Vaporization of heated materials into discharge plasmas

The vaporization of condensed materials in contact with high-current discharge plasmas is considered. A kinetic numerical method named direct simulation Monte Carlo (DSMC) and analytical kinetic approaches based on the bimodal distribution function approximation are employed. The solution of the kinetic layer problem depends upon the velocity at the outer boundary of the kinetic layer which varies from very small, corresponding to the high-density plasma near the evaporated surface, up to the sound speed, corresponding to evaporation into vacuum. The heavy particles density and temperature at the kinetic and hydrodynamic layer interface were obtained by the analytical method while DSMC calculation makes it possible to obtain the evolution of the particle distribution function within the kinetic layer and the layer thickness.

Measurement of Mechanical Properties of MEMS Materials

微电子机械系统（MEMS）技术的迅速崛起,推动了所用材料微尺度力学性能测试技术的发展.首先按作用方式将实验分成压痕/划痕、弯曲、拉伸、扭转四大类,系统介绍检测MEMS材料微尺度力学性能的微型试样、测试方法及其实验结果.测试材料主要有硅、氧化硅、氮化硅和一些金属.实验结果主要包括基本的力学性能参数如弹性模量、残余应力、屈服强度、断裂强度和疲劳强度等.最后,简要分析了未来的发展需求.

Effects Analysis on the Mechanical Properties of Nanocrystalline Materials

Nanocrystalline materials are characterized by a typical grain size from 1 to 100nm. In order to study the nanocrystalline properties of nanocrystalline materials, we chose nanocrystalline coppers as the research object. The uniaxial tensile deformation of computer produced nanocrystalline coppers is simulated by using molecular dynamics with Finnis-Sinclair potential. The mean grain size of simulated nanocrystalline coppers is varied within the 5.38 to 1.79 nm range. The strength, Young's modulus and stress-strain are strongly depended on the grain size and nanocrystalline structure. The simulated nanocrystalline coppers show a reverse Hall-Petch effect.

Experimental Study on the Thermal Argon Plasma Generation and Jet Length Change Characteristics at Atmospheric Pressure

The generation, jet length and flow-regime change characteristics of argon plasma issuing into ambient air have been experimentally examined. Different torch structures have been used in the tests. Laminar plasma jets can be generated within a rather wide range of working-gas flow rates, and an unsteady transitional flow state exists between the laminar and turbulent flow regimes. The high-temperature region length of the laminar plasma jet can be over an order longer than that of the turbulent plasma jet and increases with increasing argon flow rate or arc current, while the jet length of the turbulent plasma is less influenced by the generating parameters. The flow field of the plasma jet has very high radial gradients of plasma parameters, and a Reynolds number alone calculated in the ordinary manner may not adequately serve as a criterion for transition. The laminar plasma jet can have a higher velocity than that of an unsteady or turbulent jet. The long laminar plasma jet has good stiffness to withstand the impact of laterally injected cold gas and particulate matter. It could be used as a rather ideal object for fundamental studies and be applied to novel materials processing due to its attractive stable and adjustable properties.

The Influence of Grain Size and Temperature on the mechanical Deformation of Nanocrystalline Materials: Molecular Dynamics Simulation

Nanocrystalline (nc) materials are characterized by a typical grain size of 1-100nm. The uniaxial tensile deformation of computer-generated nc samples, with several average grain sizes ranging from 5.38 to 1.79nm, is simulated by using molecular dynamics with the Finnis-Sinclair potential. The influence of grain size and temperature on the mechanical deformation is studied in this paper. The simulated nc samples show a reverse Hall-Petch effect. Grain boundary sliding and motion, as well as grain rotation are mainly responsible for the plastic deformation. At low temperatures, partial dislocation activities play a minor role during the deformation. This role begins to occur at the strain of 5%, and is progressively remarkable with increasing average grain size. However, at elevated temperatures no dislocation activity is detected, and the diffusion of grain boundaries may come into play.

Combined Effects of Silica Filler and Its Interface in Epoxy Resin

In this paper, mechanical properties of silica-filled epoxy resin are tested. The tests show that at elevated temperatures, the material’s properties (e.g. yield stress, flow stress, etc.) vary immonotonically with filler volume fraction. Nanoindentation test results suggest that an interface region, stronger than the matrix, is formed in the materials. The formation of the interface has positive effects on the yield strengths of materials. The addition of particles in the matrix produces a large disturbance in stress distribution, leading to stress concentration in the matrix. The stress concentration has negative effects on the yield strengths of materials. The calculation demonstrates that the maximum stress in samples varies immonotonically with particulate concentration. So, the immonotonic variation of mechanical behavior of materials may be rooted in the contradictory effects of the interface region and the stress concentration caused by particulate addition.

Theermal Stress and Fracture in Transparent Optical Materials due to Laser Energy-Absorption by a Defect

利用热弹性理论分析了在光学材料中由于缺陷吸收激光能量引起的温度和热应力分布,并且针对一个简单的裂纹模型分析了热应力产生的应力强度因子,给出了一些主要参数对于应力强度因子的影响的规律。

Nonuniformity Effect of Surface-Nanocrystalline Materials in Nanoindentation Test

In the present research, microstructures of the surface-nanocrystalline Al alloy material are observed and measured based on the transmission electron microscopy (TEM) technique, and the corresponding mechanical behaviors are investigated experimentally and theoretically. In the experimental research, the nanoindentation test method is used, and the load and microhardness curves are measured, which strongly depend on the grain size and grain size nonuniformity. Two kinds of the nanoindentation test methods are adopted: the randomly selected loading point method and the continuous stiffness method. In the theoretical modeling, based on the microstructure characteristics of the surface-nanocrystalline Al alloy material, a dislocation pile-up model considering the grain size effect and based on the Mott theory is presented and used. The hardness-indent depth curves are predicted and modeled.

Asymptotic analysis for a crack on interface of damaged materials

An asymptotic analysis for a crack lying on the interface of a damaged plastic material and a linear elastic material is presented in this paper. The present results show that the stress distributions along the crack tip are quite similar to those with HRR singularity field and the crack faces open obviously. Material constants n, mu and mo are varied to examine their effects on the resulting stress distributions and displacement distributions in the damaged plastic region. It is found that the stress components sigma(rr), sigma(theta theta), sigma(r theta) and sigma(e) are slightly affected by the changes of material constants n, mu and m(0), but the damaged plastic region are greatly disturbed by these material parameters.

Modeling and Simulation of Liquid Pulsed Particulate Fluidized Beds

Pulsed fluidization is of considerable interest in process engineering for improving fluidization quality. Quantitative understanding of the pulsed two-phase flow behaviors is very important for proper design and optimum operation of such contactors. The

Crack problems of piezoelectric materials

This paper presents an analysis of crack problems in homogeneous piezoelectrics or on the interfaces between two dissimilar piezoelectric materials based on the continuity of normal electric displacement and electric potential across the crack faces. The explicit analytic solutions are obtained for a single crack in piezoelectrics or on the interfaces of piezoelectric bimaterials. A class of boundary problems involving many cracks is also solved. For homogeneous materials it is found that the normal electric displacement D-2 induced by the crack is constant along the crack faces which depends only on the applied remote stress field. Within the crack slit, the electric fields induced by the crack are also constant and not affected by the applied electric field. For the bimaterials with real H, the normal electric displacement D-2 is constant along the crack faces and electric field E-2 has the singularity ahead of the crack tip and a jump across the interface.

Deformation and Failure of Polymer Bonded Explosives under Diametric Compression Test

The tensile deformation and failure of polymer bonded explosives (PBXs), a particulate composite, is studied in this paper. Two HMX-based PBXs with different binder were selected for study. A diametric compression test, in which a disc-shaped specimen is loaded diametrically, was chosen to generate tensile failure in the materials. The quasi-static tensile properties and the tensile creep properties were studied by using conventional displacement transducers to measure the lateral strain along the horizontal diameter. The whole-field in-plane creep deformation was measured by using the technique of high resolution moire´ interferometry. Real time microscopic examination was conducted to monitor the process of deformation and failure of PBXs by using a scanning electron microscope equipped with a loading stage. A manifold method (MM) was used to simulate the deformation and failure of PBX samples under the diametric compression test, including the crack initiation, crack propagation and final cleavage fracture. The mechanisms of deformation and failure of PBXs under diametric compression were analyzed. The diametric compression test and the techniques developed in this research have proven to be applicable to the study of tensile properties of PBXs.

On failure modes and strength characterization of brittle disordered materials under uniaxial compression and tension

With a newly developed Material Failure Process Analysis code (MFPA(2D)), influence of hetero geneity on fracture processes and strength characterization of brittle disorder materials such as rock or concrete is numerically studied under uniaxial compression and tension conditions. It is found th at, due to the heterogeneity of the disordered material, relatively more diffused micro-fractures appear in the early stage of loading. Different from homogeneous materials such as glass, macro-crack nucleation starts well before the peak stress is reached and the crack propagation and coalescence can be traced, which can be taken as a precursory to predict the macro-fracture of the material. The presence of residual strength in the post-peak region and the resemblance in the stress-strain curves between tension and compression are significant results and are found to be dependent on the heterogeneity of the specimens. Examples showing the tentative applications of MFPA(2D) in modeling failure of composite materials and rock or civil engineering problem are also given in this paper.

An incremental micromechanical scheme for nonlinear particulate composites

A general incremental micromechanical scheme for the nonlinear behavior of particulate composites is presented in this paper. The advantage of this scheme is that it can reflect partly the effects of the third invariant of the stress on the overall mechanical behavior of nonlinear composites. The difficulty involved is the determination of the effective compliance tensors of the anisotropic multiphase composites. This is completed by making use of the generalized self-consistent Mori-Tanaka method which was recently developed by Dai et al. (Polymer Composites 19(1998) 506-513; Acta Mechanica Solida 18 (1998) 199-208). Comparison with existing theoretical and numerical results demonstrates that the present incremental scheme is quite satisfactory. Based on this incremental scheme, the overall mechanical behavior of a hard-particle reinforced metal matrix composite with progressive particle debonding damage is investigated.

Interacting multiple cracks in piezoelectric materials

In this paper, a method is presented to calculate the plane electro-elastic fields in piezoelectric materials with multiple cracks. The cracks may be distributed randomly in locations, orientations and sizes. In the method, each crack is treated as a continuous distributed dislocations with the density function to be determined according to the conditions of external loads and crack surfaces. Some numerical examples are given to show the interacting effect among multiple cracks.