采用机械钻铣轻量化技术的轻型镜研制

以φ400mm轻型平面镜为例,介绍了采用机械减重法制造轻型镜的工艺技术。镜坯的轻量化采用了刚度虽不是最优,但工艺性最好的圆孔结构,并采用专门设计的钻头钻孔,再用氢氟酸消除微裂纹。加工结果表明,该镜的减重比达44．5％,但仍具有足够的刚度,且没有影响加工精度的变形。     

MPiSIM: Massively parallel simulation tool for metallic system

We report a domain decomposition molecular dynamics (MD) for simulation on metallic systems based on distributed parallel computers. The method is a development of a spatial decomposition in 3-D space with the combination of link-cell and neighbor list techniques for enhanced efficiency. The algorithm has been successfully implemented on the Origin2000, Wolf, HP Examplar etc. various platforms. The scaling performance on these platforms will be discussed and several applications in metallic systems will also be given in the paper. This revised version was published online in June 2006 with corrections to the Cover Date.     

Integrated computational chemistry system for catalysts design

The understanding of valuable catalytic and adsorptive properties of heterogeneous catalysts at atomic and electronic levels is essential for the design of novel catalysts. Computer simulation studies can significantly contribute to provide a rational interpretation of the observed experimental results and suggest modification of new catalysts. Our recent work on the application of integrated computer simulation methods to investigate the structure and catalytic properties of solid surfaces including zeolites, transition metals and their oxides have been reviewed in this paper. We have emphasized the effectivity and applicability of integrated computer simulation system to solve the problems in a variety of targets of industrial and academic importance.     

Manufacturing of lightweight aggregates with carbon fiber and mineral wastes

The use of carbon fiber wastes (FC) as a component to manufacture lightweight aggregates (LWAs) for concrete has been studied. Amounts of 0, 2.5, 5 and 10% (w/w) of powdered FC were added into a mineral matrix composed by 90% of granite-marble sludge (COR) plus 10% of sepiolite rejection (SEP). The mixtures were milled, kneaded with water, extruded, shaped into pellets, oven-dried and finally fired at 1100, 1125 and 1150 °C for 4, 8 and 16 min in a rotary kiln. The main technological properties of the sintered aggregates were measured. The addition of FC promoted bloating and the formation of an internal structure in which both pores and unburnt carbon fibers were present. Improvements in lightness and mechanical properties were also observed. This is the first time that carbon fibers have been embedded within aggregates, opening the way to the development of a new type of LWAs for concrete.     

Simulations of the elastic response of single-walled carbon nanotubes

Using the Tersoff-Brenner potential we have performed molecular dynamics simulations of nanotubes under axial strain, analyzing both compression and stretching forces. These large-scale simulations were carried out on a MasPar massively parallel computer. The elastic response is investigated and expressions for various elastic constants are derived from the simulations. Typical failure modes are also shown and discussed.     

Using aggregate flowability testing to predict lightweight self-consolidating concrete plastic properties

This paper presents the results of an experimental study on the flow properties of lightweight self-consolidating concrete (LWSCC) which utilizes a new test relating aggregate flow to concrete flow. Three types of LWSCC were tested containing differing proportions of lightweight and normal weight, coarse and fine aggregates, as well as a normal weight self-consolidating concrete (NWSCC) as a control. The flow properties of the aggregate mixes used in the LWSCC and NWSCC specimens were tested using a V-funnel. The concrete flow properties were also tested for comparison, as were the compressive and tensile strengths of the various mixtures. A relationship between the aggregate frictional resistance and the traditional concrete flowability tests—i.e., slump flow, J-ring, and T₅₀₀—was demonstrated. Compressive strengths were greater in LWSCC mixes that contained smaller sized coarse and normal weight aggregates. Finally, a design procedure is introduced that utilizes the aggregate frictional resistance, paste flow properties, and aggregate void ratio to predict the plastic properties of the concrete.     

Dissociated dislocations in Ni: a computational study

A systematic computational study of the behavior of a dissociated screw dislocation in fcc nickel is presented, in which atomic interactions are described through an embedded-atom potential. A suitable external stress is applied on the system, both for modifying the equilibrium separation distance d and moving the dislocation complex. The structure of the dislocation and its corresponding changes during the motion are studied in the framework of the two-dimensional Peierls model, for different values of the ratio d/a^′, where a^′ is the period of the Peierls potential. The distance between the edge and screw components of the partials, as well as their widths, undergo a modulation with period a^′, as the dislocation moves, and the amplitudes of such oscillations are shown to depend on d/a^′. The stress profile acting on the dislocation complex is analyzed and the effective Peierls stress is estimated for different values of d/a^′.     

Absorption and desorption properties of fine lightweight aggregate for application to internally cured concrete mixtures

Recently, substantial interest has developed in using fine lightweight aggregate for internal curing in concrete. Mixture proportion development for these mixtures requires the specific gravity, water absorption, and water desorption characteristics of the aggregate. This paper presents results from a recent study in which the properties of commercially available expanded shale, clay and slate lightweight aggregates (LWA’s) were measured. This research measured the time-dependent water absorption response for the lightweight aggregate. The results indicate that a wide range of 24 h water absorption values exist for commonly used fine lightweight aggregates (e.g., absorption between 6% and 31%). Desorption was measured and it was found that between 85% and 98% of the 24 h absorbed water is released at humidities greater than 93%. These properties can be normalized so that they can be efficiently used in proportioning concrete for internal curing. Normalized plots of absorption and desorption demonstrate benefits for a single function that describes a large class of expanded shale, clay, and slate aggregate for use in internal curing.     

Molecular dynamics simulations of spallation in metals and alloys

We report on the results of large-scale molecular dynamics simulations of the mechanical behavior of two-dimensional metallic systems. The specific impact phenomenon studied is that in which a flyer of mass M moving with x-velocity v impacts a target of mass 2M moving with x-velocity −v/2. Simulations of such a spallation experiment have been performed for a generic metal, modelled with an embedded atom potential and also for a Cu-Ni alloy system, modelled with truncated Lennard-Jones potentials.

Our simulations indicate cold-welding upon impact, and shock wave generation, followed by rebound from the boundaries. The alloy was less ductile than the generic metal and consequently the system came apart due to the cooperative effect of the reflected shock waves.     

Configuration design of a lightweight torpedo subjected to an underwater explosion

The response of a lightweight torpedo when subjected to an underwater explosion (UNDEX) is an important criterion for multidisciplinary design. This paper investigates the effect of structural stiffeners on the performance of a lightweight torpedo. The finite element package ABAQUS was used to model the UNDEX and the fluid–structure interaction (FSI) phenomena, which are critical for accurate evaluation of torpedo stress levels. The pressure wave resulting from an underwater explosion was modeled using similitude relations and it was assumed to be a spherical wave. Various explosive weights and explosion distances were explored to determine the critical distance both for an un-stiffened and a stiffened torpedo. Once it was established that the stiffened torpedo performed better under explosive pressure loads, various configurations were studied to determine the optimal number of ring and longitudinal stiffeners. A final configuration was obtained for the torpedo that had minimum weight and was least sensitive to small manufacturing variations in the dimensions of the stiffeners. This paper presents details of the torpedo and fluid models and the finite element analysis method for FSI.     

碳化硅反射镜轻量化结构优化设计

本文根据某型号碳化硅主反射镜的设计要求,对比了扇形、三角形、六边形轻量化孔对镜体静态、动态性能的影响,选择了综合性能较好的三角形轻量化结构方案,利用有限元方法进行参数建模,将对镜体力学性能和质量影响较大的镜体厚度、面板厚度、筋厚度等结构参数作为设计变量,运用零阶优化方法对镜体结构进行优化设计,得到了轻量化程度高、镜面变形误差完全满足设计要求的碳化硅反射镜体。     

Fabrication of lightweight structural panels through ultrasonic consolidation

A fundamental investigation of the feasibility of producing lightweight structural panels using ultrasonic consolidation (UC) was undertaken. As a novel solid freeform fabrication technology, UC utilizes both additive ultrasonic joining and subtractive CNC milling to enable the creation of complex aluminum structures with internal geometry at or near room temperature. A series of experiments were performed to understand the issues associated with sandwich structure fabrication using UC, including peel test experiments which evaluated the bond strength for various geometric configurations. The honeycomb lattice was found to offer the best core configuration due to its ability to resist vibration from the sonotrode and provide adequate support for pressure induced by the sonotrode. UC was found to be capable of producing lightweight and stiff structures, including honeycomb and other sandwich panels, without the use of adhesives. An effective manufacturing process plan for fabricating structural panels was developed. A case study was performed on a deck built for the TOROID small satellite spacecraft. The fabricated deck was tested for mechanical integrity. Finally, the cost and benefits of utilizing UC for lightweight structural panels versus traditional fabrication methods are discussed.     

Fabrication of lightweight structural panels through ultrasonic consolidation

A fundamental investigation of the feasibility of producing lightweight structural panels using ultrasonic consolidation (UC) was undertaken. As a novel solid freeform fabrication technology, UC utilizes both additive ultrasonic joining and subtractive CNC milling to enable the creation of complex aluminum structures with internal geometry at or near room temperature. A series of experiments were performed to understand the issues associated with sandwich structure fabrication using UC, including peel test experiments which evaluated the bond strength for various geometric configurations. The honeycomb lattice was found to offer the best core configuration due to its ability to resist vibration from the sonotrode and provide adequate support for pressure induced by the sonotrode. UC was found to be capable of producing lightweight and stiff structures, including honeycomb and other sandwich panels, without the use of adhesives. An effective manufacturing process plan for fabricating structural panels was developed. A case study was performed on a deck built for the TOROID small satellite spacecraft. The fabricated deck was tested for mechanical integrity. Finally, the cost and benefits of utilizing UC for lightweight structural panels versus traditional fabrication methods are discussed.     

Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures

This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 °C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO₂ for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)₂ lowered stress from rapid recrystallization after exposure to 600 °C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance.     

Pozzolanicity of finely ground lightweight aggregates

This paper examines the pozzolanic behavior of finely ground lightweight aggregates with a mean particle size between 4 and 26 μm. Cement pastes are made with a 20% mass replacement of cement with finely ground lightweight aggregates, fly ash, quartz, and limestone in addition to a control paste with no cement replacement. Isothermal calorimetry, thermogravimetric analysis, and compressive strength testing as well as thermodynamic calculations are performed on these pastes. Isothermal calorimetry and compressive strength testing are shown to not be able to clearly distinguish and quantify the pozzolanic response of the finely ground lightweight aggregates, fly ash, quartz, and limestone when they are used in cement pastes. However, thermogravimetric analysis and thermodynamic calculations clearly show that the finely ground lightweight aggregates are pozzolanic through the consumption of calcium hydroxide. A pozzolanic reactivity test based on isothermal calorimetry also confirms that the finely ground lightweight aggregates are pozzolanic. These results indicate that finely ground lightweight aggregates are pozzolanic and could be used in concreting applications.     

NMR relaxometry during internal curing of Portland cements by lightweight aggregates

The hydration of an ordinary Portland cement of low water-to-cement ratio under the influence of internal curing by addition of water-saturated lightweight aggregates (LWA) is investigated by non-destructive low-field NMR relaxometry. The methodology, which was recently developed to investigate internal curing by addition of a water-saturated polyelectrolyte gel, is applied to follow the transition of water from the mineral aggregates into hydrating cement paste and to detect the time dependence of the water consumption by the hydration reaction. By the changes of the transverse relaxation times of the physically bound water, the compaction of the pore space of the hardening cement is estimated qualitatively. The water retention potential of two types of LWA and the temporal moisture requirement of the cement during internal curing are determined without and with superplasticizer.     

Utilization of lignite power generation residues for the production of lightweight aggregates

A novel process is proposed for the utilization of lignite combustion solid residues in the production of inflammable lightweight aggregates (LWA). The process consists of two stages, pelletization and sintering, and carbon contained in BA was used as the process fuel. The main residues bottom ash (BA) and fly ash (FA) from Megalopolis power plant were characterized, mixed in different proportions and treated through pelletization and sintering process. Sintering benefits from combustion of BA carbon content and the product is a hardened porous cake. The energy required for achievement of high temperatures, in the range of 1250 °C, was offered by carbon combustion and CO₂ evolution is responsible for porous structure formation. Selected physical properties of sintered material relevant to use as lightweight aggregates were determined, including bulk density, porosity and water absorption. Bulk density varies from 0.83 to 0.91 g/cm³, porosity varies from 60% to 64% and water absorption varies from 66% to 80%. LWA formed is used for the production of lightweight aggregate concrete (LWAC). Thermal conductivity coefficient varies from 0.25 to 0.37 W/mK (lower than maximum limit 0.43 W/mK) and compressive strength varies from 19 to 23 MPa (higher than minimum limit 17 MPa). The results indicate that sintering of lignite combustion residues is an efficient method of utilization of carbon containing BA and production of LWA for structural and insulating purposes. Carbon content of BA is a key factor in LWA production. Finally, this research work comprises the first proposed application for utilization of BA in Greece.     

Parallel processing in computational stochastic dynamics

Studying large complex problems that often arise in computational stochastic dynamics (CSD) demands significant computer power and data storage. Parallel processing can help meet these requirements by exploiting the computational and storage capabilities of multiprocessing computational environments. The challenge is to develop parallel algorithms and computational strategies that can take full advantage of parallel machines. This paper reviews some of the characteristics of parallel computing and the techniques used to parallelize computational algorithms in CSD. The characteristics of parallel processor environments are discussed, including parallelization through the use of message passing and parallelizing compilers. Several applications of parallel processing in CSD are then developed: solutions of the Fokker–Planck equation, Monte Carlo simulation of dynamical systems, and random eigenvector problems. In these examples, parallel processing is seen to be a promising approach through which to resolve some of the computational issues pertinent to CSD.     

The effect of additives on the properties of lightweight aggregates produced from clay

In an attempt to improve the properties, lightweight aggregates were produced from clay with the addition of Na₂CO₃, SiO₂, Fe₂O₃, and Fe in quantities between 2 and 10 wt% and examined with respect to strength, density and expansion behavior. The additives were mixed into dry clay powder, water was added and pellets were formed by hand and fired at 1120 °C in a chamber furnace. Particle densities of the products ranged from 0.31 to 0.57 g/cm³, porosities from 78% to 89% and the solid strength from 0.54 to 1.58 MPa. The addition of Na₂CO₃ proved to decrease the viscosity of the glass phase at the surface of the pellets but resulted in a reduced expansion, irregular shape and pellets sticking together. SiO₂ addition did not give any major change in properties. The addition of Fe₂O₃ increased the pore size in the center of the pellets, however with insignificant change in strength and density. Adding 5 wt% metallic iron powder led to LWA pellets with increased porosity, reduced density, larger pores and low mechanical strength and could be a useful additive in applications where low density is more important than strength.     

Molecular dynamics in semiconductor physics

Molecular dynamics, which is a powerful method for studying both structural and dynamic properties of condensed matter, is employed as a “microscope for the motion of atoms”. Information about the motion of individual atoms that is difficult to obtain experimentally can be obtained using this method. Parallel computers with tera-flop speed and tera-byte memory will make it possible to perform hundreds of million particle simulations using empirical potential. Moreover, the macroscopic phenomena analyzed by continuum theory can be obtained by coarse-graining the atomic-level information provided by this approach. When considering the problem of data analysis, examples in semiconductor physics such as the implantation process are provided.