INTERNATIONAL JOURNAL OF ENGINEERING SCIENCE (ENGLISH)
Year:2003 | Volume:14 | Issue:2|Papers Count:13

In this paper the existing methods of strengthening for reinforced concrete beams were introduced and also using ANSYS structural software, reinforced concrete beams strengthened with external unbonded reinforcements were modeled and analyzed nonlinearly to determine their ultimate flexural strength and deformability. For this purpose, 24 beams with different dimensions and different properties were modeled, analyzed and compared with the experimental results. The effect of different parameters, such as percentage of internal reinforcement, shear span to effective beam depth, the existence of deflectors to hold in place external reinforcement, span length to effective beam depth ratio, and the percentage of external reinforcement on the ultimate flexural strength and deformability we re studied. The results of the analysis show that increase in shear span to effective beam depth ratio, will cause a decrease in the external reinforcement force, flexural strength and deformability of the beam. The use of deflectors for external reinforcement will decrease deformability but increase flexural strength of the beam. Increasing percentage of internal reinforcements will decrease both the force in external reinforcement and deformability of the beam. In spite of the decrease in deformability due to the usage of external unbonded reinforcement for strengthening of reinforced concrete beams, the method can be safely used for flexural strengthening of reinforced concrete beams. Finally the model introduced in this research can be properly used for the analysis of the beams strengthened by external unbonded reinforcement.

Hardness measurements have been used to determine the maximum amount of SiC that can be added homogeneously to aluminium using a double pressing - double sintering method. The wear behavior of the composites was studied using a pin-on-disc type machine. Large amounts of SiC exhibited high wear resistances. Data from compression and tensile tests are also reported for composites having 0-30 wt % SiC reinforcement. Optical microscopy was carried out on all the Al-SiC composites to examine the distribution of the reinforced particles. XRD and EDAX analyses have been used to reveal if any unwanted phases were obtained in the composite materials as a result of the unusual double pressing double sintering production process.

Thermal degradation behavior of sized and unsized carbon fibers in polyimide matrix was investigated. Degradation of neat resin and unidirectional laminates were investigated by thermogravimetric analysis (TGA) technique at temperatures between 470°C-650°C and up to 250 hrs. Isothermal ageing of the PMR-X composite samples under different test conditions (i.e different temperatures and prolonged aging times). Showed that oxidation and degradation occurs in stages at three different rates. Thermogravimetric analysis (TGA) showed that the cured PMR-X composite panels are more stable in an inert atmosphere (nitrogen atmosphere) than in air and the degradation of neat resin is much higher than the composite samples. However, the rate of degradation of the unsized untreated carbon fibers in nitrogen environment is much higher than that for the PMR-X composites containing sized fibers.

Effects of the initial compaction pressure of pressed power mixtures and the ambient atmosphere of the reactor for a SHS of Molybdenum Disilicide has been investigated. Using Various compaction pressures, a number of green specimens were prepared from stoichiometric mixtures of Mo and Si powders. The specimens were then introduced to a reactor in which a SHS process would be ignition-started, converting the initial power mixture to MoSi2. Higher compaction pressures had the positive effect of lowering the SHS temperature, it also increased the reaction rate retarded the occurrence of unwanted phases. Furthermore, use of Ar instead of vacuum shifted the maximum reaction temperature to higher values and increased the reaction rate.

In the persent work the effect of Al in magnesia-graphite refractories(MGR) on phase transformation has been investigated. Small disc shape sample were prepared by using proper starting materials and different amounts of Al powder. Then the samples were heat treated in a carbon bed in temperature range of 600-1400°C DTA/TG, XRD and SEM were used to analyse the sequences of the thermal enents and microstructural characterization. Results show that at temperature around 800°C carbide and nitride of Al formed due to the reaction of Al melt with carbon and nitrogen in the media. Above 1000°C spinel formed. Around 1200°Cby decreasing amount of Al__4 C__3 spinel phase increased and at 1400°C the major formed phases are spinel and ALN. ALN phase and Al__4 C__3 had needle and platelet shape respectively

In recent years several mine researchers have studied the optimization of cutoff grade with purpose of maximizing Net present Value (NPV). The best and popular one is Lane algorithm. Based upon Lane algorithm it is possible to calculate optimum cutoff grade with regard to mine, mill, and refinery capacities for single metal deposits with variable grade. In this study iterated grid search (IGS) method was used to calculate optimum cutoff grades of multiple metal deposits for the same conditions as Lane assumed in single metal deposits. Based on this method first, one grid of metal grades was defined and objective function of nodes of this grid was evaluated then node wit highest amount of objective function was determined and one new grid around this node was created. To make calculation more accuracy, the smallest size of grid around the optimum node was chosen and for each node of new grade, the amount of objective function and optimum cutoff grades were determined. These operations were continues were continued until the optimum cutoff grades with high desirable accuracy found.

The shape of niobium carbide in niobium modified cast AISI M7 high-speed steel is string like, which has negative advantages on the mechanical properties of the steel. So in this paper experiments have been carried out to evaluate the effect of titanium addition on the modifying of the shape of this carbide, and the other changes occurred in the microstructure, using EDX analysers attached to the scanning electron microscope. The volume percent of eutectic phase and eutectic cell size and also volume percent of different carbides of new steel, which is heat treated under different condition, are also determined. The results show that titanium addition modifies the cast structure of Nb-alloyed steels and reduce the size and volume of eutectic cells and increases the maximum hardness of the steel.

The Influence of metallurgical silicon carbide and ferrosilicon as silicon carriers, on the microstructure and cooling curve of hypoeutectic gray cast irons with different carbon equivalents has been studied. It was concluded that SiC modifies the microstructure of gray cast iron and also changes thermal analysis parameters, specially for cast irons with a relatively high carbon equivalent, resulting in a more uniform distribution of type A graphite, refined eutectic cells and decreased secondary dendrite arm spacing of primary austenite. In addition, results of thermal analysis revealed that addition of SiC instead FeSi to the melts resulted in increasing solidification range (in higher carbon equivalents) as well as eutectic undercooling.

The in situ melting of Fe-4wt% C and Fe-72wt% Ti caused reaction between carbon and titanium, resulted in production of TiC particles within the melt. The amount of the particles and the type of phases in the matrix were related to primary chemical composition of the melt. Results of this research indicate that by increasing the amount of Ti in the melt, the amount of secondary phase, hardness, uniform distribution of TiC particles increase while a decrease in the amount of eutectical cementite within the matrix occurred. Wear and friction coefficient of these composites were investigated. Increasing of TiC content to a decrease in both friction coefficient and weight loss due to an increase in matrix hardness.

Degradation behavior of near resin. Epoxy sized and unsized carbon fibers in polyimide matrix were investigated. Degradation of neat resin and unidirectional laminates (0__8 and 0__6) were investigated at temperatures between 470°C - 650°C By using SEM technique, voids formation in neat resin, transverse cracking and microcracking at fibre-matrix interface were investigated. The results showed that PMR-X composites are more stable in an inert atmosphere (nitrogen atmosphere) than in air, degradation of PMR-X near resin is accompanied by volatilization and microcrack formation at longitudinal and transverse direction of fiber-matrix interface within composite panels. However, it was concluded that there is good adhesion between fiber and matrix in epoxy sized composites.

Numerous large-scale open pit mines in developing countries lack suitable production planning. Production scheduling in these cases pose considerable technical and productivity in most case could be improved. Usage of this mathematical model and the solutions obtained proved to be improved. Usage of this mathematical model and the solutions obtained proved to be applicable and effective for optimum designing of open pit mines, especially production scheduling for a large-scale mining operation, In this work, an attempt is made by using the multiple criteria decision-making model and zero-one nonlinear goal programming to derive production scheduling as effective ways to achieve higher profits in pre-assumed time span under specific economic and technical consideration.