铜单晶拉伸试样表面滑移带痕迹的晶体塑性分析

采用建立在晶体塑性理论基础上的晶体塑性有限变形计算方法,针对铜单晶试样单轴拉伸过程中晶体滑移在试样表面留下的滑移带痕迹进行了数值研究.作者利用三维有限元模拟不同取向铜单晶试样的拉伸变形,通过晶体塑性滑移面与试样表面交线的几何分析,得到了试样在不同晶向拉伸下不同滑移系启动造成的试样表面滑移痕迹,并对数值计算的试样表面滑移...     

低碳钢滑移带的实验观测

针对一些文献对低碳钢滑移带描述的不准确性以及实验教学的需要,提出一种观测低碳钢滑移带的实验方法.该方法采用数字图像相关技术提取低碳钢试件在单向拉伸过程中的表面位移场和应变场,直观的显示出滑移带产生和传播的过程.实验教学表明,该实验不但能有效地揭示低碳钢滑移带的特性,而且还能激发学生的科研兴趣,教学效果良好,值得推广.     

基于平台圆环试样的无机玻璃动态拉伸性能研究

无机玻璃的拉伸强度往往远小于压缩强度,服役过程中玻璃大多会发生拉伸断裂.论文采用平台圆环(flattened circular ring, FCR)试样测试钠钙硅酸盐玻璃的拉伸性能.分别利用电子万能试验机和电磁分离式Hopkinson压杆(electromagnetic split Hopkinson pressure bar, ESHPB)开展准静态、动态单轴单向和单轴双向实验,加载过程中采用高速相机对试样的破坏过程进行观测.结果显示,该材料试样强度具有明显的正加载速率相关性.动态单轴双向加载可比单向加载更快实现应力平衡,但两种应力波加载方式下试样的动态拉伸强度大致相同.高速摄像与动态加载同步分析表明,这是因为试样的裂纹产生时刻与应力峰值时刻几乎同时产生.对三种形式的拉伸实验结果进行对比,发现拉伸强度受试样形状影响,这与试样断裂路径上的拉伸应力分布有关.     

分子动力学模拟金属玻璃板拉伸时剪切带的形成和演化过程

金属玻璃在低温高应力条件下容易生成剪切带而导致结构的破坏,大大限制了它的推广应用。本文采用分子动力学模拟研究了三种Cu64Zr36（不带缺口、一侧带缺口、两侧带缺口）金属玻璃板试样在拉伸过程中剪切带的形成和演化过程及其力学性能。结果表明：不带缺口金属玻璃板试样在低温高应力的拉伸过程中会自发出现局部剪切转变区,发生剪切局部化,继续拉伸会在与加载轴大约成45°方向上形成剪切带。剪切带的形成与剪切转变区的分布和局部化有关,带缺口比不带缺口的试样会更早出现应变局部化,即在较低的拉伸应变下便形成剪切带,其拉伸强度也相应较低。相同条件下,一侧带缺口与两侧带缺口的试样在拉伸强度上几乎相同,但两侧带缺口试样的应变局部化程度稍低,主要是两侧缺口处均出现了剪切转变区,导致其分布和局部化不够集中,这也是形成主剪切带和次剪切带的主要原因。以上结果为进一步从原子尺度理解金属玻璃剪切带的形成和演化特征提供了重要的信息。     

分子动力学模拟金属玻璃板拉伸时剪切带的形成和演化过程

金属玻璃在低温高应力条件下容易生成剪切带而导致结构的破坏,大大限制了它的推广应用。本文采用分子动力学模拟研究了三种Cu64Zr36（不带缺口、一侧带缺口、两侧带缺口）金属玻璃板试样在拉伸过程中剪切带的形成和演化过程及其力学性能。结果表明：不带缺口金属玻璃板试样在低温高应力的拉伸过程中会自发出现局部剪切转变区,发生剪切局部化,继续拉伸会在与加载轴大约成45°方向上形成剪切带。剪切带的形成与剪切转变区的分布和局部化有关,带缺口比不带缺口的试样会更早出现应变局部化,即在较低的拉伸应变下便形成剪切带,其拉伸强度也相应较低。相同条件下,一侧带缺口与两侧带缺口的试样在拉伸强度上几乎相同,但两侧带缺口试样的应变局部化程度稍低,主要是两侧缺口处均出现了剪切转变区,导致其分布和局部化不够集中,这也是形成主剪切带和次剪切带的主要原因。以上结果为进一步从原子尺度理解金属玻璃剪切带的形成和演化特征提供了重要的信息。     

孪晶诱导塑性钢的非均匀形变研究

通过原位电子背散射衍射(EBSD, Electron backscatter diffraction)和数字图像相关(DIC, Digital image correlation)技术,对孪晶诱导塑性(TWIP, Twinning induced plasticity)钢拉伸过程中的非均匀形变进行研究。结果发现,拉伸应变不均匀,应变集中于高角度晶界与大晶粒内部。滑移带的开启并不完全遵从斯密特(Schmid)定律,且滑移带与晶界作用明显。非均匀变形导致晶粒的取向梯度增大。研究表明,TWIP钢在拉伸过程中,出现明显非均匀形变现象,非均匀程度随着变形的增大而降低,且降低速率逐渐减小,应变超过12%后非均匀程度趋于稳定,但其值相对较大导致TWIP钢在应变较大时(15%),非均匀变形现象依然明显。滑移带容易在高角度晶界处产生集中,在低角度的晶界处穿过晶界。晶粒内各区域不同的变形量造成晶格朝不同方向旋转,或者朝同一方向旋转不同的角度而形成较大的取向差,且随着应变的增加而增加。非均匀变形主要是由斯密特因子(Schmid factor, SF)的大小、晶粒内滑移带可滑移的长度、滑移带间的相互作用和晶界对滑移带阻碍强度等因素共同影响。     

带裂纹板单向拉伸极限承载力有限元研究

一般金属材料的板式构件,在其使用的过程中有时会出现裂纹,而构件出现裂纹后的承载力问题是人们比较关心的.由于金属材料一般为韧性材料,在极限承载力计算中有两种模型,一种是弹塑性屈服模型,一种是弹塑性断裂力学模型.本文利用通用软件分别采用弹塑性和基于扩展有限元的裂纹扩展分析模型对单向拉伸的带裂纹板进行数值模拟,并将数值模拟的极限承载力与试验结果进行比较;结果表明,裂纹扩展分析模型计算出的极限承载力与实验结果更吻合;在此基础上,本文对裂纹角度不同的单向拉伸带裂纹板进行分析,得到其极限承载力跟裂纹与拉伸方向夹角的关系,其趋势与理论分析结果吻合.     

润滑条件下激光加工纹理的摩擦磨损

采用YAG纳秒激光器在45#钢平板试样表面制备纹理,考察了不同润滑状态下表面纹理对摩擦学性能的影响.45#钢表面纹理采用了不同间距的点、槽和网格形貌,并在UMT-Ⅱ试验机上与SiC钢球配副,分别在石蜡和机油润滑条件下进行往复摩擦试验,得出了试验条件下的Stribeck曲线.结果表明,在Hersey数较小时,纹理的引入不利于材料摩擦系数的降低,Hersey数的增加有助于摩擦学性能的提高.网格状纹理间距越小摩擦系数越大.网格状形貌试样相对于点和凹槽形貌试样,表现出更好的摩擦学性能.     

材料超高温动态拉伸SHTB实验方法的有效性分析

针对高温拉伸分离式Hopkinson杆实验技术,通过数值模拟、实验验证以及几种典型材料的高温动态拉伸性能测试相结合的方法,对此实验技术中存在的几个关键问题进行了深入研究。结果表明:对于平板状钩挂式拉伸试样,通过标距段尺寸优化后,应力分布均匀,流动应力曲线与螺纹拉伸试样一致,且应力上升段后没有剧烈跳动;通过精确气动控制,在加载脉冲到来同时,可实现有效的试样快速同步组装和加载;当试样温度为1 200 ℃时,在冷加载杆与高温试样接触以及应力波加载试样的整个过程中,试样平均温度下降约1.3%,而加载杆端温升低于180 ℃。为了验证此实验技术,对3D打印TC4、镍基单晶高温合金DD6进行了最高温度约1 200 ℃时的高温动态拉伸力学性能实验测试。     

玻璃纤维增强聚酯复合材料断裂性能的实验研究

研究纤维增强树脂复合材料的断裂韧度,通常采用弹性断裂力学的原理和方法。在研究中对复合材料的断裂韧度是否会受到试样类型和试样厚度的影响这个问题,有着不同的结论。例如文献[7]-[9]指出断裂韧度值是与试样的类型有关的。文献[8、9]还表明试样的厚度对断裂韧度值也有影响。澄清这个问题需要作深入细致的试验研究。本文采用玻璃纤维增强聚酯树脂复合材料的三点弯曲试样、单边裂缝和双边裂缝单向拉伸试样来探讨试样类型对断裂韧度的影响;同时还采用几种不同厚度的单边裂缝单向拉伸试样来探讨试样的厚度对断裂韧度的影响。     

MEASUREMENT OF LÜDERS BAND IN SMALL SIZE LOW CARBON STEEL SPECIMEN BY 3D DIGITAL IMAGE CORRELATION METHOD

吕德斯效应是多种金属和合金材料由于屈服阶段的不均匀变形而在材料表面产生条带状褶皱的现象,它会使冲压件表面质量降低. 为了防止它的出现,对吕德斯效应进行研究变得非常重要. 采用小视场(15mm×15 mm)下三维数字图像相关方法对小尺寸低碳钢试件在单轴拉伸载荷作用下的变形场进行测量,实际观测了小尺寸试件的吕德斯效应,结合理论模型解释了其形成机理,并分析了吕德斯带传播过程中应变及应变率的变化规律.实验研究表明,运用三维数字图像相关方法测量试件表面变形场,实现了对小尺寸低碳钢试件的吕德斯带演化过程以及颈缩、断裂等细观力学行为的观测,该方法是研究材料变形细观机理的一种有效测量手段.     

Lüders bands propagation of 1045 steel under multiaxial stress state

Inhomogeneous plastic deformation of 1045 steel under monotonic loading was experimentally studied. Thin-walled tubular specimens were used in the experiments and custom-made small strain gages were bonded on the specimen surface to characterize the local deformation. Experiments were conducted under tension, torsion, and combined tension–torsion. During the propagation of Lüders bands, the local deformation experienced two-stage deformation: an abrupt plastic deformation stage followed by a slower deformation process. In some area of the gage section of the specimen, a small amount of initial plastic deformation occurred before the Lüders front reached. During the propagation of Lüders bands, multiple Lüders fronts can be formed. Under tension, torsion, and combined tension–torsion with a constant axial load, the Lüders front was approximately parallel to the material plane of maximum shear stress. When the combined axial-torsion followed a proportional fashion, the stress–extensometer strain responses were dependent on the axial/torsional loading ratio, and the Lüders fronts were oriented differently and propagated along the specimen axis at a different velocity. The local strain was inhomogeneous even at the work-hardening stage. The relationships between the equivalent stress and the equivalent plastic strain were found to be practically identical for all the loading cases studied.     

On the effect of Lüders bands on the bending of steel tubes. Part II: Analysis

Part II of this study presents a modeling framework that is shown to successfully simulate all aspects of the inhomogeneous bending of tubes associated with Lüders banding reported in Part I. The structure is discretized with solid finite elements using a mesh that is fine enough for Lüders bands to develop and evolve. The material is modeled as a finitely deforming, J₂ type, elastic–plastic solid with an “up–down–up” response over the extent of the Lüders strain, followed by hardening. Regularization of the solution was accomplished by introducing a mild rate dependence of the material. Simulation of the rotation controlled bending experiments confirmed most of the experimental observations and revealed additional details of the localization. Thus, the initial uniform-curvature elastic regime terminates with the nucleation of localized banded deformation on the tensioned and compressed sides of the tube. The bands appear in pockets that propagate into the hitherto intact part of the structure while the moment remains essentially unchanged. The tube develops two curvature regimes; a relatively high curvature in the Lüders deformed section and a low curvature in the unaffected one. Simultaneously, the plasticized zone develops higher ovalization and wrinkles with a wavelength that corresponds to the periodicity of the banded pockets. For tubes with lower D/t and/or shorter Lüders strain the higher curvature eventually spreads to the whole structure at which point homogenous bending resumes. For tubes with higher D/t and/or longer Lüders strain the localized curvature, ovalization, and wrinkle amplitude are larger and cannot be sustained; the tube collapses prematurely leaving behind part of its length essentially undeformed. For every tube D/t there exists a threshold of Lüders strain separating the two types of behavior. This bounding value of Lüders strain was studied parametrically.     

On the effect of Lüders bands on the bending of steel tubes. Part I: Experiments

In several practical applications hot-finished steel pipe that exhibits Lüders bands is bent to strains of 2–3%. Lüders banding is a material instability that leads to inhomogeneous plastic deformation in the range of 1–4%. This work investigates the influence of Lüders banding on the inelastic response and stability of tubes under rotation controlled pure bending. Part I presents the results of an experimental study involving tubes of several diameter-to-thickness ratios in the range of 33.2–14.7 and Lüders strains of 1.8–2.7%. In all cases the initial elastic regime terminates at a local moment maximum and the local nucleation of narrow angled Lüders bands of higher strain on the tension and compression sides of the tube. As the rotation continues the bands multiply and spread axially causing the affected zone to bend to a higher curvature while the rest of the tube is still at the curvature corresponding to the initial moment maximum. With further rotation of the ends the higher curvature zone(s) gradually spreads while the moment remains essentially unchanged. For relatively low D/t tubes and/or short Lüders strains, the whole tube eventually is deformed to the higher curvature entering the usual hardening regime. Subsequently it continues to deform uniformly until the usual limit moment instability is reached. For high D/t tubes and/or materials with longer Lüders strains, the propagation of the larger curvature is interrupted by collapse when a critical length is Lüders deformed leaving behind part of the structure essentially undeformed. The higher the D/t and/or the longer the Lüders strain is, the shorter the critical length. Part II presents a numerical modeling framework for simulating this behavior.     

Strain Localization in Mild (Low Carbon) Steel Observed by Acoustic Emission - ESPI Coupling during Tensile Test

Two strain localization modes: the Piobert-Lüders band propagation and the development of necking, were investigated in uniaxial tensile tests for a low alloyed and low carbon steel. These two macroscopic localization phenomena were simultaneously monitored by speckle interferometry (ESPI) and acoustic emission (AE). The coupling of these two experimental techniques gives complementary information about strain localization features and mechanisms. For Lüders bands, it was found that the acoustic activity heard during the travel of the Piobert-Lüders band varies in closely correlated to the tensile force fluctuations, the relations between strain rate, band velocity, band width and plastic strain were investigated. Although the strain rate in the wake of the wave front is not always zero, the acoustic activity remains concentrated in the wave front itself. For necking, the acoustic activity is found to decrease regularly through the homogeneous plasticity stage and the diffuse necking stage and then increases when the localized necking starts, while ESPI patterns show a gradual strain concentration.     

Localization and propagation of curvature under pure bending in steel tubes with Lüders bands

The paper examines the plastic bending of steel tubes exhibiting Lüders bands through a combination of experiments and analyses. In pure bending experiments on tubes with diameter-to-thickness ratio of 18.8 tested under end-rotation control, following the elastic regime the moment initially traced a somewhat ragged plateau. At the beginning of the plateau Lüders bands appeared on the tension and compression sides of the cross section and simultaneously the curvature localized in one or two short zones while the rest of the tube maintained a much lower curvature. As the rotation of the ends was increased, one of the higher curvature zones spread at a nearly steady rate, affecting an increasingly larger part of the tube. When the whole tube was deformed to the higher curvature, the moment started to gradually increase while the tube deformed uniformly. A moment maximum was eventually attained and the structure failed by localized diffuse ovalization without any apparent effect from the initial Lüders bands-induced propagating instability. The problem was analyzed using 3D finite elements with a fine mesh. The material was modeled as an elastic–plastic solid with an up–down–up response over the extent of the Lüders strain, followed by hardening. The calculated response reproduced all major structural events observed experimentally including the initiation of the Lüders deformation, the moment plateau that followed, its extent, and the curvature localization and propagation associated with it. As in the experiments, once the high curvature extended over the whole tube length, the response of the tube became stable and the curvature uniform. With further bending the increasing ovalization induced a limit moment at a very high curvature.     

Buckling of steel bars with Lüders bands

Experiments and simulations are presented for the study of interaction between material and structural instabilities that occur in mild steel bars under axial compression. The material instability consists of Lüders bands that nucleate and propagate along the specimens. The structural instability involves lateral deflections of the bar leading to collapse. The study includes an investigation of bars of several different lengths. The mechanical response in the experiments were monitored through measurements of axial load, axial and midspan lateral displacements, and full field imaging of a brittle coating showing the Lüders deformation. Interesting interactions exist between the localized deformation due to the material-level instabilities and the global collapse of the bars. Finite element simulations, using a constitutive model with a nonmonotonic stress–strain behavior, showed good agreement with the experiments and helped to explain the variety of collapse modes seen in the experiments. The symmetry of imperfections and/or loading misalignments have dramatic effects on the evolution of Lüders deformation and the eventual collapse mode. Certain imperfections lead to deformation modes that delay structural collapse.     

Piobert–Lüders plateau and Portevin–Le Chatelier effect in an Al–Mg alloy in simple shear

The jerky flow in an Al–Mg alloy is studied during simple shear tests at room temperature and various strain rates. Direct observations of the sample surface using digital image correlation allow the study of the type and the dynamics of bands associated to plastic instabilities as a function of shear strain and shear strain rate. The paper features that both Piobert–Lüders and Portevin–Le Chatelier phenomena can be observed for a simple shear stress state at room temperature. The nucleation, growth and movement of the bands are described: it is shown that the kinematics of the bands is similar to those observed in tension but that the orientation of the bands varies with the shear strain.     

Heat sources,energy storage and dissipation in high-strength steels: Experiments and modelling

Tensile tests on three high-strength steels exhibiting Lüders band propagation are carried out at room temperature and under quasi-static loading conditions. Displacement and temperature fields on the surface of the flat samples are measured by digital image correlation and digital infrared thermography, respectively. The true stress versus true strain curves were calculated from the displacement data, while the thermal data were used to estimate the heat sources using the local heat diffusion equation. Based on these measurements the stored and dissipated energies were estimated up to diffuse necking. A thermodynamically consistent elastic-plastic constitutive model including the von Mises yield criterion, the associated flow rule and two non-linear isotropic hardening variables is applied to describe the behaviour of the high-strength steels. It is shown that this simple model is able to reproduce both the local behaviour, such as the power associated to heat sources, and the global behaviour, such as Lüders band propagation and stored and dissipated energies. It is further shown that the ratio of dissipated power to plastic power varies during plastic straining and that this variation is captured reasonably well in the numerical simulations.