مجله مهندسی مکانیک امیرکبیر
سال پنجاه و یکم شماره 3 (امرداد و شهریور 1398)

Interference fit process, as a popular method, is used in aerospace and automotive industries to improve fatigue life of the joints. Investigating studies on interference fit process demonstrates that the effect of temperature variations has not been studied as an effective factor on fatigue life. In the present study, the effect of short time exposure to thermal cycle before cyclic loading on fatigue life of interference fitted fastener holes have been evaluated by experimental and numerical methods. So, to investigate the effect of temperature variations two different temperatures (i.e. 60°C and 120°C), aside from room temperature, were selected to accomplish this study. Fatigue tests were carried out on the specimens of Al-alloy 7075-T6 to obtain S-N curves. In relation to fatigue tests, three dimensional finite element analyses were used to explain the experimental results. The interference fit process, and subsequent thermal cycles and remote cyclic loading were simulated using Ansys package. The results obtained from the finite element analyses of the interference fit were employed to predict the fatigue life.

Advances in Additive Manufacturing (AM) techniques have made the design, control and modification of bone scaffolds inner architectures and their mechanical properties possible. CAD bone scaffolds based on triply periodic minimal surfaces (TPMSs) have attracted attentions, due to their high surface area to volume ratio pore interconnectivity which enhance cell migration and attachment. The mechanical stimuli acting while fluid is flowing through scaffold pores can influence on proliferation, migration, differentiation and fate of mesenchymal stem cell. In the present study, the interaction between 2 TPMS-based bone scaffolds, termed G and I, with fluid in the presence 8.5 μm-cell layer (as mesenchymal stem cell accumulation) have been evaluated computationally. The results demonstrated that the scaffold G can modulate the cells more adequate due to producing homogenous distribution of mechanical stimuli comparing to scaffold I. The range of shear stress and von Mises stress for scaffold G are not wide which means the cells are sensing roughly the same mechanical stimuli. For both scaffolds in inlet velocities less than 50 μm/s, the magnitude of stresses is negligible. In addition, for scaffold I, there are dead zones which mechanical stimuli are approximately zero which prevents dynamic cell culture and homogenous signaling.

Dual phase (DP) steels contain of soft ferrite and hard martensite phases. The features of these steels are high strength and ductility that make them attractive especially in the automotive industries and related fields. In the past, researches were conducted to analyze the behavior of DP steels based on the micromechanical and macro mechanical simulation using two-dimensional RVEs . The researchers have analyzed the failure mechanism of DP steel using simulation of uniaxial tensile test based on 2D RVE. Analysis of 3D RVE can be considered for more detailed investigation of failure mechanisms and modification of 2D RVEs. In this paper, prediction of deformation pattern and damage mechanisms of DP steels and failure of specimens are performed at micro scale using finite elements (FE) method. Metallographic and SEM images at three different loads ((i) necking, (ii) after necking, and (iii) failure point) were used to investigate the deformation pattern and failure mechanisms at micro scale. On the other hand the effect of various factors such as 2D and 3D RVEs, mesh size, consideration of large and small deformation theories in FE analyses and also effect of volume fraction of martensite phase on the mechanical properties are examined.

The friction stir spot welding of Al 6061-T6 alloy is experimented to determine the behavior of micro-hardness, static tensile strength and fatigue behavior of the lap shear specimens by changing of rotating speed at three different speeds. Micro-hardness results show similarity in the regions far from shoulder indentation region. By the static strength and fatigue results, The optimal behavior of the rotational speed is determined. Therefore, the speed of 1000 rpm shows much better mechanical behavior than the other conditions of this research. The fatigue results of different welding conditions demonstrate, in the same cycles, more divergences in the lower cycles than, the higher ones. Two different failure modes have been observed at different load levels. At high load levels, final failure was nugget pull out. At low load levels, the final failure was as separation from the plate. At medium load levels, although the final failure was similar to high load levels, the growth of the crack in the sheet outside the stir zone, just like low load levels, was also observed.

In this paper, using modified couple stress theory, a new cylindrical shell element is introduced. Since classical continuum theory is unable to correctly compute stiffness and account for size effects in micro/nanostructures, higher order continuum theories such as modified couple stress theory have taken on great appeal. In this paper, using modified couple stress theory and using shell model in place of beam model, buckling of nanotubes is investigated via the finite element method. The new cylindrical shell element based on super element’s shape function defined and the mass-stiffness matrix has been developed. In addition to modified couple stress cylindrical shell element, the classical cylindrical shell super element can also be defined by setting size effects parameter to zero in the equations. In special cases, in order to investigate the application of the equations developed, the cylindrical nanoshell buckling is studied using modified couple stress cylindrical shell element and the results are validated using the analytical method. In addition, the effects of parameters such as size effects parameter, length, and thickness on cylindrical shell buckling are investigated.

In this study, an innovative method for crack identification in buckled plates using differential quadrature element method (DQEM) and sequential quadratic programming (SQP) method is proposed. This study consists of two steps. In first step, a numerical method is applied to calculate the frequencies of buckled cracked plates. Cracks are assumed to be open and modeled as linear rotational spring. Governing equations are extracted considering effects of shear deformations and initial geometric imperfection. Then, considering the solution as summation of static solution (postbuckling) and dynamic solution, the governing equations are converted into two different equation sets; postbuckling equations and vibration equations. The natural frequencies can be obtained solving these equation sets. In second step, SQP optimization method and the method used in first step combined to make a new method for identification of crack specification using natural frequencies. In this step, a weighted sum of square of differences between calculated frequencies and experimental frequencies considered as cost function and used to identify crack properties. Finally, the accuracy and precision of proposed method verified using some experimental and numerical case studies.

The aim of this study is an investigate the effect of design parameters and profile modification on static transmission error and load sharing factor of helical gears using analytical method with Matlab software. In the first step, we define the concept of transmission error that is the source of noise in the gear pair. Then load sharing factor and static transmission error are calculated using the gear mesh stiffness. In this paper the total stiffness of helical gear’s and the load sharing factor is determined using an accumulated integral potential energy method. In the next, results are verified and the effect of design parameters on avarege and pick to pick of the helical gears transmission error and load sharing factor is investigated. This parameter is called macro-geometric parameters of gear pairs. At the end, we write a Matlab code to modify the tip relif of tooth to optimize the load sharing factor and static transmission error. The results of this study show that the simultaneous correction of macro-geometric modifications and profile modification (micro-geometric modifications) cause a more uniform load distribution and significantly reduce the transmission error of the helical gear and consequently reduce the noise and vibration of the gearboxes.

In this work, using a three-dimensional constitutive model and implicit solution through a user defined subroutine (UMAT) in Abaqus software, mechanical behavior of shape memory alloy (SMA) cables and their constituents are investigated. Material parameters of shape memory alloy cables are identified by numerical simulations and available experimental data. Finite element (FE) method is first employed for analysis of an elastic steel cable and subsequently for a superelastic (SE) cable. The simulation results for these two steel and SE cables show good agreement when compared with experimental data which also validates the simulation approach. The wire rope is then simulated for shape memory effect (SME) cables and investigating mechanical behavior and several diagrams including normal stress, shear stress, strain and temperature for both superelastic and shape memory effect cables are presented. Moreover, utilizing the design of experiments method, shape memory effect cable is optimized to achieve the maximum specific energy. The method proposed in this study can be used for the design and optimization of shape memory alloy wire ropes.

Recently, to improve the mechanical properties of polymeric composites, the inclusion of various nano particles into the polymer matrices has received much attention. But, it is not well established whether nano particles can enhance the mechanical properties of fiber metal laminates (FMLs). The present study aims to investigate the effect of multi-walled carbon nano tubes (MWCNTs) on the flexural behavior of a FML made up of alternating layers of Al 2024 along with basalt fibers reinforced epoxy. Results showed that the flexural strength and flexural modulus of samples have an upward trend up to 0.5 wt. % loading, but beyond that a downward trend was observed. Therefore, in this research, the optimal content of MWCNTs for the best flexural properties was the 0.5 wt. % and comparing to the samples without MWCNTs, the flexural strength and flexural modulus values was found to 36.62 % and 60.16 % improvement, respectively. Also, Scanning Electron Microscopy (SEM) observations showed that the mechanisms of CNT-pull-out and CNT-bridging was the main reason for this findings.

In this paper, the free longitudinal vibration of single-walled coiled carbon nanotubes (SWCCNTs) with various boundary conditions (BCs) is investigated via Molecular Dynamics (MD) simulation method. Subsequently the detection of carbon nanotubes, their uses advantage to a wide range of engineering, biophysics and materials areas, because of their great mechanical and electrical properties. Heretofore vibration behavior of the single-walled coiled carbon nanotubes had not been studied with this technique, so using this method, longitudinal fundamental frequencies of single-walled coiled carbon nanotubes has been obtained by applying Reactive Empirical Bond Order (REBO) potential without considering thermal effects. In order to parametric study, the influence of the coiled carbon nanotubes diameter, number of pitch and boundary conditions on the fundamental frequencies is evaluated. The results indicated that increasing the tubes diameter and number of pitch (or length of single-walled coiled carbon nanotubes) lead to reducing the fundamental frequencies. Furthermore, the clamped–clamped single-walled coiled carbon nanotubes’s fundamental frequency is always higher than cantilevered one. The results of this study can be used in vibration analysis of the Nano sensor and Nano actuator with coiled carbon nanotubes elements.

In this research, free vibration analysis of functionally graded carbon nanotube reinforced composite (FG-CNTRC) conical shells subjected to high temperature environment is investigated. The material properties of FG-CNTRC are assumed to be graded through the thickness direction. Two kinds of carbon nanotube reinforced composites including uniformly distributed (UD), CNTs are distributed uniformly through the shell thickness, and functionally graded (FG), CNTs are graded with three different distribution, are considered. The effect of thermal loading is considered as an initial stress. Applying the Hamilton’s principle based on classic theory and considering Von Karman strain-displacement relation, the governing equations are obtained. The analytical Galerkin method together with beam mode shapes as weighting functions is employed to solve the equations of motion. The results are compared with those presented in literature. In addition, the effect of various parameters such as thermal loading, boundary conditions, and different geometrical conditions are studied. It is shown that the initial thermal stresses have significant effects on the natural frequencies and cannot be neglected. Moreover, the critical buckling temperature rise of the shells can be extracted from the presented diagrams of the fundamental frequency parameters verses the temperature rise.

In this paper, by using a semi analytical method the effect of dynamic vibration absorber on amplitude reduction of fluid conveying pipe is investigated. Considering the Euler-Bernoulli beam theory, the governing equations of motion are derived. By using the first four vibration modes of the fluid conveying pipe, the Galerkin method is applied to discretize the equations, and then the discretized equations are solved numerically. Moreover, simple approximate analytical expressions are proposed for prediction of the natural frequencies of the simply supported fluid conveying pipe and the dynamic vibration absorber parameters. After validating the results of the proposed method, some proper curves are plotted to characterize the effects of system parameters on reduction of pipe vibration amplitude. The results indicate that around the first critical fluid velocity, by using an appropriate vibration absorber, the vibration amplitude can be reduced by 80%. Therefore, this type of absorber due to its simplicity of installation and high energy absorption capacity, can be considered as a benefit method to reduce/eliminate unwanted vibrations of the fluid conveying pipes.

The goal of this paper is to study the large amplitude free vibration of wind turbine tower which modeled as a variable cross-section beam with eccentric mass. The effects of variable axial force due to gravity is also taken into account. The nonlinear governing equations of motion and the corresponding boundary conditions of the system are obtained using the Hamilton's principle as well as Euler-Bernoulli’s assumptions. Then a numerical finite difference scheme is utilized to find the natural frequencies and the mode shapes of the system. Using Galerkin method, the partial differential equations governing dynamic of the system are reduced to ordinary differential equations in terms of the end displacements which are coupled due to the presence of the transverse eccentricity. These temporal coupled ODEs are then solved analytically using the multiple time scales perturbation technique. The obtained analytical results are compared with the numerical ones and excellent agreement is observed. The results of this research can be used to study the effect of the eccentric tip mass, variable cross-section and gravity on the large amplitude vibration of wind turbine tower for improved dynamic performance.

With the development of urbanism and the airports, construction of highways around cities, noise pollution caused by the traffic of vehicles, landing and take off the airplane has become one of the most harmful effects on the habitants around the highways and urban areas. In order to prevent these damaging effects, or in other words, for reducing the transmission of the noises from streets to the residential buildings, it can be used from sound absorbing material around the highways. A number of 4 mixture designs have been investigated in this study in order to reduce sound transmission through concrete, including control sample and 3mixture designs of recycled rubber with sizes of 1–3 mm. The rubber has been used as a replacement of 5,10, and 15 percent of sand. First, 7,14 and 28-day strengths of concrete have been measured and then, sound transmission losses through the samples have been measured at the range of 63–6300 Hz by using impedance tube of the transfer function. The results indicate a reduction in the transmission of sound in samples containing rubber at high frequency ranges, which, given the reduction of environmental losses, makes use of this type of concrete in airports and freeways.

The complexity and cost of solid fuel engines cut-off demands a comprehensive method for guiding these missiles so current required velocity methods can not solve this problem. This paper presents an implicit cut-off insensitive guidance scheme for ballistic missiles. The main idea behind this scheme is to correct the nominal flight path angle, at arbitrary time with respect to value of disturbances and uncertainties such as wind, thrust misalignment, thrust value, aerodynamic coefficients, to satisfy the nominal burnout time conditions. Compared with preset scheme, the proposed scheme is more robust to motor performance uncertainty. The circular error probability of proposed method is calculated about 1.242 km which is 61% less than preset method's circular error probability. It is also simpler and has a lighter calculation load. It is shown that the proposed algorithm has good performance through the computer simulation.The circular error probability of proposed method is calculated about 1.242 km which is 61% less than preset method's circular error probability. It is also simpler and has a lighter calculation load. It is shown that the proposed algorithm has good performance through the computer simulation.

Rehabilitation and assistive robots have drawn a large amount of interest, related to the increase of the elderly and the increase in diseases such as stroke and spinal cord injuries as well as the high cost of rehabilitation. In this paper, a fractional order adaptive fuzzy terminal sliding mode control is proposed for a knee joint orthosis. A model integrating the human lower-limb and orthosis based on the Lagrange equations is used. To overcome the uncertainties and external disturbances, a fractional order terminal sliding mode control is designed, then in order to remove the undesirable chattering phenomenon in control signal, a fractional order adaptive fuzzy controller is designed. To improve the precision and speed of tracking and to decrease the effect of the uncertainties in muscular torque modeling on the system control, a nonlinear disturbance observer is combined with fractional order terminal sliding mode control. The stability of closed loop system is proved by new fractional order extention of Lyapunov theorem. The PSO algorithm is used to determine the coefficients of the adaptive fuzzy terminal sliding mode control and the fuzzy membership functions. Finally, the performance of the proposed controller is compared with conventional sliding mode control and PID control.

In this paper, active flutter control of a swept wing with an engine is carried out. The aircraft wing is considered as a uniform swept cantilever beam carrying an engine. The piezoelectric layers are attached to the wing to control the vibrations. To simulate aerodynamic loads, the Theodorsen model is used. The equations of motion are determined via Hamilton’s variational principle and are transformed to a set of ordinary differential equations through the assumed mode method. Lyapunov controller is used to control the system. Effects of design parameters like engine trust, location and mass and wing sweep angle, are evaluated on the flutter speed, and the control system has been applied at the flutter situation. Results show that the control system can substantially suppress the vibration in investigated cases. According to the results, the length of the piezoelectric layers affects the speed of the flutter and the flutter speed increases by increasing the length of these layers. Also, according to the influence of the Lyapunov gains on the performance of the system, it is necessary to select these values carefully to control system has the best performance for the different values of the system parameters.

Feasible sets and robust feasible sets have indispensable role in a priori stability guarantee of the constrained systems and in model predictive control. This article presents two algorithms for generating these sets for constrained linear time-invariant systems. Because the conventional algorithms for generating these sets must be applied iteratively over time, they are incapable of dealing with systems having the input vector constructed in any domain other than the time domain. The new algorithms, presented in this article, remove this limitation by treating the input vector monolithically over the time horizon. The presented algorithm for computation of the robust feasible set is capable of incorporating disturbances as well as parametric uncertainties that can be formulated as polytopes. For the verification, the results of the proposed algorithms were compared with results of the conventional methods in a similar circumstance. Finally, examples are presented to compare the computation times of the proposed algorithms with the conventional ones and to illustrate the effect of input vector parametrization, employing orthonormal functions, on the feasible region and the robust feasible region. Results showed that the parametrization improved the feasible set and robust feasible set.