مجله مهندسی مکانیک امیرکبیر
سال چهل و نهم شماره 2 (تابستان 1396)

In this article, nonlinear buckling analysis of nonlocal boron nitride Timoshenko nano beam on elastic foundation based on Modified couple stress theory, nonlocal elasticity Eringen’s model, and Von karman nonlinear geometry theory are investigated. The governing equation of motion and boundary conditions based on Hamilton’s principle are obtained. To solve the governing equation of motion, the differential quadrature method (DQM) is used to obtain the critical buckling load for two edges simply supported
(S-S) and simply supported-clamped (S-C) boundary conditions. The results of this research are compared with the obtained results by Murmu et al. that there is a good agreement between them. Finally, the effects of various parameters such as nonlocal Eringen’s parameter, slenderness ratio of nano beam, elastic foundation constants, electric field, temperature changes and material length scale parameter on the nonlocal critical buckling load of Timoshenko nano beam are illustrated. The results show that with increasing nonlocal parameter, slenderness ratio, electric field, and temperature changes, the critical buckling load decreases, while this results vice versa for elastic foundation constants and material length scale parameter. The critical buckling load for S-S boundary condition is lower than that of for S-C boundary condition.

In the present paper, a two dimensional analytical solution for temperature, stress and displacement fields in a hollow cylinder is presented. An asymmetric and time dependent heat flux is exposed on the outer surface of the hollow cylinder. Moreover, the cylinder carries a fluid that transfers heat through convection on its inner surface. The separation of variable method to obtain temperature field is used. Also, stress distribution is taken by means of thermal stress function method. Afterwards, displacement components is obtained by means of stress-strain and strain-displacement relations. The cylinder is regarded as a model for the absorber tube of parabolic trough collector. Using the analytical solution together with the actual properties of the model in solar power plant in shiraz city, results are presented for a period of twelve hours from 06:00 AM until 06:00 PM. The analytical solution is employed to extract numerical results using MATLAB software package. The results are also validated with those given by FEM, conducted via ANSYS computer code. Finally, we are concluded that differences between the results of analytical solution and output of ANSYS computer code are in reason of infirmity of MATLAB software package to calculating the kelvin functions in high accuracy.

In this study, by an inverse method, which uses the Tikhonov regularization method, traction boundary conditions on the surface of a hyper-elastic material are determined. Displacements at several points on the surface of the body are measured and used to find the unknown stress parameters on a part of the problem boundary. The inverse analysis is carried out for Mooney-Rivlin and Ogden isotropic models. An example for identification of boundary conditions on a boundary part of a two dimensional domain with a relatively complicated geometry is presented to show the effectiveness of the proposed method. Effects of different parameters are studied in this example. The results for both hyper-elastic models show that the error of the solution decreases with increasing the number of measured data and decreasing the measurement error. Moreover, it is observed that the accuracy of the solution is decreased when the nonlinear behavior of the material is increased.

The main target of this study is identification of the constitutive model of a soft tissue. For such a purpose a robotic tactile device (Robo-Tac-BMI) was used for breast tissue examinations and stress versus strain was collected for every test point during loading and unloading processes. Utilizing accurate experimental dataset for mechanical modeling of the tissue in conjunction with an optimization algorithm provides a reliable constitutive model of tissue’s mechanical behavior. Eight major hyperelastic models were adapted to the stress-strain data to find the most compatible constitutive equation applicable to the soft tissue mechanical behavior. For this purpose, a new optimization algorithm called Imperialist Competitive Algorithm (ICA) which is based on social and political strategy was used. The novelty of the present study is producing a realistic mathematical model with high accuracy of the soft tissue based on experimental data. The achieved hyperelastic model can be used for prediction of mechanical behavior of the breast tissue in surgery simulation for assistance and educational purposes. Other application of this model is clustering of healthy and cancerous tissue which facilitates the surgeon’s task in the diagnosis procedure. This application also makes the diagnosis procedure almost independent of using imaging techniques or performing biopsies. This model is useful in distinguishing cases where the soft tissue has altered from normal situation like tumors and cancer attacks.

In this research the optimization of piezoelectric fibers in a functionally graded cylindrical panel with PFRC layers as sensor and actuator under dynamic load and electrical excitation with various types of supports including: simple, clamped and combination of free and clamped supports is provided. The main goal is to obtain the volume fraction of piezoelectric fibers in a PFRC layer so that the radial displacement of this layer across the circumferential direction is equal to that of piezoelectric layer. For the optimization, Genetic algorithms were used and in each case, the algorithm parameters are obtained by using Parameter Tuning method. In order to saving time and reducing the use of memory, Artificial Neural Networks are trained and employed. In the end, the results for the stresses and displacements of the panel with piezoelectric layers and the one with PFRC layers are presented and compared. The effect of support conditions and on the optimization process and the obtained volume fractions are examined. Results show that with the introduction of clamped supports, increment for maximum Von mises stress in the structure and the volume fraction of piezoelectric fibers can be seen.

As the cracks and notches in structures may exist in various angles with respect to the loading direction, crack growth and final fracture occur under combined condition of loading i.e. mixed mode fracture condition. Therefore, studying factors that affect material’s fracture strength under mixed mode loading is important. In this study, In order to investigate the effect of bolt clamping force, resulting from torque tightening, on fracture strength, stress intensity factors and T-stress of a rectangular center cracked specimen under mixed mode I/II loading condition, experimental and numerical studies have been carried out. Fracture experiments were conducted on four types of polymethyl methacrylate (PMMA) specimens with crack length ratio a/w of 0.5. Complete range of mode mixities from pure mode I to pure mode II was created by placing the specimens in a modified Arcan fixture and their fracture strength under different loading directions were obtained using a static tensile machine. To give explanations for the obtained results from fracture tests by using mixed mode fracture theories, finite element simulations of the experimental tests were performed to compute mode I and mode II stress intensity factors and also T-stress. The results revealed that bolt clamping force has positive effect on fracture strength of the specimens in all loading directions. Also it was shown that bolt clamping force has a considerable effect on increasing T-stress values.

In this article, an analytical method is used to study surface and geometrical defect effects on free vibrations behavior of circular nanoplates. Due to production process and constrains conditions, nanoplates may be opposed to structural defect. Some of the defects can be modelled as an eccentric hole. Gurtin-Murdoch and thin plate theories are employed to model the eccentric circular nanoplate. In order to solve equation of motion, separation of variables method as well as additional theorem for the regular and modified of first and second kinds of Bessel functions are used. To validate the approach, present results are compared to those obtained by literature. Both of symmetric and antisymmetric vibration modes are analyzed. Some mode shapes are illustrated to make the better physical sense. Finally, effects of various geometrical and material properties on natural frequencies of the nanoplates are investigated. Also, effects of various boundary conditions as free, clamped and simply supported on the natural frequencies are investigated using a wide range of results. Results show that surface effects and eccentric circular defect play an important role in vibrational behavior of an eccentric circular nanoplate. It is observed that the free boundary condition has no more effect on the fundamental natural frequency.

In this study, surface effects on the free torsional vibration of nanobeams are investigated. To this end, equations of motion of nanobeams incorporating the surface effects are derived using the Hamilton’s principle and based on the surface elasticity theory. Then, equations of motion are analytically solved for three types of boundary conditions: clamped-clamped, clamped-free, and free-free, and associated mode shapes and natural frequencies are obtained. Nanobeams made of aluminum and silicon are selected as case studies. A detailed study is performed to examine the surface effects on the free torsional vibration of nanobeams for various nanobeam lengths, nanobeam radii, and mode numbers. In addition, influences of each of the surface parameters on torsional natural frequencies are separately investigated. The results show that influences of the surface effects on the free torsional vibration of nanobeams are completely different from those on the free transverse vibration of nanobeams. Results of the present study can be useful in design of nano-electro-mechanical systems like nano-bearings and rotary servomotors.

In this paper, nonlinear free vibration of magneto-electro-elastic rectangular thin platebased on classical plate theory is investigated. The plate is supported by a nonlinear foundation and simply-supported boundary condition is assumed along all edges. The plate is considered in two forms; uniformly distributed one-layered plate and the functionally graded one. The plate is subjected to electric and magnetic potentials between top and bottom surfaces. Equations of motion of this smart plate are obtained by using classical plate theory along with the Gauss laws for electrostatics and magnetostatics. Then the partial differential equations has been converted to an ordinary differential equation using Galerkin method. The obtained nonlinear equation of motion is solved analytically by using multiple time scales method and an analytical relation for nonlinear natural frequency has been obtained. The accuracy of this relation has been validated by comparing the results of literature.Using this relation, the effects of several parameters like plate's dimension, foundation parameter and electric and magnetic potentials on the nonlinear response of the plate are studied.

Dynamic instability and control of reentry capsules is one of interesting issues for researcher because of its complex flow structure and interaction of very parameters effects in unsteady flow. One of the main problems for reentry capsules is their dynamic instability in some velocities and environmental conditions. Thus, determination of dynamic stability criteria of this kind of capsules is one of key parameters of its design. In this paper firstly, 3D numerical simulation of unsteady flow is performed around a reentry capsule in forced pitching oscillation using slip mesh technique. After validation of the results and study of grid independency, effects of environmental parameters including the angle of attack, Mach number and frequency on the capsule dynamic stability are determined. For this aim, the results are captured by Fluent software and flow structure around the capsule are studied at different conditions. The longitude dynamic derivatives, which are needed for stability analyzing of a body in pitching oscillation, are calculated. Finally, effect of mentioned parameters on value of dynamic derivative is investigated. In the present study, variation range for the mean angle of attack is between 0 to 15 degree, for the Mach number is between 0.8 to 2 and for the frequency is between 10 to 40 hertz. Using the sliding mesh in simulation of pitching oscillation of the Muses-C capsule which decreases computational cost, study of multi-parameter on dynamic stability of this capsule and investigation of variation trend of dynamic stability in different conditions are some goles of this paper. Results of analysis of various parameters influence indicate that increasing of mean angle of attack from zero angle enhances dynamic stability. Furthermore, dynamic instability of the capsule is more critical at subsonic flow rather than supersonic flow. In addition, dynamic stability of Muses-C capsule increases by rising of oscillation frequency.

Rotary systems application in aerospace, power stations, automotive industries and many others is prevalent. Maintenance of rotating system based on traditional logics is very expensive in different industries. Therefore, intelligent fault detection of engineering systems, especially mechanical ones, is important and growing issues in the industries all around the world.
In this research which done on power transmission system in tail gearbox of helicopter, an intelligent fault detection system for gear wear in tail gearbox of helicopter represented. For designing an intelligent testing system, an experimental set-up consisted of tail gearbox of helicopter with its related shafts and a real fix support condition designed and developed. Simulated fault on considered system is pinion wheel input wear, which was created in three stages¡ various conditions of gearbox in intact and damage state was studied .In designing this intelligent fault detection system time-domain signal analysis, discrete wavelet transform, Principal component analysis and automatic decision making techniques such as k nearest neighbor recognition method used.

The aim of this research is to compute the allowable ranges of the uncertain parameters, affecting the torques and forces working on the tires of an articulated vehicle, to maintain stabilty. A seven-degree-of-freedom model of the vehicle is adopted, while it moves on a straight track. A nominal output feedback controller is designed for the model of the vehicle with nominal parameters, in order to attain acceptable performance in the presence of external disturbances.
Due the multi-linear structure of the characteristic equation of the linearized model of the vehicle, the polynomial method is used to compute the stability margins for the uncertain parameters. The computed margins are verified by plotting the root-locus of the closed-loop poles of the system when the parameters are perturbed inside the computed margins. Also, a more realistic model of an articulated vehicle is built in the environment of ADAMS software to verify the computed stability margins. It is observed that the computed bounds of parameter perturbations are relatively exact and the perturbations out of the computed ranges result in instability.

In this paper, the transient behavior of heavy duty gas turbines in the starting regime is analyzed and its characteristics are evaluated. Starting a gas turbine is quasi-transient process. So, the integral form of unsteady conservation equations and the ideal gas state equation are used in order to model the system. A new method is proposed to analyze the compressor performance of gas turbine during start-up condition. Compressor stages are grouped into three categories (front, middle, rear), which each of them have a different performance curve in low-speed. Also, the effects of the inter-stage bleed valves modulation are investigated. Finally, the dynamic behavior of a 160 MW MAPNA gas turbine is simulated during start-up condition. The simulation results are compared with the field data and a good agreement is between them. This research has shown the important rule of an exact estimation of start-up procedure (fuel, blow off valves, power of starter motor) for stable operation of compressor and, eventually, of the gas turbine.

This paper presents a new method to improve the lane tracking performance and to create automatic lane keeping system for Active Front Steering (AFS) vehicles. To achieve full tracking and stabilize the lateral position of the vehicle, a new approach based on the Fast Terminal Sliding Mode Control (FTSMC) is proposed which is responsible for reducing the rate of convergence and getting finite-time tracking control. Moreover, it is shown that the proposed controller is robust against vehicle mass uncertainties and external disturbances. In this research, to design a lane keeping controller, a lateral dynamic model of vehicle is firstly extracted based on three-dimensional motion equations. To verify the effectiveness of our method, an especial configuration for virtual prototyping is utilized. The results taken from co-simulation of ADAMS CAR and MATLAB show the efficiency of the control method to track the desired patch and to guarantee the yaw stability under uncertain conditions.

Fiber-Reinforced Polymer (FRP) composites are an important category of advanced engineering materials. These materials are widely used in the industry due to their unique characteristics such as high specific strength and stiffness. In FRP composite materials, there are several failure mechanisms such as matrix cracking, fiber breakage, fiber/matrix debonding and delamination. Delamination is the most striking one. In this paper, mechanical and acoustic emission (AE) behaviors of delamination are investigated under the actual operation loading modes (i.e. mode I, mode II and the combination of these pure modes). The composite laminates have been fabricated with 14 layers of woven carbon/epoxy prepregs. DCB, ENF and MMB specimens were prepared according to ASTM D5528 and D6671 standards and subjected to the different loading modes. First, inter-laminar fracture toughness of the specimens is calculated using the represented methods in ASTM D5528 and ASTM D6671 standards. Then, fracture toughness of the specimens is determined using the AE-based methods. The results indicate that, the AE-based methods have good applicability to determine the fracture toughness of the composite materials.

In this study the possibility of producing a part of tractor (tractor’s hood) with anisotropic low-carbon steel sheet, ST14, through hydroforming method has been investigated which was not possible through traditional deep-drawing method. It must be mentioned that this has been done both experimentally and through finite element simulation. Due to the shape of the work piece, two of them are placed beside each other to be formed through hydroforming process, then after producing they are cut into two separate halves. Because of the big size of the work piece, the smaller simulated scaled size has been used for doing the investigation. The simulation results were compared and validated to the experimental scaled work pieces. To reduce the production costs and increase the quality of manufactured pieces, the original dimensions of the blank optimized with sensitivity method at four stages. In addition, the influence of four pressure paths were examined on the quality of the produced pieces. The results show that through using optimal pressure paths and optimizing the shape of the sheet , the part can be produced with high quality and accuracy just in one stage.

In this paper, the performance of electric hot incremental forming process is investigated on the Ti-6Al-4V sheet by FE simulation and experimental method. Ti-6Al-4V titanium alloy sheet has extensively used in aerospace industries. Therefore, forming of this lightweight alloy is very important. Ti-6AI-4V sheet has poor formability at the room temperature and can be well-formed in high temperature. In this paper, the geometrical shape of final part was a cone frustum that obtained from forming of rectangular sheet, and the effect of processing parameters, such as wall angle, step size and tool diameter, on the forming forces were investigated. The results demonstrated that the forming force are increased by increasing of step size and decreasing of tool diameter. Also, to form parts with larger wall angle, the required force is lower. The process was simulated in the ABAQUS software to investigate the process using finite element analysis. The modeling was performed using local heat on the sheet. It showed a good agreement between the experimental and numerical results of forming force and deference between both of them is about 5.7%.

Geometric defects of the roll forming process are affected by amount and situation of the strains distribution. In this work, simulation of the round section roll forming process has been implemented using the finite element method. Effective factors on the strain distribution have been investigated. Roller diameter, distance between the stations and line speed of the sheet as input parameters and maximum longitudinal strain and strain distribution uniformity of across the sheet was considered as the response functions. Design of experiments was conducted using the central composite design (CCD). They were executed by finite elemet method and then values were extracted for response functions. The effect of input parameters to the function response was investigated using the analysis of variance. The results of tests were indicated and discussed by the main and interactions effects diagrams. The results demonstrated that according to the analysis of variance and main effect diagrams, with increasing the distance of stations and roller diameter, the maximum longitudinal strain is reduced. According to the analysis of variance On the other hand, the line speed of sheet and roller diameter affecte on the transverse strain distribution, but variation domain is very low. Also distance between the stations has not efficient effect on the transverse strain distribution.

In this paper, temperature distribution, melt flow and depth of heat affected zone (HAZ) in LASER welding of Ti6A14V Titanium alloy platen of 1.7 mm thickness have been studied. Due to high costs of practical LASER welding experiments, finite volume method was employed to predict the weld behavior on the specimen. Simulation is an superior method in finding optimized settings which reduces the costs as well. Fluid finite volume method and Open Foam software were employed in simulation. In order to verify the simulation results, experimental data obtained from weld geometry and temperature distribution were used. Buoyancy and Marangoni forces and boussinesq assumptions, were considered intently in simulation process. Moreover, thermodynamic properties were assumed independent of temperature and Gaussian heat source was employed for mechanical properties. Numerical results have good agreement with experimental results. The developed model can predict temperature distribution, melt flow in different parts of plate and melt penetration depth properly. This model can also be used for design and evaluation of welded parts.