钛珠涂层和羟基磷灰石涂层与皮质骨界面的切向微动损伤研究

通过体外模拟切向微动行为,探究两种生物固定材料-钛珠涂层和羟基磷灰石涂层与皮质骨界面之间的微动摩擦磨损机理,建立钛珠涂层和羟基磷灰石涂层与骨组织生物固定界面的微动-松动-骨损伤的关系.结果表明,界面处于部分滑移区时,弹塑性协调作用起主导,固定界面不易产生松动,皮质骨表面损伤以剥落为主,表面损伤较小.增大微动位移幅值,界面摩擦系数增大,皮质骨的损伤增大.同时,划分出了钛珠涂层和羟基磷灰石涂层部分滑移区的工况分界线.通过对比两种材料的摩擦磨损机理,钛珠涂层与皮质骨界面固定效果较好,羟基磷灰石涂层与皮质骨界面损伤较小.结果对人工关节生物固定起到一定的参考作用.     

Propagation of nitromethane detonations in porous media

The characteristics of the propagation of a detonation in chemically sensitized nitromethane in a dense porous medium are investigated. By introducing liquid NM+15% (by weight) DETA into densely packed beds of solid spherical glass beads 66μm to 2.4 mm in diameter, a highly heterogeneous explosive mixture is obtained. The critical (i.e., failure) charge diameter of this mixture is systematically measured in unconfined charges over a wide range of bead sizes. Velocity measurements are also made for the various charges. It is found that there exists a critical bead size above which the critical diameter decreases with increasing bead size and below which it decreases with decreasing bead size. This result indicates an abrupt change in the mechanism of propagation at the critical bead size. Velocity measurements further support this by emphasizing the different behavior above and below the critical point.     

Bifurcations of relative equilibria of a heavy bead on a rotating parabolic bowl with dry friction

The motion of a heavy bead on the surface of a parabolic bowl rotating at a constant angular velocity about its axis, which coincides with the vertical, is considered. It is assumed that the dry friction force acts between the bead and the bowl. The sets of nonisolated relative equilibria of the bead on the bowl are determined, and their dependence on the problem parameters is studied. The results are illustrated in the form of bifurcation diagrams.     

应力调整对石英玻璃珠低速冲击破碎行为的影响

结合高速摄影技术,应用SHPB加载装置,分别使用钢制、铝制和有机玻璃制3种透射杆,对直径约7.90、11.80、15.61 mm 3种尺寸的石英玻璃珠进行了低速冲击实验。根据不同透射杆条件下的玻璃珠破碎过程中的载荷-位移曲线,结合有限元软件计算玻璃珠在冲击作用下载荷的变化情况以及实验过程中玻璃珠的应变,探讨了应力调整对玻璃珠破碎过程的影响。结果表明:相同冲击条件作用下,改变透射杆的材料,会改变玻璃珠破碎过程中的载荷分布,即透射端边界波阻抗的改变会导致反射波发生改变,从而导致玻璃珠内部载荷发生变化;透射杆为铝材和有机玻璃材质时,玻璃珠在破碎过程中的载荷明显下降,在加载过程中伴随着垫块的变形,玻璃珠内部的应力调整时间变长;透射杆为钢杆时,玻璃珠的应变主要表现为两端最大,越靠近中间应变越小,对于透射杆为铝杆和有机玻璃杆的玻璃珠,透射端局部出现了卸载行为。采用有机玻璃透射杆之后,局部应力和变形降低的结果使得玻璃珠在经受较大的变形之后发生破碎,表明玻璃珠的破碎行为由局部变形和局部变形梯度共同控制。     

Biological membranes from the perspective of smart materials – A theoretical study

The unique properties and diverse functionality of biological membranes make them excellent candidates for nano-scale applications, such as sensors and actuators. Taking the view of biological membranes as smart bio-materials, we study the behavior of a simply supported beam made from a biological membrane-like material. Equilibrium configurations are derived by calculating the first variation of a generalized Helfrich energy, and their stability is examined by means of the second variation. Our numerical results demonstrate the richness of phenomena exhibited by these structures, in accordance with experimental observation of multi-component vesicles. Further, we demonstrate that the intriguing behavior of biological membrane beams, which is fundamentally different from standard beams and from standard Cahn Hilliard systems, can be utilized for actuation and sensing. For example, temperature and also pressure difference across the membrane can be indirectly measured by gauging the fluorescence intensity of the membrane components.     

Interaction of a high-speed combustion front with a closely packed bed of spheres

The head-on collision of a combustion front with a closely packed bed of ceramic-oxide spheres was investigated in a vertical 76.2 mm diameter tube containing a nitrogen diluted stoichiometric ethylene–oxygen mixture. A layer of spherical beads in the diameter range of 3–12.7 mm was placed at the bottom of the tube and a flame was ignited at the top endplate. Four orifice plates spaced at one tube diameter were placed at the ignition end of the tube in order to accelerate the flame to either a “fast-flame” or a detonation wave before the bead layer face. The mixture reactivity was adjusted by varying the initial mixture pressure between 10 and 100 kPa absolute. The pressure before and within the bead layer was measured by flush wall-mounted pressure transducers. For initial pressures where a fast-flame interacts with the bead layer peak pressures recorded at the bead layer face were as high as five times the reflected Chapman–Jouget detonation pressure. The explosion resulting from the interaction developed by two distinct mechanisms; one due to the shock reflection off the bead layer face, and the other due to shock transmission and mixing of burned and unburned gas inside the bead layer. The measured explosion delay time (time after shock reflection from the bead layer face) was found to be independent of the incident shock velocity. As a result, the explosion initiation is not the direct result of the shock reflection process but instead is more likely due to the interaction of the reflected shock wave and the trailing flame. The bead layer was found to be very effective in attenuating the explosion front transmitted through the bead layer and thus isolating the tube endplate. This paper is based on work that was presented at the 21th International Colloquium on the Dynamics of Explosions and Reactive Systems, Poitiers, France, July 23–27, 2007.     

Combustion in a horizontal channel partially filled with a porous media

Experiments were carried out to investigate the combustion propagation phenomenon in a horizontal channel partially filled with ceramic-oxide spherical beads. A 1.22 m long, 43 mm nominally thick layer of spherical beads is located at the ignition end of a 2.44 m long, 76 mm square channel. Tests were performed with 6.4 and 12.7 mm diameter beads. A flame is ignited at the bead end wall by an automotive spark ignition system. Flame propagation and pressure measurements are obtained via ionization probes and piezoelectric pressure transducers mounted on the top and bottom surfaces of the channel. High-speed schlieren video was used to visualize the structure of the explosion front. Experiments were performed with a 31% nitrogen diluted stoichiometric methane–oxygen mixture at room temperature and at an initial pressure in the range of 15–50 kPa. For initial pressures of 15 and 20 kPa the flame accelerates to a velocity close to the speed of sound in the combustion products. For initial pressure of 30 kPa and higher DDT occurs in the gap above the bead layer. An explosion front propagating at a velocity just under the CJ detonation velocity is detected in the bead layer even though the bead layer pore size is much smaller than the detonation cell size. It is demonstrated that flame propagation within the bead layer is the driving force behind the very rapid flame acceleration observed, however the DDT event occurring in the gap above the bead layer is not affected by the bead layer porosity. Schlieren video indicates that the structure of the explosion front varies across the channel height and with propagation distance down the channel.     

A validated thermal model of bead-on-plate welding

In this paper, finite element model is used to carry out thermal analysis of bead-on-plate welding. The model followed the proposed five step strategies which were then built into a model to obtain temperature history at the positions of thermocouples. Temperature field was also evaluated by comparing predicted weld bead with the actual weld bead. Using these proposed strategies, well matched temperature histories and temperature field have been obtained.     

Motion of a spherical particle under the influence of osmotic forces

One of the basic mechanisms through which biological cells preserve the consistency of their structure is the regulation of material exchange between cell and surrounding medium. The regulation is performed by a thin semipermeable membrane surrounding the cell; it passes some materials freely, while allowing only slight transmission of others, or completely blocking passage. Water shows the maximum penetrating power. If there exists on both sides of the membrane an aqueous solution of some material which does not penetrate the membrane, then under the influence of osmotic pressure water flows into that region where the material concentration is higher, as long as the difference in hydrostatic pressure does not suppress the motion [1, 2]. The viscous flow which develops may lead to displacement of a particle.     

Decohesion of a rigid punch from non-linear membrane undergoing finite axisymmetric deformation

Adhesion of flexible thin film or membrane plays an important role in many biological and industrial applications. In many physical systems, large deformation is often expected, especially for biological membranes, which suggests that non-linear analysis might be required. In this paper the adhesion and decohesion between a rigid flat punch and a non-linear membrane undergoing finite axisymmetric deformation is studied. The decohesion is assumed to obey the Griffith fracture criterion. The cases of initially stress-free and prestressed membrane are analyzed. A comparison with published works using the linear theory shows that the non-linear theory substantially deviates from the linear one, qualitatively and quantitatively. These results advocate the use of non-linear consideration when the membrane undergoes large deformation prior to and during the debonding process.     

Simulation and prediction of membrane fusion dynamics

Membrane fusion is an important process by which biological membranes perform their life activities. Simulations show that the membrane fusion process happens mainly through three pathways, where the Stalk-Pore hypothesis, in which two membranes come into close contact to form a stalk to a hemifusion intermediate, and then the fusion pore opens to achieve completely fusion, is widely accepted, and there exist two free energy barriers that break the current structural steady state for lipid rearrangement. Factors of lipid composition, mechanical environment, protein and ion have regulatory roles in the membrane fusion process by effecting membrane curvature structurally and the free energy barriers from energetic perspective. Meanwhile, many theoretical models, represented by the Helfrich model, have been proposed to predict the membrane fusion process. In this paper, we review the research process of membrane fusion and mainly introduce the dynamics of membrane fusion, regulation factors and typical theoretical models.     

Application of DEM modified with enlarged particle model to simulation of bead motion in a bead mill

We applied the discrete element method （DEM） of simulation modified by an enlarged particle model to simulate bead motion in a large bead mill. The stainless-steel bead mill has inner diameter of 102 mm and mill length of 198 mm. The bead diameter and filling ratio were fixed respectively at 0.5 mm and 85%. The agitator rotational speed was changed from 1863 to 3261 rpm. The bead motion was monitored experimentally using a high-speed video camera through a transparent mill body. For the simulation, enlarged particle sizes were set as 3-6 mm in diameter. With the DEM modified by the enlarged particle model, the motion of enlarged particles in a mill was simulated.The velocity data of the simulated enlarged particles were compared with those obtained in the experiment. The simulated velocity of the enlarged particles depends on the virtual frictional coefficient in the DEM model. The optimized value of the virtual frictional coefficient can be determined by considering the accumulated mean value. Results show that the velocity of the enlarged particles simulated increases with an increase in the optimum virtual frictional coefficient, but the simulated velocity agrees well with that determined experimentally by optimizing the virtual frictional coefficient in the simulation. The computing time in the simulation decreases with increased particle size.     

Hydrodynamic interactions of dilute polymer solutions under shear flow in a narrow channel

Hydrodynamic interactions on dilute solutions of spherical beads under shear flow are calculated with the method of induced forces. The Navier-Stokes equation is considered in the Stokes approximation. Hydrodynamic interactions cause the drag to be anisotropic in space.Numerical solutions are obtained for the added stress, caused by polymeric molecules in solution in a narrow channel under shear flow. The polymeric molecules are considered as Hookean spring-dumbbells.Slip velocity and the effective viscosity are obtained taking different dumbbells' bead radii. Transversal migration in the channel is obtained for different bead radii.     

Stress distribution in L-shaped butt joint: Welded or bonded

The stresses in L-shaped butt joint, welded or bonded, are obtained by the finite element method. Correction for kinematic nonlinearity is made by including an upward movement of the joint. The local stress field behavior depends on the bead dimension, applied load and the governing material properties. The ways with which these parameters affect the joint behavior are investigated by comparing results obtained from a linear and nonlinear analysis.     

Application of DEM modified with enlarged particle model to simulation of bead motion in a bead mill

We applied the discrete element method (DEM) of simulation modified by an enlarged particle model to simulate bead motion in a large bead mill. The stainless-steel bead mill has inner diameter of 102 mm and mill length of 198 mm. The bead diameter and filling ratio were fixed respectively at 0.5 mm and 85%. The agitator rotational speed was changed from 1863 to 3261 rpm. The bead motion was monitored experimentally using a high-speed video camera through a transparent mill body. For the simulation, enlarged particle sizes were set as 3–6 mm in diameter. With the DEM modified by the enlarged particle model, the motion of enlarged particles in a mill was simulated. The velocity data of the simulated enlarged particles were compared with those obtained in the experiment. The simulated velocity of the enlarged particles depends on the virtual frictional coefficient in the DEM model. The optimized value of the virtual frictional coefficient can be determined by considering the accumulated mean value. Results show that the velocity of the enlarged particles simulated increases with an increase in the optimum virtual frictional coefficient, but the simulated velocity agrees well with that determined experimentally by optimizing the virtual frictional coefficient in the simulation. The computing time in the simulation decreases with increased particle size.     

泡沫铝-复合泡沫冲击韧度及黏弹性分析

泡沫铝-复合泡沫互穿相复合材料是一种新型的结构功能一体化材料,在机械、交通、建筑等领域具有重要的应用前景。本文制备了纯泡沫铝及空心玻璃微珠四种体积分数的泡沫铝-复合泡沫试件,通过摆锤冲击实验研究了其冲击韧度,讨论了其断口形貌与结构性能的关系,通过单轴压缩的应力松弛实验研究其黏弹性性能。研究表明：空心玻璃微珠体积分数为10%的泡沫铝-复合泡沫试件的冲击韧度最大,随后其冲击韧度随空心玻璃微珠体积分数的增加而减小;当空心玻璃微珠体积分数较小时,泡沫铝的结构形貌决定了冲击断口的形貌,但当其体积分数达到30%时,空心玻璃微珠的破坏是形成断口形貌的主要原因;复合材料的黏弹性性能随着玻璃微珠体积分数的增加而增大。     

Molecular shape in homogeneous flows: predictions of the rouse model

An analysis of the Rouse model is used to predict the shape of flexible, free-draining polymers in homogeneous flows. The derivation is based on the Lodge-Wu/Booij-Van Wiechen solutions [1,2] for the configurational distribution function of a Rouse chain, which consists of a free-draining string of beads connected by Hookean springs. Much of the theoretical development parallels the one used by Saab et al. [3,4] to predict link orientations for the same molecular model; rather than solving for link orientation, however, we solve for bead location. The function P_v(U, t), which describes the spatial distribution of bead v about the chain center of mass in the presence of hydrodynamic forces, is a key development of this contribution. Various averages evaluated from this function can be employed to study details of a deformed molecule's configuration. The model predicts that spatial distortion in flow is a highly nonuniform function of location in the chain; hydrodynamic forces have a greater influence on the beads near the ends of the chain than on those near the center.     

Experimental Study of Strongly Nonlinear Resonances and Anti-Resonances in Granular Dimer Chains

Previous theoretical works considered the intrinsic dynamics of one-dimensional uncompressed granular dimer (diatomic) chains composed of pairs of dissimilar spherical elastic beads in Hertzian interaction. Such ordered granular media exhibit essentially nonlinear acoustics and have been characterized as ‘sonic vacua’ due to the fact that the speed of sound in these media (as defined in classical acoustics) is zero. Yet, depending on the mass ratios of the pairs of dissimilar beads of these dimers, it was proven that they may possess countable infinities of anti-resonances leading to solitary waves (this in spite of their high inhomogeneity), or countable infinities of strongly nonlinear resonances leading to passive strong attenuation of propagating pulses through energy radiation by means of excitation of traveling waves. The aim of this work is to experimentally verify the existence of these strongly nonlinear dynamics through a series of experiments involving granular dimer chains supported by flexures. By carefully designing the supporting flexures so that their dynamics is sufficiently ‘soft’ and thus separate from the ‘stiff’ dynamics governing the bead to bead interactions, we overcome a basic limitation for the experimental realization of such dimer systems, namely the construction of one-dimensional dimer chains with beads of different radii. Our results confirm experimentally the occurrence of nonlinear resonances and anti-resonances in dimer chains, and conclusively prove the capacity of appropriately designed granular dimers for passive strong attenuation of propagating pulses due to nonlinear resonance. Moreover, we validate the theoretical prediction that within the elastic range of bead to bead dynamical interactions the results are fully re-scalable with respect to energy. This work provides the first experimental evidence of strongly nonlinear resonances and anti-resonances in essentially nonlinear ordered granular media.     

Finitely extensible bead spring chains in time-dependent shear flows

The configurational and rheological properties of bead spring chains in time-dependent shear flows are calculated. The finite extensibility is incorporated through the constraint of constant contour length of the chain. Start-up of shear flow yields a stress overshoot, whereas oscillatory shear flow yields the same frequency dependence of the dynamic moduli as the simple bead spring model. The results show that finite extensibility can lead to non-linear rheological behavior of dilute polymer solutions. The influence of preaveraged hydrodynamic interaction on the obtained results is discussed.     

Dynamics of biological soft tissue and rubber: internally pressurized spherical membranes surrounded by a fluid

The behavior of a family of dynamical systems representing the elastodynamic response of an internally pressurized, non-linearly elastic spherical membrane lying in an incompressible external fluid is governed primarily by the strain energy function for the membrane, the specific forcing function due to the internal pressure, and the viscosity of the external fluid. It is shown that such systems with an inviscid external fluid and having a constant internal pressure are integrable but not Hamiltonian. Under periodic internal loading, and for a small spherical radius and constitutive relations typical of many biological soft tissues, a periodic orbit in phase space exists near a static equilibrium. A viscous external fluid causes the periodic orbit to be an attractor. The dynamical system is robust under small loading perturbations common in normal biological systems. Rubber models, on the other hand, may admit structural catastrophes. For small initial sphere radii, a jump from one periodic orbit to another is possible for rubber models but not for the classical soft tissue models. It is dangerous, therefore, to model soft biological tissue as a rubber either mathematically or physically in experiments because the predicted instabilities may not exist in tissue.