Showing 18 results for rahmatabadi
Asghar Dashti. rahmatabadi, Ardshir. Karami Mohammadi, Reza Rashidi,
Volume 10, Issue 2 (9-2010)
Abstract
This paper presents the effect of tilt angle on static and dynamic performance characteristics of two gas-lubricated noncircular journal bearing configurations, namely two and four lobe bearings. The linearized system approach using finite element method is used to obtain both steady state and dynamic characteristics.
The results of the investigation show that tilt angle has a significant effect on static and stability characteristics. With an increase in tilt angle, power loss is decreased while stability margins are increased.
Asghar Dashti rahmatabadi, Mahdi Zaree Mehrjardi, Reza Rashidi Meybodi,
Volume 15, Issue 5 (7-2015)
Abstract
In this work, using finite element method (FEM) the effects of preload factor on the dynamic stability of noncircular two lobe hydrodynamic micropolar lubricated journal bearing based on the linear and nonlinear analytical dynamic models are presented. Assuming that the rotor is solid, the governing Reynolds equations for incompressible lubrication of journal bearing have been modified using micropolar theory. Later, the linear and nonlinear dynamic models, including a certain harmonic disturbances and time dependent trajectory of rotor center are applied to obtain the stability performance of bearing. The 4th order Rung-Kutta method has been used to solve the time dependent equations of rotor motion. Finally, the numerical results for the critical mass parameter and whirl frequency ratio of rotor as the stability characteristics of bearing are evaluated for different values of preload factor and compared together. Results show that the stability performance of two lobe bearing enhances by increasing the amount of bearing noncircularity in terms of the critical mass parameter increase and decrease of the whirl frequency ratio. Also, by comparing two dynamic analysis methods, it is obvious that the results of linear dynamic model are more cautious in different investigated cases. The results of nonlinear dynamic analysis reveal that by increasing the value of preload factor the dynamic response of rotor center involves return to steady state equilibrium position, limit cycle periodic motions and contact between rotor and bearing's shell.
Mahdi Zare Mehrjardi, Asghar Dashti rahmatabadi, Reza Rashidi Meybodi,
Volume 15, Issue 7 (9-2015)
Abstract
Oil journal bearings are one of the most common parts of high load carrying rotating machine. Stability of these bearings can be affected by various stimulus such as changes in loading and lubrication conditions. Therefore, identification of the dynamic response of journal bearings can improve the control and fault detection process of rotor-bearings systems and prevent them from placing in critical operation condition. Since past, the mass unbalance of rotor is proposed as an effective factor on the dynamic behavior and long life of bearings. For this reason, in this research the effects of this parameter on the stability of hydrodynamic two lobe noncircular journal bearing with micropolar lubricant is investigated based on the nonlinear dynamic model. To achieve this goal, the governing Reynolds equation is modified with respect to micropolar fluid theory and the equations of rotor motion are derived considering the mass unbalance parameter. The static and dynamic pressure distributions of the lubricant film and the components of displacement, velocity and acceleration of the rotor are obtained by simultaneous solution of the Reynolds equation and the equations of rotor motion. Investigation of results in terms of dynamic trajectory, power spectrum, bifurcation diagram and Poincare map show that the dynamic behavior of two lobe bearings appears in different manner with variation of mass unbalance of rotor. The response of analyzed dynamic system include converge oscillations to the equilibrium point, periodic, KT periodic and quasi periodic behavior and also divergent disturbances which leads to collision between the rotor and bearing.
Asghar Dashti rahmatabadi, Abolfazl Rasoolizadeh Shooroki, Mahdi Zare Mehrjardi,
Volume 16, Issue 5 (7-2016)
Abstract
Noncircular lobed journal bearing performance, in comparison with circular types, depends on various design parameters such as tilt and mount angles. Mounting orientation of this kind of bearings with respect to machine frame (mount angle) and also the way of setting their lobes with respect to each other (tilt angle), can change the bearings configuration and as the result their performances. In present study the thermo-hydrodynamic performance of noncircular two, three and four lobed journal bearings for different values of tilt and mount angles, using generalized differential quadrature (GDQ) method, are investigated. The results show that the thermal effects on these bearings performance are considerable and that the thermal consideration makes the results closer to real performance situations. The results of bearings performances due to rise in temperature in rotor, lubricant fluid and bearing shell, when compared to their isothermal conditions, show that viscosity of lubricant as well as load carrying capacity of bearings are decreased, depending on tilt and mount angles especially in case of two lobed bearings. The results also show that the effects of tilt and mount angles on bearing performance are periodic and so it is possible to select these angles suitably for bearings to be optimum.
Davood rahmatabadi, Ramin Hashemi,
Volume 16, Issue 10 (1-2017)
Abstract
Accumulative roll bonding is new method of severe plastic deformation that in the last decade, utilized to produce many materials. In present study, investigated mechanical properties and fracture mode of microstructure and multi-layered Al-Al fabricated during accumulative roll bonding process. Accumulative roll bonding process applied without using lubricant, in the ambient temperature repeated in seven cycle continuously and without heat treatment between cycles of process that the value of reduction thickness is 50% in each cycle. The evaluation of mechanical properties and fracture mode performed by uniaxial tensile test, micro hardness and scanning electron microscope (SEM) and reveled that by increasing number of ARB cycles, micro hardness and tensile strength increased that increasing rate at initial cycles more than last cycles. Also elongation after first cycle decreased and then increased. The variation for mechanical properties during ARB due to governing cold work and high strain hardening at initial cycles and improve microstructural and grain refinement at last cycles. Maximum value of tensile strength and micro hardness achieved in last cycle (seventh cycle) that compared with primary annealed sheet 241.4 and 106% increased, respectively. Also results of SEM demonstrated that by rising the number of ARB cycles, viewed dimples with shallower and smaller than the initial sample and changed fracture mechanism from ductile to shear ductile.
Davood rahmatabadi, Ramin Hashemi, Bijan Mohammadi, Taghi Shojaee,
Volume 17, Issue 2 (3-2017)
Abstract
Cold roll bonding process is one methods for production of multi-layered sheets with same and dissimilar materials that, in the past decade has been attention of many researchers. In this research, for the first time and according to ASTM-E561 and using compact tension specimens investigated plane stress fracture toughness for thin for two layers of aluminum sheets produced by Cold Roll Bonding Process. The fracture toughness is an important parameter in the design that their analysis can predict crack growth and life for material has crack. In addition to the fracture toughness, mechanical properties and tensile fracture surfaces were evaluated by using of uni-axial tensile test, micro hardness and scanning electron microscopy, respectively. Results of these test demonstrated that value of tensile strength, micro hardness and plane stress fracture toughness for two layers of aluminum sheets produced by Cold Roll Bonding process is far more than annealed sample that compared to the initial sheet respectively 150%, 80% and 51% improved. But value of elongation decreased. Also results of SEM demonstrated that dimples shallower and smaller than the initial sample that this factor confirmed increasing strength and reducing ductility. Of course the main role of this variation, applying high strain and cold working plays.
Davood rahmatabadi, Ramin Hashemi,
Volume 17, Issue 3 (5-2017)
Abstract
Cold roll bonding process, as a solid phase method of bonding same or different metals by rolling. In this study, for the first time, formability of two-layer aluminum strips fabricated by the CRB process are investigated by Nakazima tests and experimental. To produce two-layer aluminum strips using a rolling machine and apply thickness reduction was %50 at room temperature. Mechanical properties, tensile fracture surfaces were studied and compared. It was observed that strength and microhardness 149.5 and 80% increased respectively, but elongation and ductility decreased compared to the initial strip due to strain hardening and cold work. Also results of SEM demonstrated that after CRB process, ductile fracture accompanied by dimples samples and shear zones were observed.
Davood rahmatabadi, Bijan Mohammadi, Ramin Hashemi, Taghi Shojaee,
Volume 17, Issue 5 (7-2017)
Abstract
In recent years, metallic multilayer material have been attention of many researchers and different industries. Cold roll bonding is one of the method for produce layered composite that compared to other composite manufacturing methods are more economically and have the ability to produce layered composite with different material. In this research, for the first time and according to ASTM-E561 and using compact tension specimens investigated plane stress fracture toughness for thin three-layer Al/Cu/Al composite sheets produced by Cold Roll Bonding Process. The fracture toughness is an important parameter in the design that their analysis can predict crack growth and life for material has crack. In addition to the fracture toughness, mechanical properties and tensile fracture surfaces were evaluated by using of uni-axial tensile test, micro hardness and scanning electron and optic microscopy, respectively. Results of carried out tests, showed the value of tensile strength, microhardness and fracture toughness for Al/Cu/Al layered composite compared to initial Al 5052 and pure Cu, increased that the main cause of this increase is applied high strain and cold working. Value of fracture toughness for Al/Cu/Al layered composite received 38.7MPa.m1/2 that compared to initial Al5052 and pure Cu, 81% and 165% enhanced, respectively. Results of SEM demonstrated that ductile fracture mechanism govern for Al/Cu/Al composite such as initial samples, but the difference is that dimples for composite layers shallower and smaller compared to initial samples.
Davood rahmatabadi, Moslem Tayyebi, Ramin Hashemi, Beitallah Eghbali,
Volume 17, Issue 7 (9-2017)
Abstract
In this investigation, for the first time, used from Cross Accumulative Roll Bonding to produced Al/Cu/SiC composite. Microstructure and mechanical properties of produced composite were evaluated by using of optic microscopy, scanning electron microscopy, uni-axial tensile test, microhardness, respectively. Results of microstructure showed that after eight pass, has a perfectly uniform distribution of reinforcing and strong bond between particles. Tensile strength fell at the first and then continuously increased. Also trend of tensile strength and elongation was similar. Microhardness value for aluminum and copper layers of produced composite 1469% and 163% enhanced compare to annealed samples, respectively. Investigation of tensile fracture surface after eight pass showed that the mechanism of fracture in shear ductile for Al matrix.
Shahab Shojaei, Ramin Hashemi, Davood rahmatabadi,
Volume 17, Issue 10 (1-2018)
Abstract
FLDs, in fact are the range of strain combinations which identify the beginning of local necking. Different parameters such as sheet thickness, structural defects, temperature, loading direction, forming speed and etc. have influence on these diagrams and one of the most effective parameters is forming speed and it has a direct connection with press speed in sheet forming. In this research FLD are calculated for aluminum 6061 sheets with 3mm thickness in rates of 20, 100 and 200 mm/min experimentally and simulated in the rates of 20, 100, 200, 500 and 800mm/min. In order to do the experimental tests, bulge test is conducted in sheets in six different sizes according to standard by hydraulic press and built steel die. Also numerical modeling was done using the Abaqus finite element software and the maximum strain gauge criterion by entering the Johnson Cook data. Experimental and modelling results verify is studied by surveying the tearing location and errors between FLDs and result showed that experimental and numerical data are compatible with acceptable errors. It was observed that by increasing forming speed FLD increases, in a way that by increasing the press speed from 20 mm/min to 200 mm/min, FLD increases for 30 percentage. This variation can have different reasons such as friction effect and interaction effects between die and sheets, because at the low forming speed (of less than 100 1/s for strain rate) and at the room temperature, the effect of strain rate and mass inertia are minimal.
Mohammad Zamansani, Asghar Dashti rahmatabadi, Reza Rashidi Meybodi, Mahdi Zare Mehrjardi,
Volume 17, Issue 12 (2-2018)
Abstract
Today, oil journal bearings are widely used as an efficient support for rotary systems in various industries. When these bearings are used by loading in high speed conditions, whirling disturbances in the rotor motion status leading to collisions and abrasion is probable. Designing specific geometric shapes or applying industrial lubricants with different new combinations can affect the journal bearings ability to maintain their dynamic stability in critical situations. From this view, the use of non-circular bearings and non-Newtonian fluids in the field of lubrication has recently been heavily taken into consideration. In the present study by choosing non-Newtonian lubricant simulated by power law fluid model, the effects of design parameters such as eccentricity ratio, aspect ratio and power law index on dynamic stability of noncircular two, three and four lobe bearings are investigated. For this purpose, assuming the limited cycle oscillations of the rotor around the equilibrium point after damping the effects of initial imposed disturbances and using finite element numerical method to solve the governing equations, stability range of the system in form of linear dynamic analysis characteristics is determined based on the whirl frequency ratio and critical mass parameter. The results indicate that by increasing the power law index and decreasing aspect ratio, the dynamic range of bearing support will be developed. Also, by increasing the number of noncircular bearings lobes with power law lubricant and providing the system's positioning conditions in high values of eccentricity ratio, more ability to damping dynamic disturbances can be achieved.
Masoud Aliheidari, Asghar Dashti rahmatabadi, Mahdi Zare Mehrjardi,
Volume 18, Issue 2 (4-2018)
Abstract
Use of oil journal bearings in recent decades has grown considerably because of their desirable performance in light and heavy loading condition and also for reducing noise pollution, as a suitable supports in different industrial equipment such as turbomachines, combustion engines and nuclear reactors .Due to the influence of the geometry of these bearings on their performance, a variety of models such as elliptical, lobed, waved, pivoted pad and axial grooves have been introduced to market for purposeful improvement in their steady-state and dynamic operating conditions. In recent decade, with the development of advanced non-traditional machining equipment, the ability to create textures on the bearings shell has been provided by manufacturers. Cubic, cylindrical, spherical and cone shaped textures can have a different effect on the performance of journal bearings. In this study, the performance of two lobe bearings with cylindrical textures is evaluated. For this purpose, the governing Reynolds equation of Newtonian lubrication has been investigated, regarding to the changes in the lubricant film thickness according to the geometry and position of textures, by the FEM using the Reynolds boundary condition for determining the cavitation zone. Then, the bearing performance is evaluated based on the pressure distribution of the lubricant film and the location of the textures. The results show that the location of the textures, to achieve a more favorable performance, is different for various values of noncircularity index. Also, with increasing the bearing noncircularity, the effect of textures formation on the bearing performance will be more noticeable.
M. Maleki Varnoosfaderani, A. Dashti rahmatabadi, A.a. Dehghan,
Volume 19, Issue 1 (January 2019)
Abstract
In recent years, due to the increase in the speed of rotary machineries, demands for enhanced lubrication and bearing design to overcome this challenge has increased. To satisfy these need, researchers have proposed additive contained lubricants such as Nano-lubricants and bearings with different designs such as noncircular lobed bearings. In this article, effects of preload and aspect ratio on static performance of noncircular lobed journal bearings of finite length lubricated with lubricant containing TiO2 Nano-particles for particle volume fraction of 0.01 are studied. Using finite element method, the steady-state film pressure is obtained by solving the modified Reynolds equation based on the Nano-lubricants and Couple Stress model theories. With the help of film pressure, attitude angle, friction coefficient, friction force, and side leakage of noncircular lobed journal bearings are obtained. The results show that using lubricants containing TiO2 Nano-particles can enhance the performance of static characteristics of two, three, and four lobed journal bearings. According to results, increase in preload and bearing length will increase load carrying capacity noncircular lobed bearings. Based on results, choosing proper design parameters can have great impact on static performance of noncircular lobed journal bearings.
D. rahmatabadi, A. Shahmirzaloo, M. Farahani, R. Hashemi,
Volume 19, Issue 2 (February 2019)
Abstract
The cold roll bonding (CRB) is a solid state welding process for bonding similar and dissimilar metals. The use of materials produced by the CRB method for different applications and the prediction of their behavior in simulation software requires the complete and accurate identification of their mechanical properties. Digital image correlation (DIC) is a powerful non-contact method for measuring the field of material deformation. Recently, the DIC method has been developed and widely used in various studies due to its advantages. In this research, two-layered aluminum alloy 1050 was produced via CRB process with applying 50% reduction of thickness at ambient temperature and then using the 2D-DIC system to extract distribution of the strain field during the uniaxial tensile test at rolling direction. Strain in two directions of length and width was calculated, using DIC and strain in terms of thickness, effective strain, and anisotropy coefficient, using plasticity relationships. Moreover, for the first time, using the virtual field methods (VFM), elastic and plastic parameters such as elastic modulus, Poisson ratio, strength coefficient, strain hardening exponent, and yield stress were calculated. The results showed that the strength and microhardness were significantly increased due to the work hardening and increasing the density of dislocations, and the elongation and strain hardening exponent were reduced. The strength for the two-layered aluminum was 113MPa, which improved more than three times of the initial aluminum. Also, changes in the elastic parameters were very small and the modulus of elasticity for the primary aluminum and two-layered aluminum was 69.3 and 70GPa, respectively.
A. Rasoolizadeh Shooroki , A. Dashti rahmatabadi, M. Zare Mehrjardi,
Volume 19, Issue 10 (October 2019)
Abstract
Improvement of behavioral indicators of oil journal bearings has particular importance due to the increasing development of their application as support of rotary components in industrial machinery. Creation of regular roughness (texture) with various geometries on the inner surface of a bearing shell is one of the newest methods proposed by the lubrication researchers to enhance the performance of the hydrodynamic journal bearings. In this study, the comparison of the performance of circular bearings with variable cubic, cylindrical and ellipsoid textures of different depths arranged in a different zone of the shell has been evaluated. For this purpose, the governing Reynolds equation on hydrodynamic lubrication of oil journal bearing was modified considering the changes of the film thickness affected by the geometry and position of the textures. This equation was solved by finite element numerical method, applying the assumption of the Reynolds boundary condition for determining cavitation zone. After obtaining the lubricant pressure profile, the parameters of steady-state performance of the bearing with different texture types were calculated and compared together. Results indicate that the creation of textures with any geometry reduces the lubricant pressure and changes the parameters of the bearing performance. Also, the placement of textures in the maximum pressure area leads to significant changes in performance components while their positioning in the lubricant cavitation region has a weak effect on the bearing behavior. Further, the results show that the difference in characteristics of bearing performance with shallow textures is more considerable and with the increase of textures depth the effect of geometry form on the performance will be reduced.
M. Zare Mehrjardi, A. Dashti rahmatabadi, A. Rasoolizadeh Shooroki,
Volume 20, Issue 5 (May 2020)
Abstract
The lubricant's ability to maintain the dynamic stability of rotor particularly in special conditions such as operating at critical speeds and instantaneous turbulences in loading or lubricant properties is always one of the most prominent characteristics of the journal bearings. Aspect or length to diameter ratio of bearing is an important factor that in different loading conditions will have an obvious effect on the performance of the trapped lubricant film between the rotor surface and bearings shell. So, the effects of aspect ratio on the damping of rotor disturbances with linear and nonlinear dynamic analysis approaches are studied in this research. Initially, the static equilibrium point of the rotor center in noncircular two, three and four lobe bearings space is obtained using the governing Reynolds equation of micropolar lubrication for different values of aspect ratio. Later, assuming the rotor perturbation as the limit cycle oscillations around the equilibrium point, critical mass and whirl frequency ratio are determined as the linear dynamic stability indexes for recognizing the converging disturbances. In nonlinear analysis model, the simultaneous solving of the lubrication and the rotor motion equations in successive time steps with Runge-Kutta method is done to differentiate the converging or diverging rotor perturbations. Results show that decreasing the aspect ratio improves the stability and the chance of controlling disturbances and returning the rotor center to static equilibrium position. Comparison of linear and nonlinear dynamic analysis results also indicates more cautious behavior and limited stability range of linear model in most of investigated cases.
Kiandokht Mirasadi , Davoud rahmatabadi, Esmaeil Ghasemi , Majid Baniassadi , Mostafa Baghani ,
Volume 23, Issue 10 (October 2023)
Abstract
In this research, processing and 3D printing of PETG-ABS- Fe 3 O 4 nanocomposites reinforced with iron oxide nanoparticles in three different weight percentages of iron oxide nanoparticles with PETG70-ABS30 polymer matrix was done. This research was carried out with the aim of strengthening the shape memory properties, thermal properties, mechanical properties and adding the ability to indirectly stimulate the background matrix through the addition of iron oxide nanoparticles. SEM images confirmed that the mixture of PETG-ABS is immiscible and adding nanoparticles does not change the compatibility and miscibility of the base polymer, and this result is consistent with the DMTA analysis was also checked and confirmed. With increasing amount of iron oxide, the tensile strength and elongation decrease, and this decrease in mechanical properties is more pronounced in the sample of 20% by weight of iron oxide compared to the sample of 10% by weight. Nevertheless, the final strength of the samples is around 25 to 32 MPa, which indicates a suitable and acceptable distribution of nanoparticles up to 15% by weight in the polymer field. By increasing the amount of iron oxide nanoparticles, the amount of shape recovery increases and the nanocomposites containing 10, 15 and 20% by weight show shape recovery of 63.77%, 88.48 and 93.33%, respectively.
Kiandokht Mirasadi , Davoud rahmatabadi , Esmaeil Ghasemi , Majid Baniassadi , Mostafa Baghani ,
Volume 23, Issue 10 (October 2023)
Abstract
Smart materials can react to environmental changes like living organisms and adapt themselves to environmental conditions and changes such as changes in temperature, electric current, magnetic field, light, humidity, etc. Using 3D printing to process smart materials is a new approach known as 4D printing. In this research, processing, manufacturing and 3D printing of PETG-ABS in three weight percentages of 70/30, 50/50 and 30/70 were done. The results of SEM also confirmed the compatibility of these two polymers. In all PETG-ABS mixtures, a combination of sea-island and drop-matrix morphology was observed, and for the 30/70 and 30/70 blends, phase droplets dispersed in the matrix were clearly observed. The results of mechanical properties also showed that as the percentage of ABS in the mixture increases, the tensile strength increases and the elongation decreases. The results obtained from the shape memory test indicate the existence of the ability to program the shape memory property in 4D printing mixtures. As expected, the increase in the weight percentage of ABS was associated with the disorder in the recovery of the mixtures, so the mixture with 70% by weight of PETG and 30% by weight of ABS showed the most favorable shape memory properties.
