FeCrAl合金材料织构对其宏观力学本构关系的影响研究

FeCrAl合金具有优良的高温抗氧化性和耐辐照性能,是事故容错核燃料包壳的重要候选材料. 其在加工过程和热处理过程中易形成α纤维织构（<110>//RD）和γ纤维织构（<111>//ND）,会影响材料的宏观力学性能与深加工成形能力. 本研究针对具有不同织构的多晶FeCrAl合金,建立了代表性体元模型, 使用晶体塑性有限元方法,在ABAQUS/Explicit中模拟材料单轴加载下的宏观应力应变曲线,分析不同织构对FeCrAl合金宏观力学本构关系的影响. 计算结果表明,对于具有α织构、γ织构和晶粒无择优取向的材料,在轧向上的应力应变曲线差异较小. γ织构会引起材料强烈的各向异性,在轧面法向上的屈服强度远高于轧向和横向上的屈服强度,这是因为晶粒的<111>方向平行于加载方向,滑移系难以启动. 提高γ纤维织构的比例,将增大轧面法向上的屈服强度. 本研究可以为优化FeCrAl合金材料织构、加工条件和材料力学性能提供参考.     

猪后腿肌肉的冲击压缩特性实验

波形整形技术使试样满足应力均匀条件和恒定应变率加载,石英片检测试样两端的应力均匀性, 获得微弱的透射信号。对开始加载阶段径向惯性效应带来的轴向应力附加值给予了计算修正,得到了肌肉在 不同加载方向和不同应变率下的应力应变曲线。结果发现:肌肉材料的应力应变曲线呈现粘弹性材料所具有 的凹向上特征,由最小显著差数法进行统计分析,发现猪后腿肌肉的力学性能对应变率和加载方向都敏感。 沿纤维方向压缩时肌纤维易发生压缩失稳,强度比垂直纤维方向低。     

梯度多孔材料单轴压缩弹塑性变形的宏微观数值分析

针对闭孔的密度梯度多孔材料,建立含球形孔洞的三维数值分析模型,研究其单轴压缩力学行为。首先,研究密度梯度对多孔材料宏观力学行为(如弹性模量和屈服强度)的影响;其次,研究密度梯度与材料局部力学性能的关系,得到了沿梯度方向弹性模量和屈服强度的分布规律;最后,讨论梯度多孔材料单轴压缩变形局部化机制。结果表明:当梯度材料与均质材料的总体相对密度相同时,梯度材料的宏观弹性模量和屈服强度均低于均质材料水平,其宏观应力-应变关系曲线降低;梯度多孔材料沿梯度方向的力学性能发生急剧变化,等效弹性模量沿梯度方向呈线性分布,屈服强度呈非线性曲线分布,导致沿梯度方向应力、应变呈现高度的不均匀性;多孔材料的变形局部化产生于孔隙率较大的薄弱位置,再逐渐向孔隙率较小的位置发展。由此可知,孔隙率的梯度变化影响多孔材料的力学性能,通过改变孔隙率的分布可实现材料预期的力学性质。     

近片层γ-TiAl基合金宏观屈服的微观表征

将近片层γ-TiAl基合金视为以等轴γ颗粒为基体,PST颗粒为夹杂的两相复合材料,基于细观力学自洽理论,对合金的有效弹性模量及基体和夹杂中的应力和应变场进行了解析分析计算,并结合细观力学的宏细观关联方法,确定了近片层γ-TiAl基合金的宏观屈服的微观表征.结果表明:夹杂颗粒中的应力和应变场与外载及夹杂的体积分数f和椭球长细比ρ有关,软取向PST夹杂颗粒的微变形屈服导致近片层γ-TiAl基合金材料的整体宏观屈服.     

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。     

基于晶体塑性理论研究铝材料高压高应变率下的强度特性

为了了解金属材料在极端加载下复杂动态响应过程中的多种机制和效应,重点针对Al材料在高压、高应变率加载下的塑性变形机制,在经典晶体塑性模型的基础上,对其中的非线性弹性、位错动力学和硬化形式进行改进,建立适用于高压、高应变率加载下的热弹-黏塑性晶体塑性模型。该模型可以较好地描述单晶铝和多晶铝材料屈服强度随压力的变化过程,相比宏观模型,用该模型还获得了多晶Al材料在冲击加载下的织构演化规律,揭示了织构择优取向行为和压力的关系。     

三种加载方向下云杉静动态力学性能研究

利用INSTRON和Hopkinson压杆对含水率为12.72%, 密度为413kg/m^{3}云杉木材试件沿顺纹、横纹径向和横纹弦向进行准静态和动态压缩实验, 获得了云杉木材3个方向的抗压模量、准静态压缩应力应变曲线和3种应变率下的动态应力应变曲线. 结果表明云杉木材沿顺纹方向加载破坏形式表现为木材纤维轴向屈曲、褶皱; 横纹径向和弦向加载失效行为表现为木材纤维间的滑移破坏. 云杉顺纹方向抗压弹性模量最大, 分别约为横纹径向抗压弹性模量的21倍和横纹弦向抗压弹性模量的32倍; 横纹径向和弦向准静态压缩屈服应力基本相等, 试件沿顺纹方向准静态压缩屈服应力约为横纹径向和弦向屈服应力的9倍; 动态压缩屈服强度具有率敏感性, 在应变率为500-1000s^{-1}动态压缩实验中顺纹、横纹径向和弦向动压屈服强度均随着应变率的增加而显著提高. 同时对不同方向压缩下木材胞壁失效行为进行了理论分析, 表明产生完全压缩失效的平均极限载荷与胞壁屈服强度、胞元结构和产生的褶皱长度相关.      

软化Mohr-Coulomb材料强度参数的测定方法

为了得到试件的粘聚力和内摩擦角随轴向塑性压应变变化的曲线提出本方法。试件的弹塑性本构关系遵循相关联的Mohr-Coulomb强度准则;对常规三轴试验,试件受力进入塑性状态后,处在棱椎状屈服面的棱上,加载过程遵循Koiter流动法则。按经典塑性力学理论,推导得到轴向塑性压应变与轴向应力与轴向应变的关系;在常规三轴试验机上获得不同围压下试件的全程应力-应变曲线,进而可得到各自围压下轴向塑性压应变随加载过程的变化曲线;把来自不同围压下对应同一轴向塑性压应变的应力分别代入屈服面方程,即可求得对应的粘聚力和内摩擦角。结果表明,Mohr-Coulomb材料的两个强度参数的变化由轴向塑性压应变确定。轴向塑性压应变可以作为塑性变形的状态参数,它和试件的受力过程可以唯一确定试件的变形过程。     

聚氨酯泡沫塑料在应力波加载下的压缩力学性能研究

通过ＳＨＰＢ冲击实验装置研究了硬质聚氨酯泡沫塑料在应力波加载下的动态力学性能，得到了泡沫塑料在较高应变率下的应力－应变曲线；确定了泡沫塑料的动态屈服强度和动态弹性模量等力学参数，并同落锤冲击实验及准静态压缩实验的结果进行了比较。     

大理岩试样循环加载强化作用的试验研究

利用不同晶粒大理岩块加工成完整和含孔道的圆柱试样,在伺服实验机上分别进行单轴和常规三轴循环加载试验,其中三轴循环加载包括维持围压恒定和卸载围压两种情况.结果表明,循环加载确实可使试样的强度增加,强度大致提高5%-10%.岩石的材料强度和承载能力是两个不同的概念.大理岩晶粒之间多为裂隙,轴向压缩加载时晶粒局部接触应力远高于名义应力,在岩样未达到峰值应力之前,接触处已经产生很大的变形乃至出现局部破坏,形成的碎屑在卸载时可以脱落充填到附近的空隙,提高岩样承载能力.这种强化特征是与摩擦相关的承载能力,并非材料强度;与多次循环加载造成材料强度逐步劣化不同.从卸载点附近卸载与再加载应力应变曲线的关系,可以确认卸载是否引起岩石承载能力的增加.     

Incorporation of twinning into a crystal plasticity finite element model: Evolution of lattice strains and texture in Zircaloy-2

A crystal plasticity finite element code is developed to model lattice strains and texture evolution of HCP crystals. The code is implemented to model elastic and plastic deformation considering slip and twinning based plastic deformation. The model accounts for twinning reorientation and growth. Twinning, as well as slip, is considered to follow a rate dependent formulation. The results of the simulations are compared to previously published in situ neutron diffraction data. Experimental results of the evolution of the texture and lattice strains under uniaxial tension/compression loading along the rolling, transverse, and normal direction of a piece of rolled Zircaloy-2 are compared with model predictions. The rate dependent formulation introduced is capable of correctly capturing the influence of slip and twinning deformation on lattice strains as well as texture evolution.     

Mechanical response and texture evolution of AZ31 alloy at large strains for different strain rates and temperatures

In order to study the behavior of material under finite deformation at various strain rates, the responses of AZ31 Mg sheet are measured under uniaxial (tension and compression) and multiaxial (simple shear) loadings along rolling direction (RD), 45° to rolling direction (DD), 90° to rolling direction (TD), and normal to the sheet (ND) to large strains. The material exhibits positive strain rate sensitivity (SRS) at room and elevated temperatures; the SRS is more pronounced at high temperatures and lower strain rates. The r-value of the material under tensile loading at room temperatures is higher in TD at lower strain rate. Texture measurements on several failed specimens are reported under tension and simple shear after finite plastic deformation of about 20% equivalent strain. The as-received material exhibits a strong fiber with equal fractions of grains having the c-axis slightly tilted away from the sheet normal towards both +RD and −RD. Pole figures obtained after tensile loading along the rolling direction (RD) show that the texture of the material strengthens even at low strains, with c-axis perpendicular to the sheet plane and prism planes lining up in a majority of grains. However, the tensile loading axis along TD does not lead to similar texture strengthening; the c-axis distribution appears to be virtually unchanged from the virgin state. The pole figures obtained after in-plane compression along RD brings the c-axes of the grains parallel to the loading direction. The pole figures after simple shear loading show that the c-axis rotates to lie on the sheet plane consistent with a compression axis 45° away on the sheet plane.     

Continuum modeling of the response of a Mg alloy AZ31 rolled sheet during uniaxial deformation

Lightweight magnesium alloys, such as AZ31, constitute alternative materials of interest for many industrial sectors such as the transport industry. For instance, reducing vehicle weight and thus fuel consumption can actively benefit the global efforts of the current environmental industry policies. To this end, several research groups are focusing their experimental efforts on the development of advanced Mg alloys. However, comparatively little computational work has been oriented towards the simulation of the micromechanisms underlying the deformation of these metals. Among them, the model developed by Staroselsky and Anand [Staroselsky, A., Anand, L., 2003. A constitutive model for HCP materials deforming by slip and twinning: application to magnesium alloy AZ31B. International Journal of Plasticity 19 (10), 1843–1864] successfully captured some of the intrinsic features of deformation in Magnesium alloys. Nevertheless, some deformation micromechanisms, such as cross-hardening between slip and twin systems, have been either simplified or disregarded. In this work, we propose the development of a crystal plasticity continuum model aimed at fully describing the intrinsic deformation mechanisms between slip and twin systems. In order to calibrate and validate the proposed model, an experimental campaign consisting of a set of quasi-static compression tests at room temperature along the rolling and normal directions of a polycrystalline AZ31 rolled sheet, as well as an analysis of the crystallographic texture at different stages of deformation, has been carried out. The model is then exploited by investigating stress and strain fields, texture evolution, and slip and twin activities during deformation. The flexibility of the overall model is ultimately demonstrated by casting light on an experimental controversy on the role of the pyramidal slip 〈c + a〉 versus compression twinning in the late stage of polycrystalline deformation, and a failure criterion related to basal slip activity is proposed.     

镁合金晶体塑性本构模型与非均匀孪生变形分析

微结构演化对镁合金材料力学性能有着显著的影响,为了揭示镁合金宏观塑性各向异性特性与非均匀孪生变形的关系,开展了不同路径下的单轴加载试验以及采用含滑移、孪生机制的晶体塑性本构模型对试验条件下的镁合金变形行为进行数值模拟研究.文中本构模型描述了滑移与孪生变形机制以及晶格转动的机制,同时研究采用三维微结构代表性有限元模型,其包含晶粒尺寸、晶向和晶界倾角等微结构参数.研究结果表明,轧制镁合金具有强烈的宏观塑性各向异性行为,并对这种镁合金塑性各向异性行为的模拟结果以及多晶织构的模拟演化结果与试验测量进行对比,结果都基本吻合.对孪生非均匀变形模拟分析表明,镁合金宏观塑性各向异性行为与滑移、孪生变形机制的不同启动组合紧密相关,同时多晶体内应力的非均匀分布受到孪生变形的严重影响.而不同晶粒尺寸的晶粒所发生的孪生变形有比较大的差异,造成孪晶变体在晶粒内的分布极不均匀.该研究可为通过微结构的合理配置来设计和控制材料的力学性能提供理论依据.     

考虑晶体滑移面分解正应力的细观损伤模型

材料内部的解理、滑移面剥离等细观损伤是引起宏观失效的根源,从细观尺度研究损伤的发生和发展有助于深入认识材料的变形和失效过程.本文基于晶体塑性理论,从滑移系的受力和变形出发研究材料的细观损伤,建立了考虑滑移面分解正应力的细观损伤模型,为晶体材料解理断裂的分析提供了新方法.首先,在晶体弹塑性变形构型的基础上引入损伤变形梯度...     

Deformation texture prediction: from the Taylor model to the advanced Lamel model

The paper reports on a recent effort to develop a statistical (or Monte-Carlo) model for quantitative deformation texture prediction which is yet fast enough for implementation in every Gauss point of an FE simulation of a metal-forming process. The principles of Taylor-type models for the prediction of deformation textures of polycrystalline materials are reminded. This includes the full-constraints Taylor theory (every grain of a polycrystal undergoes the same plastic deformation), classical Relaxed Constraints Taylor theory (one or two of the components of the local velocity gradient tensor need not be the same for all grains) and multi-grain models (LAMEL model; mentioning of GIA model). The primal–dual structure of the equations relating strain rates with slip rates, and those relating stresses and resolved shear stresses on slip systems, is made clear. It is then possible to describe the basic philosophy and the mathematical implementation of a new model, called “advanced Lamel model” (ALAMEL). This model is more generally applicable than the previously developed LAMEL model, which is only valid for rolling. Both take interactions between neighbouring grains into account. Finally, quantitative comparisons are given between experimentally observed rolling textures and the predictions of the new model, as well as of other models: full-constraints and relaxed constraints Taylor, LAMEL, GIA, visco-plastic self-consistent and crystal plasticity finite element (CPFEM) models. This was done for IF steel (one thickness reduction) and for two aluminium alloys: AA1200 (five thickness reductions) and AA5182 (one thickness reduction). It was found that for AA1200, the new model is on average the best; for the two other cases, it is among the best models, but the LAMEL or CPFEM models are better. These results suggest that in spite of all simplifications, the ALAMEL model captures (and identifies) the domination mechanisms controlling the development of deformation textures in cubic metals.     

沿指定应力路径加载的有限元RVE模型研究

在细观尺度上建立能反映材料微观组织结构又能反映统计意义上宏观力学性能的代表性体积单元（Representative Volume Element, RVE）,对其进行复杂加载下的数值研究,是目前预测材料宏观力学性能较有效的方法。本文从理论上分析并提供了对正六面体RVE在任意应力状态及任意应力路径下加载及宏观应力、应变计算的方法,用有限元软件ABAQUS实现了数值计算过程,并用此方法对循环加载下缺口圆棒颈部中心和边缘位置进行了RVE分析。结果表明：（1）此方法能准确的控制并实现正六面体RVE在任意应力状态及应力状态路径下加载;（2）通过RVE分析,可用于复杂加载下试样局部细观结构变化的研究。     

Modeling the evolution of anisotropy in Al–Li alloys: application to Al–Li 2090-T8E41

It is widely reported in current literature that the precipitation hardened Al–Li sheet alloys exhibit extremely high anisotropy in yield (and ultimate tensile) strength, which is well beyond what can be explained as purely a consequence of the strong crystallographic texture in the material (e.g. J. Mater. Sci. Eng. A265, 1999, 100). This paper presents a crystal plasticity based modeling framework that will (i) facilitate the segregation of the contributions to the overall anisotropy from crystallographic texture and precipitation hardening, and (ii) correlate the contribution from precipitate hardening to either co-planar slip activity or the non-coplanar slip activity in the cold-working step prior to the aging heat treatment. More specifically, a Taylor-type (fully-constrained) crystal plasticity model was formulated to predict the yield strength of the fully processed sheet and its anisotropy, while accounting for the initial texture in the hot-worked sheet, its evolution during the cold-working step prior to aging, and the inhomogeneous nucleation of the T₁ phase platelets (these are known to form on {111} planes, but not usually in equal amounts on the different {111} planes in a given crystal). In an effort to illustrate the methodology developed in the study, a limited set of experiments was conducted on Al–Li 2090-T8E41 alloy sheet. Off-axis stretches were applied on the sheet at room temperature prior to the aging treatment, and the mechanical anisotropy in the fully processed sheets was characterized by performing tension tests on coupons cut from the sheet at 0, 30, 45, 60 and 90° to the original rolling direction (RD). Both the initial texture in the sheet and its evolution during the different off-axis stretches were characterized. The alloys processed in this study showed pronounced anisotropy. The application of the methodology developed in this study revealed that much of the observed anisotropy in this particular data set could be explained by accounting for the texture in the sample in the processed condition. Although the data set available was inadequate to establish clear correlations of the anisotropy with preferential hardening mechanisms arising from either co-planar or non-co-planar slip activity during the off-axis stretch, there were indications favoring the latter. This case study, however, illustrates the application of the methodology developed in this study to obtain better insight into the nature of the anisotropy in these sheets and its physical origin.     

Rate-independent crystalline and polycrystalline plasticity,application to FCC materials

This paper deals with the simulation of the mechanical response and texture evolution of cubic crystals and polycrystals for a rate-independent elastic–plastic constitutive law. No viscous effects are considered. An algorithm is introduced to treat the difficult case of multi-surface plasticity. This algorithm allows the computation of the mechanical response of a single crystal. The corresponding yield surface is made of the intersection of several hyper-planes in the stress space. The problem of the multiplicity of the slip systems is solved thanks to a pseudo-inversion method. Self and latent hardening are taken into account. In order to compute the response of a polycrystal, a Taylor homogenization scheme is used. The stress–strain response of single crystals and polycrystals is computed for various loading cases. The texture evolution predicted for compression, plane strain compression and simple shear are compared with the results given by a visco-plastic polycrystalline model.