IRANIAN JOURNAL OF SCIENCE AND TECHNOLOGY TRANSACTION B- ENGINEERING
Year:2004 | Volume:28 | Issue:B2|Papers Count:12

By applying shortest path analysis in stochastic networks,we introduce a new approach to obtain the reliability function of time-dependent systems. We assume that not all elements of the system are set to function from the beginning. Upon the failure of each element of the active path in the reliability graph, the system switches to the next path. Then, the corresponding elements are activated, and consequently, the connection between the input and the output is established. It is also assumed that each element exhibits a constant hazard rate and its lifetime is a random variable with exponential distribution. To evaluate the system reliability, we construct a directed stochastic network called E-network, in which each path corresponds with a minimal cut of the reliability graph. We also prove that the system failure function is equal to the distribution function of the shortest path of E-network. The shortest path of this new constructed network is determined analytically by using continuous time Markov processes.

A special hybrid genetic algorithm (GA) is designed to solve the long-term coordination of hydro-thermal power systems with cascaded reservoirs and stochastic inflows. Since decision variables are continuous, in the proposed GA we employ real number rather than binary encoding. To create superior children we introducedynamic tuning of the weights of operators. An exponential normalization is also developed such that better cl1romosomeshave more chance to reproduce. To test the proposed method, ]6 GAs are investigated which differ based on real or binary encoding, dynamic tuning or fixed weights for operators, inverse or exponential normalization and mixed or pure random initial populations. By applying the data of a real power system, the performance of these algorithms are compared. We also compare the performance of the proposed GA with that of the conventional Lagrangian relaxation method. The results show that the proposed GA gives promising performance for situations, which if not impossible, are very difficult to handle using conventional optimization methods.

In this paper, a new approach for optimizing the expansion policy (EP) of power plant networks has been developed based on the recursive Tabu search technique. The objectives of the EP model are minimizing the fixed investment and the operationaI--cost over certain network reliability in the long term. The EP problem is the NP-complete mixed integer-programming model. The model deals with optimizing the region allocation to power plant centers and the power plant center capacity over a specified planning horizon (years) by considering the 3Es (Energy, Environment, Economic) and the network reliability constraints. Examples are given to illustrate that the approach is able to resolve the EP model in short CPU time in comparison with an analytic approach.

In this studythe effectof Sr and Sb onsiliconmodificationwereinvestigated.Casting properties, Le. tendency to produce the porosity of both Sr and Sb treated alloys were evaluated. Finally, the effect of heat treatment on tensile and fatigue properties and silicon morphology was examined. Modification by Sr can convert the silicon phase from acicular to fibrous. The optimum amount of Sr content for achieving a modified fibrous structure in a sand mould is 0.013% Sr (by weight), whereas the amount for Sb is around 0.1%. Castings modified with Sr develop a significant increase in porosity, whereas little effect was observed in alloys treated with Sb. Mechanical properties of the modified A356 alloy were superior to the unmodifiedalloy. Solution heat treatment at 540°C for 6 hours, water quenching and aging at 150°Cfor 3 hours significantly enhance these properties

Al-AI4C3 dispersion strengthened material was prepared by a powder metallurgy method of reaction milling. Interparticle distances of Al4C3 particles in the Al-AI4C3 composite materials were evaluated. In this paper, nearest neighbour distribution function, polygonal methods and the quadrate counts method are used for characterization of particle arrangement.

The tensile behavior of AI-Cu-Mg-Mnalloy matrix composites produced by a powder metallurgy process was investigated as a function of particle size and volume fraction in a naturally agedcondition(T4). Microstructural examinationshaveenabledidentificationof particlesandgrain structure of the materials. The results indicated that tensile properties of the composites significantly improved in the yield strength, UTS and elastic modulus, on incorporation of hard, brittle ceramic particles as compared to unreinforced counterparts. On the other hand, ductility decreased considerably due to the brittle ceramic particles. The results were discussed in the light of microstructures.

Polycrystalline Ni]Al with a columnar grained structure was grown unidirectionally by the floating zone method at a rate of 20 mm/h. Grain boundary structure was characterized by 2.: values determined from grain orientations of two adjacent grains using a coincidence lattice theory. Two types of specimens PO and NO, whose cold-rolling directions were parallel and perpendicular to the crystal growth direction, were prepared. Workability and fracture behavior of PO and NO specimens were examined focusing on the effect of the grain boundary character on fracture. PO specimens showed the better workability than NO specimens.Low angle and coincidence boundaries exhibited a strong resistance to grain boundary fracture, while random boundaries were harmful to crack initiation and propagation.

Permeation tests were conducted by Devanathans electrochemical method and a newly invented electro-vacuum method. Experiments illustrated that as the microstructure changed, the diffusion constant, D, changed. The steel SS 4360 when quenched, decreased its D value by 3.7 times that of the original plate. Comparing the behavior of AISI 4340 with API X52, it is noted that the D value of the latter alloy is 2.23 times that of API X52, due to the presence of less C and Ni content. Ingress of hydrogen increased with the reduction of voltage and pH. For alloy SS 4360, when the charging voltage was dropped by -400 mv,the D value increased by 62%. Thickness would not have a realistic effect, but cold work reduced D. The same alloy when cold worked by 72%, its D value decreased by 15%.The difference in D measuredby the two methods of permeation was due to the variation in material, apparatus design and operation. Hydrogen degradation during stress corrosion and corrosion fatigue tests is one of the two processes of crack propogation. Immersion, dynamic polarization, stress corrosion (dynamic and static), corrosion fatigue tests, cathodic protection and fractographic studies were conducted in seawater and sour (hydrogen sulfide generatedby sulfate reducing bacteria) environments. Overall, it was concluded that with regards to Nemests equation, as the partial pressure of hydrogen increased, the value of D increased. Furthermore, the speed of hydrogen diffusion increased. In the sour environment, the overtaking mechanism of failure was found to be hydrogen embrittlement rather than anodic dissolution.

We have proposed the novel etching technology of Pt using a hard mask in reactive ion etching plasmas. By the insertion of a Ti mask layer and increasing the wafer temperature in O2 plasma by a dual frequency reactive ion etcher (RIE), we have obtained a higher Pt etching slope, This result not only enlightens the next generation of DRAM and FRAM technology, but also develops the basic technology ofpatterning inert materials.

Micro-structural failure analysis of the heat resisting HP40 Nb modified alloy was studied by light and electron methods. Samples from the failed reformer furnace tube were cut and prepared for metallographic examination. Examination with electron microscope was carried out with secondary and backscattered electron detectors; x-ray analysis was conducted at the grain boundary areas. In tubes, which are filled with a supported nickel catalyst, methane reacts with steam, carbondioxide and oxygen into synthesis gas. The overall heat of reactions may be positive, zero, or negative, depending on the process conditions.The catalyst plays a key role in developing overheating in the tubes. The catalyst deactivation caused by feeding is heavier than that designed hydrocarbon. Using an unsuitably designedhydrocarbon causes a thin layer of carbon coating on the catalyst surface. Overheating due to catalyst poisoning caused creep failure. The presence of intergranular voids in the microstructure of the failed tube seemsto be the result of creep phenomenon.

The association of elements in a regional stream-sediment geochemical exploration was investigated by using R-mode factor analysis on 233 samples in the arid environment of the Shahkouh area, east of Iran. A five-factor model with a cumulative variance of 70.60% clearly indicated the group associations of the elements, which coincide precisely with the lithology and the mineralization present in the study area. Factor scores were calculated and mapped in order to delineate the anomalous locations of the relevant association across the whole area. This multivariate statistical method can be used in arid environments, despite the occurrence of some drawbacks in such climate condition.

The effect of the addition of nucleating agents such as Zr02 and Ti02 on setting, working times and the compressive strength of a Si02-AI2O)-MgO-CaF2 restorative glass ion om er cement has been investigated using differential thermal analysis and X-ray diffraction. The addition of 2.5% Zr02 resulted in the formation of anortite, MgF2 and CaF2 crystalline phases more than the addition of the same amount of Ti02, and hence the setting, working times and compressive strength of the glass ionomer cement were notably increased. An increase in the heat treatment time caused an increase in the formation of the named crystalline phases.