مجله مکانیک سازه ها و شاره ها
سال پنجم شماره 3 (پاییز 1394)

The necessity of being regular in plan and elevation of concrete tunnel form buildings in spite of accelerating the manufacturing process and high quality assurance, following several limitations in architectural design and puts limits on the application of this system in areas where there is no possibility of symmetrical construction. Loss of R-factor in the current earthquake regulations is a major challenge in design of these structures. In this study, the seismic behavior of two-tunnel form structures with 5 and 10 stories with irregular floor plans is investigated and the demand/ capacity behavior factor, based on seismic demand and capacity of structure have been calculated respectively. The most distinctive feature of this study is multi-level definition of the behavior factors and their extraction with respect to seismic intensity, and accepted damage level as expected performance levels in designing structure. Moreover surveying literature showed that the lack of adequate study of seismic characteristic of this type of tunnel form buildings. Also, uncoupled frequency ratios and fragility curves are determined for studied models by incremental dynamic analysis (IDA). The results show high capacity of the system and flexible torsional behavior of the structures due to their irregularity. Since both structures are placed in the immediate occupancy performance level in design earthquake, it seems that criteria which necessitate tunnel form structures to be regular in plan, are strict and prudent for the studied structures.

Actuators of robot operate in the joint-space while the end-effector of robot is controlled in the task-space. Therefore, designing a control system for a robotic manipulator in the task-space requires the jacobian matrix for transforming joint-space to task-space. Using an imprecise Jacobian matrix and the presence of uncertainties degrade the control performance. Uncertainties include the parametric uncertainty, unmodelled dynamics and external disturbance. In this paper, a novel decentralized adaptive fuzzy sliding mode control approach in the task-space is presented using the voltage control strategy. The control design is simple, free from model and robust agains uncertainties. These advantages are because of using voltage control strategy instead of commonly used torque control strategy. The case study is an articulated robot manipulator driven by permanent magnet DC motors. The stability analysis, simulation results and comparison with a torque based control method are presented to verify the effectiveness of the control method. Simulation results show the effectiveness of proposed control method.

The wind turbine blades are made of composite materials and subjected to severe operational loadings. The complexity of composite materials behavior along with variable amplitude loadings dictates the need for experimental setups which can conduct real part test as close as possible to in service loadings. In this regard, wind turbine blade manufacturers are obliged to perform standard ultimate and fatigue tests on their products. In this research a cost effective Blade Testing Machine (BTM) is proposed which is capable of conducting ultimate static and fatigue tests according to wind turbine blade standards. A new control unit is designed and implemented to track fatigue block loading in the frequency range of 1 to 3Hz. The main focus is on designing a controller to perform desired block loading fatigue tests with proper performance. PI and robust feedback controllers are designed and analyzed. Due to the poor robust performance, an adaptive feed forward controller is proposed based on the gain scheduling algorithm. The proposed robust controller compensates un-modeled high frequency dynamics and rejects measurement noises while the adaptive controller compensates low frequency uncertainty and improves reference tracking. Extensive experimentations in the desired frequency range confirm proper performance of the developed controller.

In this article, the high strain rate, dynamic plastic response of materials in the standard plate impact test is numerically studied. To get the closest results to the experimental data available in the literature, different types of hardening as well as dynamic fracture models are employed. With the aid of this standard plate impact test, one can verify the new models in the ideal condition of uniaxial-strain cases. To properly model this test, to discrete the field, von-Neumann Finite-Volume method is utilized. Jonson-Cook and perfectly elastoplastic hardening models as well as the Zerilli-Armstrong model are used beside the dynamic fracture model of modified Tuler-Butcher, to predict the spall phenomenona. In this work, the impact of two plates (the flyer plate and the target plate) is analyzed. Results of the simulation is compared with the experimental data as well as the other numerical results reported in the literature. The results of the present work are in a better correspondence comparing to the experimental data. Among investigated models, employing JC constitutive model, accompanying with the modified Tuler-Butcher fracture model and Steinberg model for the elastic modulus gives the most accurate results compared to other model combinations.

Target function in vehicle crash, is about the suitable deformation of vehicle body structure in the case of do not harm occupant’s space. As a consequence the vehicle body structure should be deformed to absorb the impact’s energy and decrease the transmitted forces to occupants. In this paper at design process the vehicle’s body in white has been simulated in CATIA software firstly, and then the element has been created by the Hyper mesh pre-processor software and finally in Pam Crash software, after introducing appropriate materials, standards joints and contacts between vehicle’s parts, and then loads has been created by initial velocity for able to deform barrier toward side panel of vehicle. The vehicle body structure is designed in Vehicle Research Center, based on the style of B-class vehicle platform’s product and the analysis results of this paper is part of new product development process of national platform’s project. As a result, the requirements out puts such as displacement, velocity, acceleration, and energy absorption and section forces have been extracted. After all the results has been compared with the targets which are predicted. Then the suitable thickness for B-pillar illustrated based on the out puts which are extracted.

In this paper, one-dimensional design of centrifugal compressor with inverse design of 90-degree bend duct between radial and axial diffuser is accomplished using an iterative method. In the design procedure, all overall dimensions of the centrifugal compressor including impeller, vanned radial diffuser, 90-degree bend duct and axial diffuser are computed. To evaluate the results, after the geometry modeling and grid generation, 3-D flow field in the whole compressor are numerically analyzed using CFX software. The numerical simulation shows that there is a high loss region through the 90-degree bend between the radial and axial diffuser because of its high curvature. In order to reach a minimum loss in the 90-degree bend duct, the aerodynamic design of the 90-degree bend duct is carried out using an inverse design method. At the first step, the shape modification capability of the 90-degree bend duct is evaluated by linking up the Ball-Spine inverse design algorithm and CFX software. Then, the geometry is modified by improving the hub and shroud pressure distribution and applying it to the inverse design code. Finally, the designed compressor with the modified 90-degree bend duct is analyzed. The results show that, the total pressure ratio and overall efficiency increases about 3 and 4 percent, respectively.

In this research, for the first time, the net of control points in the isogeometric analysis has been improved by employing an error estimator based on a stress recovery method. First, an error estimation algorithm based on stress recovery is used to obtain the energy norm for each element. Then, artificial rods are defined between control points and the estimated values of errors in the control points located at the vicinity of a typical point is assigned to each rod as a thermal gradient. Now, by analyzing this hypothetical truss problem under temperature changes a new arrangement of control points and consequently the knot vectors can be obtained. Repeating this process in isogeometric analysis will lead to a better distribution of errors in the domain of the problem and results in an optimal net of control points to calculate the integrals. To evaluate the efficiency of this method, the results of modeling and analysis of two elasticity problem is presented. The obtained results show that this innovative approach has a good performance and can be employed for reducing the analysis errors.

The present paper investigates the effects of grid configuration on the buckling and vibration behavior of grid structures. Hence, four similar simply supported plates with equal weights and different grid patterns (Ortho grid, Angle grid, Iso grid and orthotropic grid) are considered. Using, bending stiffness matrix, the buckling load and free vibration frequency of the plates are computed. To investigate the effects of the grid orientation on the mechanical behavior, the orientation of the grids is changed in the plates. The Rayleigh–Ritz method is applied to obtain the axial and shear buckling loads and free vibration frequencies. The results are compared with an angle-ply laminated composite plate with similar in-plane dimensions and equal weight. The results show, changing the grid pattern and orientation, will affect bending stiffness and consequently, significantly change the buckling and vibration behaviors of the grid plates.

In this paper, the stress intensity factor for a circumferential crack in a thick-walled cylinder is derived analytically and numerically which is subjected to the non-Fourier (hyperbolic) thermal shock. The uncoupled thermoelasticity governing equations for an uncracked cylinder are solved analytically. The weight function method is implemented to obtain the stress intensity factor. The non-dimensional hyperbolic heat equation is solved using finite Hankel transform and separation of variables method. Results show the different behavior of the crack under hyperbolic thermal shock. For relatively short cracks, the maximum stress intensity factor of Fourier and hyperbolic models is closed. But for longer cracks, the stress intensity factor of the hyperbolic model is significantly greater than Fourier model. Moreover, the maximum stress intensity factor in hyperbolic model occurs for a crack the peak of stress wave reaches to its tip. According to the results, assumption of adequate heat conduction model for structure design under transient thermal loading is critical.

In this paper a collaborative simulation between Matlab/Simulink and Fluent softwares is done to active control of an elastically mounted circular cylinder, free to move in in-line and cross-flow directions. The control goal is reduction of the two-dimensional vortex-induced vibrations (VIV) of cylinder. The natural oscillator frequency is complemented with the vortex shedding frequency of a stationary cylinder. A parallel simulation scheme is realized by linking the PID controller employed in Matlab/Simulink to the plant model constructed in Fluent, aiming at calculation of the control force necessary for total annihilation of the transverse cylinder vibrations. The simulation results reveal the high performance and effectiveness of the adopted control algorithm in diminishing the VIV of elastic cylinder. Once the control algorithm is turned on, there is a extreme reduction in the transverse and in-line cylinder oscillation amplitudes as well as lift and drag coefficients values. In particular, it is observed that the coalesced vortices in the far wake region of the uncontrolled cylinder are seprated and displaying wake vortices of weaker strengths.