踝关节外骨骼人机耦合动力学与助力性能分析

踝关节在人体下肢运动过程中提供了最大的关节力矩, 因此在下肢增强型外骨骼的研究中, 踝关节外骨骼受到了重点关注. 穿戴外骨骼的人体的行走是典型的动力学问题, 但目前人机耦合动力学的相关研究还处于早期阶段. 本文以绳驱踝关节外骨骼为研究对象, 融合机器人正运动学方法和拉格朗日方程建立了考虑足?地交互力、人体关节力矩和外骨骼力矩的人?机耦合动力学模型. 模型中, 足?地交互力由Kelvin-Voigt模型结合库伦摩擦模型描述, 人体关节力矩由基于粒子群优化的PD控制生成, 外骨骼期望力矩由上层控制器依据人体步态周期确定. 通过基于模型的动力学仿真, 本文从人体踝关节角度、踝关节力矩、踝关节功率和踝关节做功多个角度系统分析了踝关节外骨骼对人体行走的助力效果. 研究表明, 在2.0 km/h到6.5 km/h的人体步行速度下, 穿戴外骨骼可以实现至少24.84%的人体踝关节平均力矩下降和至少24.69%的踝关节做功下降. 本文也开展了基于SCONE平台的肌肉骨骼建模和预测仿真. 仿真结果表明, 在3.6 km/h的步行速度下, 穿戴外骨骼可以有效降低比目鱼肌的激活度峰值, 并使肌电信号的RMS值下降了6.21%, 从而从生理学的角度证实了踝关节外骨骼的助力效果. 本文的结果进一步完善了人体下肢?外骨骼耦合系统的动力学建模和分析方法, 从动力学和生理学角度证实和解释了踝关节外骨骼对行走的助力机制, 也为今后下肢外骨骼的实验研究提供了理论支撑.      

Effect of disturbances and sensorimotor deficits on the postural robustness of an ankle–hip model of balance on a balance board

This paper investigates the effect of postural disturbances and sensorimotor deficits on the robustness of the upright posture (UP) for a human body model balancing on a balance board (BB). The robustness is investigated by computing the gradient field along the basin of attraction (BoA) of an asymptotically stable equilibrium point. The human model is modeled as a double-inverted pendulum (hip and ankle joints). The human-BB system is assumed actuated by torques at the hip, BB hinge, and ankle joints. Postural disturbances induce an initial joint angle velocity either at the ankle or at the hip joint. Moreover, either proprioceptive or visual and vestibular deficits are considered in the human-BB model. The nonlinear dynamic equation of motion of the human-BB system is numerically solved to obtain the BB, ankle, and hip joint angle position and velocity profiles. The BoA of the human-BB UP equilibrium is built as the set of initial conditions whose resulting time-series profiles converge to the equilibrium. It was shown that UP is more robust to disturbances that induce hip joint initial angle velocity. That is probably due to the fact that disturbances that induce ankle joint initial velocity affect the whole body, while disturbances that induce hip joint angle initial velocity only affect the trunk. Whenever visual and vestibular deficits are considered, the UP is more robust if proprioceptive gain and BB stiffness are small. Contrarily, whenever proprioceptive deficits are considered, the UP is more robust if visual and vestibular gain and BB stiffness are large. The method proposed here (the BoA and the gradients) can be used to systemically provide understanding about the robustness of the human-BB UP to external disturbances, which may help to identify people with a higher risk of fall.     

基于TCK模型的非贯通节理岩体动态损伤本构模型

提出在岩体动态损伤本构模型中应同时考虑宏、细观缺陷;基于能量原理和断裂力学理论推导得出了同时考虑节理几何及力学特征的宏观损伤变量(张量)的计算公式;基于综合考虑宏、细观缺陷的复合损伤变量(张量)及完整岩石动态损伤Taylor-Chen-Kuszmaul(TCK)模型,建立了相应的单轴压缩下节理岩体动态损伤本构模型;利用该模型讨论了节理内摩擦角及节理长度对岩体动态力学特性的影响规律。研究表明,试件动态峰值强度随着节理内摩擦角的增大而增大,随着节理长度的增加而减小。     

单轴压缩下断续节理岩体动态损伤本构模型

断续节理将对工程岩体的强度及变形等力学特性产生显著影响,损伤力学中视节理为岩体的一种宏观损伤,因而采用损伤张量来刻画其对岩体的影响。目前学术界提出了用节理的几何、强度及变形等3类参数来描述节理的物理力学性质,而目前的岩体损伤张量计算方法都只涉及前2类参数,均没有涉及其变形参数即法向及切向刚度。为此,在前人研究的基础上,基于断裂及损伤理论提出了考虑节理法向及切向刚度的单轴压缩下单条断续节理引起的损伤张量计算公式,进而通过考虑节理间相互作用给出了单组单排或多排节理岩体损伤张量计算公式。其次,以岩石细观动态损伤模型为基础,结合宏细观损伤耦合观点提出了一个能够同时考虑节理几何、强度及变形参数的断续节理岩体动态损伤本构模型。最后,利用该模型讨论了节理参数及载荷应变率等对岩体动态力学特性的影响,认为节理长度减小及摩擦角增大将导致岩体动态峰值强度及弹性模量增大;岩体动态峰值强度及弹性模量则随着节理法向及切向刚度的增大分别减小或增大;而当节理法向及切向刚度按照同一比例增大时,岩体动态峰值强度及弹性模量则是增大的。岩体动态峰值强度与载荷应变率呈正相关。     

Nondestructive evaluation of damage in composite structures using modal parameters

The problem of using measured modal parameters to detect and locate damage in structures made of fiberreinforced composites is investigated. Recent work in this area using modal sensitivity equations is used in conjunction with internal-state variable constitutive theory to derive a set of damage-detection equations which are used to predict, from changes in measured modal parameters, the current value of the internal-state variables in each finite element. The value of the internal-state variable determines the extent of damage at a given location. Numerical examples involving damaged composite beams are used to demonstrate the capability of the theory to predict the exact location and the severity of damage. To provide experimental evidence to support the theory, mechanical and modal tests are performed on a [0,90₃] _s laminated composite beam in the undamaged state and in three additional states of progressive damage. At each stage of damage, edge replications are taken to determine the crack density along the length of the beam. The predicted values of the internal-state variables, obtained from the modalsensitivity equations using measured modal information, are compared with the values of the internal-state variables obtained from crack-density measurements along the length of the beam. Good agreement between the predicted and the measured values is found.     

A coarse-grained model of the myofibril: Overall dynamics and the evolution of sarcomere non-uniformities

A theoretical framework for predicting the macroscopic behavior of a muscle myofibril based on the collective behavior of sarcomeres is presented. The analysis is accomplished by rigorously transforming the nonlinear dynamics of an assemblage of sarcomeres into a partial differential equation for the probability distribution function of sarcomere lengths in the presence of stochastic temporal fluctuations and biological variability. This enables the study of biologically relevant specimens with reasonable computational effort. The model is validated by a comparison to quantitative experimental results. Further, it reproduces experimental observations that cannot be explained by standard single sarcomere models, and provides new insights into muscle function and muscle damage during cyclic loading. We show that the accumulation of overstretched sarcomeres, which is related to muscle damage, depends on a delicate interplay between the dynamics of a large number of sarcomeres and the load characteristics, such as its magnitude and frequency. Further, we show that biological variability rather than stochastic fluctuations are the main source for sarcomere non-uniformities.     

A statistical theory of fatigue crack growth at damage cumulation

Summary A statistical theory of the fatigue crack growth at damage cumulation is proposed. The theory gives the average of fatigue crack length at any time t, and deduces the evolution of failure probability with time varying. Furthermore, the variance and relative error of fatigue crack length at any time t are acquired. The Paris equation for the average of the crack length at any time t is derived from the statistical theory. Therefore, the prediction of the probability distribution of the crack length can be given for any time t. Actual applications of the theory are given, which conform to the experiments. Accepted for publication 17 June 1996     

固体火箭发动机柔性接头摆动密封可靠性研究

柔性接头由弹性件与增强件交替粘接而成,是固体火箭发动机进行推力矢量控制的重要装置,因而柔性接头的摆动密封性能对固体火箭发动机而言至关重要.为研究固体火箭发动机柔性接头摆动过程中的密封可靠性,以内聚力模型作为粘接界面的本构模型,通过计算柔性接头各界面的损伤情况及界面间的接触应力,并定义界面节点单元间、界面间以及柔性接头的...     

A single strain-based growth law predicts concentric and eccentric cardiac growth during pressure and volume overload

Adult cardiac muscle adapts to mechanical changes in the environment by growth and remodeling (G&R) via a variety of mechanisms. Hypertrophy develops when the heart is subjected to chronic mechanical overload. In ventricular pressure overload (e.g. due to aortic stenosis) the heart typically reacts by concentric hypertrophic growth, characterized by wall thickening due to myocyte radial growth when sarcomeres are added in parallel. In ventricular volume overload, an increase in filling pressure (e.g. due to mitral regurgitation) leads to eccentric hypertrophy as myocytes grow axially by adding sarcomeres in series leading to ventricular cavity enlargement that is typically accompanied by some wall thickening. The specific biomechanical stimuli that stimulate different modes of ventricular hypertrophy are still poorly understood. In a recent study, based on in vitro studies in micropatterned myocyte cell cultures subjected to stretch, we proposed that cardiac myocytes grow longer to maintain a preferred sarcomere length in response to increased fiber strain and grow thicker to maintain interfilament lattice spacing in response to increased cross-fiber strain. Here, we test whether this growth law is able to predict concentric and eccentric hypertrophy in response to aortic stenosis and mitral valve regurgitation, respectively, in a computational model of the adult canine heart coupled to a closed loop model of circulatory hemodynamics. A non-linear finite element model of the beating canine ventricles coupled to the circulation was used. After inducing valve alterations, the ventricles were allowed to adapt in shape in response to mechanical stimuli over time. The proposed growth law was able to reproduce major acute and chronic physiological responses (structural and functional) when integrated with comprehensive models of the pressure-overloaded and volume-overloaded canine heart, coupled to a closed-loop circulation. We conclude that strain-based biomechanical stimuli can drive cardiac growth, including wall thickening during pressure overload.     

Changes in Intracellular Calcium during Compression of C2C12 Myotubes

In recent years, damage directly due to tissue deformation has gained interest in deep pressure ulcer aetiology research. It has been shown that deformation causes muscle cell damage, though the pathway is unclear. Mechanically induced skeletal muscle damage has often been associated with an increased intracellular Ca²⁺ concentration, e.g. in eccentric exercise (Allen et al., J Physiol 567(3):723–735, 2005). Therefore, the hypothesis was that compression leads to membrane disruptions, causing an increased Ca²⁺-influx, eventually leading to Ca²⁺ overload and cell death. Monolayers of differentiated C2C12 myocytes, stained with a calcium-sensitive probe (fluo-4), were individually subjected to compression while monitoring the fluo-4 intensity. Approximately 50% of the cells exhibited brief calcium transients in response to compression, while the rest did not react. However, all cells demonstrated a prolonged Ca²⁺ up-regulation upon necrosis, which induced similar up-regulations in some of the surrounding cells. Population heterogeneity is a possible explanation for the observed differences in response, and it might also become important in tissue damage development. It did not become clear however whether Ca²⁺-influxes were the initiators of damage.     

Micromechanical design of hierarchical composites using global load sharing theory

Hierarchical composites, embodied by natural materials ranging from bone to bamboo, may offer combinations of material properties inaccessible to conventional composites. Using global load sharing (GLS) theory, a well-established micromechanics model for composites, we develop accurate numerical and analytical predictions for the strength and toughness of hierarchical composites with arbitrary fiber geometries, fiber strengths, interface properties, and number of hierarchical levels, N. The model demonstrates that two key material properties at each hierarchical level—a characteristic strength and a characteristic fiber length—control the scalings of composite properties. One crucial finding is that short- and long-fiber composites behave radically differently. Long-fiber composites are significantly stronger than short-fiber composites, by a factor of 2^N or more; they are also significantly tougher because their fiber breaks are bridged by smaller-scale fibers that dissipate additional energy. Indeed, an “infinite” fiber length appears to be optimal in hierarchical composites. However, at the highest level of the composite, long fibers localize on planes of pre-existing damage, and thus short fibers must be employed instead to achieve notch sensitivity and damage tolerance. We conclude by providing simple guidelines for microstructural design of hierarchical composites, including the selection of N, the fiber lengths, the ratio of length scales at successive hierarchical levels, the fiber volume fractions, and the desired properties of the smallest-scale reinforcement. Our model enables superior hierarchical composites to be designed in a rational way, without resorting either to numerical simulation or trial-and-error-based experimentation.     

Load enhancement effects due to polymer thickening in a short model journal bearing

A model bearing is described which is 20.0 mm in diameter and 2.5 mm in length; a short bearing of diameter to length ratio eight. The clearance is large (500 μm) and the rotor may be run true or eccentric on its own shaft; in each case the mean load and frictional (tangential) force is measured as the centreline eccentricity is varied.Comparison is made between the lubricating performance of Newtonian and highly elastic liquids; the latter give load enhancement ratios of up to 300 and reductions in coefficient of friction by factors of the order 30. These effects are greatly in excess of those obtained when dealing with bearing of diameter to length ratio close to unity; possible reasons for this are discussed.A Newtonian oil and a polymer-thickened oil are tested in the same way, the latter oil is found to give load enhancement ratios of 1.4 (true rotor) and 3.5 (eccentric rotor) with corresponding reductions of coefficients of friction by factors of 1.5 (true rotor) and 3.0 (eccentric rotor). Such effects had not previously been observed when using oils in the internal cylinder geometry (journal bearing type) although somewhat similar effects have been found in the external cylinder and squeeze film geometries.The rheological properties of the polymer-thickened solutions are measured and the relevance of the results to friction and load bearing discussed.     

细颈倾角对挠性接头力学性能影响分析

为确定某单平衡环挠性接头的最佳细颈倾角,对挠性接头进行了力学性能分析。首先给出了挠性接头的离散动力学模型,然后利用ANSYS仿真分析软件对不同细颈倾角结构的挠性接头进行了抗冲击能力和等刚度的力学性能分析,得出了细颈倾角在30°～60°范围内的冲击响应和等刚度性能。最后结合两种力学性能分析可知:当细颈倾角为44°时,该挠性接头在加速度幅值为40g、脉宽为11ms冲击条件下的轴向最大响应应力为448MPa,径向最大响应应力为584MPa;在该倾角情况下挠性接头由不等刚度造成的g2项误差优于0.06(°)/h。综合两方面的性能指标可最终确定挠性接头的最佳细颈倾角为44°。     

Nonlinear dynamic behavior of a bio-inspired embedded X-shaped vibration isolation system

To improve the vibration isolation performance and bandwidth, loading capacity and supporting stability of passive vibration isolation system by utilizing nonlinearity, a bio-inspired embedded X-shaped vibration isolation (BIE-XVI) structure is proposed considering muscle/tendon contractile functions, joint rotational friction and connecting rod mass simultaneously. Furthermore, the dynamic model with pure linear elements and geometric relationship are established and the nonlinear variation properties are investigated. The effects of the key parameters of the BIE-XVI structure on frequency response characteristics and vibration isolation range are analyzed thoroughly by incremental harmonic balance method in various working conditions. From the parametric investigations, it can be found that the sensitivities of the nonlinear resonance properties are markedly different with respect to the different structure parameters. For longer rod length, larger assembly angle and higher stiffnesses, the hardening nonlinearity is weakened, but the resonance peak does not necessarily decrease. Besides, the softening nonlinearity and hardening nonlinearity can be interconverted with changing isolated mass and excitation amplitude. The BIE-XVI structure can widen the isolation frequency range and reduce the resonance peak to improve the vibration isolation properties by adjusting/designing the structural parameters, which could realize quasi-zero-stiffness property for vibration isolation.

     

基于真实工况下的工业机器人关节转角辨识与分析

本文基于多目视觉测量系统,对真实工况下连续运动的工业机器人进行关节转角的实时重构．该方法通过机器人运动前后的坐标集,在对刚体运动进行最优拟合的条件下,采用最小二乘法获得了各关节的旋转矩阵与平移向量．在此基础上,在考虑相邻关节牵连运动的前提下,获得了各关节的相对旋转矩阵．结合罗德里格斯变换理论通过相对旋转矩阵,确定了各关节转角．仿真与实验分析,验证了该方法的有效性与正确性．在该测量与辨识体系下,初步确定了各关节转角随机器人运动的真实状态．2与3杆臂由于物理尺寸呈细长形状,连杆挠度较大,这时变形误差与振动建立了关系,角振动幅度很大,曲线随机性较强．其余杆臂由于刚度较大,关节转角曲线呈光滑状态．     

人工膝关节置换中的生物力学研究进展

膝关节是人全身最大最复杂的关节, 它的任何一个主要组成部分的损坏都会引起膝关节的反常运动, 久之软骨和半月板发生磨损、变性而形成骨性关节病, 从而影响人的日常生活. 通常采用的方法是进行膝关节矫形或置换, 对严重病变的膝关节, 则采用全膝置换手术.随着人工膝关节置换成为非常普遍的外科手术, 与膝关节假体相关的研究也越来越多的被人所关注. 从生物力学角度对人工膝关节假体的类型和材料、假体生物力学性能的理论和实验研究、骨重建的理论模型、骨整合的理论和实验、与理论和实验相关的有限元分析模型等几个主要方面进行了详尽的综述. 同时, 指出了人工膝关节置换和目前研究中存在的问题,并对其未来的发展方向进行了一定的预测.      

Influence of thermally induced chemorheological changes on the torsion of elastomeric circular cylinders

When an elastomeric material is deformed and subjected to temperatures above some characteristic value T _cr (near 100^∘C for natural rubber), its macromolecular structure undergoes time and temperature-dependent chemical changes. The process continues until the temperature decreases below T _cr. Compared to the virgin material, the new material system has modified properties (reduced stiffness) and permanent set on removal of the applied load.A new constitutive theory is used to study the influence of the changes of macromolecular structure on the torsion of an initially homogenous elastomeric cylinder. The cylinder is held at its initial length and given a fixed twist while at a temperature below T _cr. The twist is then held fixed and the temperature of the outer radial surface is increased above T _cr for a period of time and then returned to its original value. Assuming radial heat conduction, each material element undergoes a different chemical change. After enough time has elapsed such that the temperature field is again uniform and at its initial value, the cylinder properties are now inhomogeneous. Expressions for the time variation of the twisting moment and axial force are determined, and related to assumptions about material properties. Assuming the elastomeric networks to act as Mooney-Rivlin materials, expressions are developed for the permanent twist on release of torque, residual stress, and the new torsional stiffness in terms of the kinetics of the chemical changes.     

Damage accumulation and fracture initiation in uncracked ductile solids subject to triaxial loading

A damage plasticity model for ductile fracture is proposed. This model is established on the cylindrical coordinate system of principal stress space. Experimental results show that fracture initiation in uncracked ductile solids is sensitive to the hydrostatic pressure and dependent on the Lode angle. The joint effects of pressure and Lode angle define a fracture envelope in principal stress space. Plastic deformation induced damage is calculated by an integral of the damage rate measured at current loading and deformation status with respect to the fracture envelope. A power law damage rule is proposed to characterize the nonlinearity in damage accumulation. A damage-related weakening factor is adopted to describe the material deterioration. The material parameters are calibrated from standard laboratory tests. The proposed model is numerically implemented. Four simulations with emphasis on crack path prediction are presented.     

<Emphasis Type="Italic">In-situ</Emphasis> AFM Experiments with Discontinuous DIC Applied to Damage Identification in Biomaterials

Natural materials (e.g. nacre, bone, and spider silk) exhibit unique and outstanding mechanical properties. This performance is due to highly evolved hierarchical designs. Building a comprehensive understanding of the multi-scale mechanisms that enable this performance represents a critical step toward realizing strong and tough bio-inspired materials. This paper details a multi-scale experimental investigation into the toughening mechanisms in natural nacre. By applying extended digital image correlation and other image processing techniques, quantitative information is extracted from otherwise prodominantly qualitative experiments. In situ three point bending fracture tests are performed to identify and quantify the toughening mechanisms involved during the fracture of natural nacre across multiple length scales. At the macro and micro scales, fracture tests performed in situ with a macro lens and optical microscope enable observation of spreading of damage outward from the crack tip. This spreading is quantified using an iso-contour technique to assess material toughness. At the nanoscale, fracture tests are performed in situ an atomic force microscope to link the larger-scale damage spreading to sliding within the tablet-based microstructure. To quantify the magnitude of sliding and its distribution, images from the in situ AFM fracture tests are analyzed using new algorithms based on digital image correlation techniques which allow for discontinuous displacement fields. Ultimately, this comprehensive methodology provides a framework for broad experimental investigations into the failure mechanisms of bio- and bio-inspired materials.     

An empirical statistical constitutive relationship for rock joint shearing considering scale effect

The scale effect of rock joint shearing is of great significance in rock engineering. Most existing shear constitutive models could describe the pre- and post-peak deformation of rock joints, but only in one particular scale, that is, those existing models will fail to depict the rock joint shearing in different length scales. Therefore, this study aims to establish a constitutive relationship for rock joints with considering the scale effect. Based on the assumption of a random statistical distribution of rock material strength and statistical mesoscopic damage theory, damage variables are defined as the ratio of the number of damaged elements to the total number in the shear process. Together with the nonlinear relationship between the microelement failure and the joint scale, an empirical statistical constitutive relationship for joint is established. And then, the determination method of constitutive relationship parameters and the variation laws with the scale are discussed. Results show that the predicted results of the proposed empirical relationship not only agree well with the experimental results but also fully describe nonlinear deformation, pre-peak softening, post-peak softening, residual stage, and other mechanical properties of the shear deformation of joint with different dimensions, thereby demonstrating the rationality of the constitutive relationship. The physical meaning of the constitutive relationship parameters is clear, and the expressions of the constitutive relationship parameters can be deduced from the experimental results. In addition, the influence of scale effect on the shear deformation of rock joints can be quantified using parameters, which help accurately describe the action form of scale effect.