An analytical study on prediction of effective elastic constants of perforated plate

In this study, the validity of the Eshelby-type model for predicting the effective Young’s modulus and in-plane Poisson’s ratio of the 2-dimensional perforated plate has been investigated in terms of the porosity size and its arrangement. The predicted results by the Eshelbytype model are compared with those by finite element analysis. Whenever the ratio of the porosity size to the specimen size becomes smaller than 0.07, the effective elastic constants predicted by finite element analysis are convergent regardless of the arrangement of the porosities. Under these conditions, the effective Young’s moduli of the perforated plate can be predicted within the accuracy of 5% by the Eshelby-type model, which overestimates and underestimates the effective Poisson’s ratios by 10% and 6% for the plates with periodically and non-periodically arranged porosities, respectively.     

A new heat transfer model to predict cooling rates for rapid freezing fixation

A simple heat transfer model is presented to show that convection from the cryogen to the specimen is the limiting step in rapid freezing of small samples for electron microscopy when the Biot modulus, which measures the ratio of convective to conductive heat transfer, (hd/k) < 1. In comparison to the commonly accepted conduction-limited model, the convection-limited model predicts that cooling rates are proportional to the surface area/volume ratio, independent of the sample thermal conductivity, and inversely proportional to the product of sample density and heat capacity. Literature values of experimentally measured cooling rates fit the convection limited model. Simple analogies to predict the heat transfer coefficient as a function of cryogen properties, specimen geometry and cryogen velocity are presented.     

三维编织复合材料几何建模及有效弹性模量预报

重点介绍了三维四向编织复合材料的有效弹性模量预报方法,将三维四项编织复合材料的单胞划分为一些矩形的体单元,然后利用高斯积分点处的材料参数计算单元的刚度矩阵,最后由单元的刚度矩阵形成单胞的整体刚度矩阵,进而对三维四项编织复合材料的有效弹性模量进行预报。     

The ploughing friction: analytical model with elastic recovery for a conical tip with a blunted spherical extremity

The current analytical models are insufficient to describe the ploughing part of the friction coefficient when there is elastic recovery at the rear part of the contact after passage of a moving tip. A solution for the case of a perfectly conical tip was proposed recently. This paper presents a model to describe the ploughing friction for a perfectly spherical tip and then develops this analysis for the case of a conical tip with a blunted spherical extremity. The major difficulty resides in taking into account the shape of the contact area in relation to the elastic, elastoplastic or plastic behaviour of the contact.     

A model to predict scuffing failures of a ball-on-disk contact

This study aims at establishing the criterion for scuffing to occur. A point contact thermal elastohydrodynamic lubrication (EHL) model is combined with a heat transfer model to predict the friction coefficient, surface bulk temperature and surface local temperature distribution. An experimental program is conducted to generate ball-on-disk scuffing failures under different rolling and sliding speeds and normal loads. Simulating these experiments, the model predictions are shown to be in good agreement with the measured ones. The probability distributions of the instantaneous maximum surface temperature under different operating conditions are constructed and observed to be bell shaped normal distributions. It is found the distribution median can be used as the measure to establish the scuffing limit for a lubricant-material (steel) combination.     

A mixed model to predict friction forces in lubricated contact with non-newtonian fluids

A mixed model to predict friction forces in thermohydrodynamic lubricated contacts with non-Newtonian fluids is proposed, with an integral formulation and a finite element method resolution. The mixed model is used here for the first time in lubrication. It takes into account the implicit rheological laws. The shear stress field appears in this formulation as a nodal unknown and eases the use of rheological laws with yield criteria.     

Prediction of thermal conductivities of laminated composites using penny-shaped fillers

A new model is proposed to predict the thermal conductivities of laminated composites, where the Eshelby method modified with Mori-Tanaka’s mean field approach is employed to consider the interaction effect. Based on the equivalency of composites with penny-shaped fillers and composites with layers of components, each lamina is considered as a penny-shaped filler and its thermal conductivities are computed by modified Eshelby method. The laminated composites are then simulated as the matrix and penny-shaped fillers of different thermal conductivities. By comparing the results of the laminated composites predicted by the present model and conventional approach combined with the potential theory and electrical analogy, the applicability of the present model to predict the thermal conductivities of the laminated composites is validated. This paper was recommended for publication in revised form by Associate Editor Dongsik Kim Jae-Kon Lee received a B.S. degree in Mechanical Engineering from Seoul National University in 1985. He then went on to receive his M.S. degree from KAIST and Ph.D. degrees from University of Washington in 1987 and 1996, respectively. Dr. Lee is currently a Professor at the School of Mechanical and Automotive Engineering at Catholic University of Daegu in Kyeongsan, Korea. Dr. Lee’s research interests are in the area of design and analysis of smart composite materials using mechanical, thermal, and piezoelectric properties.     

A new constant life diagram model for the longitudinal fatigue of unidirectional composites

Journal of Mechanical Science and Technology - A new constant life diagram (CLD) model featuring asymmetric bilinear constant-life curves was proposed to better describe the longitudinal fatigue...     

Mechanical and tribological properties of phenolic resin-based friction composites filled with several inorganic fillers

The calcined petroleum coke (CPC), talcum powder (TP) and hexagonal boron nitride (h-BN) were used as the friction modifiers to improve the mechanical and tribological properties of phenolic resin-based friction composites (the resin matrix was coded as PHE). Thus the composites filled with the inorganic particulates of laminar structures were prepared by compression molding. The hardness and bending strength of the friction composites were measured. The tribological properties of the composites sliding against cast iron were evaluated using a pin-on-disc test rig. The morphologies of the worn surfaces of the composites and the transfer films on the counterpart cast iron disc were analyzed by means of scanning electron microscopy, and the elemental plane distributions on the transfer films were analyzed using energy-dispersive X-ray analysis (EDXA). It was found that the friction composites of different compositions showed different friction and wear behaviors, which was highly dependent on the volume fractions of the friction modifiers in the composites. Namely, the inclusion of CPC, h-BN, and TP at a volume fraction of 10% helped to greatly increase the bending strength and wear resistance of the composites, and in these cases the coefficients of friction for the composites were ranged within 0.43–0.47. In particular, the PHE-based composite with 10% h-BN had excellent friction stability at various testing conditions and showed the best wear resistance above 125 °C, which was attributed to the formation of a compact friction film (third-body-layer) on the rubbing surface of the composite and of a durable transfer film on the rubbing surface of the counterpart cast iron. The PHE-based composite with 10% CPC showed the best wear resistance below 125 °C, which was ascribed to the same reasons mentioned above. The different actions of various friction modifiers in terms of their effects on the friction and wear behavior of the phenolic resin-based friction composites could be related to their different bonding strengths with the resin matrix and their different abilities to form friction films (third-body-layer) on the surfaces of the composites and transfer films on the counterpart cast iron surface as well.     

小试件材料弹性常数超声测量系统的研制

本文针对小试件块体纳米材料力学性能测试的特殊需求，设计开发了一套小试件材料弹性常数超声精密测量系统。系统由线聚焦PVDF探头、四轴精密运动平台、NI PXI总线嵌入式控制器、超声脉冲产生／接收仪以及数字记忆示波器组成。系统基于声学显微镜技术，利用同时测定纵波和漏表面波波速来表征材料的弹性常数等，既适用于各向同性材料，又适用于各向异性材料。试验结果表明本测量系统工作良好，能够满足试验要求，为进一步评价小试件纳米材料制备工艺和表征纳米材料的力学性能奠定了基础。     

A variationally based nonlocal damage model to predict diffuse microcracking evolution

We explore a variationally based nonlocal damage model, based on a combination of a nonlocal variable and a local damage variable. The model is physically motivated by the concept of “nonlocal” effective stress. The energy functional which depends on the displacement and the damage fields is given for a one-dimensional bar problem. The higher-order boundary conditions at the boundary of the elasto-damaged zone are rigorously derived. We show that the gradient damage models can be obtained as particular cases of such a formulation (as an asymptotic case). Some new analytical solutions will be presented for a simplified formulation where the stress–strain damage law is only expressed in a local way. These Continuum Damage Mechanics models are well suited for the tension behaviour of quasi-brittle materials, such as rock or concrete materials. It is theoretically shown that the damage zone evolves with the load level. This dependence of the localization zone to the loading parameter is a basic feature, which is generally well accepted, from an experimental point of view. The computation of the nonlocal inelastic problem is based on a numerical solution obtained from a nonlinear boundary value problem. The numerical treatment of the nonlinear nonlocal damage problem is investigated, with some specific attention devoted to the damageable interface tracking. A bending cantilever beam is also studied from the new variationally based nonlocal damage model. Wood’s paradox is solved with such a nonlocal damage formulation. Finally, an anisotropic nonlocal tensorial damage model with unilateral effect is also introduced from variational arguments, and numerically characterized in simple loading situations.     

Effective elastic constants of two-dimensional cellular materials with deep and thick cell walls

In order to analyse the elastic constants of cellular materials with deep and thick cell walls, finite element analysis using two kinds of unit cell approach (stiffness matrix method and compliance matrix method) is performed which is applicable to any orthotropic cellular materials. Comparison between results from the FEA, the theories presented in this paper and experiments of previous investigators indicate that the elastic constants of cellular materials with thick cell walls depend not only on the relative density but also on the joint stiffening effect. Approximate formulae under generalised plane strain conditions are also presented for the purpose of obtaining the effective elastic constants for cellular materials with deep and thick cell walls. A satisfactory agreement was found with experimental results obtained on a deep and thick cellular material. The results indicate that the previous models in which the wall of cellular materials is treated as a simple beam are not adequate to evaluate the effective elastic constants of cellular materials with deep and thick cell walls. In addition, considerable attention needs for the measurement of effective Young's modulus of square cellular materials in the two soft directions because it is strongly affected by misalignment errors.     

A theoretical model to predict the effect of localized defect on vibrations associated with ball bearing

The paper presents an analytical model for predicting the effect of a localized defect on the ball bearing vibrations. In the analytical formulation, the contacts between the ball and the races are considered as non-linear springs. The contact force is calculated using the Hertzian contact deformation theory. A computer program is developed to simulate the defect on the raceways with the results presented in the time domain and frequency domain. The model yields both the frequency and the acceleration of vibration components of the bearing. The effect of the defect size and its location has been investigated. Numerical results for 6305 deep groove ball bearing have been obtained and discussed. The results obtained from the experiments have also been presented.     

Two-dimensional nonstationary contact of elastic cylindrical or spherical shells

Nonstationary contact problems with moving boundaries for two elastic cylindrical or spherical Timoshenko-type shells were considered. A system of resolvent equations has been obtained using the superposition principle. Influence functions for the shells in the form of Fourier expansions have been found. A numerical-analytical solution algorithm has been constructed and implemented.     

Finite expansion of a thick compressible spherical elastic shell

A method is proposed for the analysis of the finite symmetrical expansion, due to internal pressure, of a thick-walled sphere of compressible hyperelastic material. Solutions can be obtained by this method for any admissible strain energy function and numerical results are presented for three strain energy functions.     

Non-linear dynamic analysis of shallow spherical shells on elastic foundations

The present study investigates the effect of Winkler-Pasternak elastic foundation parameters on the nonlinear dynamic response of shallow spherical shells. The values of foundation parameters ( and ) have been determined for the minimaximum central response of the shallow shells for both the clamped as well as simply supported immovable edge conditions. Donnell type partial differential equations governing the moderately large amplitude behaviour of shallow spherical shells resting on Winkler-Pasternak elastic foundations under step pressure loading, have been analysed. The space and time-wise integrations of governing equations have been carried out using Chebyshev series and Houbolt techniques, respectively. It is also shown that the present analysis can be extended to study the dynamic buckling of shallow shells resting on elastic foundations.     

Predicting the elastic properties of randomly reinforced polymer composites with antifrictional additives

The elastic properties of composites (the Young’s modulus, Poisson’s ratio, Lame constant, and shear elastic modulus) are determined in model calculations, when the variation in concentration of the components is taken into account.     

弹性关节冗余度机器人轨迹规划的一种新方法

对于大多数工业机器人来讲,由传动系统、谐波减速器和伺服系统产生的关节弹性是机器人弹性的主要来源。当机器人高速运动时,由于关节弹性而产生的末端变形将会影响机器人的跟踪精度。并针对弹性关节冗余度机器人提出了一种使关节弹性变形极小化的轨迹规划新方法。该方法同时考虑了关节加速度和关节速度对关节弹性变形的影响,在保证关节弹性变形极小化的同时,也使关节加速度和关节速度明显减小。平面３Ｒ机器人的仿真研究表明,该