基于非共轴理论的各向异性砂土应变局部化分析

针对各向异性砂土应变局部化分析中本构模型存在的不足,采用非共轴理论进行改进。传统塑性位势理论采用各向同性假设,导致其模型不能描述非共轴特性和不能较好描述各向异性的不足,为克服传统塑性势理论的局限性,引入非共轴塑性理论建立了砂土的三维非共轴临界状态各向异性本构模型。考虑细观组构张量和应力张量的几何关系,改进模型即可描述主应力轴旋转条件下砂土材料状态的改变,材料状态变化直接导致模型的硬化规律和剪胀性发生变化,从而描述了原生各向异性的影响。非共轴修正后模型可以描述应力诱发各向异性和非共轴特性,结合分叉理论模型可以对不同沉积角度随围压变化的应变局部化特性进行分析。Toyoura砂的单剪试验和平面应变试验验证表明模型改进效果较好。     

考虑刚度非线性软化的修正硬化土模型

基于硬化土模型(HS模型)只能描述土体应变硬化行为和刚度线性软化现象,提出了考虑刚度非线性软化的修正硬化土模型。修正模型采用与分段幂函数关系描述初始骨干曲线,并考虑了初始静偏应力和加载速率对应力-应变关系的影响。修正HS模型能较好地描述不同初始静偏应力和加载速率下土体的应变硬化、应变软化和刚度非线性软化等特征。最后,基于原状杭州软粘土的三轴试验,验证了修正模型的合理性。     

考虑温度影响的饱和软黏土应变软化研究

循环荷载作用下饱和软黏土将产生应变软化现象,通过对宁波饱和软黏土开展温控动三轴试验,研究了温度、频率、初始偏应力、动应力和含水率对土体应变软化的影响,并在试验基础上建立了一个能够考虑温度影响的应变软化模型。结果表明:随着振动次数、初始偏应力、动应力和含水率的增加,土体软化加快,软化指数逐渐减小;随着频率和温度的提高,土体软化程度降低,软化指数增大;建立的软化模型可以较为合理地描述试验温度、频率、初始偏应力、动应力和含水率对土体应变软化特性的影响。     

含盐冻结粉质粘土单轴抗压强度试验研究

通过对饱和含盐冻结粉质粘土单轴抗压强度试验,得到了试验条件下土体的单轴抗压强度与含盐量、负温的关系;讨论了应变速率一定,温度和含盐量变化时,含盐冻结粉质粘土的屈服应力和抗压强度间的关系;提出了相对温度这一新概念,并给出抗压强度与相对温度间的线性关系。相对温度的引入建立了盐渍土和非盐渍土力学性质的关系,从而可以用非盐渍土力学性质预测盐渍土的力学性质。     

深部岩石渐进破损本构模型及其应用

深部岩石力学特性的数理描述与深部岩体的灾害控制密切相关。从岩石破坏过程和破坏机制出发,将代表性体元(RVE)划分为弹性区及剪切局部化带两个部分;并把剪切局部化带内的变形过程抽象为胶结强度弱化及摩擦强度增强两个阶段,重点考虑了这两个阶段一前一后发挥作用的破坏本质;同时采用细观链式模型及均匀化方法将破坏过程的细观特征与宏观力学特性相结合,建立了岩石局部化渐进破损本构模型的理论公式。模型计算结果与Yumlu和Ozbay试验结果吻合较好,证明了所提出模型的正确性。通过变化模型中的参数,进一步对深部岩石的局部化渐进破坏特征进行了分析,揭示了试样呈现"尺寸效应"、"形状效应"、应变软化及II型应力-应变曲线的内在原因及影响因素。     

大应变测量的密栅云纹方法及应用

介绍了一种适宜大应变测量的密栅云纹法实验装置及其试验技术。通过对材料的力学性能及其加工工艺参数的研究和实际应用表明，根据试验数据及云纹图片，可迅速、简便、可靠地获得测量结果，为材料的大应变测量提供了一种有效的手段。     

竹木复合层合板疲劳与循环蠕变的累积损伤研究

为了对疲劳与循环蠕变交互作用下的损伤进行定量描述,采用一种考虑应变比率和弹性模量2个参数共同描述损伤的变量,并建立了损伤力学模型,该模型能够综合表现疲劳循环过程中塑性应变变化和弹性模量变化规律。通过对竹木复合层合板在80%、75%和70%3种应力水平下的纯疲劳弯曲试验,得出其损伤变量变化规律的实验曲线。通过对力学模型进行分段求解的方法,得到了竹木复合层合板在疲劳与循环蠕变交互作用下的累积损伤的拟合曲线。结果表明,该损伤参量及损伤模型可以较准确的描述在疲劳与循环蠕变交互作用下竹木复合层合板的损伤累积变化规律。     

高应变率下岩石本构特性的研究

高应变率下岩石的动态本构关系和力学特性,目前主要是利用SHPB装置等来获得．基于统计学和损伤力学的观点,建立岩石在高应变率下的动态本构模型是进一步研究爆破机理、爆炸应力波传播、爆破参数优化的基础。理论和实验证明,本文推荐的4种动态本构模型与试验结果有较好的一致性,对进一步研究岩石的动态力学性质奠定了基础．     

基于数字图像相关技术的砂土全场变形测量及其DEM数值模拟

岩土材料在力学性能上表现出各向异性与非线性特征,不同土体的受力变形规律也不相同。为了更真实地反映平面应变状态下土的受力变形特性,研制了一种新型的平面应变加载设备,该设备通过对试样的侧向（围压方向上）施加柔性荷载来降低常规平面应变试验中刚性加载所造成的边界约束影响。同时,搭建了能够得到表面变形识别的数字图像采集系统。在此基础上,利用研制的平面应变设备结合二维数字图像相关技术（2D-DIC）根据获得试验过程中的全场变形来分析福建标准砂在不同围压下的变形特性。另外,通过数字图像相关法得到的平面应变试验结果来确定砂土基于抗滚动摩擦模型的细观参数,并对试验过程进行了离散元分析。结果表明:基于数字图像相关测量技术的新型平面应变试验设备可以准确获得福建标准砂的局部变形规律和变形过程的非线性行为,由此确定的砂土细观参数也能够较为真实地反映试验材料的应力-应变关系。     

低应变率下聚乙烯醇缩丁醛的拉伸力学性能

通过27种工况下的聚乙烯醇缩丁醛(PVB)低应变率直接拉伸试验,研究了低应变率下PVB拉伸力学性能,并考虑了厚度、应变率和温度对其的影响。试验结果表明,试件厚度和加载应变率对PVB的力学行为影响较小,温度对其力学行为影响较大,说明PVB是一种温度敏感材料。基于不同温度下得到的试验结果,拟合了PVB应力-应变关系。采用拉伸松弛试验得到的松弛模量,应用时-温等效原理和弹性回复对应原理,建立了考虑松弛效应的PVB拉伸本构关系。     

Meso-structure evolution in a 2D granular material during biaxial loading

Multi-scale approaches of constitutive modeling require an intermediate scale linking the variables in macroscopic scale (incremental stress and strain) to variables in microscopic scale (contact force and contact displacement). In this paper, we introduce a mesoscopic scale, in which the granular material is tessellated into small loops by contact network. Then numerical biaxial tests from different initial states by DEM modeling, is performed to investigate how the meso-structure (mesoscopic loops) evolves along the drained biaxial loading path. Results suggest that the procedure of the biaxial test is accompanied with the exchange between small, dense structures and big, loose structures. The macroscopic dilatancy primarily originates from this exchange. In dense and intermediate specimens the meso-structure evolution is found not to be consistent with the evolution of the macroscopic volumetric strain during contractancy phases. This inconsistency has led to interpret the elastic and the plastic parts of the volumetric strain from a meso-scale viewpoint. It is shown that the initial contractancy in dense and intermediate specimens is largely an elastic process, which is highly dependent on elastic parameters of the material.     

Finite element analysis of non-isothermal multiphase geomaterials with application to strain localization simulation

Finite element analysis of non-isothermal elasto-plastic multiphase geomaterials is presented. The multiphase material is modelled as a deforming porous continuum where heat, water and gas flow are taken into account. The independent variables are the solid displacements, the capillary and the gas pressure and the temperature. The modified effective stress state is limited by the Drucker-Prager yield surface for simplicity. Small strains and quasi-static loading conditions are assumed. Numerical results of strain localization in globally undrained samples of dense, medium dense and loose sands and isochoric geomaterial are presented. A biaxial compression test is simulated assuming plane strain condition during the computations. Vapour pressure below the saturation water pressure (cavitation) develops at localization in case of dense sands, as experimentally observed. A case of strain localization induced by a thermal load where evaporation takes place is also analysed. Dedicated to Professor S. Valliappan in occasion of his retirement     

Biaxial Yield for Nonlinearly Viscoelastic Materials with a Strain Clock

A constitutive equation with a dilatation dependent reduced time is used to model the mechanical response of solid amorphous polymers such as polycarbonate. Such constitutive equations have the property that stress relaxation occurs faster with increasing dilatation. In previous work, it has been shown that this constitutive equation can account for yield in materials undergoing uniaxial strain or stress control histories. In the present work, yield is discussed when materials described by this constitutive equation undergo homogeneous biaxial and triaxial strain histories. Four sets of conditions are considered: in-plane biaxial constant strain rate histories and in-plane biaxial constant stress rate histories, for both plane stress and plane strain states. Yield is defined in a manner analogous to that in the corresponding strain and stress control conditions in the uniaxial case.     

双轴受压状态下的高延性纤维增强水泥基复合材料本构模型

基于Darwin和Pecknold考虑混凝土双轴力学行为的方法,建立一个同时考虑双轴受压状态下非线性力学行为和抗压强度变化的高延性纤维增强水泥基复合材料(ECC)二维正交各向异性本构模型。在因双轴加载而产生的正交各向异性的2个方向上引入等效单轴应变,建立非线性应力-等效单轴应变关系以考虑ECC的双轴非线性行为,并采用一条双轴强度包络线确定2个方向上的抗压强度。推导模型的显式数值算法,编写包含该算法的用户自定义材料子程序UMAT,并嵌于有限元计算程序ABAQUS v6.14中。通过对两组不同配合比的ECC试件在不同应力比下的双轴受压加载试验进行数值分析验证本模型的有效性。数值计算得到的主压应力方向上的应力-应变曲线及预测的抗压强度与试验结果吻合较好,表明该文提出的本构模型能够有效地预测ECC在双轴受压状态下的非线性力学行为和破坏强度。     

含孔天然纤维织物复合材料力学性能

研究了含孔天然苎麻纤维织物/异氰酸酯复合板在双轴向拉伸载荷下的力学行为。对0.5、1.0、2.0、4.0mm 4种孔径板进行了单向和双轴向载荷拉伸试验, 同时采用数字散斑相关方法对全场位移及孔径大小对应变的影响进行了表征。结果表明: 当载荷线性变化时孔周围的位移场分布较为均匀; 随着载荷接近破坏值, 位移场呈非线性分布并出现高应变值点, 破坏以极快的速度沿孔边在这些点首先发生。随着孔径的增大, 在1000~2000N双轴向载荷下孔周围相同面积内x、y方向正应变的平均值减小, 应变值波动小但范围增大。材料在单向和双轴向拉伸时表现出不同的力学特征: 双轴向载荷下失效强度要比单向拉伸时低, 降低比例为14%～27%, 且随着孔径的增加而增加。      

Study of elastomers behavior at rupture based on the first Seth strain measures invariant

In this work, we are interested in the study of the mechanical behavior of the elastomers materials. We analyze mainly their ultimate properties. This study is based on the first Seth strain measures invariant [1] and the literature experimental data [2] and [3], allowing to take into account of the multiaxial loading effects. With this intention, the step that we adopted is translated by the division of this work into two parts:In the first time, we developed the relation of Seth–Hill based to the strain measures. In the second time, we exposed the principal experimental results used in this work, including versus loading modes (uniaxial tension, equal biaxial tension and biaxial tension), realized on two types of elastomers: a natural rubber (NR) and a styrene butadiene rubber (SBR).Indeed, the installation the first invariant of the Seth strain measures and the experimental results data made it possible to obtain an exploitable data base to determine the non-linear evolution of elastomers behavior at rupture in order to identify its parameter’s materials after.     

First-principles study on the formation of a vacancy in Ge under biaxial compressive strain

The effects of the biaxial compressive strain on the atomic relaxation and the formation energy of a neutral vacancy in Ge were investigated using first-principles calculations. Prior to this, the effects of the supercell size and Brillouin zone sampling were tested. The vacancy formation energy and atomic configuration around a vacancy are strongly affected by the inter-vacancy distance determined by the supercell size, due to the periodic boundary condition. The biaxial compressive strain reduced the formation energy of the vacancy nearly linearly by up to 1.34 eV as the magnitude of the biaxial compressive strain increased to the “Ge on Si (GoS)” condition. This was explained in terms of the bond strength characterized by the spatial electron density. The behavior of the vacancy in Ge was also compared with that in Si.     

An investigation on loose cemented granular materials via DEM analyses

This paper presents the results of a numerical study carried out by 2D discrete element method analyses on the mechanical behavior and strain localization of loose cemented granular materials. Bonds between particles were modeled in order to replicate the mechanical behavior observed in a series of laboratory tests performed on pairs of glued aluminum rods which can fail either in tension or shear (Jiang et al. in Mech Mater 55:1–15, 2012). This bond model was implemented in a DEM code and a series of biaxial compression tests employing lateral flexible boundaries were performed. The influence of bond strength and confinement levels on the mechanical behavior and on the onset of shear bands and their propagation within the specimens were investigated. Comparisons were also drawn with other bond models from the literature. A new dimensionless parameter incorporating the effects of both bond strength and confining pressure, called BS, was defined. The simulations show that shear strength and also dilation increase with the level of bond strength. It was found out that for increasing bond strength, shear bands become thinner and oriented along directions with a higher angle over the horizontal. It also emerged that the onset of localization coincided with the occurrence of bond breakages concentrated in some zones of the specimens. The occurrence of strain localization was associated with a concentration of bonds failing in tension.     

Nonlinear Analysis of Woven Fabric-Reinforced Graphite/PMR-15 Composites under Shear-Dominated Biaxial Loads

An elastic-plastic, time-independent, macroscopic, homogenous model of an 8HS woven graphite/PMR-15 composite material has been developed that predicts the nonlinear response of the material subjected to shear-dominated biaxial loads. The model has been used to determine the response of woven composite off-axis and Iosipescu test specimens in nonlinear finite element analyses using a multilinear averaging technique. The numerically calculated response of the specimen was then compared to experimentally obtained data. It has been shown that the numerically calculated stress - strain diagrams of the off-axis specimens are very close to the experimentally obtained curves. It has also been shown that the numerically determined shear stress - strain and load-displacement curves of the woven Iosipescu specimens are close to the experimentally obtained curves up to the point of significant interlaminar damage initiation and propagation. The results obtained in this study clearly demonstrate that the nonlinear material behavior of the graphite/polyimide woven composites subjected to shear-dominated biaxial loading conditions cannot be ignored and should be considered in any stress analysis. The linear-elastic approach grossly overestimates the loads and stresses at failure of these materials in the off-axis and Iosipescu tests. It can be assumed that the same discrepancies will arise in the numerical analysis of the woven composites tested under other biaxial shear-dominated loading conditions using other biaxial test methods.