Modares Mechanical Engineering
Year:2015 | Volume:14 | Issue:14|Papers Count:40

In this research, formability of two layer sheet metals of Al1050 and St12 in single point incremental forming (SPIF) has investigated using numerical and experimental approaches. In order to study the sheet metal formability in this process, the tool paths defined in ABAQUS and CNC machine so that an increasing wall angle is created until the sheet metal reaches its maximum allowable angle and fracture is occurred. Since in this process, the tool exerts local stresses on the sheet metal, 3D simulation of the process is needed. In order to study the effect of process parameters, the analysis is done in three levels of tool radius and vertical step size. In order to derive fracture depth of sheet metal, the force diagram is considered in simulations. It is shown that the outer sheet subjected to higher plastic strains and therefore failure occurred initially at the outer layer. Results also showed that increasing the tool radius and vertical step size speed up process but they have inverse effect on the forming limit angle. For experimentally study and also to validation of simulation results, full factorial experiments with respect to forming speed up to three levels designed and carried out. The difference between FEM and experimental results is about %2.1 in forming limit angle.

The Chaboche kinematic hardening model is generally used for modeling the plastic behaviour of material under quasi-static cyclic and monotonic loadings. This model is independent of strain rate and its constants are normally determined through quasi-static tests. Therefore, it cannot predict material behavior under high strain rate condition. On the other hand, the dynamic behaviour of materials even in some cyclic loadings is usually strain rate sensitive. In this investigation, the constants of Chaboche model are identified at various strain rates through quasi-static and dynamic tests and using these constants the effect of strain rate is incorporated in the Chaboche model. Moreover, the stress-strain diagrams at different strain rates are predicted using artificial neural network (ANN) and the results are compared with the experimental data. The results from the strain rate dependent Chaboche model shows reasonable agreement with the experimental data and the prediction from ANN. It is also shown in this work that the constants of Chaboche plasticity model are strain rate dependent and if the neural network is trained properly, it can be used for interpolating between the experimental data.

In recent years many investigations have been performed on design and fabrication of micro mechanical manipulators. These manipulators have a wide application in industry specifically medical applications. A practical usage of this manipulator is endoscopy system. In the endoscopy system, we need a small manipulator with high maneuverability and flexible body to make the probe’s movement into a colon easier than classic manipulators. In this paper a basic flexible module is developed for use in this structure. Connecting three wires of shape memory alloy uniformly distributed in circumference of a compressive spring with angle of 120 degrees, it is possible to make an almost large displacement in the end planes with small strain of wires. In this paper, a model is developed to define the spring deformation which in the following is coupled with the model of SMA wire presented by Brinson to describe the module behavior definitely. Dynamic modeling and simulation is implemented in MATLAB-Simulink and module performance in addition to proper geometry for the considered application is extracted. Through the results of simulation verified by experiment, proper parameters of module for providing more deflection and rotation when activated by SMA are extracted.

Low swirl burners present an effective approach to increase stability in lean premixed combustion. Effects of swirl number as a key parameter in the performance of these burners have been investigated in several studies with different conditions of pressure, bulk velocity equivalence ratio and geometrical specifications. Swirler distance from the exit, called recess length is another key parameter, which affects the performance of the burner and there are a few studies about its effects on the performance of the burner. In this study by design and fabrication of a low swirl burner and setup a rig test, several combustion parameters include flame temperature; flow rate, pressure and temperature of the air and fuel, and analysis of combustion products have been measured. And the effects of recess length and equivalence ratio variations on the performance of the low swirl burner have been studied. In addition, the exergy analysis has been done in order to investigate the performance of these burners. Results reveal that increasing recess length would result in wider range of lifted flame for different equivalence ratios. In addition, results also show that although low swirl combustion is working on lean condition, it has about 17 percent lower irreversibility ratio in comparison with diffusion flame from second law of thermodynamic point of view. Besides, the heat transfer ratio has been increased about 14 percent in the lifted flame in comparison with the attached flame.

This paper focuses on a class of continuum robot manipulators that uses cables for actuation. In order to realize more natural and various motions like human musculoskeletal, tendon-driven manipulators is studied. It is challenging to design the manipulator structure which consists of bones and redundant muscles. A comprehensive study is presented including the theoretical analysis of the mechanical design, kinematics, dynamics and tracking control of a planar continuum backbone robot. Lagrange's equation is applied to the dynamic problem and the system is controlled by a computed torque/time delay approach. This paper explores the fundamental limitations of dynamic problem for different loading conditions and the behavior is formulated based on the motion constraints. For example, the cable forces are computed considering the yield stress. Moreover the effects of cable configuration are examined by comparing the model performance. Meanwhile, the geometrical parameters have an important effect on manipulation. The analysis is applied on two main robot structures considering geometrically constrained deformable continuum body. The simulation results illustrate the efficiency of the proposed design and controller. Nevertheless, the field of continuum and hyper-redundant manipulation holds great promise also in the experimental domains.

AISI4340 hardened steel have a vast functionality in industries. Hard machining of this steel have several benefits such as, higher productivity, lower production cost and improved workpiece properties. In machining operation, ultimate surface roughness is the most important characteristic of machined surface and plays an important role in workpiece life. One of the effective factors on surface integrity is cutting fluid used in machining operation, which have health and environmental problems is spite of positive effects. As a result, using minimum quantity lubrication is considered as an alternative method. In present study, relations between milling parameters and final surface quality in milling of AISI4340 hardened steel, in the presence of lubrication systems including; dry, wet and minimum quantity lubrication have been investigated. Cutting speed, feed rate, axial and radial depth of cut have been considered as main parameters of milling operation. Totally, 90 experiments have been done using response surface method to analyze the effects of process parameters on surface roughness. Results revealed that feed rate and cutting speed have the most Influences on surface roughness. Also higher values of cutting speed and lower values of feed rate are necessary to reduce surface roughness. In addition, compared to other lubrication methods, minimum quantity lubrication have the best performance in surface quality, especially in high cutting speed and depth of cut.

The accurate evaluation and experimental investigation of the gear dynamic response have indicated some interesting nonlinear phenomena such as bifurcation and chaotic behavior on some system parameters. The chaotic motion is an unusual and unpredictable behavior and has been considered as an undesirable phenomenon in the nonlinear gear vibration systems. Therefore, in order to design and develop an optimal gear transmission system, it is important to control and eliminate these nonlinear phenomena. This paper presents the design of a gear system in order to control and suppress the chaos. A generalized nonlinear dynamics model of a spur gear pair including the backlash and the static transmission error is formulated. The idea behind the design of this control system is applying an additional control excitation force to the driver gear. The parameter spaces of the control excitation force are obtained analytically by using the Melnikov approach. The numerical simulations including the bifurcation diagram, the phase portrait, and the time history are used to confirm the analytical predictions and show effectiveness of the proposed control system for chaos suppression in nonlinear gear systems.

Fracture phenomenon in orthotropic materials, generally associates with fracture process zone (damaged zone) in crack tip vicinity. Determination of Mechanical properties in this region can help to predict the value or even the direction of crack growth in orthotropic materials. This area contains a multitude of micro cracks which cause difficulties in analytical process of the region Also cause energy waste in damaged zone that can affect the material fracture properties. So far, several models have been proposed to determine the mechanical properties of this region, but due to the immense complexity of this region, the results have not been expressed the behavior of this region properly. Moreover, the existence methods have not been verified with new experimental and numerical data, yet. In present research, a new approach based on experimental and numerical results proposed to investigate the orthotropic damaged zone properties. This model, unlike previous models by offering a range for effective elasticity modulus, can determine the mechanical properties of this region for the presence or absence of micro-cracks interaction among them. The proposed model also, validated and compared with experimental and numerical results.

Curved thin-walled structures are extensively used in many industrial applications including aviation industries. These parts have a specific role in manufacturing of helicopter main rotor blade. Fracture mechanics parameters are needed for life estimation and life extension of curved thin-walled structures. Standard methods of fracture mechanics testing are not applicable to these parts because of curvature and thinness of the specimens. So new test methods for these specimens is necessary to be investigated and developed. Vast trying has been done by scientists to overcome these problems and some theories and experimental methods such as the theory of fracture of thin-walled curved plates, the conventional burst test method and some new non-standard test methods has been initiated and developed. In the present paper, first of all, the theory of thin-walled curved plates has been briefly presented to link the behavior of curved specimens to the flat ones and the conventional burst test (BT) method has been accordingly introduced. Then, the newly developed non-standard test methods such as compact curved tension (CCT), pin loading tension (PLT), three points bending (TPB), double edge notched tension (DENT), internal conical mandrel (ICM) and X-specimen have been reviewed. Finally, a comparison between mentioned methods had been done to determine the most appropriate one.

The Stewart platform with six degree of freedom (three translational and three rotational motions) consists of two rigid bodies, lower plate (base) and upper one (mobile). These two bodies are connected together by six extensible legs between three pairs of joints on each of the bodies. This platform can be used to isolate the top plate of the platform and its payload from the applied motions to the base. Since the passive isolation methods are not effective in elimination of the high amplitude (and usually) low frequency motions, this paper practically investigates the possibility of using the 6DOF Stewart platform as an active vibration isolator. In this study, a Stewart platform was designed and constructed based on electric actuators (servo-motors). And then it was practically utilized to isolate its top plate from the applied pitch and roll rotations to the base plate. MEMS sensors including two accelerometers and one rate gyro along with Kalman filter and kinematic relations were utilized for measuring the pitch and roll motions. A PI controller was implemented to keep the top plate at level position using the MEMS sensors installed on the bottom plate. The experimental results indicated that the platform can effectively isolate the pitch and roll motions while the frequency of these motions is in the working speed range of the electric actuators.

Friction stir welding (FSW) has become a technology of widespread interest because of its numerous advantages, most important of which is its ability to weld otherwise unweldable alloys. In this study, friction stir welding process has been used to join A441 AISI steel and AA1100 aluminum alloy. Optical microscopy, X-ray diffraction analysis (XRD), Energy-dispersive X-ray spectroscopy (EDS) and Vickers microhardness tests were employed to study on the joint microstructure evolution and hardness. The results showed that after welding process, head affected zone (HAZ) and stir zone (SZ) were formed in steel base metal side and head affected zone (HAZ), thermo-mechanical affected zone (TMAZ) and stir zone (SZ) were formed in aluminum alloy side. Mg2Al3 spherical particles formed with the ferrite and pearlite constituents in the junction. These particles were formed between the aluminum grain boundaries and due to the difference in contraction coefficient with aluminum base metal, were causing hot cracks in stir zone during solidification. Due to the generated frictional heat, small grains of ferrite and pearlite with very fine grain size of aluminum were formed in stir zone. Base metals dynamic recrystallization and formation of intermetallic compounds led to stir zone microhardness became higher than other areas.

To investigate, understanding and predicting dynamic fracture behavior of a cracked body, dynamic stress intensity factors (DSIFs) are important parameters. In the present work interaction integral method is presented to compute static and dynamic stress intensity factors for three-dimensional cracks contained in the functionally graded materials (FGMs), and is implemented in conjunction with the finite element method (FEM). By a suitable definition of the auxiliary fields, the interaction integral method which is not related to derivatives of material properties can be obtained. For the sake of comparison, center, edge and elliptical cracks in homogeneous and functionally graded materials under static and dynamic loading are considered. Then material gradation is introduced in an exponential form in the two directions in and normal to the crack plane. Then the influence of the graded modulus of elasticity on static and dynamic stress intensity factors is investigated. It has been shown that, material gradation has considerable reduce influence on DSIFs of functionally graded material in comparison with homogenous material. While, static stress intensity factors can decrease or increase, depend on the direction of gradation material property.

Control of seismic shake table in order to track the predefined earthquake profile is a key concern in design of seismic shake tables. This paper proposes a vision-based real time displacement measurement system using image processing techniques to control a laboratory-scale seismic shake table. The shake table is controlled via a fuzzy-supervisory controller, an inner PID loop and a Fuzzy outer one, whose feedback is provided by the vision-based measurement system. To minimize tracking errors, the fuzzy controller uses displacement and acceleration responses as its feedbacks. For this purpose, a camera and an image processing application are utilized to measure the motions directly in real time. Results are sent to a host PC through a network as the controller feedback. Proposed system performance is compared with an alternative system which utilizes a linear encoder as displacement sensor and controller feedback. Test results prove effectiveness of the proposed fuzzy system in cutting back the tracking errors. In addition, the vision-based system uses a very low cost camera to measure the displacement directly. It has appropriate accuracy, works in real time, and doesn't need any contact with the table, comparing to the encoder version.

In this paper, the behavior of multi-layered ceramic armor and ceramic armor with ductile backing against armor piercing APM2 projectile has been considered numerically. Multi-layered armors in accordance with BR7 ballistic protection class should protect against AP 7.62-mm projectiles with impact velocity of 830m/s. Results show that unlike high strength steel, ceramic resists against initial penetration of brass jacket and lead filler and erodes them at initial stages. This enables higher resistance in ceramic armor with similar mass in comparison with the steel one. It is illustrated that ceramic armor with ductile backing beside above characteristics has the capability of bullet jacket strip and capturing brass jacket while the core penetrates through the armor. This characteristic is not observed in multi-layered ceramic armor without the backing plate. Ceramic armor with backing plate reduces projectile's exit velocity one ninth the residual velocity of multi-layered ceramic armor and one nineteenth the residual velocity of high strength steel armor with similar mass. Another point discussed in this paper is the effective ceramic mass resisting against the projectile. The more mass involved, the more ballistic resistance gained.

Nowadays, composite materials are used in different applications. Some of these applications involve components subject to cyclic loading. Fatigue is the dominant failure mechanism for structures under this type of loading. Hence, proper prediction of fatigue life is essential for safe design and operation of structures, maintenance, repair and replacement of components. Many of the existing models in this field have not assessed the degradation of material properties such as stiffness and strength during fatigue damage. In this paper, a stiffness-based model is initially evaluated for fatigue damage analysis of composite structures. The model is validated with two sets of experimental data. A residual strength model is coupled to the choice model and a modified model is developed. Then, residual fatigue life of fiber reinforced polymeric composites is predicted for three sets of experimental data under two-stage loading. The results demonstrate that the proposed model has an improvement on accuracy in the estimation of residual fatigue lives. For better evaluation of the developed model, experimental results and some existing models are compared with the present study predictions. It is concluded that in most cases, the predicted values by the proposed model is closer to experimental values in comparison with other models.

In this paper, a new forward extrusion process is proposed for producing large-diameter tubular components. At the beginning of the process, a round billet is located in the container and then extruded into a preliminary die with three bean-shaped holes forcing a hole in the original billet. The material is then entered into another die with a diverging and converging surfaces designed to weld the material and decrease the tube thickness. Material flow behavior, applied strain and the required process load were predicted using finite element (FE) simulations. The results showed that the new extrusion process had three important advantageous namely a lower process load and a container with a smaller diameter while applying much higher plastic strain compared to the conventional methods.

Friction stir welding is a novel, economical and high quality technique among aluminum welding and joining methods. In this present paper, by introducing the friction stir welding the affecting parameters on thermal, micro structural and mechanical properties of 7000 series aluminum aerospace alloys joining using Friction Stir Welding is investigated. Results show that the sizes of nugget grains are between 5 through 12 micrometer and the least grain size constructed in nugget zone which the thermal effect is responsible for. By using thermal analysis and experimental results, the amount of loosed energy during FSW process can be drawn. Furthermore the effect of process parameters on loosed energy can be investigated. In this work and in the beginning and during welding, the appropriate temperature near to the tool shoulder is 330 degrees centigrade which can be used as a norm for quality control of weld ability. Cylindrical tool with plain pine surface made a welding joint with strength up to 80 percent of base metal. Welding quality is strongly affected by defects in joining zone which came from selecting non proper welding parameters.

The forces due to the unbalance in a system cause undesired vibration and noise, and reduce the system life. One of the new and efficient methods used to reduce the unbalance is the use of automatic dynamic ball balancer. The automatic dynamic ball balancer is a typical passive balancer which doesn't need control systems to balance the rotor. Because these devices are used in the systems which have variable unbalance according to the operating conditions and the system may be switched on and off several times a day, therefore reducing the balancing time becomes a necessary task. In practice, the gyroscopic effect is created for several reasons, e.g. when the rotor is located offset from the shaft midspan. Previous studies have not determined the optimum values of the damping ratio and mass of balls of the automatic dynamic ball balancer under the gyroscopic effect. In this study, the effect of damping ratio and the mass of balls of the automatic dynamic ball balancer on the stability and balancing of the system under the gyroscopic effect have been investigated and, using the Nelder-Mead simplex algorithm, the optimum values of these parameters to minimize the time of balancing and converging the Euler angles to zero are obtained.

Recently, providing a model in order to investigate human body vibrations and also finding a methodology to simulate the functionality of central nervous system in muscle tuning while pre and post fatigue conditions during running has been discussed in numerous papers. In the current study, a 4 degree of freedom vibration model of human body and an objective function that mimics the functionality of central nervous system with bounds for pre-fatigue and post fatigue conditions were considered. Firstly, it was shown that the optimization method used in the literature leads to inaccurate results. Then, in order to achieve more accurate results, the optimization problem was solved using Particle Swarm Optimization method. A comparison between the results of current study with those in literature showed that our results are more consistent with experiments. Our findings indicated that an increase in running time followed by an increase in fatigue leads to a significant decrease in the safe region and increase in vibration amplitude of lower body wobbling mass. Moreover, a parametric study was conducted to investigate the effects of mass distribution and variations in touchdown velocities on the area of the safe region for both pre and post fatigue conditions. A high dependency of this area to the mass distribution and touchdown velocities of lower extremities was also indicated. The results of this study can be useful in the shoe design for running and similar activities.

In this article, an analytical procedure is presented for prediction of linear buckling load of a waffle cylinder stiffened by an array of equilateral triangles. The grid stiffened shell is subjected to axial loading condition. The shell has simply supported boundary conditions at its two edges. The equivalent stiffness of the stiffener and skin is computed by superimposing between the stiffness contributions of the stiffeners and skin with a new method. Total stiffness matrix of the shell is composed of stiffness matrix of skin and grids with special volume fractions. In this analysis, using energy method, equilibrium equations of the grid stiffened shell are extracted based on the thin shell theory of Flugge. The Navier solution is applied to solve the problem. A 3-D finite element model was also built in ANSYS software to show the accuracy and validity of the present solution. The results show that the present new approach has high accuracy and precision. The effect of various geometrical parameters on the critical buckling load is investigated. Due to the stability and accuracy, the present method can be used by many designers and engineers to improve their design quality.

New trends have been observed in recent years for rapid prototyping of sheet metal parts by Incremental Forming process particularly at low quantity production. Recent ideas have been presented for a new type of this process known as Incremental sheet metal hammering (ISMH) method. In ISMH process, by sequence moving of a hammering punch over a clamped sheet metal, a three-dimensional work piece is produced without using a die. In this paper, the effect of tool parameter on the formability of Al-1100 will be studied. To investigate this issue, the sheet is clamped. Then by considering the cone angle, hammering is applied at a certain diameter and frequency until the failure happens. By recording the angle and the height values at failure point, a correlation has been extracted between the diameter and the frequency. Analysis of the results shows that by decreasing the diameter of the punch, maximum strain in the direction of thickness is observed at higher height. Also, by increasing the diameter of the punch, formability of Al-1100 increases. Also, it is shown that by increasing the impact frequency, the formability of the sheet will be decreased.

This paper concerns with a similarity solution for mixed-convection boundary layer copper-water nanofluid flow over a horizontal flat plate. Appropriate similarity variables are used to convert the Governing PDEs to ODEs and the resultant equations with the nanofluid properties relations are discretized and solved simultaneously using finite-difference Keller-Box method. The effects of change in plate temperature, the volume fraction of nanoparticles, and the mixed-convection parameter, on friction coefficient, Nusselt number and velocity and temperature profiles are investigated. The results show that, the Nusselt number increases as the mixed-convection parameter and the volume fraction of nanoparticles increases. This enhancement is about 10 percent for the nanofluid with 4% volume fraction of nanoparticles, compared with the pure water. In this range, moreover, the friction coefficient parameter increases about 20 percent. However, the lower the mixed-convection parameter is, the effect of nanoparticles on the friction coefficient increment is more. The results also illustrate that the effect of the surface temperature on the increment of Nusselt number and on the reduction of friction coefficient is more considerable in higher mixed-convection parameter and volume fraction of nanoparticles. Also by increasing surface temperature, the temperature of nanofluid decreases at any surface distance.

In this study a safe and smooth path planning containing the slightest risk is considered for an Unmanned Underwater Vehicle (UUV). To do so, three smooth and continues functions resembling the three dimensional path are introduced and then their parameters are optimized using the particle swarm optimization method to find the safest possible path. For each point in space a numeric value is considered as vulnerability and the objective function is the integral of the vulnerability over the path produced. This path forms controlling signals which through a TSK fuzzy controller, the UUV is guided. The new arrangement of the propulsion vehicle subsurface was modeled. Since for the design of the controller, the parameters of the Under Water Vehicle dynamic system not used, so the control system is robust with respect to parameter Uncertainties. In the last section three environments with different complexities are considered to illustrate the creating process’s performance of the path and it is concluded that this method demonstrates desired performance in the development of a safe and smooth path through a harmful environment and the design of an adequate controller.

The aim of this study is to investigate analytical and experimental energy absorbing capacity for a hat shape structure with three different boundary conditions. Four layered unidirectional (UD) E-glass fiber /polyester resin was used to construct hat shape beam energy absorber. The length of the composite hat shape was 1m and the thickness was 3mm. Result shows good coloration between experimental energy absorption and the values obtained from the model. The best coloration between experimental and the model is related to [75, 0, 0, -75] fiber stacking configuration with 0.23% accuracy in clamp-free boundary condition, and the worst coloration between experimental and the model is related to [30, 60, -30, -60] fiber stacking configuration with 19.88% accuracy in clamp-free boundary condition.

In this paper, two-phase air–water flow was investigated experimentally and simulated numerically using VOF method. The tests are conducted in Multiphase Flow Lab. of Tarbiat Modares University. In order to evaluate the rib effect on flow regimes, experimental investigation was conducted with ribs of different width and pitch where assembled on front and back side walls (side walls) of the duct during different test runs. The rib width and pitch were held constant during each test. The experimental work considered for different regimes of wavy, plug and slug which generated in the ducts with and without rib applying various phase velocities. The effects of using ribs on regime boundaries are presented in the flow diagrams and discussed in details. Compared to the smooth duct, the ribbed duct affects the different regime boundary positions noticeabily. The results showed that in the duct with small sizes ribs, the first slug initiates at longer time and distance in compare to the duct equipped with bigger size ribs. The results show that for normal operational flow velocities, the ribbed duct decreases the slug area on flow diagram map in compare to smooth duct. However, ribs facilitate the slug regime initiation for phase velocities in accordance with slug generation, which is not benefit of operational condition.

In this paper we developed and modeled elastic - plastic contact theories for soft spherical nano - bacteria to be applied in manipulation of various micro/nanobio particles based on atomic force microscopy. First, we simulated elastic contact for three types of nano - bacteria: S. epidermidis, S. salivarius and S. aureus, using Hertz contact model and finite element. Comparing simulation results of elastic contact with experimental data showed that considering elastic contact for simulating the contact of nano - bio particles is not appropriate and will yield incorrect results. Therefore, in this research, we tried to develop and simulate Chang elastic - plastic contact theory to be applied in simulation of contact mechanics for application in simulating manipulation. Comparing simulation of Chang contact theory with available experimental data and the results from contact simulation of Chen et al showed that Chang’s complete elastic - plastic theory yields desirable results. Comparing the diagram of contact radius in terms of indentation in Hertz and Chang theories showed that the created contact radius in elastic - plastic state is larger than contact radius in elastic state.

This paper investigates the effects of constructional elements on the biomechanical behavior of desert locust hind wing. First, the microstructure of the insect wing is investigated using scanning electron microscope. The results of the scanning electron microscopy are used to develop finite element models of the wing with different constructional elements. The presented models are studied under the inertial and aerodynamic loads applied during flight and the obtained stresses and displacements are assessed. The results show that longitudinal veins, longitudinal and cross veins, corrugations, corrugations and longitudinal veins and finally a combination of corrugations and longitudinal and cross veins cause averagely 4, 25.75, 4.34, 184.54, 768.5 times decrease of the achieved principal stresses in comparison with a wing without the mentioned constructional elements. Constructional elements of the locust wing play an important role to uniform the pattern of stress distribution in the wing during flight. Further, the existence of the mentioned constructional elements causes a decrease in the variation of the stress within a stroke-cycle. In addition, it is shown that the inertia and aerodynamic forces have less effect on the wing deformation than the elastic ones. The results of this research may be helpful in the development of lightweight structures with high strength.

In this paper, a new formulation is developed for nonlinear dynamic analysis of 2-D truss structures. This formulation is based on dynamics of co-rotational 2-D truss. The idea of co-rotational approach is to separate rigid body motions from pure deformations at the local element level. Using this approach, internal force vector and tangent stiffness matrix, inertia force vector and the tangent dynamic matrix are derived. Furthermore, the inertia force vector, tangent dynamic matrix, mass matrix and gyroscopic matrix are directly derived from the derivation of current orientation matrix with respect to global displacements or orientation matrixes. Using this new formulation, nonlinear response of any 2-D truss structures can be examined. Here, for example the response of tensegrity structures under dynamic loads are investigated. Tensegrity structures are a class of structural system composed of cable (in tension) and strut (in compression) components with reticulated connections, and assembled in a self-balanced fashion. These structures have nonlinear behaviour due to pre-stress forces. And their integrity is based on a balance between compression and tension. Two numerical examples are presented to illustrate the new formulation and results show that the new formulation has more convergence rate than the existing models.

In this article, effects of Friction stir welding tool rotational and traverse speeds were studied on the temperature distribution, material flow and formation of defects in the welding zone. Computational fluid dynamics method was used to simulate the process with commercial CFD Fluent 6.4 package. To enhance the accuracy of simulation in this Study, the welding line that is located between two workpieces, defined with pseudo melt behavior around the FSW pin tool. Simulation results showed that with increase of FSW tool rotational speed to linear speed, the material flow in front of tool became more and dimensions of the stir zone will be bigger. The calculation result also shows that the maximum temperature and stir of the material was occurred on the advancing side. The computed results showed that with incompetent heat generation, insufficient material flow caused around the pin and defects formed in weld root. The computed results were in good agreement with the experimental results of other researchers. Based on the welding parameters that used in this simulation, the maximum strain rate is predicted between -4 (S-1) to +4 (S-1) in the stir zone.

The single point incremental forming, which is appropriate for low volume production, is one of the simplest varieties of incremental sheet metal forming process. One of the critical issues with single point incremental forming is excessive thinning which affects the strength of the part and confines the applicability of the process to produce only parts with small wall angle. In this paper, multistage single point incremental forming of a truncated cone with 70° wall angle made from an aluminum alloy sheet is studied to alleviate excessive thinning. By proposing a new two-stage forming strategy and obtaining the corresponding parameters using an appropriate algorithm, it is shown that thinning and forming time can be improved through a systematic design of multistage forming. The implementation of the designed two-stage single point incremental forming leads to less thinning in the part when compared to either the two or three-stage single point incremental forming based on a conventional strategy. The bulging at the bottom of the part, which is one of the drawbacks of multistage single point incremental forming, can also be controlled by using the proposed strategy.

In this article, a novel intelligent online tip-over avoidance algorithm is presented considering the interactions between the mobile base and manipulator arm. To this end, the newly suggested dynamic stability margin measure named Moment-Height-Stability (MHS) is adopted. Additionally, a function representing the increment of postural stability margin metric is defined based on MHS. The system dynamic equilibrium is then enhanced using a fuzzy logic approach. The response of the suggested method of this paper is compared with that of a previously Force-Angle based proposed one considering a planar mobile manipulator. First the dynamics of the robot is derived using Newton-Euler method via MAPLE 16 and it is verified through the model provided in SimMechanics toolbox of Simulink. The efficiency of the suggested method is illustrated in comparison to the previous one on a destabilizing robot path. Besides, the performance of proposed method of the present study is investigated in the presence of external disturbances. The obtained simulation results reveal the effectiveness of the performance of the suggested technique for stability improvement of wheeled mobile manipulators once encountering unexpected disturbing situations.

An analytical method is developed to determine the effects of the frame angle on the behavior of multi span Timoshenko beams with flexible constraints subjected to a two degrees-of-freedom moving system. In a multi-span beam, there are discontinuities in each frame angle in addition to discontinuities in flexible constraints. The correlation among every segment can be obtained considering the compatibility equations in frame angles and cracks. Eigensolutions of the serial-frame system can be calculated explicitly, using the analytical transfer matrix method. The forced responses can be determined by the modal expansion theory using the eigenfunctions. The orthogonality of the mode shapes is applied to calculate the forced response. A new formulation is introduced and confirmed for the orthogonality of the mode shapes for the case of a beam with the frame angle. It is observed that the natural frequencies will be increased by increasing the frame angles, while the maximum deflection of the beam will be decreased. The crack modeling leads to lower natural frequencies and higher maximum deflection for the beam in compared with the no crack model. The validity of the developed technique is probed by comparing the results with the analytical results reported in other articles and those from numerical solutions.

The measurement of hydrodynamic loads on submerged bodies is one of the principal uses of water tunnels. Due to the limitations of the water tunnel, an accurate force balance is necessary. This paper describes the design, fabrication and calibration of a new six-component force moment balance system for measuring the forces and moments acting on the model, in static and dynamic water tunnel testing. A balanced team performed many areas for designing balance system such as structural design, balance technology, design of calibration mechanism, balance calibration etc. A six-component balance, ability to measure the three elements of force and three components of moment simultaneously and instantly on cavitating and non- cavitating models in a water tunnel. The concept used in the balance design is the bending beam and the strain gage principle. The electrical signals are proportional to the forces applied to the model. By considering the relationship between the applied force and the balance’s output signal and by using the calibration models, the forces and moments exerted on the model in the water tunnel can measure directly. For calibrate multi component balance, a new six-degree of freedom calibration rig designed and constructed. The system is designed based on applicability of formal experimental design techniques, using gravity for balance loading and balance position and alignment relative to gravity. The six-component balance was calibrated by this rig. The standard error between the measured values and the values obtained from calibration model is less than 0.1 percent of maximum loading was achieved.

This paper presents a comparative study on the dexterity and manipulability of three planar 1-RRR, 2-RRR and 3-RRR manipulators. After derivation of the Jacobian matrix of three manipulators, they are transformed to a homogeneous form to include components with homogenous physical units. The condition number of each Jacobian matrix is calculated. Since the condition number changes in general between 1 to ∞, its inverse is used for definition of the local condition index. A zero value for this index indicates that the Jacobian matrix is singular and consequently, the robot has the worse manipulability. On the other hand, the robot indicates the best manipulability and dexterity when this index is close to one. Furthermore, the effect of changing the platform angle on the local condition index is investigated. The manipulability of three manipulators is then compared to each other for the platform angle for which the maximum local condition index is resulted. The results show that the 3-RRR manipulator has a better dexterity than other manipulators at the platform angles less than 90 degree. For the angles greater than 90 degree, the 1-RRR manipulator has a greater local condition index which indicates more dexterity. Finally, the maximum local condition index is compared for each robot at its own workspace and common workspace of three robots. The results verify that this index has almost identical values for the already mentioned workspaces for each robot. Based on the manipulability distribution, the global condition index of theses robots are checked. The results confirm the superiority of the 3-RRR robot.

In this study, experimental tests were performed to evaluate the effects of axisymmetric cylindrical projectile nose shapes and initial velocities on ballistic performance of laminated woven glass epoxy composites. Projectile initial velocity and nose sharpness changes, absorbed energy, delamination area, etc. are investigated by six blunt, hemispherical, conical and ogival projectiles. Hand lay-up method has been used to manufacture composite targets with 18 layers of 2D woven glass fibers of 45% fiber volume fraction. The epoxy system is made of epon 828 resin with jeffamine D400 as the curing agent. The results show that the maximum influence of projectile geometry on target behavior, occurs in ballistic limit area. In this range of initial velocity, ogival (CRH=2.5) and Blunt projectiles show the best and the worst ballistic performance. The delamination area decreases as the projectile nose sharpness increases or its initial velocity decreases. Ballistic curves for different projectiles show that the difference between projectiles behavior decreases in higher impact velocities. Because of target shear failure in blunt projectile impact, the amount of target absorbed energy for this projectile is less than other projectiles in higher impact velocities away from ballistic limit velocity.