纯铜后继屈服面的测试与晶体塑性模型模拟

通过纯铜薄壁圆管试样的实测和晶体塑性模拟,用单试样法和多试样法对分别经历拉伸、扭转和组合拉扭变形的试样后继屈服面进行研究.考虑预变形方式、测点数目、测试顺序和指定平移应变等不同条件,对后继屈服面测定结果差异及屈服面内凹现象进行探讨.在此基础上,比较了单试样和多试样两种方法的合理性与有效性.数值模拟采用能反映Bauschinger效应的晶体塑性模型,试样有限元模型每个单元的晶体取向均随机生成,能反映多晶材料变形特征.模拟试验加载过程与真实试验一致.研究表明:(1)采用本文方法可再现真实试验过程,模拟后继屈服面测试展示的现象与实测相近,证实了方法的有效性和合理性;(2)模拟测试与实测均发现,薄壁圆管组合拉扭加载测得的后继屈服面可能出现内凹,单试样法测得屈服面的内凹现象尤为显著;(3)若试验材料的材质比较一致,用多试样法测试后继屈服比用单试样法更合理.     

动态弹塑性断裂韧性测试方法的比较研究

利用示波冲击试验,对动态弹塑性断裂韧性测试的不同方法（单试样法和多试样法）进行了研究．结果表明,单试样法（柔度变化率）测定材料的动态断裂韧性是一种简单、准确、适合于工程应用的好方法     

单轴载荷下X80钢的包申格效应研究

本文通过单轴拉伸和压缩试验研究了X80管线钢的包申格效应(BE)。采用正向与反向加载方法研究材料变形历史特性。测定了X80钢的简单拉伸试验曲线,其应力-应变关系表明,该材料具有理想弹塑性特点。为了得到X80钢的BE,在不同预变形下对几个试件分别进行加载,并当给定的预应变值分别达到0.63%,0.67%,0.95%,1.27%和1.55%时就卸载。随后再进行反向加载实验,并记录应力应变曲线。该钢材反向加载时出现加工硬化,且屈服强度比正向加载时要低。正反向加载之间的屈服强度差值随着预应变增加而增大;当预应变超过0.95%时,反向屈服强度达到恒量。实验表明,X80钢的反向加载特性可用Remberg-Osgood关系拟合。最后给出了屈服强度降和预塑性应变之间的经验公式。     

Q235钢缺口试样拉压低循环疲劳的实验研究

以Q235钢制U型缺口板试样为研究对象,用有限元方法计算其缺口根部等效应变幅对应的试样标距段位移,以此控制试验机进行拉压循环疲劳试验。然后用局部应力应变法对试验测得的寿命结果进行分析。结果表明：无论用有限元还是修正Neuber公式计算缺口根部的应力应变,局部应力应变法的疲劳寿命评估只适用于缺口半径较大的试样;对缺口半径较小试样的估计寿命明显低于实测值,且有限元法比修正Neuber法更保守。进而又对试样缺口区域应变梯度的影响进行了探讨：参照有限元计算的应变梯度,利用Taylor模型估算了缺口根部的屈服应力和流动应力;在此基础上重新计算应变分布并估计试样的疲劳寿命,结果证实考虑应变梯度影响可改善缺口试样的疲劳寿命估计。     

基于ANN-GA协同寻优的动态拉伸试样尺寸优化方法

材料动态拉伸力学性能测试中,动态拉伸试样的几何尺寸对测试结果的准确性与有效性有着较大影响。为对动态拉伸试样的结构进行优化设计,以使其在动态拉伸过程中更好地满足一维应力与变形均匀等基本假设。首先,建立了量化的试样测量准确度指标,即应力平衡达到时间、变形均匀度、非轴向应力相对水平、过渡段相对变形。然后,对试样结构参数进行正交试验设计,通过数值模拟的方法得到了关于试样尺寸与测量准确度指标的正交试验数据库,并对正交试验数据库进行多目标正交试验矩阵分析,得到了试样结构参数对各测量准确度指标影响的主次顺序和规律。最后,以正交试验数据库为训练集,采用人工神经网络（artificial neural network, ANN）协同遗传算法（genetic algorithm, GA）的全局寻优方法对试样的结构尺寸进行优化设计,得到了试样的最优结构尺寸,并对最优尺寸的有效性进行了验证。结果表明,优化后的试样结构在材料动态拉伸力学性能测试精度上的表现明显得以提升。因此,采用ANN-GA协同优化的方法对动态拉伸试样的结构进行优化具有可行性和有效性。     

三点弯曲试样动态应力强度因子计算研究

利用Hopkinson压杆对三点弯曲试样进行冲击加载,采集了垂直裂纹面距裂尖2mm和与裂纹面成60°距裂尖5mm处的应变信号。根据裂尖附近测试的应变信号计算试样的动态应力强度因子,并与有限元计算结果进行比较,结果表明由于裂尖有一段疲劳裂纹区,通过裂尖附近应变信号来计算动态应力强度因子时,如果裂尖位置确定不准及粘贴应变片位置不够准确对计算结果将带来很大影响。因此利用应变片法计算动态应力强度因子时,为了获得更准确的计算结果,在实验后应对试件裂纹面进行分析测量,重新确定裂尖位置,必要时需对应变片至裂尖距离进行修正后再计算动态应力强度因子值。     

双屈服面法测量金属材料动高压屈服强度的若干改进

在J.R.Asay等提出的双屈服面法（简称AC方法）测量动态屈服强度的基础上,对实验设计和数据处理做了部分改进。在实验设计方面,在满足实验物理设计的前提下,将 AC方法的卸载实验装置(双层飞片和双台阶样品)简化为单飞片和单台阶样品,以简化波系作用,并减少了声速测量不确定度的引入因素。在数据处理方面,发现由Lagrange声速和工程应变关系曲线外推求预冲击态的体积声速容易引入较大的不确定度,而通过Lagrange声速和粒子速度关系曲线可以更清晰地得到塑性卸载起始点,使图解法和积分法得到的屈服强度值趋于一致。利用改进后的AC方法,得到了低温退火后LY12铝在20.0 GPa预冲击压力下的屈服强度为0.6 GPa。     

基于数字散斑相关法的紧凑拉伸试样断裂韧性实验研究

本文采用数字散斑相关方法对2A12T4铝合金紧凑拉伸试样的断裂韧性进行了实验研究。应用数字散斑相关方法计算了实验过程中试样的应变场、应力场以及位移场。针对实验所得的结果以及紧凑拉伸试样的裂纹特征,采用了矩形积分路径。选择沿裂纹方向和垂直裂纹方向的J积分路径,并且推导出各方向上J积分的数值计算公式。根据推导得到的公式选择不同的积分路径进行J积分的计算,得到了断裂韧性J0积分路径的合理选择范围,同时验证了J积分的路径守恒性。然后根据所得的路径选择标准,选择合理的积分路径,计算出2A12T4铝合金断裂韧性J0的值。将所得结果与国标计算的J0值对比,误差为1.22%,说明了此种方法的正确性。从而为数字散斑相关方法在紧凑拉伸试样断裂韧性的测试研究中提供参考。     

基于球形颗粒几何排列的离散元试样高效生成方法

在球体离散元数值模拟中,颗粒的初始排列状态是影响计算效率和计算结果的重要环节。本文采用前进面几何构造算法,提出了一种基于网格搜索的球形颗粒随机排列高效算法。通过求解空间三边方程,满足了粒径设置的任意大小的颗粒依次置入前进面的外侧,并与构成前进面的三个颗粒相互接触。为获得高体积分数的颗粒簇,该算法允许颗粒改变其粒径大小。采用颗粒网格化方法可以简化前进面的搜索,并由此提高排列效率。通过计算平均配位数、体积分数和二阶结构张量的特征值,对不同粒径比下得到的立方体试样进行了分析,得到试样配位数及体积分数均随着粒径比的增大而增大,且得到的试样为各向同性。此外,空间网格的大小和初始颗粒的生成点对随机排列的效率均会产生显著的影响。最后,对非规则铁路道砟进行了精细构造及压碎模拟,发现DEM模拟得到的应力-应变曲线与试验结果基本吻合,验证了该算法得到的颗粒试样在模拟道砟裂纹起裂、扩展等过程的有效性。     

结构钢光试样的等寿命图

提出结构钢光试样等寿命曲线方程,并提出用等寿命曲线方程绘制结构钢光试样等寿命图的新方法．只要测得材料在对称循环（r＝－1）下的S-N曲线,就可以用等寿命曲线方程绘制材料的等寿命图．用等寿命曲线方程绘制等寿命图与传统的方法相比,可减少60％～70％的疲劳试验．     

YIELD SURFACES AND PLASTIC FLOW OF 45 STEEL UNDER TENSION-TORSION LOADING PATHS

An experimental analysis on the subsequent yield-surfaces evolution using multiple specimens is presented for a 45 steel after a prescribed pre-strain loading in three different directions respectively, and the yielding is defined by a designated offsetting strain. The size of the subsequent yield surface is found smaller than the initial yield surface; the negative cross effects are observed in the normal loading direction, its shape is not a Mises circle but has a rather blunt nose in loading direction and flat in the opposite. These results strongly depend on the loading path and the prescribed offset plastic strain. The plastic flow direction to the subsequent yield surface is investigated, and it is found that the plastic flow direction deviates from the normal flow rule. The deviation differs from preloading case to preloading case. And the plastic flow direction would have a larger deviation from the normal of the yield surface, if the subsequent yield was defined by a smaller offset strain. Furthermore, the experiments are simulated using the Chaboche model, and the results show that it can rationally predict yield-surface only when yield is defined by a fairly large offset strain.     

Experimental determination of strain-induced anisotropy during nonproportional straining of an A1/Mg alloy at room temperature

A servohydraulic, computer controlled MTSn axial-torsion testing machine with a bi-axial clip-on extensometer is used to test thin-walled tubes of an A1/Mg alloy under strain control. A plastic offset strain of 10⁻⁴ determines the yield surfaces. Straining and yield surface probing is governed by a computer program which also controls digital data acquisition. Yield surfaces in stress and in strain space as well as the axial and shear stress-strain diagrams can be reconstructed from the digitally recorded data. The specimens were subjected to a strain path in the form of a regular 16-sided polygon which was followed on some specimens by a square path. The total inelastic strain path length can exceed 15% while the equivalent strain excursion is less than 2%. It is shown that yield surfaces measured on specimens withclose initial stress-strain diagrams are very consistent and that yield surface probing has an insignificant effect on subsequent yield surfaces. Yield surfaces are shown to translate, changein shape and size and to exhibit a cross effect. A post processor which includes a least square smoothing routine calculates the area and the centroid of each yield surface. The size increase is initially rapid but the rate of increase decreases as a saturation is approached. After strining for less than 1% in a fixed direction a characteristic yield surface shape is established. Yield surfaces obtained at the same point in strain space with identical prestrain direction of at least 1% but with increased amounts of accumulated plastic strain have the same shape but show an increase in size. The yield surfaces differ in shape and size when the same strain point is reached from different directions. The centroid of the yield surface in stress space moves almost in a circular path for a polygonal strain path. All stress space yield surfaces contain the origin but this is not the case for the surfaces in strain space.     

预应变对中子辐照高纯铝拉伸性能的影响

金属材料的中子辐照硬化和脆化一直都是核能安全领域十分关注的重要问题之一. 为了进一步认识预应变对中子辐照金属材料塑性形变和最终断裂特性的影响规律, 及其微观机理, 本文研究了10%拉伸预应变高纯铝的拉伸应力-应变曲线、失稳应力和失稳应变等随辐照剂量的变化规律. 结果表明, 辐照剂量越高, 预应变高纯铝内部孔洞的尺寸和数密度越高, 导致屈服强度和极限拉伸强度越高, 均匀延伸率和失稳应变越小, 表现出典型的辐照硬化和脆化效应, 但失稳应力与辐照剂量几乎无关. 相同辐照剂量条件下, 预应变引入的高密度位错能够显著降低辐照孔洞的尺寸和数密度, 加之辐照退火效应的综合影响, 导致预应变能够降低高纯铝屈服强度的增长率和失稳应变的下降率, 从而表现出一定的抑制辐照硬化和脆化的能力, 预应变还能够提高高纯铝的失稳应力, 但整体而言预应变并不能提高高纯铝的延性. 最后, 基于J-C本构模型的中子辐照退火态金属材料的脆化模型能够直接应用于预应变金属材料, 且模型预测结果与实验结果吻合较好.      

Plastic spin associated with a non-normality theory of plasticity

A plasticity model using a vertex-type plastic flow rule on a smooth yield surface for an anisotropic solid has been proposed recently. This model is here completed by incorporating the effect of plastic spin. Simple shear with a large shear strain is one of the hardest tests on finite strain anisotropic plasticity models, and it is here shown which plastic spin expression is needed to avoid unrealistic oscillatory behavior of the shear stress under large shear strains. The idea of using non-normality with a smooth yield surface originates from a recent proposal of using an abrupt strain path change to determine the subsequent yield surface shape. For this method both polycrystal plasticity calculations and experiments have shown a vertex-type response on the apparently smooth yield surface.     

Description of yield surface evolution using a convected plasticity model

The convected plasticity model proposed by Wu, 2003a, Wu, 2003b, Wu, 2005, Wu, 2007, making use of convected coordinate system, is applied to discuss the evolution of yield surface. It is shown that this constitutive model is capable of describing all experimentally observed features of subsequent yield surface: isotropic hardening, kinematic hardening, distortion, and rotation of yield surface. The rotation of yield surface in 3D stress space has not been much discussed in the literature, but recent experiments at National Taiwan University (Sung et al., 2011) have shown that it is an important property of subsequent yield surface. In particular, the rotation of subsequent yield surface is pre-strain path dependent. It does not rotate, when the pre-strain is tensile; but the yield surface rotates about the axial stress axis when the pre-strain is torsional.     

Contribution of Kinematical and Thermal Full-field Measurements for Mechanical Properties Identification: Application to High Cycle Fatigue of Steels

The purpose of this work is to extend the use of unconventional tests and full field measurements (kinematical and thermal) to the identification of the effect of a wide plastic pre-strain range on the high cycle fatigue properties of a dual-phase steel. An unconventional specimen is designed. The geometry of this specimen permits a constant gradient of pre-strain to be obtained after a monotonic tensile test. Then, a self-heating test under cyclic loading is carried out on the pre-strained specimen. During this cyclic test, the thermal field is measured using an infrared camera. Finally, a suitable numerical strategy is proposed to identify a given thermal source model taking into account the influence of plastic pre-strain. The results show that, with the unconventional test and the procedure developed in this work, the influence of a plastic pre-strain range on fatigue properties can be identified by using only one specimen when, for a classical fatigue campaign, a great number of specimens is required.     

Laser speckle decorrelation as a noncontact method for brittle-ductile fracture differentiation

Quasi-static fracture in four-point-bend specimens of both brittle and ductile materials was examined using a method which required only photographic access to the specimen surface. Decorrelation of laser speckle patterns was used as a means to map out two-dimensional regions of high surface strain associated with crack propagation. ASTM A515 grade 70 steel was tested, at temperatures above and below its brittle-ductile transition temperature, by double-exposure speckle photography of the area ahead of the crack tip. The regions where the two speckle patterns were uncorrelated, determined by pointwise spatial filtering of the speckle interferograms, have been observed and are related to plastic deformation of the specimen surface near the crack tip. A subsequent comparison of the decorrelation zones resulting from brittle versus ductile states showed differences as expected in both zone size and shape.     

Plastic deformation of aluminium alloy under abruptly-changing loading or strain paths

The effect of deformation history on the plastic behaviour of thin-walled tubular specimens of aluminium alloy 5056 was examined for three types of abruptly-changing loading or strain paths: namely, reverse loading after pre-loading, orthogonal straining after pre-strain, and orthogonal reloading after pre-loading and then perfect unloading, by applying combined loadings of axial force and torque. The experimental results revealed the following trends. The relation between the magnitudes of stress and strain after the comer is expressed for every pre-strain by a single curve parallel to the extension of the pre-loading curve, when the strain after the corner exceeds 1.2 per cent. Moreover, the relations between the stress reduced by pre-stress and strain after the corner for various values of the same type of pre-strain agree with each other; however, the relation for the tensile pre-strain differs clearly from that for the torsional pre-strain. Thus, A.A. Il'yushin's postulate of isotropy does not hold accurately for the above loading histories for the aluminium alloy even if the effect of the third invariant of stress deviator is eliminated.     

脆性材料的扩展裂纹前缘形态与三维J积分

本文利用超声波断口图技术对不同类型的PMMA试件进行了测试。这些试件为承受均匀拉伸或三点弯曲变形的带单侧贯穿裂纹的PMMA板，其中拉伸试件又分为光面和带表面沟槽的两种。得到了这些试件断口上的超声波线。发现它们为平面线．其凸凹方向与试件的表面状态有关。基于有限元法，计算了试件的三维J积分，考察了杨氏模量和泊松比对其的影响．结果发现泊松比影响J积分曲线形状，而杨氏模量只影响其绝对数值，不影响其曲线形状。得到的试件在不同裂纹长度下的三维J积分与它们断口图上的超声波线的形状和凸凹方向相似。说明在脆性材料中，扩展裂纹前缘曲线形态是三维J积分的作用的结果。