Our country over the past 20 years experimental curriculum material mechanics theory and practice of teaching reform

根据国内正式发表的文献,从教学理念改革与探索,教学内容改革与创新,教 学方法改革与实践,教学考核改革与配套,教学设施改善与计算机应用等5个方面, 综述了20年来我国材料力学实验课程教学改革理论与实践,并提出了作者的建议.     

上海市力学学会60年

上海市力学学会成立于1959年9月20日,到今年正好是一个甲子．60个春秋真实写照了上海市力学学会的建立、成长、发展与壮大的过程．60年来,在上海市科协的领导和中国力学学会的指导下,在各团体成员单位的大力支持下,上海市力学学会一代代力学工作者薪火相传、开拓耕耘,历届理事会及下属工作委员会和专业委员会团结奋进、不断创新,广大会员踊跃参与,为上海力学大力发展、力学人才精心培养、力学知识科学普及、力学服务国家发展战略和区域经济建设中做出了重要贡献,学会持续发展壮大．在庆祝力学学会成立六十周年之际,让我们共同抒怀上海力学六十年薪火相传、回顾近10年重要发展,展望上海力学新时代．     

Cardiovascular Mechanics: Investigation of Two Components,Tissue Heart Valves and Blood Cells

A fundamental anatomical composition of the heart valves is presented along with its relationship to the tissue mechanical behavior. During the loading and unloading phases of the tissue different stress strain pathways are followed with the curves composing the characteristic hysteresis loop, exhibiting the viscoelastic mechanical behavior of valvular tissue. The storage modulus and the phase shift (tan ) as well as the collagen modulus of human heart valves were measured in orthotropic directions using uniaxial dynamic tensile tests at 10 Hz. Viscoelastic properties of human erythrocytes are presented as calculated from micropipette aspiration experiments. Employing the hemorheometre, from filtration experiments an index of rigidity (IR) of erythrocytes is estimated. A relationship between the global parameter IR and the shear elastic modulus of erythrocyte membrane, , is established. The same two techniques adapted for leukocytes and their subpopulations have been used and a relationship between the rigidity index of leukocytes (ILR) and their apparent bulk viscosity (_app), has been found.     

制动用有机摩擦材料的研究进展

随着社会的不断发展和进步,人们对车辆的运行速度、安全性、舒适度和环保化等提出了更高的要求,迫切需要进一步提高制动用摩擦材料的综合性能.本文综述了有机粘结剂、增强纤维、摩擦性能调节剂和填料等有机摩擦材料4大类组分的研究进展,介绍了该领域的一些理论研究工作,希望为我国高性能有机摩擦材料的研制提供一些资料积累和思路.     

增强纤维含量对汽车摩擦材料性能的影响

研究了混杂纤维增强材料对汽车摩擦材料的机械性能及摩擦磨损性能的影响．结果表明：摩擦材料的冲击强度、三点弯曲断裂强度及硬度随纤维含量的增加而提高,并均达到使用要求;纤维含量对摩擦系数和磨损率影响较大,含体积分数１０％ 混杂纤维的材料具有较高的摩擦系数和较低的磨损率;ＳＥＭ 及ＥＤＡＸ分析表明,其磨损机理亦与增强纤维含量密切相关．     

聚酰亚胺复合材料与不同对偶材料滑动摩擦磨损性能

本文考察了碳纤维（CF）和固体润滑剂（LW）填充热塑性聚酰亚胺（TPI）复合材料与45^#钢、镍铬合金以及铜对摩时的摩擦磨损性能,利用SEM和光学显微镜观察磨损表面形貌.研究表明：与硬度高的钢和镍铬合金摩擦时,由于CF的增强和承载作用降低了材料的摩擦系数与磨损率,LW的加入增强了转移膜与对偶面间的结合力,进一步降低TPI的摩擦系数;与较软的铜摩擦时,由于铜偶件存在较大磨损,破坏了转移膜的稳定性,TPI及其复合材料的摩擦性能较差;TPI与钢和镍铬合金摩擦时以黏着磨损为主,与铜摩擦时以磨粒磨损为主.薄而均匀的转移膜有利于摩擦系数与磨损率的降低.     

天然牙及几种牙科修复材料的摩擦磨损性能比较研究

在改进的微动摩擦磨损试验台上,模拟人体牙齿摩擦副,考察了天然牙以及牙科用高分子、金属和陶瓷修复材料与GCr15钢对摩的摩擦磨损性能,结果表明：天然牙的摩擦学性能优良;拜尔牙、热固塑料、铜基合金、钛及钛合金是较为理想的牙科修复材料;尤其是钛及钛合金,不但生物相容性优异,而且与天然牙摩擦学性能匹配,是最具有发展前景的牙科修复材料。     

层片式复合材料摩擦性能与表面形态的研究

粉末冶金材料摩擦面上多组分的混合以及第三体的存在,不利于澄清材料中不同组分对摩擦性能的贡献程度.本文采用机械组合方法制备了铜-钢-铝层片式摩擦材料,通过定速摩擦试验机,在干、湿两种条件下,观察了3种组分的摩擦表面微结构随摩擦速度的变化过程,测试了不同条件下的摩擦性能.结果表明:铜良好的塑性和焊合性,易形成与基体黏附性良好的第三体,使表面的粗糙程度增加;钢较高的强度及其氧化物的脆性,形成的第三体流动性好且与基体的结合强度有限,容易发生开裂和脱落;铝形成的富氧化铝的第三体,其颗粒间较差的黏合程度易在表面弥散分布,使表面平整度好.在摩擦速度低于900 r/min条件下,水分的润滑作用使湿摩擦条件下的摩擦系数低于干摩擦;摩擦速度高于900 r/min时,水分的冷却和清理微细第三体颗粒的作用,降低了材料的软化程度和第三体的流动性,使湿摩擦系数大于干摩擦系数.     

Mechanics of robust and releasable adhesion in biology: Bottom-up designed hierarchical structures of gecko

Gecko and many insects have evolved specialized adhesive tissues with bottom-up designed (from nanoscale and up) hierarchical structures that allow them to maneuver on vertical walls and ceilings. The adhesion mechanisms of gecko must be robust enough to function on unknown rough surfaces and also easily releasable upon animal movement. How does nature design such macroscopic sized robust and releasable adhesion devices? How can an adhesion system designed for robust attachment simultaneously allow easy detachment? These questions have motivated the present investigation on mechanics of robust and releasable adhesion in biology. On the question of robust adhesion, we introduce a fractal gecko hairs model, which assumes self-similar fibrillar structures at multiple hierarchical levels mimicking gecko's spatula ultrastructure, to show that structural hierarchy plays a key role in robust adhesion: it allows the work of adhesion to be exponentially enhanced with each added level of hierarchy. We demonstrate that, barring fiber fracture, the fractal gecko hairs can be designed from nanoscale and up to achieve flaw tolerant adhesion at any length scales. However, consideration of crack-like flaws in the hairs themselves results in an upper size limit for flaw tolerant design. On the question of releasable adhesion, we hypothesize that the asymmetrically aligned seta hairs of gecko form a strongly anisotropic material with adhesion strength strongly varying with the direction of pulling. We use analytical solutions to show that a strongly anisotropic elastic solid indeed exhibits a strongly anisotropic adhesion strength when sticking on a rough surface. Furthermore, we perform finite element calculations to show that the adhesion strength of a strongly anisotropic attachment pad exhibits essentially two levels of adhesion strength depending on the direction of pulling, resulting in an orientation-controlled switch between attachment and detachment. These findings not only provide a theoretical foundation to understand adhesion mechanisms in biology but also suggest possible strategies to develop novel adhesive materials for engineering applications.     

Mechanics of very fine-grained nanocrystalline materials with contributions from grain interior, GB zone, and grain-boundary sliding

In this paper, we formulated an atomically-equivalent continuum model to study the viscoplastic behavior of nanocrystalline materials with special reference to the low end of grain size that is typically examined by molecular dynamic (MD) simulations. Based on the morphology disclosed in MD simulations, a two-phase composite model is construed, in which three distinct inelastic deformation mechanisms disclosed from MD simulations are incorporated to build a general micromechanics-based homogenization scheme. These three mechanisms include the dislocation-related plastic flow inside the grain interior, the uncorrelated atomic motions inside the grain-boundary region (the GB zone), and the grain-boundary sliding at the interface between the grain and GB zone. The viscoplastic behavior of the grain interior is modeled by a grain-size dependent unified constitutive equation whereas the GB zone is modeled by a size-independent unified law. The GB sliding at the interface is represented by the Newtonian flow. The development of the rate-dependent, work-hardening homogenization scheme is based on a unified approach starting from elasticity to viscoelasticity through the correspondence principle, and then from viscoelasticity to viscoplasticity through replacement of the Maxwell viscosity of the constituent phases by their respective secant viscosity. The developed theory is then applied to examine the grain size- and strain rate-dependent behavior of nanocrystalline Cu over a wide range of grain size. Within the grain-size range from 5.21 to 3.28 nm, and the strain rate range from 2.5 × 10⁸ to 1.0 × 10⁹/s, the calculated results show significant grain-size softening as well as strain-rate hardening that are in quantitative accord with MD simulations [Schiotz, J., Vegge, T., Di Tolla, F.D., Jacobsen, K.W., 1999. Atomic-scale simulations of the mechanical deformation of nanocrystalline metals. Phys. Rev. B 60, 11971–11983]. We have also applied the theory to investigate the flow stress, strain-rate sensitivity, and activation volume over the wider grain size range from 40 nm to as low as 2 nm under these high strain rate loading, and found that the flow stress initially displays a positive slope and then a negative one in the Hall–Petch plot, that the strain-rate sensitivity first increases and then decreases, and that the activation volume first decreases and then increases. This suggests that the maximum strain rate sensitivity and the lowest activation volume do not occur at the smallest grain size but, like the maximum yield strength (or hardness), they occur at a finite grain size. These calculated results also confirm the theoretical prediction of Rodriguez and Armstrong [Rodriguez, P., Armstrong, R.W., 2006. Strength and strain rate sensitivity for hcp and fcc nanopolycrystal metals. Bull. Mater. Sci. 29, 717–720] on the basis of grain boundary weakening and the report of Trelewicz and Schuh [Trelewicz, J.R., Schuh, C.A., 2007. The Hall–Petch breakdown in nanocrystalline metals: a crossover to glass-like deformation. Acta Mater. 55, 5948–5958] on the basis of hardness tests. In general the higher yield strength, higher strain rate sensitivity, and lower activation volume on the positive side of the Hall–Petch plot are associated with the improved yield strength of the grain interior, but the opposite trends displayed on the negative side of the plot are associated with the characteristics of the GB zone which is close to the amorphous state.     

Driving Potentials of Heat and Mass Transport in Porous Building Materials: A Comparison Between General Linear,Thermodynamic and Micromechanical Derivation Schemes

In systems of coupled transport processes the question of the appropriate driving potentials is a point of discussion. In this article, three different approaches to derive models for transport currents are systematically compared. According to a general linear approach, an arbitrary full set of independent state variables and material properties is sufficient to describe any transport current. This approach is derived here from a symmetry principle. Thermodynamic and micromechanical approaches are more complex and even less general, but they allow additional statements about the transport coefficients and they reduce the number of transport processes. In the thermodynamic approach the additional information stems from the calculation of the entropy production rate; the micromechanical approach involves a microphysical model of the considered porous system. As a practical example, the three derivation schemes are applied to the often-encountered case of non-hysteretic heat and moisture transport in homogeneous building materials. It is shown, how the general state variables of a porous system are reduced to only two. Then from the general linear approach it can be seen, that all equations for the moisture transport current using a main driving potential (e.g. moisture content, vapour pressure, chemical potential) and an independent secondary driving potential (e.g. temperature, liquid pressure) are equivalent, without recurrence to the thermodynamic or micromechanical approach. However, the transport coefficients are arbitrary phenomenological functions depending on the two state variables. Based on a literature survey it is shown, which additional statements can be made in the thermodynamic and in the micromechanical approach. The latter yields the pressure-driven model (vapour and liquid pressure as the two driving potentials). Finally it is shown, what is to be expected, if in more complex systems the number of state variables increases.     

Nonaxisymmetric Deformation of Solids of Revolution Made of Elastic Orthotropic Materials with Different Tensile and Compressive Moduli

A method is proposed to allow for the difference of the tensile and compressive moduli of compound orthotropic bodies of revolution subject to nonaxisymmetric loading and heating. The compliance matrix is symmetrized by introducing weighting coefficients that take into account the influence of the sign of stresses in two mutually perpendicular directions on the corresponding coefficients of this matrix __________ Translated from Prikladnaya Mekhanika, Vol. 41, No. 7, pp. 47–57, July 2005.     

On Anisotropic Elastic Materials for which Young''s Modulus E ( n ) is Independent of n or the Shear Modulus G ( n , m ) is Independent of n and m

It is shown that, among anisotropic elastic materials, only certain orthotropic and hexagonal materials can have Young modulus E(n) independent of the direction n or the shear modulus G(n,m) independent of n and m. Thus the direction surface for E(n) can be a sphere for certain orthotropic and hexagonal materials. The structure of the elastic compliance for these materials is presented, and condition for identifying if the material is orthotropic or hexagonal is given. We also study the case in which n of E(n) and n, m of G(n,m) are restricted to a plane. When E(n) is a constant on a plane so are G(n,m) and Poisson's ratio ν(n,m). The converse, however, does not necessarily hold. A plane on which E(n) is a constant can exist for all anisotropic elastic materials. In particular, existence of such a plane is assured for trigonal, hexagonal and cubic materials. In fact there are four such planes for a cubic material. For these materials, not only E(n) is a constant, two other Young's moduli, the three shear moduli and the six Poisson's ratio on the plane are also constant.     

New Transforms for the Resolving Equations in Elastic Theory and New Integral Transforms, with Applications to Boundary-Value Problems of Mechanics

This paper reviews results from an analysis of exact boundary-value solutions of static and dynamic elasticity obtained by the method of integral transforms. Consideration is given to the solutions of problems for a half-space and a blunted hollow cone, the heat conduction problem for a cone, and the diffraction problem for a cone with a hole along the generatrix. Solutions of mechanics problems are analyzed and the method of integral transforms is generalized     

两种杂环有机物的分子结构与其在离子液体中缓蚀、减摩抗磨性能的构效关系研究

本文作者通过电化学和摩擦学测试手段,对比考察了苯并三氮唑(BTA)和糖精(SAC)两种杂环有机物在[HMIM][Cl]离子液体体系中对铜锡合金的缓蚀性能和减摩抗磨性能.然后,利用扫描电子显微镜对铜锡合金表面的腐蚀形貌和磨损形貌进行了分析表征.结果表明:BTA能够吸附在铜锡合金表面形成一层致密的聚合物保护膜,阻止Cl-对金属表面的进攻,降低离子液体的腐蚀作用,有效地抑制Cl-引发的孔蚀现象;同时,BTA还进一步提高了基础油的减摩抗磨效果.在铜锡合金表面,SAC与离子液体存在竞争腐蚀作用,低浓度时,以离子液体中Cl-的孔蚀为主,高浓度时,以均匀腐蚀为主;同时,由于促进腐蚀作用的存在,SAC的引入会延长磨合期,加剧磨损.对[HMIM][Cl]离子液体体系而言,杂环分子BTA是一种兼具优异缓蚀性能和良好减摩抗磨性能的添加剂;杂环分子SAC作为添加剂,则不具有缓蚀和减摩抗磨作用.     

Development and implementation of an IOT based instrumentation system for computing performance of a tractor-implement system

A high precision and compact IOT based digital instrumentation setup to measure, display and record various tractor and implement system performance parameters was developed and installed on a 28.3 kW Tractor. The setup was capable of continuous monitoring and wirelessly transmitting tractor-implement performance parameters on a cloud platform such as engine speed, radiator fan speed, fuel consumption, draft, forward speed, lift arm angle, wheel slip, wheel slip, PTO speed, geo-location/position of the tractor, choking of seeds in the implement and vibrations experienced by the implement. For precision measurements, commercial transducers used in the system were calibrated and assessed under both static and dynamic conditions. The average calibration constant for fuel consumption, forward speed, lift arm angle and load cell were 0.00009804 L/pulse, 0.01610306 km/h/pulse, 0.056 mA/degree and 0.2575 mV/kN respectively. The system based on DataTaker DT 85 Data logger connected to a micro-computer through transducers capable of transferring data wirelessly was installed on John Deere 5038 tractor and was tested with a Spatially Modified No-Till Drill in agricultural field with varied implement depth.