种解法方程(3)是非线性的.对于一般问题,精确求解比较困难.目前,一般都用近似法来求解.下面介绍几种解法.

基板预热对激光金属沉积成形过程热应力的影响

为降低沉积过程的热应力,抑制成形过程中裂缝的产生,研究基板预热对激光金属沉积成形(Laser metal deposition shaping,LMDS)过程热应力的影响具有非常重要的意义。根据有限元分析中的"单元生死"思想,利用APDL(ANSYS parametric design language)编程建立多道多层激光金属沉积成形过程的数值模拟模型,深入探讨基板未预热和预热到400℃时对成形过程热应力的影响。计算结果表明,基板预热到400℃可以显著降低成形过程中试样的热应力变化波动性,试样的Von Mises热应力最大值可降低10%左右,其中x方向热应力最大值可降低8.5%左右,z方向热应力最大值可降低8.1%左右。在与模拟过程相同的条件下,利用自行研制的激光金属沉积成形设备进行了成形试验,成形试验的结果与模拟结果基本吻合。

A Lagrangian for von Karman Equations of Large Deflection Problem of Thin Circular Plate

By the semi-inverse method proposed by He, a Lagrangian is established for the large deflection problem of thin circular plate. Ritz method is used to obtain an approximate analytical solution of the problem. First order approximate solution is obtained, which is similar to those in open literature. By Mathematica a more accurate solution can be deduced.