Evaluation of effective parameters in metal bellows forming process

Metal bellows have wide applications in aerospace, micro-electromechanical and industrial systems. Forming process of the metal bellows is very sensitive to increasing the ratio of crown to root diameter. In this state, precise control of the parameters is very important in order to form high-quality metal bellows with good thickness distribution and desirable dimensions and resilience. In this paper, a new method has been proposed for manufacturing of the metal bellows and important parameters such as initial length of tube, internal pressure, axial feeding and velocity, mechanical properties and the type of materials were investigated by finite element (FE) analysis (LS-Dyna) and experimental tests. The explicit time integration method is used for modeling the tube-bulging and folding processes. Meanwhile, the implicit time integration method is used for the spring back stage. Finally, the results of finite element method (FEM) and experiments show a very good agreement. The results of the present work could be used as a basis of designing a new type of the metal bellows.     

The impacts of capabilities and constraints of generating units on simultaneous scheduling of energy and primary reserve

Frequency control, as an ancillary service, is usually provided by generation reserves. Modern generating units have special technical capabilities; e.g., their governor operation mode can be selected to be either active or passive, their ramp rate can be selected to be either normal or fast, etc. On the other hand, generating units have some technical constraints; e.g. some generating units cannot participate in primary frequency control at their capacity limits. In this paper, operational capabilities and constraints of generating units are incorporated in a “simultaneous scheduling of energy and primary reserve” problem. Furthermore, a heuristic iterative method (based on genetic algorithm) is proposed to obtain the optimal scheduling. The impacts of capabilities and constraints on scheduling are investigated through simulation studies. Simulation results depicts that taking these capabilities and constraints of generating units into account, not only reduces the total operational cost of generating units, but also will end up with a feasible solution for the system, even in cases where the previously proposed methods fail.     

Pressureless sintering of boron carbide

The effect of small Al addition on pressureless-sintering and mechanical properties of B₄C ceramic was analyzed. Different amounts of aluminium powder, from 0% to 5 wt%, were added to the base material and pressureless-sintering was conducted at 2050 and 2150 °C under argon atmosphere. Microstructure, crystalline phases, density evolution, fracture strength, elastic modulus, hardness and fracture toughness were analyzed and correlated to Al additions and firing temperature. Density and grain size of sintered samples increased significantly with Al load while the effect of sintering temperature was less evident; 94% dense material was obtained by adding 4 wt% Al. Bending strength, hardness and fracture toughness of sintered B₄C samples were shown to increase for Al content up to 4 wt% while further additions resulted in a decrease of the mechanical resistance. Conversely, elastic modulus showed an increase with Al load especially between 1 and 3 wt%.     

A probabilistic model for assessing the reliability of wind farms in a power system

Modeling the generation of a wind farm and its effect on power system reliability is a challenging task, largely due to the random behavior of the output power. In this paper, we propose a new probabilistic model for assessing the reliability of wind farms in a power system at hierarchical level II (HLII), using a Monte Carlo simulation. The proposed model shows the effect of correlation between wind and load on reliability calculation. It can also be used for identifying the priority of various points of the network for installing new wind farms, to promote the reliability of the whole system. A simple grid at hierarchical level I (HLI) and a network in the north-eastern region of Iran are studied. Simulation results showed that the correlation between wind and load significantly affects the reliability.     

Recycling of aluminium alloy turning scrap via cold pressing and melting with salt flux

Among the various steps of aluminium production from liquid metal, a lot of scrap is generated due to machining operations. Therefore, recycling of aluminium scrap is an interesting subject because of the broad applications of this metal and low efficiency of processes used to recycle metal scrap. In this paper, the recyclability of aluminium alloy AA 336 turnings with different cold compacting pressures and a protective salt flux (NaCl–KCl–KF) has been experimentally studied. Various categories of compacted samples were melted at 750 °C in molten aluminium alloy AA 336 and also in the protective salt flux to recover aluminium alloy. In order to understand the amount of recycling of different samples, weight loss measurement was applied. From recyclability stand point it is shown that using protective salt flux is the best route, from the point of view of recyclability. Mechanical properties and chemical analysis of samples were approximately the same as the primary material produced by conventional casting process.     

Code-division multiple-access techniques in optical fiber networks - part III: optical AND logic gate receiver structure with generalized optical orthogonal codes

In this paper, we present a deep insight into the behavior of optical code-division multiple-access (OCDMA) systems based on an incoherent, intensity encoding/decoding technique using a well-known class of codes, namely, optical orthogonal codes (OOCs). As opposed to parts I and II of this paper, where OOCs with cross-correlation /spl lambda/=1 were considered, we consider generalized OOCs with 1/spl les//spl lambda//spl les/w, where w is the weight of the corresponding codes. To enhance the performance of such systems, we propose the use of an optical and logic gate receiver, which, in an ideal case, e.g., in the absence of any noise source, except the optical multiple-access interference, is optimum. Using some basic laws on probability, we present direct and exact solutions for OOCs with /spl lambda/=1,2,3,...,w, with the optical and logic gate as receiver. Using the exact solution, we obtain empirical solutions that can be easily used in optimizing the design criteria of such systems. From our optimization scheme, we obtain some fresh insight into the performance of OOCs with /spl lambda//spl ges/1. In particular, we can obtain some simple relations between P/sub emin/ (minimum error rate), L/sub min/ (minimum required OOC length), and N/sub max/ (maximum number of interfering users to be supported), which are the most desired parameters for any OCDMA system design. Furthermore, we show that in most practical cases, OOCs with /spl lambda/=2,3 perform better than OOCs with /spl lambda/=1, while having a much bigger cardinality. Finally, we show that an upper bound on the maximum weight of OOCs are on the order of /spl radic/2/spl lambda/L where L is the length of the OOCs.     

Dynamic analysis of carbon nanotubes under electrostatic actuation using modified couple stress theory

Modified couple stress theory is a size-dependent theorem capturing the micro/nanoscale effects influencing the mechanical behaviors of the micro- and nanostructures. In this paper, it is applied to investigate the nonlinear vibration of carbon nanotubes under step DC voltage. The vibration, natural frequencies and dynamic pull-in characteristics of the carbon nanotubes are studied in detail. Moreover, the effects of various boundary conditions and geometries are scrutinized on the dynamic characteristics. The results reveal that application of this theory leads to the higher values of the natural frequencies and dynamic pull-in voltages.     

Mcchanical and Microstructural Properties of Ultra-fine Grained AZ91 Magnesium Alloy l ubes Processed via Multi Pass Tubular Channel Angular Pressing (TCAP)

Ultra-fine grained(UFG) cylindrical tubes were produced via recently developed tubular channel angular pressing(TCAP) process through different passes from as-cast AZ91 magnesium alloy.The microstructure and mechanical properties of processed tube through one to four passes of TCAP process at 200 ℃ were investigated.Microhardness of the processed tube was increased to 98.5 HV after one pass from an initial value of 67 HV.An increase in the number of passes from one to higher number of passes has no more effect on the microhardness.Yield and ultimate strengths were increased by 4.3 and 1.4 times compared to those in as-cast condition.Notable increase in the strength was achieved after one pass of TCAP while higher number of passes has no more effect.Microstructural investigation shows notable decrease in the grain size to around 500 nm from the primary value of ~ 150 μm.Dissolution and distribution of hard Mg_(17)Al_(12) phase in the grain boundaries of dynamically recrystallized UFG AZ91 with a mean grain size of ~500 nm was an interesting issue of TCAP processing at 200 ℃ compared to other severe plastic deformation processes.     

Heat Treatment of Ni-Ti Alloy for Improvement of Shape Memory Effect

Nickel-Titanium alloys with stoichiometric single phase and non-stoichiometric dual phase structures of NiTi and NiTi+Ni3Ti are produced through high speed induction melting and combustion synthesis of pure Ti/Ni elements. Both alloys are homogenized at 1273 K for two hours, rolled into thin strips of 0.3 mm thickness, solution treated at 1273 K for two hours under vacuum and finally quenched in water. Effect of ageing on austenite/martensite and intermediate phase transformation temperatures are investigated. Results show that transformation temperatures and reversible shape memory properties comparable with those required for bioengineering applications such as manufacturing of artificial hand prostheses can be obtained through careful control of the chemical composition and the heating processes.