二硫化钼改性热塑性聚酰亚胺复合材料的摩擦磨损性能研究

采用热压成型工艺制备MoS2 填充热塑性聚酰亚胺复合材料,测定了其弯曲强度和压缩强度,采用MPX-2000型摩擦磨损试验机评价MoS2 填充热塑性聚酰亚胺复合材料在干摩擦和水润滑2种工况下的摩擦磨损性能,利用扫描电子显微镜观察和分析材料磨损表面形貌和元素分布.结果表明:与聚酰亚胺树脂相比,加入MoS2后复合材料的弯曲强度和压缩强度有所降低,随着MoS2含量增加,材料的弯曲强度和压缩强度趋于稳定;在干摩擦条件下,MoS2逐步在磨损表面富集,相应的摩擦系数有所降低;MoS2含量为10%时磨损表面仅出现局部熔融,磨损率最低,当MoS2含量为20%时磨损表面出现深度熔融,磨损率较大;在水润滑条件下,MoS2仍起到良好的润滑作用,摩损率较干摩擦条件下降低1个数量级,表现出疲劳磨损特征.     

混凝土材料动态性能的经验公式、强度理论与唯象本构模型

混凝土是一种应用广泛的结构工程材料, 其材料组份复杂、变化因素多, 因而力学 特性也复杂多变. 动\linebreak 态/强冲击载荷作用下, 还涉及了材料应变率敏感效应和静水 压力相关特性等诸多影响因素, 使得其本构理论的研究更加困难. 本文中, 回顾了 近20多年来混凝土材料动态力学特性和本构关系研究方面的进展状况, 主要总结了 混凝土材料动态本构特性研究中的经验公式、强度理论和本构模型, 并在分析 比较的基础上给出了相应的讨论和评述.     

无约束修正Timoshenko梁的冲击问题

介绍了修正后的Timoshenko梁运动方程，并比较了修正Timoshenko梁与经 典Timoshenko梁的运动方程. 推导了考虑剪切变形引起的转动惯量的修正Timoshenko 梁的正交条件，推导了集中质量对无约束修正Timoshenko梁的正碰撞对梁所引起的瞬态冲 击响应公式，并用算例进行了分析，且与集中质量对经典的无约束Timoshenko梁的正碰撞 对梁所引起的冲击响应进行了比较，另外还用算例分析了梁的刚度的变化和冲击质量比对其 冲击响应产生的影响.     

Extension of the static shakedown theorems to softening materials

A generalization of the static shakedown theorems for elastic plastic hardening solids with isotropic [Mech. Res. Commun. 29 (2002) 159] and anisotropic [Acta Mechanica, 2004] damage accounting for the possibility of material softening is proposed.     

Asymptotic Behavior of Quasi-Autonomous Expansive Type Evolution Systems in a Hilbert Space

We introduce the notion of almost expansive sequences and curves and study their ergodic and asymptotic properties in a Hilbert space H. We apply our results to study the asymptotic behavior of solutions to the quasi-autonomous expansive type evolution system (du/dt)(t) + f(t) ∈ Au(t) on [0, ∞).     

心脏与血管的相互影响

心脏与血管的相互影响早就引起生理学家们的兴趣，但其在临床医学上的广泛应用则是近几年的事．这主要归功于两大生物力学成就，一是应甩泵功能曲线和时变弹性元描述心脏功能，另一是应用输入阻抗来描述血管功能．本文系统地总结和评述了２０多年来国际上关于心脏与血管相互影响的研究发展概况．     

三氧化二铝短纤维对ZA22合金干摩擦磨损性能的影响

针对无油润滑场合的实际需要，采用挤压铸造法制取了Ａｌ２Ｏ３短纤维强化ＺＡ２２合金复合材料，并对其在干摩擦条件下的摩擦磨损性能进行了试验研究，同时还用扫描电子显微镜对试样磨损表面形貌进行了观察，进而对材料的磨损机理作了分析与讨论。结果表明，随着Ａｌ２Ｏ３短纤维含量的增大，ＺＡ２２／Ａｌ２Ｏ３复合材料的耐磨性能提高，但摩擦性能降低；当纤维取向垂直于摩擦而时，复合材料的摩擦磨损性能比纤维平行于摩擦面取向     

微孔洞生长对板材拉伸过程中变形局部化的影响

本文在作者提出的含孔洞材料下限本构方程的基础上，采用了初始缺陷带模型对微孔洞生长及分布对板材拉伸过程中变形局部影响进行了，分析着重研究了细观损演化规律对变形局部化模式及临界应变的影响，并成功预测了ＡＩＳＩ４３４０钢板材拉伸试件变形局部化换稳为及失稳方向。     

End Effects for Anti-Plane Shear Deformations of Periodically Laminated STrips with Imperfect Bonding

The axial decay of Saint-Venant end effects is investigated for anti-plane shear deformations of semi-infinite generally laminated anisotropic strips. Imperfect bonding conditions are imposed at the interfaces. The analytical approach, using a displacement field which decays exponentially in the axial direction, gives rise to a transcendental equation for the real eigenvalues. The decay rate for the stresses is given in terms of the smallest positive eigenvalue. Laminated strips with periodic layout are then considered. In the presence of imperfect bonding, the effective shear elastic moduli, computed through a homogenization method, depend on the total number of slipping interfaces in the laminate. Numerical examples confirm that the decay lengths computed with effective shear moduli represent the asymptotic values (for an increasing number of layers) for those of periodically laminated strips. This revised version was published online in July 2006 with corrections to the Cover Date.     

A terminally anchored polymer chain in shear flow: Self-consistent velocity and segment density profiles

The behavior of a terminally anchored freely-jointed bead-rod chain, subjected to solvent shear flow, was investigated via Brownian dynamics simulations. Previous calculations have been improved by computing the segment density and fluid velocity profiles self-consistently. The segment density distributions, components of the radius of gyration, and chain attachment shear and normal stresses were found to be sensitive to low values of shear rate. Additionally, it was found that the thickness of a model polymer layer was a strong function of the shear rate, and that the functional dependence on shear rate changed dramatically as the chain length increased. For the longest chains studied, the thickness of the model polymer layer first increased as the shear rate increased, passed through a maximum, and then decreased at high shear rates, in accordance with experimental results in theta solvents. These results suggest that a dilute or semi-dilute layer model may explain hydrodynamic behavior previously thought to be due to the entanglements that occur in dense surface bound polymer layers.Nomenclature a _i acceleration of bead i - b radius of the beads - d length of the rods connecting the chain beads - d _i vector from bead i to bead i + 1 - F _i external force applied to bead i - F _i ^b external force on bead i due to Brownian motion of surrounding fluid - F _i ^h external force on bead i due to viscous drag - F _i ^s external force on bead i due to surface interactions - f Stokes drag coefficient - Boltzmann's constant - L _h effective hydrodynamic thickness - m _i mass of bead i - N number of beads on a model chain - n number of chains anchored to the surface per unit surface area - P segment density distribution P pressure - Q flow in a tube with no surface bound polymer layer - Q _a flow in a tube with a surface bound polymer layer - R _g vector representation of the radius of gyration - R tube radius - r radial coordinate in the tube geometry - S _ij pair hydrodynamic interaction tensor for beads i and j - T _i internal chain force in rod i connecting beads i and i + 1 - T _X component of the surface attachment force in the direction of the fluid flow - T _y component of the surface attachment force perpendicular to the surface - T temperature - v _i velocity of the center of mass of bead i - V _if average fluid velocity at the location of bead i - v _if ⁰ fluid velocity in the absence of a polymer chain - v _if perturbation to the fluid velocity due to hydrodynamic interactions - V _b bead volume = 4 b ³/3 - scalar fluid speed in the axial direction down the tube - x axial coordinate in the tube geometry Greek symbols _w apparent shear rate - fluid viscosity - polymer layer permeability - volume fraction of space occupied by chain beads - (_w)_a chain attachment stress perpendicular to the surface - (_w)_a chain attachment stress in the plane of the surface and in the direction of fluid flow