Time-dependent mechanical behaviour of the periodontal ligament

The process of tooth displacement in response to orthodontic forces is thought to be induced by the stresses and strains in the periodontium. The mechanical force on the tooth is transmitted to the alveolar bone through a layer of soft connective tissue, the periodontal ligament. Stress and/or strain distribution in this layer must be derived from mathematical models, such as the finite element method, because it cannot be measured directly in a non-destructive way. The material behaviour of the constituent tissues is required as an input for such a model. The purpose of this study was to determine the time-dependent mechanical behaviour of the periodontal ligament due to orthodontic loading of a tooth. Therefore, in vivo experiments were performed on beagle dogs. The experimental configuration was simulated in a finite element model to estimate the poroelastic material properties for the periodontal ligament. The experiments showed a two-step response: an instantaneous displacement of 14.10 +/- 3.21 microns within 4 s and a more gradual (creep) displacement reaching a maximum of 60.00 +/- 9.92 microns after 5 h. This response fitted excellently in the finite element model when 21 per cent of the ligament volume was assigned a permeability of 1.0 x 10(-14) m4/N s, the remaining 97 per cent was assigned a permeability of 2.5 x 10(-17) m4/N s. A tissue elastic modulus of 0.015 +/- 0.001 MPa was estimated. Our results indicate that fluid compartments within the periodontal ligament play an important role in the transmission and damping of forces acting on teeth.     

Anisotropic response of the human knee joint meniscus to unconfined compression

The response of meniscal tissue to axial, radial and circumferential compressive forces was measured at physiologically relevant levels of load in eight pairs of human knee joint menisci. Compression was unconfined and uniaxial. Stress strain data were fitted to a two-parameter exponential model. The tissue was found to be significantly stiffer to axial compressive forces than to radial and circumferential forces. No significant difference was found between the responses to circumferential and radial forces.     

超高压力温度状态液压油体积弹性模量测量装置故障分析

液压油的体积弹性模量和热膨胀系数是分析研究液压系统动、静态特性的两个不可或缺的重要参数。在超高压力及温度状态下,液压油就不能再视为不可压缩流体,其压缩和膨胀将对液压系统的动、静态特性产生极其重要的影响。因此,测量超高压力及超高温度状态液压油的体积弹性模量和热膨胀系数对设计研究超高压力温度液压系统有着重要的理论和实际意义。针对所设计的超高压力温度液压油体积弹性模量测量装置,结合测量过程中出现的实际问题,进行故障原因分析研究,并提出改进措施,以利于后续对不同高温高压液压油进行体积弹性模量测量研究。     

Elimination of the friction effects in unconfined compression tests of biomaterials and soft tissues

The mechanical properties of biomaterials and soft tissues are determined conventionally using unconfined compression tests. In such tests, frictionless specimen/platen contact in unconfined compression tests has to be assumed in determining the material properties of the materials. Previous theoretical analysis demonstrated, however, that the effects of the friction at the specimen/platen contact interface on the measured stress responses are non-negligible. In this study, a computational approach was proposed to eliminate the effects of friction. The friction coefficient between the specimen and the compression platens is measured first. Using a finite element model, the stress-strain relationship, without the influence of the friction effects, can be derived from the experimental data obtained in conventional unconfined compression tests. In order to validate the proposed approach, unconfined compressive tests of rubber have been performed.     

硬质聚氨酯泡沫在多轴压缩试验下的力学特性研究

硬质聚氨酯泡沫广泛应用于吸能装置。为研究硬质聚氨酯泡沫在多轴载荷下的力学响应,应用静水压试验和围压试验对其进行多轴压缩试验,分析其力学特性和吸能特性。结果表明：在静水压试验中,泡沫材料经历了弹性区域、平台区域和密实化区域三个阶段,相应的体积弹性模量、平台应力和单位体积吸能量均比单轴压缩试验的高;而在围压试验中,泡沫仅经历弹性阶段和峰值后阶段,无明显的平台区域和密实化区域。峰值偏应力随着围压增加而减少,但当围压超过一临界值时,偏应力-轴向应变曲线表现出一定的随机性,丧失了进一步承受轴向载荷的能力。基于试验数据和微分硬化屈服面模型,获得不同塑性应变下硬质聚氨酯泡沫的屈服面。     

Elastohydrodynamic lubrication analysis of a functionally graded layered bearing surface,with particular reference to 'cushion form bearings' for artificial knee joints

Elastohydrodynamic lubrication of a functionally graded layered (FGL) bearing surface, whose elastic modulus increases with depth from the bearing surface, was investigated in this study. The finite difference method was employed to solve the Reynolds equation, simultaneously with the elasticity equation of the bearing surface, under circular point contacts. The finite element method was adopted to solve the elasticity equation for the FGL bearing surface. The displacement coefficients thus obtained were used to calculate the elastic deformation of the bearing surface, required for the elastohydrodynamic lubrication analysis. Good agreement of the predicted film thickness and pressure distribution was obtained, between the present method and a previous study for a single layered bearing surface with a uniform elastic modulus. The general numerical methodology was then applied to an FGL bearing surface with both linear and exponential variations in elastic modulus, with particular reference to the 'cushion form bearing' for artificial knee joints. The predicted film thickness and pressure distribution were shown to be quite close to those obtained for a single layer under typical operating conditions representative of artificial knee joints, provided that the elastic modulus of the single layer was chosen to be the average elastic modulus of the graded layer.     

声空化条件下传动液中空气析出与溶解过程的研究

传动液中空气的析出与溶解影响传动系统的控制精度。传动液起到传递动力的作用,本身会溶解少量空气,溶解的空气随着压力的变化产生溶解和析出过程,破坏了液流的连续性,造成传动性能的下降,甚至影响传动系统的使用寿命。为此,基于斜压流模型,引入气体析出与溶解的气泡模型,建立传动管内的气液两相流含气率模型,考虑空气质量分数和体积分数,得到空化流动相关方程式。采用特征线法和一维有限差分法求解,获得了气液两相流主要参数的变化,包括空气析出与溶解时间常数、压力和温度对含气率的影响,并研究了含气率对体积弹性模量的影响。结果表明：传动管内的传动介质压力越大,空化程度越少;空气析出速率越小,含气率越低。传动介质受压引起其温度的变化;在空化区域,温度变化较小。当传动管内压力高于空气分离压时,传动介质的压力和体积弹性模量随含气率增大而减小;当压力低于空气分离压时,发生空化现象,含气率增加较快,但对体积弹性模量影响较小。     

Probing elastic modulus and depth of bottom-supported inclusions in model tissues using piezoelectric cantilevers

We have experimentally investigated the depth sensitivity limit of a piezoelectric cantilever tissue elastic modulus sensor and simultaneously determined the elastic modulus and the depth of a tumor directly. Using model tissues consisting of bottom-supported modeling clay inclusions of various depths in a gelatin matrix, we empirically determined that the depth sensitivity limit of a piezoelectric cantilever sensor was twice the linear dimension of the indentation area (or the cantilever width). Knowing the depth sensitivity limit of the individual cantilever sensor as input and treating a model tissue that has the gelatin matrix on top and the modeling clay inclusion at the bottom as two springs in series, we showed that the elastic moduli and depths of the hard inclusions could be simultaneously determined with the elastic modulus profiles measured by two cantilevers with different widths as input.     

集成式电子液压制动系统位移压力特性理论研究

集成式电子液压制动系统（Integrated electro-hydraulicbrakesystem,IEHB）的位移压力特性对于现有的IEHB主缸压力控制算法及未来可期的主缸压力估计算法均至关重要，现有研究多以实测和曲线拟合为主，缺乏理论依据。为此，从含气制动液等效体积弹性模量和制动回路变形特性分析入手，首先基于合理假设，提出制动回路简化模型；之后通过由制动回路变形表示的等效体积弹性模量与制动液自身的等效体积弹性模量相等，推导出主缸活塞位移与压力的函数关系，即位移压力模型；最后通过台架试验对位移压力模型进行参数辨识和模型验证，结果表明，在0～10MPa的压力范围内，所提出的位移压力模型与试验数据的误差最大不超过0.255MPa。     

Compressibility of Thin Film Lubricants Characterized Using Atomistic Simulation

Compressibility of thin lubricating films remains incompletely understood. Molecular dynamics simulations of a polymeric fluid confined in a nanoscale channel are used to investigate the effects of pressure and temperature on a thin film’s response to compression. The observed trends are analyzed and compared to predictions of traditional, bulk fluid compressibility models. At ambient temperatures and pressures, molecular dynamics and bulk fluid models predict consistent trends for both the volume ratio and the bulk modulus. However, small deviations between the models suggest that molecular-scale confinement may, under some conditions, affect a fluid’s compressibility such that it is not necessarily predictable using bulk fluid theories.     

Three-dimensional dynamic model of the knee

A three-dimensional dynamic model of the knee was developed to study the interactions between the articulating surfaces of the bones and the geometrical and mechanical properties of the ligaments. The contact-surface geometry of the distal femur, proximal tibia, and patella was modeled by fitting polynomials to each of the eight articular surfaces. Twelve elastic elements were used to describe the function of the ligamentous and capsular structures of the knee. The origin and insertion sites of each model ligament were obtained from cadaveric data reported for an average-size knee. The response of the model to both anterior-posterior drawer and axial rotation suggests that the geometrical and mechanical properties of the model ligaments approximate the behavior of real ligaments in the intact knee. Comparison of the model’s response with experimental data obtained from cadaveric knee extension indicate further that the three-dimensional model reproduces the response of the real knee during movement.     

The influence of the viscoplastic and viscoelastic character of a lubricant on elastohydrodynamic lubrication

The non-newtonian constitutive equation proposed by Johnson and Winer is discussed. Derivations of the lubrication equations suitable for the viscoplastic rheological model and the viscoelastic model are presented. The full numerical elastohydrodynamic lubrication solutions for the viscoplastic model and the viscoelastic model are obtained in order to observe the effects of the elastic shear modulus of the fluid and the limiting shear stress on the pressure distribution, film thickness and coefficient of friction.     

基于Hirsch模型的复合材料弹性模量的改进算法

基于颗粒增强型复合材料的特征，在Hirsch纤维状复合材料弹性模量预测模型的基础上，修正并提出了一种新的复合材料弹性模量计算表达式。为验证该算法的实用性，引用了文献试验数据进行预测计算。结果表明：对于由喷射共沉积法制备获得的6066/SiC复合材料，计算结果处在Voigt模型和Reuss模型的上下限之间，并与试验值相一致，从而验证了所提出计算模型的正确性。      

SiCp/2024Al复合材料高应变率热变形行为的新本构模型#br#

通过分离式霍普金森压杆(SHPB)动态压缩试验研究了体积分数为45%的铝基碳化硅颗粒增强复合材料（SiCp/2024Al）在大应变率和变形温度范围内的热变形行为,分析了热变形参数（变形温度和应变率）对流动应力的影响。研究发现：变形温度和应变率对复合材料的流变应力、抗压强度、弹性模量、应变率敏感性有显著影响;抗压强度、弹性模量随变形温度的增大而减小,而抗压强度、弹性模量、应变率敏感性随应变率的增大出现了拐点。根据试验结果,结合热力学和统计损伤力学理论,建立了描述SiCp/2024Al复合材料动态热变形行为的连续损伤本构模型,预测的流动应力与试验结果吻合较好,表明所建立的模型能够准确地描述SiCp/2024Al复合材料动态热变形行为。     

The elastic moduli and fatigue properties of canine trabecular bone tissue

The elastic modulus and fatigue properties of canine and human trabecular bone tissues (single trabeculae) were experimentally determined on a microstructural level using four-point bending cyclic test, and they were compared based on microstructural characteristics and mineral density. The results showed that canine trabecular bone tissue had significantly lower modulus and lower fatigue strength than human tissue. The observed microstructural differences between the two tissues may be more responsible for the differences, although the lower mineral density in canine tissue might also have contributed to the lower modulus and fatigue strength.     

Analysis of design parameters for shunt valve and anti-siphon device used to treat patients with hydrocephalus

The present study investigated design parameters of shunt valves and anti-siphon device used to treat patients with hydrocephalus. The shunt valve controls drainage of cerebrospinal fluid (CSF) through passive deflection of a thin and small diaphragm. The anti-siphon device (ASD) is optionally connected to the valve to prevent overdrainage when the patients are in the standing position. The major design parameters influencing pressure-flow characteristics of the shunt valve were analyzed using ANSYS structural program. Experiments were performed on the commercially available valves and showed good agreements with the computer simulation. The results of the study indicated that predeflection of the shunt valve diaphragm is an important design parameter to determine the opening pressure of the valve. The predeflection was found to depend on the diaphragm tip height and could be adjusted by the diaphragm thicknesss and its elastic modulus. The major design parameters of the ASD were found to be the clearance (gap height) between the thin diaphragm and the flow orifice. Besides the gap height, the opening pressure of the ASD could be adjusted by the diaphragm thickness, its elastic modulus, area ratio of the diaphragm to the flow orifice. Based on the numerical simulation which considered the increased subcutaneous pressure introduced by the tissue capsule pressure on the implanted shunt valve system, optimum design parameters were proposed for the ASD.     

Muscle-ligament interactions at the human knee (I)

A three-dimensional dynamic model of the knee was developed to study the interactions between the articulating surfaces of the bones and the geometrical and mechanical properties of the ligaments. The contact-surface geometry of the distal femur, proximal tibia, and patella was modeled by fitting polynomials to each of the eight articular surfaces. Twelve elastic elements were used to describe the function of the ligamentous and capsular structures of the knee. The origin and insertion sites of each model ligament were obtained from cadaveric data reported for an average-size knee. The response of the model to both anterior-posterior drawer and axial rotation suggests that the geometrical and mechanical properties of the model ligaments approximate the behavior of real ligaments in the intact knee. Comparison of the model’s response with experimental data obtained from cadaveric knee extension indicate further that the three-dimensional model reproduces the response of the real knee during movement.     

拉压弹性模量不同材料圆环内的热变形

研究了拉压弹性模量不同材料圆环内的热应力问题。采用结构力学弹性中心法得到了拉压弹性模量不同材料圆环在温度作用下热变形的正则方程,求得了圆环截面的弯矩和轴力表达式。通过算例分析,讨论了温度变化对圆环截面弯矩和轴力的影响。     

主动弹支干摩擦阻尼器控制转子突加不平衡响应的研究

建立了弹支干摩擦阻尼器——转子系统突加不平衡响应的计算模型,理论分析和实验验证了转子系统突加不平衡响应特性。结果表明,主动弹支干摩擦阻尼器能够有效控制转子系统突加不平衡的振动响应;并且,当动、静摩擦片之间压紧力增大时,将随之使振动幅值减小、瞬态过程缩短。实验与理论分析结果吻合较好。     

Lateral Contact Stiffness and the Elastic Foundation

A model for the lateral contact stiffness for an elastic foundation was developed. The model was evaluated using a low force and low contact pressure microtribometer capable of performing indentation and reciprocated sliding experiments. The slope of lateral force versus the lateral displacement was used to fit the shear modulus. When complementary elastic indentation measurements are made to determine the composite modulus of the elastic foundation, there is sufficient data to fit elastic modulus, shear modulus, and Poisson ratio for these thin films. Using these models, the elastic properties for a thin (~65 μm) vertically aligned multiwall carbon nanotube film were evaluated. The experiments were performed with a silicon nitride indenter (radius = 1.6 mm) over a range in loads from 100 to 800 μN. The resulting values of the elastic modulus, shear modulus, and Poisson ratio were E = 429 kPa, G = 156 kPa, and ν = 0.37, respectively.