准脆性单晶材料动态压痕试验研究

本研究设计了由钛合金压杆与金刚石压头组成的动态压痕试验装置,研究准脆性单晶材料(冰糖和RDX含能晶体)等的高应变率响应。试验装置引入了动量缓冲块,可以避免压杆造成的多次压痕加载,对样品施加产生80μs脉冲宽度的动态压痕载荷。对比了多次与单次加载压痕加载的冰糖单晶压痕坑,其动态硬度值约为5.18MPa。RDX单晶在压痕载荷作用下非常容易发生断裂破碎,测得动态硬度值为1.304MPa。相比RDX单晶,冰糖单晶的塑性变形能力更好。用光学显微镜观察到了压痕坑的径向裂纹和侧边裂纹,试验研究给出了准脆性晶体在动态压痕载荷下的裂纹扩展模式。     

沸石分子筛单晶体弹塑性双线性本构关系的实验研究与有限元确定

为研究两种沸石分子筛方钠石SOD和镁碱沸石FER的力学性能,采用纳米压痕技术,测得随载荷连续变化的位移,得到载荷-位移曲线图.根据Olive算法,利用接触刚度连续测量技术,得到这两种沸石分子筛的硬度及弹性模量.基于弹塑性双线性本构关系假定,用ANSYS有限元程序模拟纳米压痕实验过程,利用搜索法得到沸石大单晶SOD和FER的双线性本构关系.     

单轴拉伸条件下脆性岩石微裂纹损伤模型研究

利用断裂力学、损伤力学和均匀化原理,对脆性岩石单轴拉伸条件下的力学特性进行分析,建立了脆性岩石的微裂纹损伤本构模型.首先对岩石内部微裂纹的统计分布规律进行分析,给出了理论分析过程中微裂纹分布的假设条件,在此基础上,参考已有研究成果,得到含细长微裂纹脆性岩石有效弹性参数的计算公式.然后,对岩石内部单一微裂纹进行断裂力学和损伤力学分析,得到了扩展裂纹尖端的应力强度因子计算公式,在一定微裂纹断裂扩展准则和断裂扩展速率的假设基础上,利用积分原理,得到了岩石整体的损伤变量和损伤演化方程,由此建立单轴拉伸条件下脆性岩石的微裂纹损伤本构模型.最后,通过一花岗岩的单轴拉伸试验结果对微裂纹损伤本构模型进行了验证.     

含微裂纹和椭球颗粒介质的强度及本构关系

针对含随机分布微裂纹及椭球颗粒的复合材料，通过考虑椭球颗粒内的本征应变及其与微裂纹的相互作用，利用等效夹杂方法研究了微裂纹损伤对材料有效模量和强度的影响，推导了复合材料的细观应力场及本构关系，并导出了材料破坏的临界条件．     

基于近场动力学理论的准脆性材料的本构力函数的构建

近场动力学理论(PD)是基于非局部思想的连续介质力学新理论,用于研究材料破坏问题。根据准脆性材料破坏的线性和非线性的力学行为,在初始微观弹脆性材料(PMB)的本构力函数中引入了键的损伤模型,将键的断裂过程分成了线性的弹性变形阶段和非线性的损伤变形阶段,以此构建了准脆性材料的本构力函数的基本形式。以典型的准脆性材料为例构建了其本构力函数,通过在压缩载荷下对含预制不同角度单裂纹缺陷的类岩材料的裂纹扩展进行PD数值模拟仿真,裂纹起裂位置和扩展方向与试样试验结果在一定程度上保持了一致,证明了该基于近场动力学理论的典型准脆性材料的本构力函数可用于该类材料的破坏分析。     

复合材料刚度性能表征的协同损伤力学模型

针对复合材料层合板的弥散型损伤,提出一个刚度性能表征的协同损伤力学模型. 该模型兼顾了微观物理损伤响应和宏观材料刚度性能表征. 从微观角度,建立细观RVE 模型求解裂纹表面张开位移和滑开位移,以此定义损伤张量,并在宏观上通过对材料应变和损伤表面位移进行均匀化处理,建立单向板或层合板的损伤刚度矩阵和损伤张量之间的联系. 以基体裂纹为例,详细分析并建立了横向裂纹和纵向裂纹的损伤本构. 计算了[±θ/90₄]_S 铺层层合板中基体横向裂纹对刚度性能的影响,结果表明该方法能够准确地预测复合材料层合板由损伤导致的刚度性能衰减.      

内爆炸载荷下圆管变形、损伤和破坏规律的研究

以物理统计和唯象研究相结合的方法,建立了微孔洞型的损伤函数模型和损伤演化方程,以变形热力学,Ｄｒｕｃｋｅｒ公设和内变量理论为基础建立了含损伤热塑性材料的增量型本构关系,用所建立的本构关系及损伤演化方程对内部爆轰作用下的圆管破坏过程完成了系列数值模拟,数值结果与实验结果相比较,在圆管变形过程,破坏时间,破坏速度,破坏应变,破坏应变率等信息方面都是基本一致的。     

方沸石大单晶及多晶粉饼力学性能的实验研究

报道了采用纳米压痕硬度计和Instron仪器分别对方沸石ANA大单晶和方沸石ANA多晶粉饼进行的力学性能测试工作。在纳米压痕实验中可实时测得连续载荷和随载荷连续变化而变化的位移,得到载荷-位移曲线图。根据W.C.O live算法,利用接触刚度连续测量(CSM)技术,实现了ANA沸石的硬度、弹性模量随压痕深度变化的连续测量,得到了ANA沸石单晶的硬度、弹性模量分别为0.25GPa和4GPa。用Instron仪器测得ANA多晶粉饼的弹性模量值为125MPa。对ANA沸石大单晶和ANA沸石多晶粉饼的力学性能进行了比较。     

考虑微裂纹相互作用的的岩石微观力学弹塑性损伤模型研究

本文建立基于微裂纹扩展的岩石弹塑性损伤微观力学模型。用自洽方法考虑裂隙间相互影响,压缩载荷下微裂纹尖端翼裂纹稳定扩展表征岩石的微观损伤,基于应变能密度准则用Newton迭代法求复合型断裂的翼裂纹扩展长度,并采用微裂隙统计的二参数Weibull函数模型反映绝对体积应变对微裂纹分布数目影响,进而用翼裂纹扩展所表征的应力释放体积和微裂纹数目来表示含有微裂隙的岩石损伤演化变量;宏观塑性屈服函数采用Voyiadjis等的等效塑性应变的硬化函数,反映塑性内变量对硬化函数的影响;建立岩石的弹塑性损伤本构关系及其数值算法,并用回映隐式积分算法编制了弹塑性损伤模型的程序。从围压和微裂隙长度等因素分析弹塑性损伤模型的岩石的损伤和宏观塑性特性。     

压缩载荷作用下岩石的细观损伤和塑性研究

建立岩石微裂纹扩展的细观力学模型,研究了岩石的细观损伤和塑性性质.压缩载荷下微裂纹尖端翼裂纹稳定扩展表征岩石的细观损伤,采用应变能密度准则求解复合型断裂的翼裂纹扩展长度,微裂隙统计的二参数Weibull函数模型反映绝对体积应变对微裂纹分布数目影响,进而用翼裂纹扩展所表征的应力释放体积和微裂纹数目来表示含有微裂隙的岩石损伤演化变量;宏观塑性屈服函数采用Voyiadjis等的等效塑性应变的硬化函数,反映了塑性内变量对硬化函数的影响;建立岩石模型的本构关系及其数值算法,并用回映隐式积分算法编制了模型的本构程序.分析弹塑性损伤模型的围压对岩石损伤的影响,并从围压和短微裂隙长度等因素分析模型的岩石的损伤和宏观塑性特性.     

延性单晶非均匀变形的三维有限元模拟

对延性单晶在拉伸载荷作用下的应变局域化和颈缩等非均匀变形过程进行了三维有限元数值模拟。将相关晶体塑性本构模型及一种新的数值积分方法补充到ABAQUS6.1商用有限元软件中。该方法的特点是，利用晶体塑性的动力学方程，获得一个关于晶体弹性变形梯度的演化方程，采用半隐式积分方案进行求解。本文推导出一种新的应力变本构矩阵。按此方式更新本构矩阵，计算速度和计算稳定性大大提高。加载方式，边界条件和变形程度等因素影响着滑移系的启动状况，这是平面模型所不能预测的。本文利用三维有限元方法模拟了不同取向下滑移系的启动状况，全面地考虑了FCC单晶材料12个可能滑移系在变形过程中的启动状况，合理地模拟了FCC面心立方单晶沿不同取向加载时晶轴旋转导致的应变局域化和颈缩等非均匀变形过程。     

A plasticity and anisotropic damage model for plain concrete

A plastic-damage constitutive model for plain concrete is developed in this work. Anisotropic damage with a plasticity yield criterion and a damage criterion are introduced to be able to adequately describe the plastic and damage behavior of concrete. Moreover, in order to account for different effects under tensile and compressive loadings, two damage criteria are used: one for compression and a second for tension such that the total stress is decomposed into tensile and compressive components. Stiffness recovery caused by crack opening/closing is also incorporated. The strain equivalence hypothesis is used in deriving the constitutive equations such that the strains in the effective (undamaged) and damaged configurations are set equal. This leads to a decoupled algorithm for the effective stress computation and the damage evolution. It is also shown that the proposed constitutive relations comply with the laws of thermodynamics. A detailed numerical algorithm is coded using the user subroutine UMAT and then implemented in the advanced finite element program ABAQUS. The numerical simulations are shown for uniaxial and biaxial tension and compression. The results show very good correlation with the experimental data.     

A combined experimental-numerical approach for elasto-plastic fracture of individual grain boundaries

The parameters for a crystal plasticity finite element constitutive law were calibrated for the aluminum–lithium alloy 2198 using micro-column compression testing on single crystalline volumes. The calibrated material model was applied to simulations of micro-cantilever deflection tests designed for micro-fracture experiments on single grain boundaries. It was shown that the load–displacement response and the local deformation of the grains, which was measured by digital image correlation, were predicted by the simulations. The fracture properties of individual grain boundaries were then determined in terms of a traction–separation-law associated with a cohesive zone. This combination of experiments and crystal plasticity finite element simulations allows the investigation of the fracture behavior of individual grain boundaries in plastically deforming metals.     

Multiscale modeling of the plasticity in an aluminum single crystal

This paper describes a numerical, hierarchical multiscale modeling methodology involving two distinct bridges over three different length scales that predicts the work hardening of face centered cubic crystals in the absence of physical experiments. This methodology builds a clear bridging approach connecting nano-, micro- and meso-scales. In this methodology, molecular dynamics simulations (nanoscale) are performed to generate mobilities for dislocations. A discrete dislocations numerical tool (microscale) then uses the mobility data obtained from the molecular dynamics simulations to determine the work hardening. The second bridge occurs as the material parameters in a slip system hardening law employed in crystal plasticity models (mesoscale) are determined by the dislocation dynamics simulation results. The material parameters are computed using a correlation procedure based on both the functional form of the hardening law and the internal elastic stress/plastic shear strain fields computed from discrete dislocations. This multiscale bridging methodology was validated by using a crystal plasticity model to predict the mechanical response of an aluminum single crystal deformed under uniaxial compressive loading along the [4 2 1] direction. The computed strain-stress response agrees well with the experimental data.     

应力三轴度的有限元计算修正

对某高强度钢制成的光滑圆棒和缺口圆棒进行了系列准静态拉伸实验,采用ABAQUS对每个试 件进行了数值模拟,得到了该材料的真实应力应变曲线,拟合出了J-C本构模型和失效模型的部分材料常数。 最后,对该高强度钢制成的平板进行了撞击实验,并用得到的J-C模型对平板撞击实验进行了数值模拟,计算 结果与实验结果吻合很好,证明利用数值模拟并修正应力三轴度的方法是可行的。     

A plasticity model for interface friction: application to sheet metal forming

Successful numerical simulations of forming operations require robust and accurate tool-workpiece interface friction models. In this paper we extend the rate-independent, isotropic, isothermal interface friction model proposed by Anand (Anand, L., 1993. A constitutive model for interface friction. Computational Mechanics 12, 197–213) to a rate-dependent formulation. Material parameters in the friction model are determined for lubricated interfaces between Al6111-T4 sheet and D2 tool steel. The lubricants used are MP404 and boric acid; the MP404 lubricant is currently used in industry, whereas boric acid has recently been proposed as a solid-film lubricant for sheet forming by Erdemir (Erdemir, A., 1991. Tribological properties of boric acid and boric acid-forming surfaces. Part i: crystal chemistry and mechanisms of self-lubrication of boric acid. Lubrication Engineering 47, 168–173). The interface friction model is implemented in the finite element code ABAQUS/Explicit (ABAQUS Reference Manual., 1999. Providence, RI), and the finite element program is used to simulate two sheet forming operations: axisymmetric cup-drawing and square pan-drawing. The predictions from the finite element simulation are shown to be in very good agreement with experimental results.     

循环硬化材料本构模型的隐式应力积分和有限元实现

针对新发展的、能够描述循环硬化行为应变幅值依赖性的粘塑性本构模型,讨论了它的数值实现方法。首先,为了能够对材料的循环棘轮行为（Ratcheting）和循环应力松弛现象进行描述,对已有的本构模型进行了改进;然后,在改进模型的基础上,建立了一个新的、全隐式应力积分算法,进而推导了相应的一致切线刚度（Consistent Tangent Modulus）矩阵的表达式;最后,通过ABAQUS用户材料子程序UMAT将上述本构模型进行了有限元实现,并通过一些算例对一些构件的循环变形行为进行了有限元数值模拟,讨论了该类本构模型有限元实现的必要性和合理性。     

边坡稳定的剪切带计算

为了解决边坡稳定分析中剪切带有限元网格的依赖性问题,采用梯度塑性理论,从本构关系中引入特征长度入手,建立计算模型。提出了一种8节点缩减积分的梯度塑性单元,并采用梯度塑性理论推导了Drucker-Prager屈服准则的软化模型的有限元格式,在ABAQUS中进行了二次开发,嵌入了本文提出的8节点单元和本构模型,并用ABAQUS软件进行了边坡剪切带的计算。计算结果表明,本文提出的方法消除了经典有限元计算的网格依赖性问题,可以得到与单元剖分无关的稳定的剪切带宽度。本文所提出的方法可适用于其他场合的剪切带计算。     

Crystal plasticity model with enhanced hardening by geometrically necessary dislocation accumulation

A strain gradient dependent crystal plasticity approach is used to model the constitutive behaviour of polycrystal FCC metals under large plastic deformation. Material points are considered as aggregates of grains, subdivided into several fictitious grain fractions: a single crystal volume element stands for the grain interior whereas grain boundaries are represented by bi-crystal volume elements, each having the crystallographic lattice orientations of its adjacent crystals. A relaxed Taylor-like interaction law is used for the transition from the local to the global scale. It is relaxed with respect to the bi-crystals, providing compatibility and stress equilibrium at their internal interface. During loading, the bi-crystal boundaries deform dissimilar to the associated grain interior. Arising from this heterogeneity, a geometrically necessary dislocation (GND) density can be computed, which is required to restore compatibility of the crystallographic lattice. This effect provides a physically based method to account for the additional hardening as introduced by the GNDs, the magnitude of which is related to the grain size. Hence, a scale-dependent response is obtained, for which the numerical simulations predict a mechanical behaviour corresponding to the Hall-Petch effect. Compared to a full-scale finite element model reported in the literature, the present polycrystalline crystal plasticity model is of equal quality yet much more efficient from a computational point of view for simulating uniaxial tension experiments with various grain sizes.