نشریه مهندسی مکانیک مدرس
سال چهاردهم شماره 11 (بهمن 1393)

The AISI D2 steel is a high-chromium and high-carbon tool steel which has good mechanical properties such as high compressive strength and good through-hardening. Despite these advantages, fracture toughness of this steel is moderate. In this study, fracture toughness of AISI D2 steel was determined using Finite Element and Acoustic Emission methods. Selected steel (AISI D2 cold-work tool steel) was heat treated and tempered at different conditions. Then Compact testing specimens were prepared according to ASTM E399 standard and fracture toughness of the specimens was specified according to the standard method. The specimens were modeled in the commercial FE software (ABAQUS) and fracture toughness of the specimens was determined using FEM. Determination of fracture toughness using AE technique was carried out according to three Acoustic Emission Energy Rate (AEER), Acoustic Emission Count Rate (AECR) and integral of sentry function. The results obtained from ASTM E399, Finite Element and Acoustic Emission methods were compared with each other. It was found that fracture toughness values which were obtained using AECR and integral of sentry function techniques are lower bound and the results obtained from FEM are upper bound values of the fracture toughness. Furthermore, fracture toughness values obtained using AEER were the most consistent with the results obtained from ASTM E399 standard method. Finally, it could be concluded that Acoustic Emission method can be used as a useful method for determination of fracture toughness of engineering materials.

In this study, the nonlinear vibration behavior of a dynamic atomic force microscope (DAFM) in the tapping mode is investigated. First, the governing differential equation of motion and boundary conditions for dynamic analysis are obtained by a combination of the basic equations of the modified couple stress theory and Hamilton principle. Regarding the nonlinear dynamics of the probe, perturbation technique has been used to solve the nonlinear equations. Afterwards, closed-form expressions for nonlinear frequency and effective nonlinear damping factor are derived. The effect of connection position of the tip on the vibration behavior of the microcantilever are also analyzed. The results obtained by couple stress theory are compared with those of classical beam theory. The results show that the nonlinear frequency and effective nonlinear damping factor are size dependant. According to the results, an increase in the equilibrium separation between the tip and the surface sample reduces the overall effect of van der Waals forces on the nonlinear frequency, but its effect on the effective nonlinear damping factor is negligible. The results also indicate that the change in the distance between tip and cantilever free end has a significant effect on the accuracy of the DAFM.

Sound is a wave pressure which propagates through an elastic or compressible medium. The pressure wave may be produced by vibrations of an elastic structure which is in contact with the medium. Therefore, the propagated wave may be used to collect some useful data about the geometrical and the physical characteristics of the sound source.In this study, a simply-supported rectangular plate stimulated by a harmonic force and attached to a viscous damper in specific locations, is considered. The governing equations of the plate are derived using Lagrange method. Then, these equations are solved employing Helmholtz–Kirchoff integral and Fourier transforms to measure near filed and far field scattered acoustic pressure. In the sequel, assuming the location of the damper and the applied force to be unknown, we have used the sound recorded by a microphone array to identify the force and the damper locations and also to find vibration stream lines, in a reverse manner. Numerical simulations confirm the applicability of the proposed method. The obtained data may be used to suggest proper mechanisms for absorption and damping of the vibration energy.

In the present article, velocity and deformation of an air bubble have been considered in quiescent liquid at different consecutive slopes from 5 to 90 degrees in respect to horizontal condition. To establish these purposes, air-water two-phase flow has been simulated numerically by using volume of fluid method. The two-phase flow interface has been traced by using Piecewise Linear Interface Calculation (PLIC) method. Surface tension force was estimated by Continuum Surface Force (CSF) model. The simulation results show that maximum bubble velocity occurred at 45 degrees which is in agreement with the previous researchers result. Simulation of bubble movement was also continued to two consecutive slopes at different angles. At slope deviation location, a vortex was generated due to liquid movement governed by gravity forces. This vortex changes the bubble velocity as well as bubble shape. This vortex also reduces the bubble velocity and changes the bubble nose shape from sharp to flatten at deviation from low to high slope values. However, at deviations from high to low slope values, the bubble nose becomes more sharpened in addition to bubble velocity increase. The maximum average velocity of bubble movement at two consecutive slopes was obtained during the condition that the first and second slopes were set to 60 and 30 degrees, respectively.

Over the past few years the issue of fire in transport tunnels has become an important concern. Fire safety is now seen as being one of the important elements in tunnel design. For this reason, many tunnels are equipped with critical longitudinal ventilation systems to control smoke in the event of fire. In this study, tunnel fire simulation is done with open-source software FDS. Also, study on the critical velocity is done in tunnel fire. In this research, the effects of geometrical parameters on the critical velocity is studied. To investigate the effects of geometrical parameters, tunnel slope and tunnel blockage is studied. For the study on the effects of tunnel slope, propagation and behavior of smoke is studied for slopes between -10 and +10 degrees. In positive slopes of tunnel, fire plume is spread to the downstream of tunnel and in negative slopes of tunnel, fire plume is spread to the upstream of tunnel. It is observed that with increasing from negative slopes to positive slopes of the tunnel, the critical velocity decreases linearly with increasing tunnel slop. Tunnel blockage is studied for one vehicle and 30 vehicles. The results of study on vehicular blockage show that the critical velocity decreases with increasing percent of blockage.

In the present study, the frequency analysis of a smart sandwich plate is investigated using the finite element method. The sandwich plate is consisted of a magnetorheological elastomer (MRE) layer between two cross ply composite elastic faces. MRE is a smart material with controllable properties and a short time response when subjected to a magnetic field. This property can be used for improvement of the dynamic behavior of the structure. To model the sandwich plate with MRE layer, a complex shear modules is used to show the pre-‌yield behavior of MRE layer. In this study, effect of imperative parameters are discussed. In the present paper, the effect of different parameters such as applied magnetic field, the stacking sequences of the cross ply laminated faces in the sandwich plate and applying different boundary conditions on the natural frequencies and modal loss factors of the smart sandwich plate with MRE is investigated. The results show that considering special value for magnetic field, the stacking sequences of the composite layers of the sandwich plate and the boundary condition of the sandwich structure can lead to the satisfactory design of the sandwich plate.

In this research, the machinability of iron-recycled grey cast iron powder metallurgy parts is investigated. For this purpose, grey cast iron swarfs were transformed to powders by target jet milling method and were then used to prepare powder metallurgy parts in combination with commercial iron powder. Green compacts were prepared with the variables of cast iron powder percentage and compaction pressure. Design of experiments was conducted by response surface method for sintered parts with the variables of cast iron powder percentage, compaction pressure, sintering temperature and sintering time each in five levels. Regression analysis and analysis of variance were used to investigate the effect of input parameters, develop the mathematical models and evaluate the validity of the models. In the green section, machinability was qualitatively investigated in drilling. For sintered parts, machinability was evaluated by measuring the thrust and torque forces and the obtained surface finish in drilling. The obtained results certificated the accuracy of the extracted regression equations for predicting the machining properties of the parts. Also, the results demonstrated that the addition of jet milled grey cast iron improves the machinability of iron-based powder metallurgy parts.

Incremental Sheet Metal Forming (ISMF) is based on localized plastic deformation. In this process, a hemispherical-head tool, controlled by a CNC milling machine, shapes a sheet metal according to a defined path. Study of the forming force is one of the most important topics in this process. Increasing of vertical step size, tool diameter, wall angle and sheet thickness together with using of high strength sheet metals and lightweight alloys, leads to an increase in the forming force. In this paper, the performance of a novel forming process, named Ultrasonic Vibration assisted Incremental Sheet Metal Forming (UVaISMF) has been investigated. The procedure of design, manufacture and test of vibratory forming tool, is presented. The occurrence of longitudinal mode and resonance phenomenon has been confirmed by the results of modal analysis and experimental test. Furthermore, the effect of ultrasonic vibration on the vertical component of forming force and spring-back has been studied. Aluminium sheet of grade Al 1050-O is used as a work material. Experimental results obtained from straight groove test, indicate that ultrasonic excitation of forming tool, will reduce the average of vertical component of forming force and spring-back in comparison to conventional process.

In this paper in order to study of effective parameters on energy and exergy efficiency, the modeling and analysis of fluidized bed dryer of Bandar Imam Petrochemical Complex is performed. For do this paper the commercial code with Euler-Euler two phase flow modeling has been used. Due to the importance of moisture content in the dryer system and method transfer between solid and gas phases, a numerical algorithm for estimating moisture content in each phase and exchange or transfer between phases in the proposed the mentioned, implement the code. With applying this algorithm in the code led to considerable correspondence between the results of modeling and the results from the actual performance of the dryer. The difference between the modeling and the experimental results is maximum 1% that represents significant fitness with similar works. The results also express that increase in inlet air and heat exchanger hot water mass flow rates, reduce efficiency while increment in the mass flow rate and temperature of products increase the efficiency. The results of this research for the mentioned petrochemical complex show that with the 15 % increase in mass flow rate of inlet product, overall efficiency of the dryer rises from 38.62 % to %42 and exergy efficiency increases from 35.16 % to 39.5 % while the product moisture decreases 18%.

In this paper the three dimensional dynamic analysis and stress wave propagation in thick functionally graded plate subjected to impact loading is studied. Material properties (elasticity modulus and density) are assumed to vary continuously through the thickness direction of the plate according to a simple power law distributions and the Poisson’s ratio is assumed to be constant. The equations of motion are based on three dimensional theory of elasticity. The three dimensional Graded Finite Element Method (GFEM) based on Rayleigh-Ritz energy formulation and Newmark direct integration method has been applied to solve the equations in time and space domains. It is assumed that in dynamic loading the upper surface of the plate is subjected to a pressure load that varies linearly with time, and suddenly is unloaded at a specified time. This unloading acts as an impact loading. Afterward, the time histories of displacement through the thickness, stresses in three dimensions and velocity of stress wave propagation for different values of power law exponents, various boundary conditions and thickness to length ratios have been investigated. The obtained results are in agreement with available data in literature.

In the present study, the mixing fluids flow in the twin and circular mixers is investigated by using an improved robust weakly compressible Smoothed Particle Hydrodynamics method. In order to remove the Smoothed Particle Hydrodynamics complications and according to a predictive corrective scheme, a robust modified algorithm which uses the advanced second order discretization, pressure velocity decoupling, kernel gradient corrections and shifting algorithm is offered. After the verification and validation of the present algorithm for the moving boundary problems, the present algorithm is applied for investigation of the mixing behaviors of the two-blade circular and twin chamber mixers. By investigation of the mixing paths, the proper geometry for the two-blade mixers is proposed and examined. The effects of the rotation direction of the blades, geometry and Reynolds number on the mixing rate are investigated. The results show that the twin chamber mixer can improve the mixing performance over 60% in comparison with the circular chamber mixer while the case with circular chamber and same direction rotation of the blades has the weakest performance among the cases which have been examined.

Incremental sheet metal forming process is considered as one of methods which able manufacturer to produce parts without dedicated die in low and rapid prototype production, and many researches have been done to improve it. Using of ultrasonic vibration is one of the modern approaches in forming processes which reduce friction and forming force. The purpose of this study is to investigate the effect of ultrasonic vibration applied to the tool in single point incremental sheet metal forming process. For this, first theory of single point incremental forming has been studied; its principle has been investigated and analytical relations have been modified then analytical relations in the case of applying ultrasonic are derived from those. To practical evaluation of applying ultrasonic to this process a set can be installed to the head of CNC milling machine is designed and manufactured. According to results of analytic compared to experimental results a reasonable approximation of forming force variation in normal single point incremental forming process and applying ultrasonic can be offered. Based on tests results forming force in applying ultrasonic compare to normal mode reduces between 33 to 63.5 percent depend on test circumstances.

Rupture of abdominal aortic aneurysm is a result of the relatively complex hemodynamic forces that are exerted along the arterial walls. In the present study, numerical simulations of blood flow in a patient-specific model are performed employing the fluid-structure interaction method. The aneurysm model is reconstructed from CT angiographic scans from a patient diagnosed with abdominal aortic aneurysm, which also contains an intraluminal thrombus. Both isotropic and anisotropic material models are considered for arterial wall. In particular, the effects of arterial wall heterogeneity with respect to its material model are examined. Results indicate that unlike the computational solid stress method, which exerts the peak systolic pressure to the inner surface of the arterial wall, FSI method predicts the time of peak wall stress between the times of peak systolic velocity and pressure. Results also indicate that the isotropic material model with uniform wall thickness as compare to the anisotropic material model with variable wall thickness significantly underestimate wall stresses. The peak wall stress in all models are located somewhere on the posterior wall near the maximum diameter of AAA and the extent of the region of higher wall stresses are larger in models with variable wall thickness as compared to the uniform wall thickness. This fact along with the higher values of wall stress for variable wall thickness models, increase the rupture risk of the variable wall thickness model as compared to the models with uniform wall thickness.

Kinematic trajectory optimization of the dual-arm cam-lock parallel robot in the different lock configuration has been done in this paper. A different path has been considered for each of lock configuration. The optimal trajectory of each joint has been calculated by minimizing an objective function in whole trajectory. According to the number of redundancy in the different configurations, an initial guess of the variables have been considered. Then the initial guesses have been modified and optimum results have been obtained by using Pontryagin’s minimum principles and determining the governing initial condition on the system. According to the optimal joint variable, optimal trajectory has been obtained for each of the joints. In all of the configurations, optimal performance index has been achieved. Also the direct kinematic equations have been considered as the constraints of the system.

Axially moving beams are extensively involved in various industries and have significant importance in many mechanical engineering problems. In this paper, the nonlinear forced vibrations of axially moving beam under harmonic force and thermal environment have been studied. In order to considering the effects of transverse shear deformation and rotary inertia, the Timoshenko beam theory has been used to model the axially moving beam. The nonlinear governing equations are derived with the help of Hamilton’s principle. Then the equations and boundary conditions are discretized through generalized differential quadrature method (GDQ) and its differential matrix operators, and accordingly the partial differential equations are converted into the ordinary differential equations. To study the frequency response of the system, the harmonic balance method is used. Also the time responses of the axially moving beam are obtained by the Runge-Kutta method. In a case study, the effects of various parameters such as the axial speed, transverse force acting on the beam, damping coefficient and temperature change on the frequency responses of the axially moving beam with both end simply supported boundary conditions are discussed. The results show that the dynamic behavior of system is significantly affected by any of the mentioned factors.

Numerical modeling of compressible two-phase flow is a challenging and important subject in practical cases and research problems. In these problems, mutual effect of shock wave interaction creates a discontinuity in fluid properties and interface of two fluids as a second discontinuity lead to some difficulties in numerical approximations and estimating an accurate interface during hydro-dynamical capturing process. The objective of this research is to increase the accuracy of numerical simulation of two-phase flow using two dimensional five-equation two-fluid model. For this purposes, MUSCL strategy was used for increasing the Godunov numerical scheme accuracy from 1st order to 2nd order. The privilege of this method is high accuracy, low numerical oscillation and low numerical diffusion. The problems considered for the verification of the results are the water-air shock tube, a square bubble with moving interface in a uniform flow and a shock wave with 1.72 Mach having interaction with an air bubble in a water pool. The obtained numerical results showed that, the results that have been obtained by second order accuracy have less diffusion in the two-phase flow interface.

In this paper, a cable actuated robot is introduced as a new rehabilitation approach. The quality improvement of human and machine interface has led to create a new device in this area. The interface between the robots with the physical characteristics of body can improve the interaction forces and the patient safety. Considering the joint compliance during the motion range can make the patient feel better and thus, bring success for the rehabilitation program. The key element "cable" makes the possibility of force transmission in this mechanism. Cable actuator is used in this project in order to achieve to maximum adaptation with elbow operation Moreover in the design of rehabilitation device, some advantages are regarded like the low-cost and light weight, smooth joint motion with adjustable stiffness, motor size reduction. The dynamic parameters related to the elbow behavior are described with amplitude and frequency investigating. The performance of the elbow rehabilitation device is examined. Stiffness variation of robot joint is effectively compatible with the elbow joint stiffness according to rehabilitation protocols. As the presented mechanism able to simulate elbow rehabilitation, it can be used more widely in the field of medical robotics.

Orthopedic plates are currently used in bone healing process. However they cause density loss because of the change in natural stress patterns.The aim of this study was to evaluate a newly developed bone plate using functional graded material in term of stress pattern. In the present study, 3D finite element models of tibial bone plate with variable stiffness of a graded material and traditional bone plates made of stainless steel and Ti alloy have been developed by using the ABAQUS software. Effects on the predicted stresses at the fracture site in the presence of a distance between the plate and fractured bone were also studied. For this purpose, a 3D model of tibia was created with the exact geometry of the real bone geometry by using CT scan images of a human left leg. Results showed that the bone plate with graded material offers less stress-shielding to the bone, providing a higher compressive stress at bone to induce accelerated healing in comparison with Ti alloy and stainless-steel bone plate. Results also showed that the use of non-contact plates provide a favorable mechanical environment for the following fracture healing.

Annular vortex tube is a vortex tube which allows the hot flow pass again over the hot tube. It is introduced for first time in this work. Hot Flow is not allowed to exit after passing conic valve in annular vortex tube, but it is redirected over hot tube. This back flow absorbs heat from outer wall of hot tube. To study temperature separation which occurs in an annular vortex tube; the performance of this type of vortex tube has been experimentally tested and compared with the performance of a typical vortex tube. Inlet test pressure is 4 bars and natural gas is being used as working fluid. For both type of vortex tubes, ratio of length to diameter of tube is 10. Cold orifice diameter of vortex generator is set to 6.4 mm. It was observed that redirecting hot flow over the hot tube in annular vortex tube improves cooling efficiency up to 24% respect to a typical vortex tube at the maximum temperature difference. The results show that cold mass fraction in which the coldest temperature occurs is lower for annular vortex tube comparing with a typical vortex tube.

In this paper, friction of the contacted surfaces with random roughness distribution in nano scale has been modeled and simulated. So, firstly the modified friction model, AMM, was derived for the contact of the flat/rough surfaces based on the JKR contact, HK friction model and random distribution of Greenwood-Williamson (GW) model. The results show, modified AMM model predict higher friction force. It is more accurate than the earlier AMM model due to considering of the surface forces. Following, the obtained model was extended for evaluating of friction between a flat particle and the rough surface, and especially for the tip of the atomic force microscopic on the rough surface. Then the effect of geometric parameters of surface such as standard deviation of the asperities height and radius of the asperities peak on the friction between tip and rough surface was done. It was observed that normal load and friction force are increased with growing of the standard deviation of the asperities height while as the friction coefficient will be reduced. Furthermore, the normal load, the friction force, and also the friction coefficient are raised by increasing of the asperity peak radius. Finally, it was observed that variation of standard deviation of the asperity height has more influence on the friction than the radius of the asperity peak.

Vibration of drillstring is the most important factor of its destruction and reduction in drilling operations efficiency. One of the main causes of excessive vibration of drillstring is drillstring-wellbore contact, therefore drillstring-wellbore contact has been studied in several researches. Indeed the contact behavior of drillstring and wellbore is of great concern to drilling companies in the oil and gas exploration industries. In this research, dynamic behavior and vibration of non-linear finite element model of a drillstring in contact with wellbore, has been investigated. By considering total length of drillstring, a three-dimensional timoshenko beam element is employed. In addition the geometric stiffening effect including nonlinear terms, the added fluid mass and the contact between drillstring and borehole wall has been considered using improved contact model with a more complete model than previous studies, that the stiffeness of contact determined form energy balance relations. The equation of motion of drillstring obtained using energy approach and lagrange’s equations and full order equations are used to drive the results. Coupling between various vibrations has been tested and the effect of recent model in analysis of dynamic behavior and vibration of drillstring have been evaluated.

Subcooled flow boiling has many industrial applications such as nuclear engineering and energy generation. The study of the subcooled flow boiling provides a lot of information about the void fraction of vapor on the wall and also the critical heat flux. In this paper, the sub cooled flow boiling in a 2D vertical corrugated channel has been investigated numerically. The two-fluid model and the finite volume method have been used in this investigation. The obtained results show that with increasing of nanoparticles in the base fluid, the void fraction and bubbles departure reduced, However, the wall temperature increases due to heat flux evaporation decreases. Also, with increasing the wavy amplitude, the wall temperature and void fraction increases. that increasing the water temperature on the wall decreases its density and the fluid velocity increases in the vicinity of the wall because the momentum of the flow is constant in the transverse cross section of the channel. Furthermore, because of the variations of the velocity due to the variations of the channel's cross section, the void fraction of the vapor increases in the converging areas of the corrugated channel.

In this paper, a new multi-objective differential evolution with a diversity preserving mechanism called the ε-elimination algorithms is used for the Pareto optimum design of gripper mechanisms. The ε-elimination diversity is used to improve the population diversity among the obtained Pareto front. In the ε-elimination diversity approach based on a threshold value all the clones or ε-similar individuals are recognized and simply removed from the current population. It should be noted that such e-similarity must exist both in the space of objectives and in the space of the associated design variables. The proposed algorithm has been used for two different configuration of robot’s gripper. The dimensions of mechanisms are considered as design variables and optimally selected by proposed algorithm to improve the efficiency of griper mechanism. Two conflicting objectives which are the difference between maximum and minimum gripping forces and the transmission ratio of actuated and experienced gripper forces, are considered for Pareto optimization. The best configuration of gripper mechanism is suggested by comparing of trade-off design points. The comparisons of the obtained Pareto front using the method of this paper with those obtained in other references shows a significant improvement.