双周期分布圆形弹性夹杂平面热弹性问题

研究了含双周期分布圆形弹性夹杂的无限弹性平面在均匀拉伸和均匀温变下的弹性响应问题．运用Isida的区域单元法和复势函数的级数展开技术,将问题转化为线性方程组的求解．数值结果表明：相邻夹杂问距过大或过小都不利于减小界面应力,当相邻夹杂中心间距与夹杂半径之比为2．2～2．8时,界面剪切应力与环向应力的极大值最小;比值为2．5～3．5时,界面最大径向应力值最小;并且该比值范围不随两相材料弹性模量之比和热膨胀系数之比而变化．     

微型计算机对全息位移与应变分析的冲击

本文的中心是微型计算机与图象处理机在全息干涉法的位移与应变分析中的作用。首先将考虑数据采集问题,即如何将条纹图转变为计算机中的数据,在这一部分将讨论从图形数据化到固态电视摄象机等一系列问题。然后考虑成像区域物点的正确辨别问题以及成像区域与对应的有限元分析的网格点之间转换问题。此后,将考虑对条纹插值,算出网格点位置的应变或位移的问题。最后,将给出由全息分析的数据得出位移或应变值的数学公式。     

基于超声散射元平均间距的生物软组织弹性测量

超声弹性成像有希望成为生物软组织病变诊断的有效手段。本文论述了软组织散射元准周期规则分布导致超声散射信号相干峰产生的机理，提出利用ＡＲ幅度倒谱估计散射元平均间距及其变化，结合组织外加应力来估计软组织内局部杨氏弹性模量。结果表明，该方法能有效地测量软组织内局部杨氏弹性模量，轴向和侧向空间分辨力分别为２．５ｍｍ和１．５ｍｍ，可用于在体软组织弹性成像。最后讨论了软组织弹性测量中的具体问题。     

63Sn-37Pb钎料合金在非比例循环加载下的非弹性流动特性研究

基于63Sn-37Pb钎料舍金在多种非比例应变循环加载下的实验结果,通过考察材料的非弹性应变率与偏应力之间的夹角随累积非弹性应变的变化规律,对63Sn-37Pb钎料合金的非弹性流动特性进行了定量分析。分析结果显示：在相同的非比例加载路径下,当加载等效应变幅值相同时,等效应变率越高,非弹性应变率与偏应力之间夹角平均水平越低,当等效应变率相同时,等效应变幅值越大,相应的夹角平均水平越低;在保持时间范围内,非弹性应变率方向与偏应力方向趋于一致;当非比例路径形状不同时,其非弹性应变率与偏应力之间的夹角随累积非弹性应变的变化趋势明显不同。研究表明,材料的非弹性流动特性强烈依赖于等效应变幅值、等效应变率、保持时间、非比例路径形状。     

A95陶瓷材料的动态压缩测试研究

为了得到陶瓷材料有效的实验结果,必须采用多种改进措施(PulseShaper、高强度和高刚度垫块、直接测试试样应变以及使用较大长径比的试样)来满足SHPB装置所要求的基本假设条件。研究以A95陶瓷的测试为例,通过数值模拟分析证明:针对陶瓷材料改进的SHPB装置,可以得到陶瓷材料的有效压缩应力 应变曲线和压缩破坏强度。但是在分析材料的压缩破坏强度时,必须排除劈开破坏的干扰。数值模拟研究发现,压缩测试中出现的劈开破坏是由卸载过程中出现的局部横向(沿直径)拉伸造成的拉伸破坏。在修正了弥散对压缩破坏强度的影响后,研究得到了A95陶瓷的压缩破坏强度为3.5GPa。A95陶瓷的高应变率压缩应力 应变曲线显示了良好的线弹性,并且曲线的斜率与杨氏模量相同。     

应用超声多普勒对软组织弹性成像的研究

超声弹性估计与成像有可能成为生物软组织病变诊断的新技术。本言语提出采用仿体外加低频振动与超声多普勒检测及成像相结合的方法,对均匀及非均匀仿体进行声弹性成像,以波动方程和彩色超声多普勒成像为基础,给出了组织振动幅度、速度和调制参数β的估算公式。初步的仿体实验结果表明软组织振动公式与超声弹性图吻合得很好,超声弹性成像比传统Ｂ超更分弹性模量变化垢区域。这些研究圣字位与诊断声阻抗及声散射特性差异不明显的小     

形状记忆合金伪弹性行为模拟

基于对伪弹性形状记忆合金(SMA)典型应力-应变曲线的特征分析,在原Graesser本构模型中增加简洁多项式来描述SMA应力诱发马氏体相变完成后在变形马氏体相下继续加载阶段的变形特征;并引入应变幅值与混相下SMA弹性模量的关系来改进不同应变幅值下卸载时SMA的应力-应变关系,从而提出了一种新的SMA一维本构关系模拟其伪弹性力学行为。该模型对直径为0.5mm的NiTi合金丝的拉伸加载、卸载试验曲线的模拟结果表明:改进本构模型与原Graesser模型相比,其能够准确地模拟SMA在不同应变幅值下加载和卸载应力-应变关系。此外,通过研究SMA本构模型的物理关系,推导出了控制SMA滞回曲线特征的关键参数fT与相变临界应力、弹性常数之间的明确关系,可利用该关系直接确定参数fT,摆脱了只靠试算获取该参数的传统做法,其准确性得到了试验验证。     

SHPB试验中岩石试件的端面不平行修正

为研究短圆柱体岩石试件端面不平行对岩石动力学特性测试结果的影响,采用有限元分析软件LS-DYNA对9种端面不平行度和5种杨氏模量的岩石试件开展SHPB(split Hopkinson pressure bar)试验数值模拟,对岩石选用HJC(Holmquist-Johnson-Cook)本构模型。数值模拟结果表明,当端面不平行度在0.40%以内时,端面不平行对动态应力测试结果的影响可忽略不计;但对动态应变测试结果的影响较大。当杨氏模量一定时,平均应变率测试误差和峰值应变测试误差随端面不平行度增大呈线性增大;当端面不平行度一定时,平均应变率测试误差和峰值应变测试误差随杨氏模量增大也呈线性增大。对数值模拟得到的平均应变率测试误差和峰值应变测试误差实施二元线性回归分析,提出了SHPB试验中端面不平行岩石试件平均应变率和峰值应变的修正公式。     

黏弹性功能梯度材料裂纹问题的有限元方法

针对组分材料体积含量任意分布的黏弹性功能梯度材料裂纹问题建立有限元分析途径. 通过Laplace变换,将黏弹性问题转化到象空间中求解,基于反映材料非均匀的梯度单元和裂纹尖端奇异特性的奇异单元计算象空间中的位移、应力和应变场,应用虚拟裂纹闭合方法得到应变能释放率,分别由应力和应变能释放率确定应力强度因子. 给出这些断裂参量在物理空间和象空间之间的对应关系,由数值逆变换求出其在物理空间的相应值. 文中分析两端均匀受拉的黏弹性边裂纹板条,首先针对松弛模量表示为空间函数和时间函数乘积的特殊梯度材料进行计算,结合对应原理验证方法的有效性. 然后分析组分材料体积含量具有任意梯度分布的情形,由Mori-Tanaka方法预测象空间中的等效松弛模量. 计算结果表明,蠕变加载条件下,应变能释放率随时间增加,其增大程度与黏弹性组分材料体积含量相关. 由于梯度材料的非均匀黏弹性性质,产生应力重新分布,导致应力强度因子随时间变化,其变化范围与组分材料的体积含量分布方式有关.     

一种从离散模拟到连续介质弹性模拟的过渡方法

提出了一种从离散分子动力学模拟(MD)到连续介质弹性有限元计算分析(FEA)的过渡方法, 简称MD-FEA方法. 首先通过MD计算获得晶体材料原子的移动位置, 然后根据晶体结构的周期性特征构造连续介质假设下的有限单元变形模型, 进一步结合材料的力学行为本构关系获得应变和应力场. 为了检验MD-FEA方法的有效性, 将该方法应用于详细分析Al-Ni软硬组合两相材料纳米柱体的拉伸变形问题和基底材料为Al球形压头材料为金刚石的纳米压痕问题. 采用MD-FEA方法获得了上述两种问题的应力?应变场, 并将计算结果分别与传统MD方法中通过变形梯度计算的原子应变以及原子的位力应力进行了比较, 详细讨论了用MD-FEA方法计算的应力?应变场与传统MD原子应变和位力应力的区别, 并对MD-FEA方法的有效性及其相较于传统MD方法所具有的优势进行了探讨. 结论显示, MD-FEA方法与传统MD方法在应力?应变变化平缓的区域得到的结果接近, 但在变化剧烈的区域以及材料的表/界面区域, MD-FEA方法能够得到更加精确的结果. 同时, MD-FEA方法避免了传统MD方法中, 需要人为选取截断半径以及加权函数所导致的误差. 另外, 当应变较大时, MD-FEA方法计算的小应变与传统MD方法计算的格林应变存在一定差异, 因此, MD-FEA方法更适合应变较小的情形.      

多晶石墨烯拉伸力学性能及其影响因素的灵敏度分析

晶粒尺寸、温度和应变率等对纳米材料的力学性能有重要影响。本文通过分子动力学（MD）数值模拟,分析了不同晶粒尺寸多晶石墨烯在不同温度、拉伸应变率下的杨氏弹性模量、极限应力、极限应变等拉伸力学性能。结果表明,晶粒尺寸、温度和拉伸应变率对拉伸力学性能有较大影响。利用正交实验法,分别分析了杨氏弹性模量、极限应力和极限应变对晶粒尺寸、温度和拉伸应变率的敏感程度。结果表明,杨氏弹性模量和极限应力对影响因素的敏感程度由大到小依次为晶粒尺寸、温度和拉伸应变率;极限应变对影响因素的敏感程度由大到小依次为晶粒尺寸、拉伸应变率和温度。研究结果可为多晶石墨烯的理论研究和工程应用提供参考。     

纳米铜丝尺寸效应的分子动力学模拟

采用EAM原子势函数对不同截面尺寸纳米铜丝拉伸性能进行了零温分子动力学模拟。研究表明截面变化对纳米丝拉伸性能有明显影响。由于表明原子弛豫，纳米丝存在表面张应力以及与之相平衡的内部应力。这种本征应力的存在是纳米丝尺寸效应的根源。纳米丝截面减小，则拉伸强度提高、屈服推迟、初始拉伸模量的软化程度增加。     

Atomistic simulation of the structure and elastic properties of gold nanowires

We performed atomistic simulations to study the effect of free surfaces on the structure and elastic properties of gold nanowires aligned in the 〈100〉 and 〈111〉 crystallographic directions. Computationally, we formed a nanowire by assembling gold atoms into a long wire with free sides by putting them in their bulk fcc lattice positions. We then performed a static relaxation on the assemblage. The tensile surface stresses on the sides of the wire cause the wire to contract along the length with respect to the original fcc lattice, and we characterize this deformation in terms of an equilibrium strain versus the cross-sectional area. While the surface stress causes wires of both orientations and all sizes to increasingly contract with decreasing cross-sectional area, when the cross-sectional area of a 〈100〉 nanowire is less than , the wire undergoes a phase transformation from fcc to bct, and the equilibrium strain increases by an order of magnitude. We then applied a uniform uniaxial strain incrementally to 1.2% to the relaxed nanowires in a molecular statics framework. From the simulation results we computed the effective axial Young's modulus and Poisson's ratios of the nanowire as a function of cross-sectional area. We used two approaches to compute the effective elastic moduli, one based on a definition in terms of the strain derivative of the total energy and another in terms of the virial stress often used in atomistic simulations. Both give quantitatively similar results, showing an increase in Young's modulus with a decrease of cross-sectional area in the nanowires that do not undergo a phase transformation. Those that undergo a phase transformation experience an increase of about a factor of three of Young's modulus. The Poisson's ratio of the 〈100〉 wires that do not undergo a phase transformation show little change with the cross-sectional area. Those wires that undergo a phase transformation experience an increase of about 10% in Poisson's ratio. The 〈111〉 wires show, with a decrease of cross-sectional area, an increase in one of Poisson's ratios and small change in the other.     

An Area Modulus of Elasticity: Definition and Properties

A new modulus of elasticity is defined to be the ratio of an equibiaxial stress to the relative area change in the planes in which the stress acts. This area modulus of elasticity is intermediate in properties between Young's modulus and the bulk modulus. Expressions for the area modulus are computed in isotropic elasticity. A simple, convenient expression for the compliance tensor of transverse isotropy is found in terms of, amongst others, the longitudinal (axial) area modulus and this leads to a new, concise condition for positive definiteness of the compliance tensor. The limits of incompressibility, inextensibility and constant area are briefly considered. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stereo-PIV study of flow inside an eye under cataract surgery

We measured velocity distributions in the anterior chamber of porcine eyes under simulated cataract surgery using stereoscopic particle image velocimetry (stereo-PIV). The surface of the cornea was detected based on the images of laser-induced fluorescent light emitted from fluorescent dye solution introduced in a posterior chamber. A coaxial phacoemulsification procedure was simulated with standard size (standard coaxial phacoemulsification) and smaller (micro coaxial phacoemulsification) surgical instruments. In both cases, an asymmetric flow rate of irrigation was observed, although both irrigation ports had the same dimensions prior to insertion into the eye. In cases where the tip of the handpiece was placed farther away from the top of the cornea, i.e., closer to the crystalline lens, direct impingement of irrigation flow onto the cornea surface was avoided and the flow turned back toward the handpiece along the surface of the corneal endothelium. Viscous shear stress on the corneal endothelium was computed based on the measured mean velocity distribution. The maximum shear stress for most cases exceeded 0.1 Pa, which is comparable to the shear stress that caused detachment of the corneal endothelial cells reported by Kaji et al. in Cornea 24:S55–S58, (2005). When direct impingement of the irrigation flow was avoided, the shear stress was reduced considerably.     

Bending of functionally graded cantilever beam with power-law non-linearity subjected to an end force

A realistic beam structure often exhibits material and geometrical non-linearity, in particular for those made of metals. The mechanical behaviors of a non-linear functionally graded-material (FGM) cantilever beam subjected to an end force are investigated by using large and small deformation theories. Young's modulus is assumed to be depth-dependent. For an FGM beam of power-law hardening, the location of the neutral axis is determined. The effects of depth-dependent Young's modulus and non-linearity parameter on the deflections and rotations of the FGM beams are analyzed. Our results show that different gradient indexes may change the bending stiffness of the beam so that an FGM beam may bear larger applied load than a homogeneous beam when choosing appropriate gradients. Moreover, the bending stress distribution in an FGM beam is completely different from that in a homogeneous beam. The bending stress arrives at the maximum tensile stress at an internal position rather than at the surface. Obtained results are useful in safety design of linear and non-linear beams.     

皮下组织对羟基磷灰石力学性质的影响

将羟基磷灰石试样植入大白鼠的皮下组织20\,d后取出,然后采用数字图像 相关方法研究纤维组织对人造羟基磷灰石的力学性质的影响. 测量了两种试样：植入大白鼠 皮下组织的羟基磷灰石试样和未经植入的羟基磷灰石试样. 得到了羟基磷灰石完整的应力- 应变曲线;还测量了压缩极限应力. 实验结果表明羟基磷灰石的应力-应变曲线在断裂前呈良 好的线性关系;皮下纤维组织能使羟基磷灰石材料的弹性模量和强度增加, 20\,d后使羟基磷 灰石的弹性模量增加了29.06{\%}, 压缩极限应力增加了28.71%.     

高分子材料机械密封磨损特性及表面织构的影响

为提高金属/高分子材料机械密封的抗磨损性能,采用光刻-电解技术在316不锈钢表面制作微凹坑阵列形式的表面织构,与5种不同弹性模量的高分子材料组成摩擦副进行磨损试验.试验结果表明,对弹性模量最小的UHMWPE材料,表面织构起到了增磨作用,对其他四种弹性模量较高的材料,表面织构起到减磨作用,而且,随着高分子材料弹性模量的增大,表面织构表现出的减磨作用也随之增大.为解释这个现象,利用ANSYS有限元分析软件对摩擦副接触面进行了应力和变形分析,结果表明:织构化表面在接触过程中会产生应力集中和表面形变,材料的弹性模量越小,凹坑引起的变形越明显,可能产生的切削作用越显著.     

Mechanical and optical characterization of cellulose acetate

To obtain the basic relations for photo-viscoelastoplastic stress analysis using cellulose acetate, the effect of strain rate as well as room temperature on mechanical and optical properties was precisely investigated by the uniaxial tension test. As a result, the nonlinear stress-strain, the non-linear-stress-fringe order and the nonlinear-fringe-order strain relations were uniquely reoresented in their nondimensional forms regardless of strain rate and room temperature. Young's modulus, yield stress and the yield fringe order were linearly related both to room temperature and to logarithm of strain rate. The effect of strain rate on these relations has caused great difficulties in experimental stress analysis with respect to photoplasticity. However, it was found in this work that the value of the strain rate at any points in the model can be determined by the fringe-order rate measured. Therefore, it is possible to estimate not only the distribution of strain rate but that of stress or strain in the photo-viscoelastoplastic model with cellulose acetate.     

EFFECTS OF SI,N AND B DOPING ON THE MECHANICAL PROPERTIES OF GRAPHENE SHEETS

Molecular dynamics(MD) simulations were performed to stretch the rectangular graphene sheets doped with silicon, nitrogen or boron atoms. Young's modulus, ultimate stress(strain) and energy absorption were measured for the graphene sheets with the doping concentration(DC) ranging from 0 to 5%. The emphasis was placed on the distinct effects of each individual dopant on the fundamental mechanical properties of graphene. The results indicated that incorporating the dopants into graphene led to an almost linear decrease in Young's modulus. Monotonic reductions in ultimate strength, ultimate strain and energy absorption were also observed. Such doping effects were found to be most significant for silicon, less pronounced for boron, and small or negligible for nitrogen. The outputs provide an important guidance for the development and optimization of novel nanoscale devices, and facilitate the development of graphene-based M/NEMS.