剪切载荷作用下植物细胞的力学特性分析

根据植物细胞的结构特点,以二维问题为研究对象,在已建立的植物单细胞力学模型的基础上,利用有限元方法和MATLAB计算软件研究了单细胞受到剪切载荷作用时,外力、应力、应变及内压间的相互关系,给出了关系曲线图。得出了在剪切情况下,外力、应力、应变及内压之间的关系是非线性的;细胞内压改变量随外力或细胞变形或细胞壁应力的增加而增加;细胞壁的应力随剪切力的增大而增大;细胞将在哪个方向破裂等6个结论。     

侧向受拉脆性岩石压缩本构及蠕变失效研究

轴向压缩作用下,脆性岩石侧向应力严重影响岩石力学特性。侧向压应力影响下的轴向压缩岩石力学行为已经得到广泛研究,然而侧向拉应力对轴向压缩岩石力学行为影响研究很少。本文基于脆性岩石翼型裂纹扩展模型中,初始裂纹面法向应力与剪切应力的正负方向为判断依据,研究了侧向拉应力对轴向压缩力学行为的影响。发现恒定的侧向拉应力作用下,轴向压缩应力渐进变化过程中,脆性岩石内部细观初始裂纹面的法向应力只能为压缩应力,不存在拉应力情况。分析了从侧向压应力到拉应力转化过程中,脆性岩石轴向压应力与细观裂纹扩展长度关系、轴向压应力与轴向应变关系、岩石峰值强度、裂纹启裂应力及初始弹性模量的变化规律。并分析了侧向拉应力对岩石蠕变裂纹长度、裂纹速率、轴向应变及应变率演化曲线,以及对蠕变失效时间及稳态蠕变应变率的影响。讨论了侧向拉压应力突变转化以及侧向拉应力分级增大对轴向压缩岩石蠕变演化行为影响。该研究为深部地下工程围岩稳定性评价提供了一定理论依据。     

考虑拉压异性徐变的高混凝土坝实测应变转换应力探讨

应变计组实测应变转换为应力与混凝土的徐变特性有关,本文探讨了基于拉压异性徐变的高混凝土坝实测应变转换为应力。首先针对施工期高混凝土坝实测应变存在监测误差,基于实测应变建立的统计模型,获得拟合应变过程线;然后采用应力增量法,基于线性叠加假设,导出了考虑拉伸压缩异性徐变的实测应变转换应力计算公式。结合某高混凝土坝工程应变计组实测应变,对比分析了拉压一致徐变、拉压异性徐变转换应力的差异。分析结果表明,考虑拉压异性徐变后,由于拉伸徐变度小于压缩徐变度,计算应力向拉应力方向偏移,且随时间的增加,偏移量缓慢增加。     

基于裂纹扩展脆性岩石动力压缩力学特性研究

动态压缩荷载作用下,脆性岩石内部动态细观裂纹扩展特性,对岩石宏观动态力学特性有着重要的影响。然而,对岩石内部动态细观裂纹扩展与宏观动态力学特性的关系研究较少。基于准静态裂纹扩展作用下的应力-应变本构模型、准静态与动态裂纹扩展断裂韧度关系、裂纹速率与应变率关系模型及应变率与动态断裂韧度关系,提出了一种基于细观力学的动态应力-应变本构模型。其中裂纹速率与应变率关系,是根据裂纹长度与应变关系的时间导数推出;应变率与动态断裂韧度关系,是根据推出的裂纹速率及应变率关系,与裂纹速率及断裂韧度关系相结合而得到。研究了应变率对应力-应变本构关系及动态压缩强度影响。并通过试验结果验证了模型的合理性。讨论了岩石初始损伤、围压、模型中参数m、ε0和R对应力-应变关系、动态压缩强度和动态弹性模量的影响。研究结果可为动态压缩荷载作用下深部地下工程脆性围岩稳定性分析提供了一定的理论支持。     

脆性颗粒材料的应变率效应机理研究

利用分离式Hopkinson压杆（SHPB）,对干燥石英砂进行了被动围压下的动态压缩实验,发现石英砂表现出了明显应变率效应。通过激光粒度分析测量了动静态压缩后的试件的级配曲线,发现同一应力水平下准静态压缩比动态压缩后的试件的颗粒破碎量更大。而通过拟合相对破碎率与外力功之间的关系,发现准静态压缩在颗粒破碎方面能量利用率更高也即破碎效率更高,而这正是脆性颗粒材料应变率效应的本质原因。     

冲击载荷下Al_2O_3陶瓷材料的动态力学行为和响应

采用分离式Hopkinson压杆实验装置和ABQAUS软件,对氧化铝陶瓷材料进行冲击压缩实验和有限元数值模拟;分析了此类材料中的应力-应变关系、应力波的传播特性和冲击动态响应。结果表明:氧化铝陶瓷的极限强度和应力波波速均随着应变率的增加而明显增加,应变率效应明显;氧化铝陶瓷材料属于递减硬化材料,随着应变的增大,应力波的波速降低,动态切线模量逐渐减小;当应变率增加50%时,此类材料的应力、应变、速度、加速度等参量的冲击动态响应分别增加约68.67%、77.42%、22.84%、25.75%。     

钢纤维活性粉末混凝土的动态力学性能

利用?74mmSHPB实验装置对钢纤维活性粉末混凝土(RPC)进行动态压缩实验和动态劈裂拉 伸实验。获得了钢纤维RPC在1~102s-1应变率加载下的动态力学参数。对试件内的动态应力分布进行数 值模拟,验证了动态实验的有效性。结果表明,钢纤维RPC的动态压缩和动态劈裂拉伸的力学性能均表现出 显著的应变率效应。随着应变率的增加,钢纤维RPC冲击压缩破坏应力、冲击压缩破坏应变、弹性模量、动态 劈裂拉伸破坏应力均有一定程度的增加,动态拉压比相对静态拉压比也有显著的提高。     

基于颗粒离散元法的连接键应变软化模型及其应用

基于颗粒间的有限接触假设,提出了可表述颗粒间力、力矩传递的连接键模型. 为了表征连接键的塑性、损伤及断裂过程,在连接键中引入了考虑应变软化效应的Mohr-Coulomb 准则及最大拉应力准则. 单一连接键的单向拉伸测试及直剪测试表明了上述连接键应变软化模型的计算精度. 研究了颗粒体系的宏观应变能与颗粒平均配位数的对应关系. 通过计算发现,对于二维颗粒体系,当平均配位数为5 时,颗粒体系的宏观应变能与相同参数下连续介质方法(如有限元等) 计算获得的应变能基本一致. 利用上述连接键应变软化模型对岩石的单轴压缩过程进行了模拟,计算结果表明:岩石单轴压缩的应力应变曲线经历了线性上升段、非线性上升段、非线性下降段及缓变段等4 个阶段,并给出了上述4 个阶段与岩石内部损伤破裂状态的内在联系. 计算结果还表明,随着断裂应变的增大,岩石的破裂模式逐渐由拉剪复合型破裂向单一压剪型破裂转化;随着断裂应变的增大,峰值应力及达到峰值应力时的应变均逐渐增大,但峰值时的破裂度及终态时的破裂度将逐渐减小.      

珊瑚砂应变率效应研究

为研究应变率对珊瑚砂力学特性的影响,用直径37 mm的分离式Hopkinson压杆（SHPB）对两类珊瑚砂进行了冲击实验,得到了460~1300 s?1应变率范围内不同密实度的一维应变压缩应力-应变关系;结合准静态（10?4 s?1）压缩的实验结果,发现珊瑚砂存在明显的应变率效应;通过对比两类砂物理特性,认为应变率敏感性与内孔隙和粒间摩擦密切相关;提出了率型本构模型中动态增强系数的计算模型,可为珊瑚砂在冲击下的数值计算提供理论依据。     

围压状态下的混凝土本构模型

利用已有不同围压下的混凝土三向压缩试验, 分析研究不同围压条件对混凝土强度和应力-应变关系的影响。试验表明, 把混凝土简单视为脆性材料的观点并不符合实际情况, 塑性特征也是混凝土材料主要力学特征。由热力学定律出发, 本文提出了多次方形式的屈服函数和与围压状态相关联的损伤标准化函数, 提出了考虑围压状态的混凝土本构模型。试验结果与数值计算比较表明, 该本构关系可以很好地描述混凝土在不同围压状态下的力学损伤发展和脆-塑性转换力学特征, 也可以描述拉伸作用下的应力-应变关系, 具有一定的普遍意义。     

全尺寸隔水管在复杂应力状态下的力学特性试验研究

为对隔水管在复杂应力状态下的力学特性进行试验研究,研制了能够模拟高液压环境下管柱力学行为的特殊试验装置——深水管柱力学模拟试验系统,并借助该系统对全尺寸隔水管在受内外压条件下进行了力学特性模拟试验。结果表明:试验系统运行良好,能够有效地按照要求进行加载、卸载。试验中通过对隔水管试件进行内外压加载,同时对系统两端的轴向活塞加压,消除了由于内外压产生的轴向力,实现了在内外压条件下的隔水管力学特性模拟试验。通过在管柱外壁粘贴电阻应变片,得到了其真实径向应变与环向应变随内外压变化的关系曲线,并通过分析计算得到了试件的真实应力状态。由于隔水管加工制造等方面的误差,真实的应变数据、应力状态与理论推导结果并不重合。为了提高深水作业的安全性与经济性,可以借用试验结果对理论推导值进行修正,也说明了用试验方法进行隔水管力学特性研究的重要性。该试验系统为深水作业管柱等的模拟试验提供了一个良好的平台,能够有效降低深水钻采作业的风险,提高深水钻采作业的经济性与安全性,具有较高的推广应用价值。     

Deformation, stress state and thermodynamic force for a growing void in an elastic–plastic material

The growth of a spherical void in an elastic–plastic body, subjected to external pressure or tension and a gas pressure as well as a surface stress at the void surface, is investigated. The deformation, strain and stress state in the full body is presented. In addition, the local and global energy terms are calculated. Finally the total thermodynamic force on the void surface as well as the total dissipation are evaluated and compared allowing the calculation of the mechanical contribution to void growth due to diffusion of vacancies generated by plastification or irradiation.     

Damage induced instability in beam bending

The effect of internal damage creation on the load carrying capacity of a beam is studied. Time independent relations are postulated for the development of strain and damage with increasing net stress. The resulting relations between load and deformation for a Bernoulli-Navier beam lamina with rectangular cross-section are then derived. A state of instability is shown to exist, characterized by unlimited rate of increase in deformation and damage with load. An instability locus in the plane of bending moment and normal force is defined. The shape of this locus is studied for varying parameters in the deformation and damage laws.     

磁致伸缩材料的非线性本构关系

给出了磁致伸缩材料的两个非线性本构关系,即标准平方型和双曲正切型。在确定一维问题的本构系数时,基于已有的实验结果,引进一个材料函数,用来描述磁致伸缩材料的压磁系数随预应力变化的关系。将非 线性本构关系的理论模型计算结果与实验曲线对比,结果表明标准平方型本构关系在中低磁场下能精确地模拟实验曲线,而双曲正切型本构关系在高磁场时能反映材料的磁致应变饱和现象。讨论了在标准平方型本构的一般三维情形,给出了确定本构系数的方法。     

Reduction of dimensions in random,elastic soil medium

The paper deals with an application of the plane strain analysis in a stochastic three-dimensional soil medium. In a framework of random elasticity theory, the geostatical state of stresses and the problem of a unit force acting in a statistically homogeneous half-space are considered. Only the modulus of elasticity is considered to be random and is modelled as a three-dimensional (3-D) homogeneous random field. As the result of imposed constrains due to the plane strain assumption the additional body and surface forces are induced. In order to determine them, additional equations must be introduced. The equations in a form of constrain relations are proposed in this paper. These equations are also valid for a case of uniformly distributed external loading.First, the two-dimensional (2-D) problem and its reduction to the uni-axial strain state, for the gravity forces and uniform, unlimited surface loading is considered. Then, it is generalised into a 2-D schematization of the 3-D state. Next, the problem of a unit force acting in a statistically homogeneous half-space is considered. For a 3-D state of stress and strain the resulting stresses are compared with those for a 2-D state. These stresses for the multidimensional state of strain and stress are presented as a sum of two components. The first one reflects plane strain state stresses and is given in a form of a 3-D random field. This term allows for incorporating a spatial, 3-D soil variability into a two-dimensional analysis. The second component can be treated as a correction term and it represents the longitudinal influence of a 3-D analysis.Some numerical results are presented in this paper. The proposed method can be regarded as a framework for further research aiming at application to a variety of geotechnical problems, for which the plane strain state is assumed.     

On the response and stability of an inflated toroidal membrane under radial loading

Two flat annular hyperelastic membranes, stacked and bonded together at both the boundaries (equators), form a closed inflatable structure of toroidal topology. The response and stability of the inflated toroidal structure subjected to a radial line force distribution at the inner boundary are studied. The forcing is considered under constant pressure and constant amount of gas inflation conditions. Two hyperelastic models described by the corresponding relaxed strain energy density functions are considered for the membrane material. The influence of geometry, material and level of inflation on the response and stability of the structure under load has been brought out. The structure exhibits pressure limit points with increasing levels of inflation. The force–deflection (stiffness) behaviour is found to be qualitatively different below and above the pressure limit points. Below the pressure limit point, wrinkling and pull-in under loading are revealed for different inflation conditions, and the stability boundaries are determined. Under certain conditions, a counter-intuitive stretch-softening behaviour is also observed.     

Failure properties of a PBAN propellant in multiaxial stress fields

Experimental data have been obtained which duplicate the stress and strain field in a solid-propellant rocket-motor grain under pressurization. Development of a hollow ellipsoidal specimen has made possible the acquisition of these data. This test also allows a more thorough evaluation of the failure mechanism in multiaxial stress and strain fields. Failure data were obtained in the tensile-tensile-compressive (++?) stress and biaxial tensile-compressive (+?) strain octants by applying pressure to the internal surface of an ellipsoidal specimen. Data were obtained in the triaxial compressive (???) stress and biaxial tensile-compressive (+?) strain octants by simultaneous application of pressure to both internal and external surfaces of the ellipsoidal specimen and a gradual reduction of external pressure until failure. A stress analysis of the specimen is presented and data-reduction techniques are discussed. Data obtained with a hollow spherical specimen and with the newly developed hollow ellipsoidal specimen are compared. These data are obtained in the same stress and strain octants; however, the relative magnitudes of the parameters and, therefore, the positions within the stress and strain octants are different. This results in a change in the magnitudes of the failure parameter. Data from uniaxial specimens of the same propellant also are presented.     

旋转壳非线性蠕变分析

基于现有的蠕变理论,本文推导出复杂应力状态下的蠕变本构方程,以内力与转角为基本未知量,得到旋转壳在轴对称载荷作用下的非线性蠕变控制方程.对基本未知量在空间上采用样条配点法离散,在时域上采用初应力法离散,求出壳体在不同时域的蠕变应变和应力,而最终可求出蠕变后的稳态应力重新分布.计算结果表明同,本文分析方法是有效的.     

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。