مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 2 (اردیبهشت 1400)

In the standard finite element method, the edges of the adjacent elements are aligned to each other, and the corner of an element does not located on the edges of another one. If this constraint is violated it is said that the mesh is non-conforming and the use of such meshes in the finite element method requires specific techniques. In the present paper, a new method is suggested for the use of non-conforming meshes which are generated using quadtree algorithm. Quadtree is a data structure that has a very fast recursive algorithm and can be used to divide a two-dimensional domain into sub-regions or elements. The elements produced by this method are non-conforming and are not usable in the standard finite element method. In general, six types of elements with different arrangement of nodes may be appeared in a quadtree mesh and the conventional methods which are using the polynomials cannot be used to derive the shape functions. In the present paper, a new idea is proposed to obtain the shape functions of such elements. In this method, a boundary value problem is defined and its solution is used as the shape functions of the non-conforming elements. To evaluate the applicability and accuracy of the proposed method, two numerical examples are solved and the results are presented. The results show that the proposed method can be used to effectively apply the non-conforming meshes in the finite element method.

In motorcycle accidents, the acceleration caused by the collision has a huge risk to the health of the motorcyclists and passengers. In this study, the finite element method was used for dynamic analysis of impact mechanics to predict the effect of the shell material and thickness on the head injury criteria of the helmeted head (including head, shell, foam, comfortable foam and strap). The open-face helmet, including three current market materials, were selected. Head orientation, in the most accidents, at the collision moment is oblique. In the simulated impact model, head is also placed obliquely. The results of this study are validated by the experimental results and valid published data. The simulation results show that there is an optimum thickness for the helmet shell regardless of its material. In order to determine the optimum thickness, there must be compromises between the various parameters such as head injury criteria, shell failure, weight, and price. According to the results obtained for the shell thickness, if the thickness increases, the weight and range of acceleration increase while the probability of shell failure decreases. If the thickness decreases, despite of decreasing the acceleration in the head, the stress in the shell increases that leads to failure.

Second strain gradient theory (SSGT) is employed to examine the spherical single/double-phase Lame-type problem. Due to the capability of SSGT to capture the effects of surface, size, and discrete nature of materials, the pertinent relaxed configuration is sought. SSGT is written in the spherical coordinate system and the corresponding governing equilibrium equations, stress and strain components, constitutive relations, and boundary tractions are derived. With spherical symmetry consideration of the problem, the relaxed configuration is obtained for both the diamond crystalline carbon and carbon coated crystalline silicon shell. Afterwards, the external symmetric loading is applied on the nanostructure relaxed configuration to analyze the mechanical response. The SSGT elastic material parameters for carbon and silicon are calculated via the quantum computations and lattice dynamics combined with the material continuum description. Analysis shows that the nanostructures mechanical response in SSGT is significantly different from that in the classical elasticity. For example, in the single-phase problem with inner and outer radius equal to two and ten lattice parameter, respectively, under an external pressure of about 0.0001 , the classic elasticity predicts an approximately constant radial stress of about -0.0001 in the nanoshell. However, in the framework of SSGT, the radial stress is varying from about -0.001 and -0.0002 in the vicinity of the inner and outer boundaries, respectively, to about 0.0003 in the middle of the hollow nanoshell. With increasing the inner radius, the difference between the two results in the middle points decreases.

Belows are one of widely used elements in various industries. These elements often contain an internal fluid flow and they are used as length compensator, vibration absorber and sealer. Usually they are under pressure and out of range increasing in pressure can cause buckling of bellows. There are two types of buckling in bellows known as column buckling and in-plain buckling. In this paper first, a comprehensive study was done on analytical solutions of bellows buckling in the literature and then both types of buckling were investigated by using FEM software ABAQUS and the results were compared with the experimental results that had been reported in references. According to the results, although elastic analysis of belows buckling gives correct mode shapes, the buckling pressure obtained by this method is several times as much the experimental one. Then, buckling pressure is computed by applying imperfection according to the primary buckling mode shapes and nonlinear analysis was done by the use of pressure-displacement curve. Finally, necessary number of buckling mode shapes applied as imperfection and the amount of the mode shape amplitude required were investigated by comparing the results with the experimental one, and good agreement between FEM and experimental results was obtained.

Bone is a mineralized biological tissue whose main components are enormously different from the mechanical aspect. Some of bone diseases like osteoporosis are due to mutations in bone structure at nano scale, while their clinical symptoms usually appear at macro scale. Therefore, the evaluation of bone at micro and nano scales is important and the obtained results could be used in fabricating the implant materials, scaffolds and bone grafting. In current study, the finite element modeling is performed to evaluate the mechanical properties and structural behavior of bone at nano scale and cohesive element with different mechanical properties is applied. After its verification, the stress distribution and elastic properties are compared with analytical Shear- Lag model. Limited studies are available on strain ratio in previous research and it is presented for different cohesive elements in current study. The influence of mineral volume fraction and mechanical properties of collagen is investigated. The comparison between FE models and the other ones demonstrate an excellent agreement. The collagen- HA interface with unknown mechanical properties is the most important parameter in mineralized collagen fibril and by analyzing different parameters in current research, the thick layer of water with Van der Waals interaction and Viscous shear is determined as the most probable cohesive layer. The result of parametric studies indicates the significant effect of non linear collagen on the model behavior. In order to decrease the cost of investigation in 3D models, the proposed unit cell with periodic boundary conditions could be employed.

The acceptable preformance of the data driven prognostics methods usually requires large amount of data, therefore the performance usually is not desirable for small amount of data conditions. Age clustering method multiplies the volume of the train data through observing data at multiple points. The advantage of the method is that it can be used for learning from a small set of data. The proposed approach is integratable with existing prediction methods and improves their results accuracy significantly. In this article, the ABC prognosis framework is described, its effectiveness for prognosis in normal conditions is illustrated in a case study on turbofan engines and a comparison with existing results on the same data is made. The paper continues with a study on the robustness of the proposed method under limited data conditions. The prognosis accuracy is compared for the case study in various conditions of available train data. The results emphasize (1) the efficiency of the method compared to other existing approaches in normally rich data condition and (2) the robustness of the results under limited data condition.

In recent years demand for mobile electrical power has been increased and due to this application, energy harvester systems have been developed to convert mechanical energy into suitable electrical energy using smart materials. In this investigation, a novel arrangement of a new energy harvester using Magnetic Shape Memory Alloys (MSMAs) is developed. Elements of MSMA are attached to a corrugated beam and their roots are fixed to the base support. The reason of using the corrugated beam is to increase the stiffness of the structure in less thickness and also to increase the effective strain field in MSMA elements. This feature reduces the length of the system and the occupied volume. The way of harvesting energy from this system is based on conversion of vibrational energy to the magnetic flux gradient. That is to say; there is a number of copper coils that wrapped around the MSMA elements in a constant magnetic field. If strain or stress field is applied to the MSMA elements, some variants in specific direction are changed and as a result, the electrical current is induced to the coils. The AC voltage is produced as a result of change in magnetic flux of the surrounding coil according to Faraday’s Law. The problem is studied with simulations in Abaqus using UMAT code for modeling behaviour of MSMA elements. Also, to simulate the material properties of MSMA substance, Kiefer and Lagoudas nonlinear model is used. It will be shown the effect of various parameter on output voltage value.

Sensors and actuators have a special place in today's science and industry world. Therefore, electromechanical analysis of piezoelectric materials is one of the topics of interest for researchers so that besides understanding the behavior of piezoelectric structures by optimizing this behavior, they could pave the road of designing and producing structures. In this study, using the first-order shear deformation theory, the first-order electrical potential theory and the application of the energy method, the governing equations are extracted for thick cylinders subjected to mechanical and electrical loading at their internal and external surfaces under various boundary conditions in two cylinder’s heads. Then an analytical solution is presented for the governing equations and using this solution, the results of electromechanical behavior of the cylinders under different boundary conditions in two cylinder’s heads are estimated and evaluated, and compared with the finite element method (FEM) results. The analytical solution in this research is not a series solution, and there is no need to investigate the convergence and also, it is of less computational volume. The obtained results from the analytical method and FEM have a good agreement, which showed the presented analytical solution can be used with higher accuracy.

The existence of defects and residual stresses in the multilayer coatings could reduce their crack resistance. By using the method of molecular dynamics (MD) simulation, the crack propagation behavior in deposited Ti/TiN bilayer is compared with its perfect structure. For this purpose, TiN was deposited on Ti substrate, then crack growth was investigated in the two structures. The growth of TiN film on Ti was island growth, and the structure has defects and residual stress. The results showed that both the biaxial and normal residual stresses in the substrate and film are tensile and compression, respectively. The cohesive energy of the interface was calculated by energy difference along the atomic layers. In the following, a crack was considered perpendicular to the Ti/TiN interface in both models, with an initial length of 15 Å. Due to the brittle behavior of the ceramic layer, crack propagates rapidly until interface. The plastic deformation of the Ti layer and the structure of the interface blunt the tip of the crack and prevents to fail. Also, the critical stress for crack growth in a regular structure is found to 2.5 times of its value in the layered structure because of defects and tension in the layered structure

Regarding the necessity of obtaining high-quality resonators in micro electromechanical systems (MEMS), recognizing and investigating the parameters that effect on the quality factor are essential and inevitable. In micro electromechanical systems, microplates are used as resonators and RF filters and so on. In this paper, the effect of thermoelastic damping, which is one of the most important factors affecting the quality factor, has been investigated for rectangular microplates. Galerkin method has been used to simplify and solve the governing equations. The result is a nonlinear algebraic expression for the quality factors of microplates of general conditions due to thermoelastic damping. Unlike previous researches, the proposed model can directly calculate the quality factor. COMSOL Multiphysics software is used for finite element simulation. After verification of the proposed model, the effect of various parameters is investigated. The proposed model can also be used to calculate the pull-in instability voltage. The results of current paper can be used to design micro electro mechanical systems (MEMS) and another applications can be defined for that.

Aerial structures under non-conservative forces especially follower loads, may be exposed to dynamic or static instabilities. Thus, it is essential to design these structures so that it would prevent this phenomenon. In this paper, for the first time, dynamic instability of a thick sandwich beam with FGM flexible core under follower force is considered using high-order theory of sandwich beams. Sandwich beam is considered as a linear elastic structure with small rotations and deformations. Equations of Motion of high-order sandwich beams under follower force, are derived using Hamilton’s principle. The Beam fluttering phenomenon is investigated by applying boundary conditions and using a generalized differential quadrature method. in addition to verification of results, effects of the beam’s geometry and mechanical parameters have been studied. These results revealed that threshold flutter force of the sandwich beam is similar to Timoshenko one.

Combustion chambers are one of the main parts of the power generation systems such as gas turbines and internal combustion engines and affect their efficiency and environmental pollutions. To reduce the high amount of the pollutions e.g. NOx, lean premixed combustion was introduced to be used instead of traditional non-premixed flames, however, this method has more tendency to become unstable. In fact, the thermal and acoustics interactions in the combustion chambers can amplify the acoustic waves and thereby produce noise and increase the vibration level of the liner. The continuation of large amplitude vibrations can lead to failure due to fatigue. Therefore, the vibration modeling of the liner is very important. In the present research, the vibration of a liner in a laboratory combustion chamber is investigated. First step to model the vibration of the liner is to extract its modal parameters in the cold and hot states. Finite element (FE) model updating is utilized to modify the initial FE model of the liner based on the experimental modal parameters. Then, using the computational fluid dynamics (CFD), the flow analysis is performed to obtain the pressure and velocity fluctuations during the analysis time. These data are then used to model the flame as a monopole acoustic source. To find the structural response of the liner due to this acoustic source, the transient analysis is performed. The results show the effectiveness of the updated FE model to predict the modal parameters and the vibration amplitude of the liner with acceptable accuracy.

In this paper an efficient and accurate solution method is developed for transient analysis and free vibration of functionally graded truncated conical shell, subjected to symmetric internal or external moving pressure. The material properties of the shell are graded continuously in the radial direction according to a Mori-Tanaka and volume fraction power-law distribution. A hybrid solution method composed of the layerwise theory, differential quadrature method, and Fourier series expansion is employed to investigate the aforementioned problem. A Fourier series expansion is used for the displacement components and dynamic pressure in the axial direction. Then the layerwise theory across the thickness direction in conjunction with the Hamilton’s principle is employed to obtain equations of motion and boundary conditions. Eventually, the differential quadrature method is implemented to discretize the governing equations in the time domain. This research shows some interesting results that can be helpful for design of functionally graded shells subjected to moving pressure. The developed results are successfully compared with the available results in the literature. The convergence study demonstrate the fast convergence rate with relatively low computational cost. The results reveal that a free vibration with significant amplitude is generated due to excitation from transition of the moving pressure. Also increasing the area of moving pressure in the case of constant load magnitude leads to reducing in the magnitudes of displacements and stresses components.

Control of impact time of missile to target has a great importance in applications such as Cooperative Attack of Multiple Missiles or Anti-Ship Missiles. This paper presents sliding mode control based guidance law against maneuvering targets with impact time constraints. Using nonlinear engagement dynamics, appropriate switching surface has been selected and sliding mode control has been designed using Lyapunov stability theorem. The sliding surface has been selected such that both the line of sight range and the error of the time remaining to target become zero to guarantee impacting the target at the desired time . By considering the non linear dynamic equations of maneuvering target , the proposed method can achieve the desired impact time against maneuvering target . In the proposed method , there is no need to consider small impact angle and non maneuvering target . Using the simulation model, the effectiveness of the proposed method in different scenarios (static and maneuvering) and taking into account different times for the impact time and its efficiency and its superiority as compared to other methods is shown.

A theoretical model is proposed to study the sound transmission loss of a truncated conical shell with porous layer. The isotropic thin-walled conical shell is excited by an oblique incident plane sound wave, which impinges on the outer surface of the shell. The governing equations of the shell motion are obtained by Love’s theory, and a convergent power series solution is applied to obtain the exact dynamic response of the shell. An equivalent fluid model based on Biot’s theory is considered to describe the wave propagation in the porous material. The model results are firstly validated against the results of prior studies. Then, the effects of several design parameters such as different boundary conditions at the ends of the shell, cone angle, incident sound wave angle and material properties of the shell are studied on the characteristics of the sound transmission loss. The proposed model can provide an effective tool in the acoustic design stage of the truncated conical shells. In addition, the transmission loss is obtained in the presence of the porous layer with two different configurations. The results generally show desirable performance of the porous layer in the sound insulation ability.

Object grasping by robot fingers with purely rolling constraints is one of the most interesting issues under consideration by many researchers. In earlier studies, the main goal was the manipulation of object under purely rolling constraints to reach the final stable configuration. In this paper, in addition to deriving kinematic and dynamic equations of the system dual fingers robot and grasping semi-circular object located on a moving hand on the plane with rigid hemi-spherical fingertips under pure rolling constrained, we investigate object manipulation on desired path maintaining dynamics stability. Modified multiple impedance control is used for object manipulation and robot fingers by considering the required reforms in this control law. In this method multiple impedance control is performed by applying the desired behavior of the entire system, including moving base, fingers and object, and dynamics stability condition is satisfied. In this way power adjustment and that these forces arrive in the right place largely effective in minimizing slip is the fingers on the surface object. The results of simulations show the eligible object manipulation and dynamics stability by robot fingers and moving base under pure rolling grasp.

Vehicle stability control is one of the most important subjects in control engineering field. Many research activities have been done to develop more comfort and safe travel for passengers. In this paper, vehicle mixed stability in longitudinal and lateral motion has been investigated. Full model of vehicle (4 wheel) is considered to extract the dynamic equations. Dugoff’s nonlinear model has been used to simulate the behavior of tyres and road, and Cho’s engine model with two state variables has been used for vehicle power system simulation, so it makes the input torque to wheels to be more realistic. Because of the good robustness properties of sliding mode control the second order sliding mode with super-twisting algorithm has been used for calculation of control inputs. This method is proved to be so appropriate and useful in the case of uncertainty in complicated vehicle dynamic model and multiple disturbances in vehicle motion. Engine throttle angle and yaw moment have been considered as longitudinal system and lateral system control inputs respectively. Longitudinal slip coefficient and yaw rate are considered as system output. Simulation results shows the effectiveness of the proposed method.

In this paper, limitation in actuator capacity has been used as a key role in the design of the flight control system. In order to guarantee the performance and stability of flight control systems in the presence of saturation, in flying high angle of attack area, the development of linear matrix inequality, optimization techniques, and numerical methods are proposed, and the integration of these methods with saturated controllers is introduced. Also, in this paper, the combination of two anti-windup method and direct saturation method in the tracking problem of flight path angle is discussed. For this purpose, the nonlinear model of the aircraft is modeled, moreover the linear model is obtained at the trim operation condition. Then the controller is designed to track maneuver flight path angle regardless of saturation. In the following, considering the maximum disturbance involved in aircraft maneuvering, a safe controller that guarantees performance and stability is designed, and gain scheduling technique to prevent conservatism in the use of controllers is employed. The results of the nonlinear and linear model of the aircraft are presented in tracking the flight path angle at high angle of attack with consideration of control and saturation constraints in unstable operation condition. Simulation results, indicating the improvement of the mentioned control method for an unstable aircraft.

The main objective of this paper is to design a fuzzy control system for path planning and controlling mobile robots in a social environment with moving obstacles. The control system, with the target position given at any moment, produces the appropriate path to reach the target without encountering obstacles. It is assumed that there are fixed and moving obstacles, including humans, in the social environment. Also, the robot moves in such a way that the emotions of the individual are considered and do not cause fear or change in the behavior of people. Robot kinematic model and obstacles are considered to reduce the computational volume and simplicity of implementation. After performing various simulations, in order to validate the proposed method, the designed control system is implemented on the Scott laboratory robot. For performing the presented algorithm, the graphical interface designed by Matlab software, the compass module added to the existed system. Finally, the results obtained from the simulation and the laboratory system are evaluated and compared for predefined path and obstacles.

Integrated modeling of multi-domain physical systems requires a common language. One of the methods which has been used to do so, is called bond graph. The bond graph provides a common and core language for describing basic elements and connections across different fields by using its elements, bonds and junctions. Also, by using genetic programming as an evolutionary method, an initial model can be evolved into a final model. The initial model of a system in bond graph called “Embryo” and must have input, output and basic elements of the desired model. By defining a series of operational functions in genetic programming, an embryo model evolves and a final model obtained in one objective and multi-objective approaches. The current research presents an optimized design tool by integration of bond graph and a Pareto multi-objective genetic programming for guiding automated topology synthesis. In order to evaluate the performance of the proposed method, obtained results were first compared to the 20-sim software and then two models of electric filter and a mass, spring and damper system compared to the reference.