Showing 16 results for Ahmadi
H. Ahmadi, Rahim Zolanwar,
Volume 8, Issue 1 (4-1990)
Abstract
K. Sadrnezhaad and H. Ahmadian,
Volume 17, Issue 2 (4-1998)
Abstract
In this research a Cu-Zn-Al alloy is produced by melting the raw materials in an electric resistance furnace and then pouring it into a steel mould. The optimum way to achieve the final analysis in the hypo-eutectoid range is determined and the influence of the alloying element, Ti on the grain size and the shape memory properties of the samples are investigated. Solution treatment (done at 850˚C) followed by quenching in ice-water mixture results in the formation of the martensitic structure and the shape memory effect. Aging at temperatures bellow 200˚C results in the reduction of transformation temperatures, while aging at temperatures between 200˚C and 350˚C results in the enhancement of these temperatures and at temperatures above 350˚C results in the destruction of the shape memory properties. Effect of super-elasticity at 40˚C (between Md and Ms) is observed and tensile tests are run at 25˚C and -55˚C to verify the influence of the prevailing phase.
M. T. Ahmadian and S.sadeghi,
Volume 19, Issue 1 (7-2000)
Abstract
A nonlinear model consisting “yaw, roll, longitudinal, lateral and pitch” has been developed in which, tire and suspension characteristics have been considered. Tire model is based on the elliptic concept and tire Calspan data. According to this tire model, cornering force and aligning moment are computed as a function of slip and camber (inclination) angles, normal load, tire adhesion characteristics and skid number. The effects of suspension systems and the component of lateral and longitudinal weight transfers, are considered. Finally the equations of motion are droven, vehicle handling behavior and effect of anti roll stiffness on handling characteristics are shown.
M. Izadi-Nia and M. T. Ahmadi,
Volume 19, Issue 2 (1-2001)
Abstract
Arch dams are generally built in cantilvever monoliths. Vertical contraction joints between monoliths control the stresses due to concrete shrinkage and temperature variations. Past studies in earthquake safety evaluation of the system often show the tensile fracture and local nonlinear deformation of joints. In this paper, some nonlinear joint element constitutive models are presented to examine the tensile and shear fractures of joints. Reservoir hydrodynamic interaction and water compressibility are considered in the finite element analysis of the system. Some primary examples were calculated. Regarding the numerical results, efficiency of the proposed models is promising for safety evaluation of arch dams under static and dynamic loads
E. Shirani, and H. Ahmadikia,
Volume 19, Issue 2 (1-2001)
Abstract
2-D and axisymmetric Navier-Stokes equations are solved using Reiman-Roe solver with different limiters for second-order accurate schemes. The results were obtained for supersonic viscous flows over semi-infinite axisymmetric and 2-D bodies. The free stream Mach numbers were 7.78 and 16.34. The stability of Roe method with different limiters and entropy conditions were considered. The results show that the limiters greatly affect the stability and accuracy of the numerical solution while the entropy conditions do not.
A. Nasirahmadi, L. Estehghari, and A. A. Soudi,
Volume 19, Issue 2 (1-2001)
Abstract
The compounds, lanthanium chromite, lanthanium magnetite and lanthanium ferric oxide were prepared through sol-gel methods in powdered form, then calcinated, peletized, and sintered at various temperatures. Electrical conductivity of these compounds was measured at various temperatures. The effectiveness of strontium as a dopant on electrical conductivity was also investigated. Electrical conductivity enhancement was observed in dopped compounds
A. R. Pishehvar and H. Ahmadi Kia,
Volume 20, Issue 2 (4-2001)
Abstract
In this study, the missile staging process by implementing a side-injected jet is simulated numerically. The problem is considered to be axisymmetric and the thin shear layer approximation of Navier-Stokes equations along with an algebraic turbulence model is used in a quasi-static form for the calculations. The free stream corresponds to a very high altitude flight condition with a Mach number of 10 and an injected jet pressure ratio of about 63000. An explicit Godunov-type scheme is used in the calculations, which is second-order in time and space. Computations are performed on the attached and separated geometry for a range of distances between the body and the warhead. The intense interactions between the jet flow and the main free-stream and its overall influences on the warhead aerodynamic loading are finally demonstrated.
Keywords: Missile Staging, Jet Interaction flow, TVD Scheme, Riemann problem
H. Ahmadikia, and G. A. Iravani,
Volume 26, Issue 1 (7-2007)
Abstract
Design of a natural dry cooling tower has been accomplished in two sections: the design of heat exchangers and the numerical solution of flow through the tower. Heat exchanger (Heller type) has been simulated thermodynamically and then coupled with a computer program, which calculated the turbulent natural convection flow through the tower. The computer program developed for this purpose can be used to obtain thermodynamic propertied of the cooling tower such as mass flow rate of air, temperature of outlet water, distribution of temperature, distribution of velocity, and distribution of pressure through the tower. Numerical results have been compared with experimental data of Shahid Montazery Thermal Power Plant under different environmental conditions. Comparison between numerical results and experimental data showed good agreement.
A. Eslami, H. Modaraei, and H. Ahmadi,
Volume 26, Issue 2 (1-2008)
Abstract
M. Bahrami, M.h. Fathi, M. Ahmadian,
Volume 32, Issue 1 (Jun 2013)
Abstract
Short life of current total hip replacement metallic implants is generally dependent on the aseptic loosening of the implant, which occurs due to mismatch of elastic modulus between bone and metallic implant materials. Decreasing in elasticmodulus of implant could be successful. Forsterite is biocompatible and bioactive ceramic which has suitable mechanical properties. In presented research the composite materials based on Co-Cr-Mo alloy with 10, 15 and 20wt% of forsteritenanopowder as reinforcement were fabricated and mechanical behavior of the composites were evaluated. Composites were fabricated by ball milling, cold pressing and sintering. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for characterization and evaluation phase composition and microstructure of the composites. Density, microhardness, compressive strength and elastic modulus of fabricated composites were evaluated. Obtained results showed elastic modulus of composite materials based on Co-Cr-Mo alloy reinforced with 10, 15 and 20wt% of forsteritenanopowder decreased significantly. Results also showed that the compressive strength of Co-base alloy composites reinforced with 10, 15 and 20 wt% forsterite were lower than cast Co-Cr-Mo alloy. With increasing in the content of reinforcement, compressive strength of the composites were decreased. Microhardness of prepared composites were higher than cast Co-Cr-Mo alloy. With increasing in content of bioceramic reinforcement, microhardness of the composites were increased.
M. Ahmadi, Sh. Ebrahimi, M. Ahmadi,
Volume 33, Issue 2 (Journal of Advanced Materials-fall 2015)
Abstract
This research aimed at producing microcellular foams (with cell size of 1-10 μm and cell density above 10 9 cell/cm3)
from PC/EPDM in order to use in medical devices. Due to the weak nucleation behavior of microcellular polycarbonate foams, it is difficult to prepare them. This research provides valuable information regarding the possibility of making microcellular foams from this polymer by using multiwalled carbon nanotubes (MWNT,s) as nucleating agents (the value of 1-3 phr). The nanocomposite samples were prepared in an internal mixer and foamed via a batch processing method using supercritical carbon dioxide as the foaming agent. The results showed that the addition of nanoparticles up to 3 phr improves the foamability of PC/EPDM blend. Furthermore, as nanoparticle content increased a decrease in cell size and hence an increase in cell density were observed. Another finding showed that cell size distribution is directly related to uniform dispersion of carbon nanotubes.
S. Ahmadi, R. Arabi Jeshvaghani, H.r. Shahverdi,
Volume 34, Issue 1 (Journal of Advanced Materials-Spring 2015)
Abstract
In this research, crystallization of Fe36Cr12Mo10 and α-Fe phases in devitrification of Fe51Cr18Mo7B16C4Nb4 amorphous alloy was studied using X-ray diffraction and transmission electron microscopy. For evaluation of crystallization kinetics, differential scanning calorimetric tests were carried out at different heating rates. Results showed that two-step crystallization led to the formation of Fe36Cr12Mo10 and α-Fe phases in the structure of alloy. Activation energy of crystallization of Fe36Cr12Mo10 and α-Fe phases measured according to Kissinger-Starink model were 747 and 880 kJ/mol, respectively. Results growth mechanism along with the decreasing nucleation rate in crystallization of Fe36Cr12Mo10 and α-Fe phases.
D. Yazdani, S.y. Ahmadi Brooghani,
Volume 35, Issue 1 (Journal of Advanced Materials-Spring 2016)
Abstract
In this study, a three-dimensional finite element (FE) model for armchair, zigzag and chiral single-walled carbon nanotubes (SWCNTs) is proposed. To create the FE models, nodes are placed at the locations of carbon atoms and the bonds between them are modeled using three-dimensional elastic beam elements. The FE model is used to investigate the influence of chirality and Stone-Wales defects on the ultimate strength (Ultimate stress and ultimate strain) of SWCNTs. Results indicate that Stone-Wales defect significantly reduces the ultimate stress and strain of armchair CNTs. But this defect has a negligible effect on the ultimate strength of zigzag nanotubes. Based on the results, the crack growth path in zigzag and armchair nanotubes have 90 and 45 degree angle to the long axis of the nanotube, respectively.
S. Ahmadi, H. R. Shahverdi,
Volume 35, Issue 2 (Journal of Advanced Materials-Summer 2016)
Abstract
In this research, Glass Form Ability (GFA) has been investigated in the new class of Fe-based amorphous
alloys. Indeed, the main purpose is to evaluate the effects of alloying with niobium on glass form ability of Fe55-xCr18Mo7B16C4Nbx (X=0, 3, 4, 5) alloys. Vacuum induction melting (VIM) was utilized for production of primary ingots and melt spinning process was used for production of thin ribbons required for kinetic and structural investigations. Kinetic analysis was done using
the data obtained from Differential Scanning Calorimetry (DSC) tests. Results showed that GFA and viscosity were enhanced by Nb alloying. It was also determined that devitrification transformation was accomplished in alloys by nucleation and growth mechanisms.
M. Mottaghi, M. Ahmadian,
Volume 36, Issue 1 (Journal of Advanced Materials-Spring 2017)
Abstract
In this research, the wear behavior of commercial grades of WC-10wt%Co (H10F), WC-40vol%Co and WC-40vol%FeAl-B composites with different amounts of boron from zero to 1000 ppm has been investigated by the pin on disk test method at high temperature. The wear tests were done under load of 40 N, a distance of 100 m and at ambient temperature, 200 ̊C and 300 ̊C. Wear surfaces were examined by scanning electron microscopy. The results showed that the wear resistance of all composites decreased with increasing temperature. The boron free WC-40vol%FeAl composite showed the lowest wear resistance at all ranges of temperature. In the presence of boron up to 500 ppm in iron-aluminide matrix, the high temperature wear resistance of these composites improves and the wear mechanisms changes from particle pullout into abrasive state. The toughness enhancement of these composites and plasticity enhancement of iron aluminide in the presence of boron, leads to better link of the interface of FeAl matrix and tungsten carbide particles, and thus increases the wear resistance of these composites. WC-40vol% FeAl-500ppmB composite has a higher wear resistance at high temperature than WC-40vol% Co and commercial WC-10wt% Co.
H. Ahmadi, S. Otroj, M. R. Nilforushan, A. Dehghani Varnamkhasti,
Volume 36, Issue 2 (Journal of Advanced Materials-Summer 2017)
Abstract
In this study, the composition of magnesium aluminate spinle and the converter mud were used as raw materials to in-situ formation of hercynite phase in magnesite-hercynite refractory bricks. The pressed samples were sintered at 1400 and 1500℃ and then, the phase composition of bricks was evaluated after firing at 1400℃. Besides, the effect of nano-magnesia particles addition on the properties of magnesia-hercynite refractory bricks was examined. Hence, the physical peroperties, thermal shock resistance and microstructure of refractory bricks were evaluated. The phase composition results showed that hercynite is well-formed in the refractory matrix, which leads to bonding formation and its increase between magnesia particles. The evaluation of results indicated that the addition of nano-magnesia particles can reduce the porosity of brick via increasing particles packing. In this relation, 1 wt. % nano-magnesia addition was determined as optimum content. Further addition of nano-magnesia leads to increasing of porosity via agglomeration of particles. Due to the high surface area of used nano-magnesia particles, the adequate sintering of refractory brick containing nano-magnesia take places at 1400℃. This leads to increasing of particles bonding and then, increasing mechanical strength, but it can not affect the thermal shock resistance of refractory bricks. The microstructural evaluations showed the lower porosity and further particles bonding with addition of nano-magnesia optimum content.
