Fundamental properties of elasto-plastic matrix and existence and uniqueness of elasto-plastic stress and displacement

Based on the assumption that the yield criterionF(,)=0 is a differentiable surface in stress space and by starting from a specially defined real Euclidean space, the fundamental properties of the elasto-plastic matrix in the incremental theory of plasticity is discussed in detail. By using these results, a convex analysis is made to prove the existence and uniqueness of 1) the distribution of incremental elasto-plastic stress for work-hardening materrials; 2) the displacement distribution for work-hardening materials. Material isotropy is assumed in all discussions of relevant problems.     

双炔酸聚合LB膜结构及其摩擦磨损行为研究

采用垂直提拉法将单层双炔酸膜转移到云母基片上,同时将13层双炔酸膜转移到经过亲水处理的单晶硅片和石英片上,使双炔酸在紫外光照射下发生拓扑聚合制备双炔酸聚合LB膜,并对双炔酸和经紫外光照射后聚合双炔酸LB膜的结构及摩擦磨损特性进行研究.结果表明:双炔酸在可见区无吸收峰,但经紫外光照射后因双炔酸聚合后形成的共轭大π键在可见区出现了吸收峰;双炔酸LB膜均匀平整,厚度约为3.5 nm,聚合后其结构发生变化,厚度降至约1.5 nm;而聚合后双炔酸LB膜的减摩性能基本保持不变,聚合前后双炔酸LB膜的摩擦系数不随载荷而变化,符合阿芒顿定律,其抗磨特性得到明显提高;磨痕结果充分表明聚合双炔酸耐磨性能较好.     

Electrogas- and electrohydrodynamic control of gas and liquid jets and flows

The physical and mathematical principles of the method of electrogas-and electrohydrodynamic conversion of electrical signals to pneumatic (hydraulic) signals and vice versa for control of gas and liquid jets and flows in electropneumohydraulic systems are considered. St. Petersburg State Technical University, St. Petersburg 195251. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 2, pp. 25–31, March–April, 2000.     

科学的统一性与力学的范围地位和方向

综述科学的统一性和统一性所决定的科学发展的方向,并从科学统一性的观战简评力不的范围,它在科学上的地位,影响和今后发展的方向。．     

Analysis of evaporative cooling and enhancement of condenser efficiency and of coefficient of performance

The graphical method to determine with the aid of a Mollier i - x diagram (psychrometric chart) combined heat and mass transfer is simulated by a computer program. Heat rejection rates from a plate-fin tube type condenser are determined for various flow rates and inlet state conditions of air and for different degrees of wetting of the heat transfer surfaces. The presence of water and the cooling by latent heat makes it possible to exchange more heat than the unwetted exchanger would even for idealized conditions of infinite heat transfer coefficient of the air. The evaporative cooled condenser also can exchange heat with ambient air which has much higher temperature than the condensing fluid. Evaporative cooling increases heat transfer by a factor of more than three for saturated inlet air and greater than five for lower inlet humidities. Wetted heat exchangers require less extended surfaces and can operate effectively with bare tubes only. Wetting the condenser of a refrigeration or heat pump system makes it possible to exchange the condenser load at lower temperatures. This yields an increase of COP of the order of 30 to 60% and therefore a substantial decrease in compressor power and its energy consumption.     

弹性与弹塑性问题的有限元与边界元耦合解法

本文提出了一种新的弹性与弹塑性问题的对称耦合解法。根据分区广义变分原理,直接导出问题的求解方程式。通过典型算例,验证了该方法的有效性,本文建议的方法与超单元形式的耦合法相比,在理论上比较直接,在计算上更为经济。     

Probability of random events of inspection and repair and maintaining reliability of structures

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Fatigue and static strength of notched and unnotched aluminum-alloy and steel specimens

This paper describes a method of presentation of fatigue data on three commonly used aircraft materials, 2024-T3 and 7075-T6 aluminum alloys and normalized SAE 4130 steel, such that variations in fatigue strength with stress-concentration factor can be shown. Comparisons of the fatigue strengths of 2024-T3 and 7075-T6 aluminum are made for the most useful range of stress-concentration factors. Static-strength results of notched and unnotched specimens of the three materials are presented to show how the strength varies with some parameters of the stress concentration. Comparison of the data with one theory for the strength of cracked specimens was made.     

纵扭复合型超声电机预压力和输出扭矩的关系

建立了纵扭复合型超声电机定子和转子之间的接触模型,根据模型讨论了预压力、纵振激振电压、扭振激振电压、摩擦力及电机能提供的最大扭矩之间的关系。指出在一定的条件下,施加某一预压力时电机的输出扭矩最大。所得计算结果和实验结果基本吻合。     

Experimental and Computational Investigation of Low-Impact Velocity and Quasi-Static Failure of PMMA

An investigation of the low velocity impact and quasi-static failure of polymethylmethacrylate (PMMA) based on global and local post-impact strain measurements was conducted. Local strains were obtained from surface mounted Fiber Bragg Grating (FBG) sensors, and they were combined with global measurements from quasi-static indentation and low-velocity impact experiments, and finite-element analyses to obtain detailed maps of how failure spatially initiates and evolves. For both loading regimes, the interactions between the host PMMA specimens and the sensors played a crucial role in the evolution of residual strains. A mapping of the strains clearly shows that strains decrease radially, from high values near the point of impact to far-field values. Sensors located in critical locations, which are near the impact region, had the highest residual strains prior to PMMA fracture. Furthermore, it was determined that strain transfer to the sensor is strongly influenced by the bonding conditions at the specimen’s surface. Due to the debonding of the sensor and the frictional effects associated with the bonding agent, compressive residual strains occurred on the rear-surface. Hence, a detailed understanding of how strain evolves due to sensor-host interactions and catastrophic fracture can be obtained, which can potentially be used to mitigate damage in PMMA for a range of strain-rates.     

Problems of Modeling and Optimization of Variable-Hardness Panels and Structures Made of Layered Composites

New ways of numerical simulation of nonstraight laying of reinforcing fibers in panels of variable rigidity and variants of setting optimization problems for them are proposed. The problem of parametric optimization of the dimensions and stacking of carbon fiber reinforced plastic layers in the design of the bracket for the installation of a star tracker was solved.     

Mechanical and control characteristics of multilayer piezoelectric transducers of nano- and microdisplacements

Mechanical and control characteristics of a multilayer piezoelectric transducer of nanoand microdisplacements in the case of parallel and encoded control are obtained. The static and dynamic characteristics of simple and multilayer piezoelectric transducers of nano- and microscopic displacements are determined for longitudinal and transverse piezoeffects.     

Gas-dynamic analogy of the motion and equilibrium of magnetizable and polarizable media

Because of the features of the basic system of equations, the steady flow of an isotropically magnetizable medium along magnetic lines of force can be associated with a certain gas-dynamic flow. A gas-dynamic analogy can also be established by considering the laws of the hydrostatics of a barotropic magnetizable medium. The “equation of state” of the gas-dynamic flow is determined by the law of magnetization of the medium whose motion or equilibrium is associated with this flow. All the conclusions about a magnetizable nonconducting medium can be transferred to the case of a liquid dielectric without space charges.     

Initiation and propagation of fracture in the models of Griffith and Barenblatt

In the setting of the simplest debonding problem we give a systematic comparison of the fracture models due to Griffith and Barenblatt. We prove that the Griffith model represents an asymptotic -limit of the Barenblatt model, when the ratio of the external and internal lengths increases indefinitely. We then illustrate the character of convergence by solving explicitly two sample problems with initially rigid and initially elastic cohesive energies. The geometrical simplicity of the setting allows us to study the small parameter dependence of both global and local minimizers of the total energy.Received: 22 April 2003, Accepted: 12 September 2003, Published online: 9 January 2004PACS: 62.20.Mk, 68.35.Md, 81.40.Jj, 83.50.LhCorrespondence to: J.-J. Marigo     

Impact of material processing and deformation on cell morphology and mechanical behavior of polyurethane and nickel foams

The cell morphology and mechanical behavior of open-cell polyurethane and nickel foams are investigated by means of combined 3D X-ray micro-tomography and large scale finite element simulations. Our quantitative 3D image analysis and finite element simulations demonstrate that the strongly anisotropic tensile behavior of nickel foams is due to the cell anisotropy induced by the deformation of PU precursor during the electroplating and heat treatment stages of nickel foam processing. In situ tensile tests on PU foams reveal that the initial main elongation axis of the cells evolves from the foam sheet normal direction to the rolling direction of the coils. Finite element simulations of the hyperelastic behavior of PU foams based on real cell morphology confirm the observation that cell struts do not experience significant elongation after 0.15 tensile straining, thus pointing out alternative deformation mechanisms like complex strut junctions deformation. The plastic behavior and the anisotropy of nickel foams are then satisfactorily retrieved from finite element simulations on a volume element containing eight cells with a detailed mesh of all the hollow struts and junctions. The experimental and computational strategy is considered as a first step toward optimization of process parameters to tailor anisotropy of cell shape and mechanical behavior for applications in batteries or Diesel particulate filtering.