Sharif Digital Repository

Friction and wear behaviors of artificially aged 2024 Al and 2024 Al/20 vol.% SiC composite prepared by powder metallurgy method were investigated in the temperature range 20250 °C. Dry sliding wear tests were conducted at a constant sliding velocity of 0.5 m/s, an applied load of 20 N, and a sliding distance of 2500m using a pin-on-disc apparatus. Worn surfaces and wear debris were also examined by using SEM and EDS techniques. All specimens showed a transition from mild-to-severe wear above a critical temperature. In the mild wear regime, the wear rate and the friction coefficient of the composite specimen were higher than those of the unreinforced alloy. The SiC particles led to an... 

The effect of Rare earths addition to AZ91 magnesium alloy and its influence on the microstructure and mechanical properties was investigated in this study. Addition of cerium rich misch metal to AZ91 alloy resulted in formation of needle shape particles, which had a very high thermal stability, providing superior mechanical properties compared to AZ91 magnesium alloy. As a result, the grain boundaries were less susceptible for grain boundary sliding at high temperatures. The steady state creep rates were specified and for the AZ91 alloy and the results indicate a mixed mode of creep behavior, with some grain boundary effects contributing to the overall behavior. However for the RE added... 

In nonlinear static (pushover) methods of analysis as an alternative to time history analysis, the capacity curve of the structure is established with respect to the roof displacement. Disproportionate increases in the roof displacement and even outright reversals of the higher modes can distort the capacity curve of the equivalent single degree of freedom system in these kinds of method, including MPA. To overcome this problem, recently, "Energy-Based" the Modal Pushover Analysis (Energy-Based MPA) method has been introduced. In this method, the absorbed energy and/or the external work in the pushover analysis is considered. Accordingly, the assessment of the Energy-Based MPA method is... 

The quantification of carbon dioxide (CO2) dissolution in oil is crucial in predicting the potential and long-term behavior of CO2 in reservoir during secondary and tertiary oil recovery. Accurate predicting carbon dioxide molecular diffusion coefficient is a key parameter during carbon dioxide injection into oil reservoirs. In this study a new model based on adaptive neuro-fuzzy inference systems (ANFIS) is designed and developed for accurate prediction of carbon dioxide diffusivity in oils at elevated temperature and pressures. Particle Swarm Optimization (PSO) as population based stochastic search algorithms was applied to obtain the optimal ANFIS model parameters. Furthermore, a simple... 

The oxidation behavior of three types of stainless steels, namely AISI 321, AISI 316, and AISI 409, was compared. In all stainless steels, oxide layers were formed and their masses and thicknesses increased with oxidation time. Among them, AISI 409 ferritic stainless steel demonstrated higher oxidation rate. According to the kinetical oxidation behavior of them at elevated temperatures, the oxidation mechanism was determined. Among them, the AISI 409 ferritic stainless steel showed the lowest and AISI 321 austenitic stainless steel demonstrated the highest oxidation resistance. Based on the experimental results, it was suggested that the kinetic of oxide growth in stainless steels was... 

The flow behaviour of Al–SiC nanocomposites prepared by mechanical milling and hot powder extrusion methods was studied at different temperatures (350–500°C) and strain rates (0.005–0.5 s−1). The flow of the Powder metallurgy nanocomposites exhibited a peak stress followed by a dynamic flow softening behaviour. It was shown that mechanical milling increased high-temperature strain rate sensitivity of ultrafine-grained (UFG) aluminium while decreasing its flow dependence to temperature. Constitutive analysis of the hot deformation process by Zener–Hollomon parameter (Z) also indicated a remarkable increase in the deformation activation energy (about 40%). Likewise, SiC nanoparticles (up to... 

Fin stabilizers are very important device for controlling the ship roll motion against the external moments due to wave. This paper presents numerical results for flow field simulation and the hydrodynamic performance of fin stabilizer attached to a ship hull with free surface effects. Combination of CFD and RANS method has been used for this study. The fin is nonrectangular NACA0015 profile section with a finite aspect ratio. The numerical results include pressure distributions and flow field around the fin which are used to calculate lift coefficients and free surface elevation as the main interest. Some results are compared with available experimental and numerical data in literature and... 

In this experimental study the effect of physical and thermal properties of various FRP bars on their performance under elevated temperatures are investigated. The parameters included the bars' diameter, type of fiber, type of resin, fiber to matrix ratio, and thermal properties were studied. Moreover, ANOVA (ANalysis Of VAriance) was performed in order to investigate the contribution of each variable on the obtained results. The results showed that in addition to the temperature, the bars' diameter, type of fiber, type of resin, and thermal properties (Tg and Td) of the FRP bars have contributions to the results, while the fiber to matrix ratio was found to be an ineffective factor. It was... 

This paper addresses numerically-derived tsunami wave loads on bridge superstructures using smoothed particle hydrodynamics (SPH), which is a type of mesh-free methods. Although there exist some relationships for the case of impinged loads on bridges exerted by regular (sinusoidal) waves, for the case of solitary waves such as tsunamis, no relation has yet been proposed in the literature. This shortcoming is partly due to the lack of understanding the mechanism of wave action on the bridge superstructures. In this study, three water depths, three wave amplitudes and four submergence depths of the deck are considered for the process of numerical investigation of tsunami-induced loads on... 

Flow stress and ductility behaviors of the annealed and severely deformed Al were investigated at warm deformation temperatures. Constrained groove pressing (CGP) method as a severe plastic deformation process was used. The tensile test was carried out at the temperature range of the 298-573 K and strain rate range of 0.001-0.1 s−1 to present the elevated temperature deformation behavior utilizing hyperbolic sine constitutive equation. The flow stress of the CGPed sample is increased with the number of CGP passes and decreased with temperature. Dynamic recovery and strain softening are found as main restoration mechanisms. Flow stress amounts are not remarkably affected by the strain rate.... 

This study examines different soot sub-models to simulate the soot nano-aerosol formation in turbulent kerosene-surrogate flames. The examinations are carried out at three different elevated pressures. We use a two-equation soot model considering the aerosol dynamics and chemistry of soot nano-particles, i.e. nucleation, coagulation, surface growth, and oxidation. Four different soot sub-models are used to predict the structures of pressurized flames. These structures are then compared with each other as well as the data collected by experiment. Our findings indicate that the acetylene soot-inception sub-model performs poorly in prediction of the flames’ structures at different elevated... 

The main objective of this study is to investigate the effect of temperature on asphaltene destabilisation (precipitation/aggregation) in live oils at elevated pressure conditions. Here, the asphaltene related experiments were performed using solid detection systems, high pressure microscope, and high pressure-high temperature filtration apparatuses in two Iranian light and heavy live oils with different characteristics and stability. The obtained results were interpreted in terms of asphaltene onset pressure, size distribution and average diameter of the aggregates, fractal analysis of the aggregates structures, and the amount of asphaltene precipitation. As well, the results of the... 

In this study, the effects of fiber type (i.e., carbon and glass fibers) and intumescent paint on the tensile performance of fiber reinforced polymer (FRP) sheets at elevated temperatures are investigated. For this purpose, a series of tensile tests were conducted on the glass and carbon fiber reinforced polymer (GFRP and CFRP) sheets, with and without intumescent fire retardant paint, at different elevated temperatures. The studied temperatures ranged from 25 °C to 600 °C. Scanning electron microscopy was also used to examine the effects of elevated temperatures on FRP sheets and the fire protecting mechanism of the intumescent paint. Based on the test results, the tensile strength of the... 

Alkali-activated binders have been proposed and have emerged as an alternative to ordinary Portland cement (OPC) binders with acceptable mechanical and durability performances in addition to positive environmental impacts. These alternative binders use a wide range of aluminosilicate precursors, with differing availabilities, reactivities, costs and CO2 emissions. Despite all the studies published on improving the characteristics of these materials in the last decades, some aspects of alkali-activated binders require further investigations. For instance, shrinkage in alkali-activated binders due to the different chemical processes being involved is more complicated than OPC composites and it... 

This paper investigates the influence of intumescent paint on the performance of FRP bars subjected to low (25–450 °C) and severe (450–800 °C) elevated temperatures. In this research, glass and carbon FRP bars with epoxy resin and coated with nitrogen-based intumescent paint were used. In addition to the temperature effects, a variety of FRP bar diameters were employed to determine its effect on the tensile behavior of FRP bars in the presence of intumescent paint. Further, Bayesian regression methods and ANOVA (ANalysis Of VAriance) were applied on the results to develop a predictive model form and quantify the contribution of the variables in the tensile performance of FRP bars,... 

Due to high vulnerability of steel structures to elevated temperatures and the need for taking adequate and effective measures to reduce human and financial losses in fires, a thorough understanding of such behaviour is of utmost importance. In this research, a half-scale 3D model, comprising moment-resisting frames, equipped with flush end-plate connections, in one direction, and braced frames in the other, was subjected to a scaled ISO 834 standard fire. The maximum attained temperature was 1055 °C. The results showed that the structure tolerates high temperatures for an appreciable amount of time before collapsing. Moreover, for thick flush end-plates, which were used in this study, the... 

The aim of this work is to study the effect of cooling rate and subsequent hot consolidation on the microstructural features and mechanical strength of Al-20Si-5Fe-2X (X = Cu, Ni and Cr) alloys. Powder and ribbons were produced by gas atomization and melt spinning processes at two different cooling rates of 1 × 105 K/s and 5 × 107 K/s. The microstructure of the products was examined using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The particles were consolidated by hot pressing at 400 °C/250 MPa/1 h under a high purity argon atmosphere and the microstructure, hardness and compressive strength of the compacts were evaluated.... 

The effect of Al content and Si addition on the microstructural and creep properties of Mg-Al-RE alloys was investigated in this study. The steady state creep rates were specified and it was found that the creep behavior of the alloy, which is dependent on the stability of the near grain boundary microstructure, was improved by the RE and Si addition. For the AZ91 alloy, the results indicate a mixed mode of creep behavior, with some grain boundary effects contributing to the overall behavior. However for the RE and Si added samples, sliding of grain boundaries was greatly suppressed and the dislocation climb controlled creep was the dominant deformation mechanism. Analysis of creep rates... 

This paper focuses on creep properties of AZ91-RE and heat treated AZ91 magnesium alloy. Influence of heat treatment and rare earths addition on microstructure and mechanical properties were investigated. Creep behavior is dependent on the stability of the near grain boundary microstructure and is improved by rare earth addition. The results show that sliding of grain boundaries has an important part in the deformation mechanism in AZ91 alloy at elevated temperatures which is greatly suppressed by the rare earths addition