摩擦系数对钛合金等通道转角挤压影响的有限元分析

以Ti-6Al-4V钛合金高温变形行为研究为基础,建立了等通道转角挤压（ECAE）的三维模型,运用DEFORM-3D有限元分析软件模拟了600 ℃等温条件下不同摩擦系数对Ti-6Al-4V合金ECAE过程中的温度场,等效应力,等效应变以及等效应变率的影响。结果表明：核心高温区以及核心应力区主要集中于转角处;随着摩擦系数增大,核心高温区面积增大,转角入口区的应力也有所增加;点迹跟踪结果表明各点应变均在经过转角处达到最大值,内角点及外角点处的变形较不稳定。     

岩石试件端面摩擦效应数值模拟研究

试件端面摩擦效应直接影响试件内的塑性等效应变、侧向位移的分布和单元应力应变曲线。本文运用ANSYS中的接触单元模拟了平面应变状态下端面摩擦效应对塑性等效应变、侧向位移和单元应力应变曲线的影响,得到了不同摩擦系数时塑性等效应变及侧向位移的渐进变化形式。当接触面摩擦较小时,塑性等效应变图案为上下两个X形网络,侧向位移上下分布均匀;当接触面摩擦增大时,塑性等效应变网络向中部靠拢并且明显增大,侧向位移上下分布不均匀,中部较上下端面位移大;当试件端面侧向位移被限制,即摩擦力很大时,塑性等效应变网络变为一个X形局部化带,侧向位移分布更加不均匀,中部明显隆起。     

双相钢板料的单向拉伸断裂失效研究(Ⅱ)——弧长法非线性有限元分析

为了获得docol800DP双相钢板料的等效塑性应变和应力三轴度在单向拉伸断裂失效过程中的变化历程,对其拉伸过程进行了弧长法非线性有限元分析。根据试验得到的材料参数建立了双相钢单向拉伸试件的有限元模型,考虑大变形引起的几何非线性和塑性强化引起的材料非线性,模拟了试件拉伸变形的全过程。计算结果表明:弧长法可较好地模拟试件变形的全程,计算得到的典型变形阶段、集中性失稳带的分布及方向与试验结果吻合很好;发生失稳变形时的应变与理论分析结果一致。最后,给出了起裂点处失效参数的变化历程,为定量化研究双相钢材料的断裂失效模型提供准确的数据支持。     

ECAP过程压头压力的分析——试验与模拟

结合试验与数值模拟探讨了等通道转角挤压（EqualChannelAngularPressing,简称ECAP）过程中影响压头压力的主要因素。通过试验考察了A、Ba、Bc、C等四种挤压路径下压头压力测试的统计平均分布特征和大小、挤压速度对于压头压力的影响、其它影响压头压力的因素;试验结果显示了不同路径、不同挤压道次对压头压力大小的影响及压力值的显著差别。另外,针对等通道转角挤压过程中模具的转角角度、转角半径、模具与挤压试样之间的摩擦条件等因素与压头中压力的关系,采用有限元方法进行了大应变数值模拟。模拟结果显示：较小的模具转角、较大的转角曲率、粗糙的接触表面会显著增加压头中压力;试验与模拟研究可为ECAP过程及类似加工过程的力学分析提供参考。     

膨胀环受力历史对应力应变关系的影响

利用两类实验装置开展了无氧铜TU1膨胀环实验研究，发现:电磁膨胀环在加载阶段，样品受体力作用，满足均匀变形的假定；而爆炸膨胀环在加载阶段，样品内壁受面力冲击作用，不满足均匀变形的假定。针对这个差异，发展了一种考虑冲击阶段变形不均匀性的新方法，利用回收样品几何变形，将冲击阶段试样环内轴向塑性应变、径向塑性应变纳入等效塑性应变的计算中，通过修正后的方法更准确地获得了材料的应力应变关系。     

钢柱抗爆响应分析单自由度模型适用性评估

为评估单自由度(SDOF)模型在结构抗爆设计中的适用性，分别采用SDOF模型和通用有限元软件ANSYS/LS-DYNA对简支钢柱承受爆炸荷载时的动力响应进行模拟；对比二者计算结果，并以有限元模拟为准，分析SDOF模型的适用范围。研究表明:可按照自由振动阶段SDOF模型位移结果的振幅大小，将其位移响应划分为有限变形阶段、临界阶段、失稳破坏阶段，有限变形阶段SDOF模型与有限元结果基本一致；截面高宽比、翼缘宽厚比对钢柱动力破坏形式有重要影响，高宽比越大、翼缘的宽厚比越小，越容易发生平面外弯扭失稳；在SDOF模型中通过假定塑性铰分布长度计算塑性阶段应变及应变率，采用随时间变化的应变率计算Cowper-Symonds本构关系中的应力放大系数是可行的。     

高分子材料平面应变拉伸变形局部化

将基于应变软化玻璃状高分子材料微观特征建立的ＢＰＡ８－链分子网络模型引入ＵｐｄａｔｉｎｇＬａｇｒａｎｇｅ有限元方法，建立了适于变形局部化分析的大变形弹塑性有限元驱动应力法．在此基础上，数值模拟了初始各向同性高分子材料平面应变拉伸变形局部化的传播过程．探讨了ＢＰＡ模型对具有加工硬化特性的结晶性高分子材料变形分析的适应性；分析了局部化传播过程中颈缩截面的非均匀应力三轴效应；最后，讨论了网格尺寸以及初始几何不均匀性对颈缩扩散以及应力三轴效应的影响     

Cr15Mn9Cu2Ni1N不锈钢的高温拉伸变形特性分析

利用热/力模拟试验机,对Cr15Mn9Cu2Ni1N不锈钢进行了950℃~1200℃高温范围内的拉伸试验;采用有限元方法对试样的均匀变形过程进行了分析。高温拉伸过程中,试样在达到最大应力后并不立即颈缩,而是还要经历一段宏观均匀变形后才颈缩。分析结果表明:最大应力之后,试样端部区域等效应力降低,横截面积收缩量减小,而中心区域横截面积收缩量增大,形成了潜在颈缩区;在应变速率敏感性的作用下,潜在颈缩区的变形抗力随应变速率的增大而增加,使变形不能在该区域集中,而转向其它位置,保持了试样的宏观均匀变形,且颈缩未在最大应力后立即发生;在高温拉伸条件下,材料应变速率敏感性的增大是颈缩延迟发生的主要原因,随着变形温度升高,应变速率敏感性增大,也使得试样颈缩前的均匀变形量增大。     

19Mn5钢循环硬化特性与累积等效微观应变分布的有限元分析

本文用改进的空间轴对称大应变有限元程序,对19Mn5钢在低周疲劳下的应力-应变滞后回线以及试样纵截面、横截面内累积等效应变分布和水静应力在纵截面内的分布进行了计算.结果表明:19Mn5钢铁素体内的累积塑性应变和水静应力均高于珠光体,这就从微观上证明了为什么19Mn5钢在低周疲劳下呈现循环硬化,同时为19Mn5钢的微观断裂机制提供了一个清晰的物理图象.     

晶体塑性变形离散滑移模型及有限元分析

基于韧性单晶体实验现象，建立了描述晶体塑性变形的离散滑移模型．该模型的主要特点是：晶体滑移变形在宏观上是不均匀的，滑移带的分布是离散的．利用晶体塑性理论对模型进行了有限变形有限元分析，计算结果揭示了晶体滑移的离散行为，模拟的应力 应变曲线与实验曲线相吻合     

Mixed mode (I and II) crack tip fields in bulk metallic glasses

Stationary crack tip fields in bulk metallic glasses under mixed mode (I and II) loading are studied through detailed finite element simulations assuming plane strain, small scale yielding conditions. The influence of internal friction or pressure sensitivity on the plastic zones, notch deformation, stress and plastic strain fields is examined for different mode mixities. Under mixed mode loading, the notch deforms into a shape such that one part of its surface sharpens while the other part blunts. Increase in mode II component of loading dramatically enhances the normalized plastic zone size, lowers the stresses but significantly elevates the plastic strain levels near the notch tip. Higher internal friction reduces the peak tangential stress but increases the plastic strain and stretching near the blunted part of the notch. The simulated shear bands are straight and extend over a long distance ahead of the notch tip under mode II dominant loading. The possible variations of fracture toughness with mode mixity corresponding to failure by brittle micro-cracking and ductile shear banding are predicted employing two simple fracture criteria. The salient results from finite element simulations are validated by comparison with those from mixed mode (I and II) fracture experiments on a Zr-based bulk metallic glass.     

The frictional sliding response of elasto-plastic materials in contact with a conical indenter

Over the past decade, many computational studies have explored the mechanics of normal indentation. Quantitative relationships have been well established between the load–displacement hysteresis response and material properties. By contrast, very few studies have investigated broad quantitative aspects of the effects of material properties, especially plastic deformation characteristics, on the frictional sliding response of metals and alloys. The response to instrumented, depth-sensing frictional sliding, hereafter referred to as a scratch test, could potentially be used for material characterization. In addition, it could reproduce a basic tribological event, such as asperity contact and deformation, at different length scales for the multi-scale modeling of wear processes. For these reasons, a comprehensive study was undertaken to investigate the effect of elasto-plastic properties, such as flow strength and strain hardening, on the response to steady-state frictional sliding. Dimensional analysis was used to define scaling variables and universal functions. The dependence of these functions on material properties was assessed through a detailed parametric study using the finite element method. The strain hardening exponent was found to have a greater influence on the scratch hardness and the pile-up height during frictional sliding than observed in frictionless normal indentation. When normalized by the penetration depth, the pile-up height can be up to three times larger in frictional sliding than in normal indentation. Furthermore, in contrast to normal indentation, sink-in is not observed during frictional sliding over the wide range of material properties examined. Finally, friction between indenter and indented material was introduced in the finite element model, and quantitative relationships were also established for the limited effects of plastic strain hardening and yield strength on the overall friction coefficient. Aspects of the predictions of computational simulations were compared with experiments on carefully selected metallic systems in which the plastic properties were systematically controlled. The level of accuracy of the predicted frictional response is also assessed by recourse to the finite element method and by comparison with experiment.     

Predicting failure modes and ductility of dual phase steels using plastic strain localization

Ductile failure of metals is often treated as the result of void nucleation, growth and coalescence. Various criteria have been proposed to capture this failure mechanism for various materials. In this study, ductile failure of dual phase steels is predicted in the form of plastic strain localization resulting from the incompatible deformation between the harder martensite phase and the softer ferrite matrix. Microstructure-level inhomogeneity serves as the initial imperfection triggering the instability in the form of plastic strain localization during the deformation process. Failure modes and ultimate ductility of two dual phase steels are analyzed using finite element analyses based on the actual steel microstructures. The plastic work hardening properties for the constituent phases are determined by the in-situ synchrotron-based high-energy X-ray diffraction technique. Under different loading conditions, different failure modes and ultimate ductility are predicted in the form of plastic strain localization. It is found that the local failure mode and ultimate ductility of dual phase steels are closely related to the stress state in the material. Under plane stress condition with free lateral boundary, one dominant shear band develops and leads to final failure of the material. However, if the lateral boundary is constrained, splitting failure perpendicular to the loading direction is predicted with much reduced ductility. On the other hand, under plane strain loading condition, commonly observed necking phenomenon is predicted which leads to the final failure of the material. These predictions are in reasonably good agreement with experimental observations.     

Three-dimensional elastic–plastic finite element analysis for orthogonal cutting with discontinuous chip of 6-4 brass

An elastic–plastic finite element model is developed for 3D orthogonal cutting of discontinuous chips. The tool is P20 while the workpiece is made of 6-4 brass. Examined under the condition of low cutting speed are the initial crack location, the direction of crack growth and variations of discrete chips. These predictions are made possible by application of the strain energy density (SED) theory. The initial crack was formed above the tool tip and grew progressively along the stationary values of the SED function until the trajectory intersects with the free surface. The plastic deformation and friction result in a high equivalent stress in the secondary deformation zone of the first longitudinal chip. Stresses are also high at the location of crack initiation. The chip node near the tool face is sensitive to the contact of the tool face. As more residual stress prevails after the first longitudinal cut, degradation of the workpiece surface prevails and should be accounted for.     

Hybrid experimental–numerical analysis of basic ductile fracture experiments for sheet metals

A basic ductile fracture testing program is carried out on specimens extracted from TRIP780 steel sheets including tensile specimens with a central hole and circular notches. In addition, equi-biaxial punch tests are performed. The surface strain fields are measured using two- and three-dimensional digital image correlation. Due to the localization of plastic deformation during the testing of the tensile specimens, finite element simulations are performed of each test to obtain the stress and strain histories at the material point where fracture initiates. Error estimates are made based on the differences between the predicted and measured local strains. The results from the testing of tensile specimens with a central hole as well as from punch tests show that equivalent strains of more than 0.8 can be achieved at approximately constant stress triaxialities to fracture of about 0.3 and 0.66, respectively. The error analysis demonstrates that both the equivalent plastic strain and the stress triaxiality are very sensitive to uncertainties in the experimental measurements and the numerical model assumptions. The results from computations with very fine solid element meshes agree well with the experiments when the strain hardening is identified from experiments up to very large strains.     

位移加载条件下微小尺寸单晶金属的应变突变模型及其间歇性塑性变形行为研究

微压缩实验发现，微小尺度单晶金属柱体在塑性变形过程中会发生显著的应变突变，呈现出特殊的间歇性塑性流动特征。本文以数百纳米直径的单晶Au柱体为研究对象，探讨其在位移加载条件下的间歇性流动行为。首先根据位移加载条件下的塑性变形特征，提出了分析其应变突变的三阶段模型。进一步结合经典晶体塑性理论框架的有限元方法，建立了以二阶功参量为基础的连续塑性力学模型。通过与实验结果相对比发现，新模型能够较好地描述位移加载条件下微小尺度面心立方单晶金属材料的应变突变现象，能够合理预测单晶柱体的特殊变形行为。此外，二阶功准则作为位移加载条件下应变突变现象的判据是有效的。进而使用该理论模型，探讨了微小金属柱体应变突变随机性、尺寸相关性以及率敏感性等问题。     

Crystal plasticity finite element modeling of mechanically induced martensitic transformation (MIMT) in metastable austenite

A new crystal plasticity model incorporating the mechanically induced martensitic transformation in metastable austenitic steel has been formulated and implemented into the finite element analysis. The kinetics of martensite transformation is modeled by taking into consideration of a nucleation-controlled phenomenon, where each potential martensitic variant based on Kurdjumov–Sachs (KS) relationship has different nucleation probability as a function of the interaction energy between externally applied stress and lattice deformation. Therefore, the transformed volume fractions are determined following selective variants given by the crystallographic orientation of austenitic matrix and applied stress in the frame of the crystal plasticity finite element. The developed finite element program is capable of considering the effect of volume change by the Bain deformation and the lattice-invariant shear during the martensitic transformation by effectively modifying the evolution of plastic deformation gradient of the conventional rate-dependent crystal plasticity finite element. The validation of the proposed model has been carried out by comparing with the experimentally measured data under simple loading conditions. Good agreements with the measurements for the stress–strain responses, transformed martensitic volume fractions and the influence of strain rate on the deformation behavior will enable the model to be promising for the future applications to the real forming process of the TRIP aided steel.