Microstructure observation and mechanical behavior modeling for limnetic nacre

In the present research, microstructure of akind of limnetic shell （Hyriopsis cumingii） is observed and measured by using the scanning electron microscopy, and mechanical behavior experiments of the shell nacre are carried out by using bending and tensile tests. The dependence of mechanical properties of the shell nacre on its microstructure is analyzed by using a modified shear-lag model, and the overall stress-strain relation is obtained. The experimental results reveal that the mechanical properties of shell nacre strongly depend on the water contents of the limnetic shell. Dry nacre shows a brittle behavior, whereas wetting nacre displays a strong ductility. Compared to the tensile test, the bending test overestimates the strength and underestimates the Young＇s modulus. The modified shear-lag model can characterize the deformation features of nacre effectively.     

Size effect in the initiation of plasticity for ceramics in nanoindentation

The indentation size effect has been observed for many years and is usually associated with increasing hardness as the depth of indentation is reduced for pyramidal indenters. The indentation size effect for spherical indenters has recently been associated with an increase in the yield stress of metals proportional to the inverse cube root of indenter radius [Spary, I.J., Bushby, A.J., Jennett, N.M., 2006. On the indentation size effect in spherical indentation. Philos. Mag. 86 (33), 5581-5593]. Here we investigate ceramic materials where the yield point is high enough to be easily distinguished in nanoindentation tests. A robust method for determining the yield point from a nanoindentation test with spherical indenters is presented. The results for a range of ceramics confirm that the increase in yield pressure is directly proportional to the inverse cube root of indenter radius. Furthermore, the yield pressure is also shown to be proportional to the inverse square root of the contact radius. Revisiting data in the literature shows that this inverse square root relationship is also true for pyramidal indenters. This implies that the indentation size effect is driven by the contact area rather than by the depth of indentation or by the indenter radius.     

Sample boundary effect in nanoindentation of nano and microscale surface structures

Nanoindentation is a widely used technique to characterize mechanical properties of materials in small volumes. When the sample size is comparable to the indent size, the indentation-induced plastic zone can be affected by the sample boundary which may cause inaccurate interpretation of the mechanical properties. In this study, the sample boundary effect is investigated by performing experiments and atomistic simulations of nanoindentation into nano- and micro-scale Au pillars and bulk Au (0 0 1) surfaces. In experiments, a more compliant deformation is observed in pillar indentations compared to bulk Au. The elastic modulus decreases with increasing indent size over sample size ratio. Atomistic simulations are performed to gain insights on the mechanisms of pillar deformation and pillar boundary effect. The reduced modulus has a similar trend of decrease with increasing indent size over sample size ratio. Significantly different dislocation activities and dislocation interactions with the pillar boundary contribute to the lower value of the reduced modulus in the pillar indentation. The presence of the free surface would allow the dislocations to annihilate, causing a higher plastic recovery during the pillar unloading process.     

网壳结构健康监测中的传感器优化布置

针对网壳结构健康监测提出了一种以损伤可识别性与模态可观测性相协调为目标的传感器优化布置的方法.由于模态数目的选取对基于损伤灵敏度分析的传感器优化布置有很大的影响,因此本文建立了一种同时包含模态独立性信息和损伤灵敏度信息的Fisher信息矩阵,并选取合适的模态数目,然后发展了一种以信息矩阵最大和条件数最小为准则的多目标优化算法.空间网壳数值算例表明,本文提出的传感器优化方法能简单、有效地为空间结构传感器优化布置提供可行方案.     

The indenter tip radius effect in micro- and nanoindentation hardness experiments.

Nix and Gao established an important relation between the microindentation hardness and indentation depth. Such a relation has been verified by many microindentation experiments (indentation depths in the micrometer range), but it does not always hold in nanoindentation experiments (indentation depths approaching the nanometer range). Indenter tip radius effect has been proposed by Qu et al. and others as possibly the main factor that causes the deviation from Nix and Gao's relationship. We have developed an indentation model for micro- and nanoindentation, which accounts for two indenter shapes, a sharp, conical indenter and a conical indenter with a spherical tip. The analysis is based on the conventional theory of mechanism-based strain gradient plasticity established from the Taylor dislocation model to account for the effect of geometrically necessary dislocations. The comparison between numerical result and Feng and Nix's experimental data shows that the indenter tip radius effect indeed causes the deviation from Nix-Gao relation, but it seems not be the main factor. The project supported by the National Natural Science Foundation of China (10121202) and the Ministry of Education of China (20020003023)     

Incremental deformation model for a shell

A nonlinear deformation model for a shell with rigid transverse fibers is proposed. A complete system of incremental equations, a variational equation equivalent to this system, and a particular equation of virtual work are formulated. Numerical analysis of the nonlinear deformation of a spherical dome is performed using the complete equation. Institute of Computer Modeling, Siberian Division, Russian Academy of Sciences, Krasnoyarsk 660036. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 5, pp. 202–207, September–October, 1999.     

Stress distribution in a glassfiber-reinforced plastic shell structure under impulsive transverse loading

The paper outlines a method and results of experimental determination of vibratory stresses in a shell structure contacting with a medium. The stresses peak at the junction between the shell and the branch pipe. It is established that when the shell is in contact with the medium, the vibratory stresses are much higher, while the vibratory accelerations are much lower     

外爆条件下冲击波与组合壳结构的相互作用

为研究冲击波与组合壳结构的相互作用,针对带防护墙的地面直立钢筋混凝土组合壳结构,考虑结构安置于地面和周边围土2种工况,开展结构爆炸实验,分析了结构外表面冲击波荷载分布及振动特性。实验结果表明:冲击波作用下,结构外表面爆炸荷载主要产生在冲击波绕射过程,确定荷载时应考虑冲击波压力在绕射传播过程中的自然衰减;整个结构中与冲击波最早接触的构件先产生振动,而后由于结构整体参与使得振动频率降低,振动幅值减小;结构周边围土可降低防护墙迎爆部分构件的振动频率,减小防护墙和组合壳的振动幅值。     

Synthesis and characterization of magnetic polymer microspheres with a core-shell structure

Non-porous magnetic polymer microspheres with a core-shell structure were prepared by a novel micro-suspension polymerization technique. A stable iron oxide ferrofluid was used to supply the magnetic core, and the polymeric shell was made of glycidyl methacrylate （GMA monomer） and ethylene dimethacrylate （cross-linker）. In the preparation, polyvinyl alcohol was used as the stabilizer, and a lauryl alcohol mixture as the dispersant. The influence of various conditions such as aqueous phase volume, GMA and initiator amounts, reaction time and stirring speed on the character of the microspheres was investigated. The magnetic microspheres were then characterized briefly. The results indicate that the microspheres with active epoxy groups had a narrow size distribution range from 1 to 10 μm with a volume-weighted mean diameter of 4.5 μm. The saturation magnetization reached 19.9 emu/g with little coercivity and remanence.     

捷联惯导系统尺寸效应分析与补偿

尺寸效应误差是捷联惯导系统一个重要误差源。从理论上分析了尺寸效应误差的产生机理和对系统精度的影响。在此基础之上,采用多项式拟合法推导出了尺寸效应误差双子样补偿算法,并且根据尺寸效应误差的产生原因和规律,提出了尺寸效应误差参数速率标定方法。理论分析和实验结果表明,尺寸效应误差是系统在角运动条件下由于系统本身物理结构所形成的固有误差,在对其进行补偿时应当采取硬件设计和软件算法补偿相结合的方式;采用双子样补偿算法对加速度测量进行尺寸效应误差补偿,补偿前加速度测量误差最大可达0.06g,而补偿后小于1×10-4g,水平定位误差也降低10%左右。     

Particle size and packing characterization by diffuse light transmission

Particle size, packing density and blend composition of glass ballotini, microcrystalline cellulose pellets and theophylline granules were studied by diffuse light transmission. Diffuse visible light was directed to the sample and the intensity of the light transmitted to the opposite side of the sample was measured through a diffusing lens using a phototransistor. Light transmission was found to decrease with decreasing particle size and with increasing packing density. There was a correlation, though somewhat irregular, between light transmission and particle size for glass ballotini and microcrystalline cellulose pellets, and a strong near-linear correlation between light transmission and particle size for theophylline granules. The effect of packing density on light transmission was significant. Differentiating the composition of binary blends of microcrystalline cellulose pellets and glass ballotini and blends of theophylline granules was found possible on the basis of light transmission. The method proposed showed potential as a rapid, simple and inexpensive analytical tool for basic process diagnostics.     

Droplet dynamics and size characterization of high-velocity airblast atomization

Airblast atomizers are especially useful and commonplace in liquid fuel combustion applications. However, the spray formation processes, the droplet dynamics and the final drop size distributions are still not sufficiently understood due to the coupled gas-liquid interactions and turbulence generation. Therefore, empirical and semi-empirical approaches are typically used to estimate the global spray parameters. To develop a physical understanding of the spray evolution, a plain-jet airblast atomizer was investigated in an atmospheric spray rig using the phase-Doppler technique. The simultaneous drop size and axial and radial velocity components were measured on radial traverses across the spray at various axial distances from the nozzle for a range of atomizing pressures. The droplet turbulent and mean kinetic energies were found to be proportional to the atomizing pressure. Hence, the scatter of the radial motion of the droplets increased with the atomizing pressure. A droplet stability analysis was performed to locate the regions characterized by ongoing secondary atomization. The volume-to-surface diameter, D₃₂, of the fully developed spray was compared with estimates provided by five published formulae. The role of liquid viscosity, hence the Ohnesorge number, was found to be negligible in the investigated regime. Three commonly used size distribution functions were fitted to the measured data to analyze their dependence on the atomizing pressure. The Gamma distribution function was found to give the best approximation to the atomization process.