مجله مهندسی مکانیک امیرکبیر
سال پنجاه و سوم شماره 10 (دی 1400)

In the networked control system, time delay and data dropout can degrade the system performance or even cause instability. Predictive control is one of the effective methods to reduce the effect of time delay and data dropout in networked control systems. In the proposed predictive control method, the control data is predicted as long as it covers the time delay and data dropout in the network, and the estimation of state variables is performed using the Kalman filter. By sending a package containing the predicted control data to the actuator and selecting the appropriate control data, the desired control performance of the system can be achieved. In this paper, the networked control system is considered as a switching system with an arbitrary switching signal. A criterion for evaluating the stability of the closed-loop system with time delay and data dropout is presented based on the theory of switching systems. Time delay and data dropout are selected as switching parameters and a subset of switching dynamic systems is created in the discrete state space. By providing the Lyapunov function for all subsets and solving matrix inequalities, the stability of the system can be investigated. The simulation results show the effect of data dropout on the stability of the system by considering the time delay in the forward and feedback channels.

Landing on Mars is one the most important space missions undergoing various system and environmental uncertainties. Subsequently, an exact model to represent the dynamic system cannot be provided in advance. In this regard, the present paper has focused on designing an integrated estimation and control algorithm bringing accurate navigation in the presence of uncertainties for the landing problem. The proposed algorithm has been shaped based on the variable structure control framework. This algorithm alleviates the requirement of Jacobian via substituting statistical linearization for the analytical one and utilizing generalized matrix inverse theory. Performance of the proposed algorithm has been investigated via Monte Carlo simulations in the presence of different uncertainties including stochastic initial conditions of the coupled Mars lander dynamics, atmosphere instability and modeling errors of the exerted forces and moments, time delay of actuators, geometric constraint of the landing site as well as the saturation limitations of actuators. In addition, the obtained results have been compared to the results obtained from combination of well-known extended Kalman filter and PID controller. This study clearly shows the precise and robust performance of the proposed integrated estimation and control algorithm and proves its superiority against existing widespread algorithms.

Nonlinear phenomena widely exist in engineering applications. A common example of this phenomenon in aerospace structures is the creation of fatigue cracks that open and close continuously under dynamic loads during the vibration cycle, causing a bilinear behavior in the structure. Late detection of such cracks can lead to a catastrophic failure. Therefore, identifying the nonlinear behavior of the cracked structure is very important for the prevention of structural failures. In this study, the nonlinear vibration behavior of a cantilever beam with a transverse breathing crack with bilinear behavior has been studied. For this purpose, we first estimate the polynomial function equivalent to the bilinear function of the beam stiffness. Then, using the well-known perturbation method, the method of multiple scales, the nonlinear equation of the beam is solved, and the frequency response equations for both harmonic and superharmonic resonances are extracted. Then, the sensitivity of the responses to the crack parameters such as depth, location, excitation force, and damping coefficient are investigated. The beam frequency response curves in the main resonance have become highly nonlinear due to the increase of the crack parameters and causes softening of the curves. Also, according to the results of the beam vibrations in superharmonic resonance, it was observed that the behavior of the beam in superharmonic resonance has a higher sensitivity to the existence of the breathing crack.

The present paper investigates the influence of higher-order boundary conditions caused by accounting the small scales effect on free vibrations of bi-directional functionally graded thick conical micro-shells. The present model accounts for the gradation of the material length scale parameter as one of the micro-shell mechanical properties along its thickness as well as its axial axis. The modified couple stress as well as the first-order shear deformable love shell theories together with the Ritz method are employed to obtain the eigenvalue eigenvector equations governing on the free vibrations of the micro-structure. These equations are solved for some different types of boundary conditions. The present findings are compared and successfully validated by the available results in the literature. The influences of small scales, higher-order boundary conditions and power law distribution indices in both the transversal and axial directions on free vibrations of conical micro-shells are then investigated. The results reveal that higher-order boundary conditions play a crucial role in dynamics of conical micro-shells especially when these boundary conditions directly affect the eigenmodes which are dominant in the dynamics of the structure. In addition, it is observed that although the dynamics of the present conical micro-shell is affected by the power law distribution indices in both the transversal and axial directions, it is more sensitive to the transversal one.

In this research non-linear vibration behavior of viscoelastic Euler-Bernoulli beam under the influence of external fluid flow has been studied. The governing equations of motion are obtained by assuming non-linear strain-displacement relations and considering the interaction between structure and fluid. To consider more realistic hypotheses, contrary to previous researches, the effect of viscoelastic behavior has been considered using a more complete and realistic model called the Standard linear solid model. After non-dimensionalizing the motion equations, the governing non-linear differential equations are discretized using the Galerkin method. Then, the system's analytical response is acquired through the method of Multiple Time Scales. After verifying the results and confirming the analytical method's accuracy with the numerical solution results, different parameters' effect on the system's dynamic behavior has been analyzed. The results indicate that the viscoelastic behavior and the non-linear model significantly affect the lock-in area and the maximum amplitude of the viscoelastic beam oscillations. The maximum vibration amplitude for the third type's viscoelastic coefficients occurs in the fluid velocity range u = 1, equal to 0.127. Furthermore, the lock-in area is created for the beam with viscoelastic coefficients of the first type in the velocity range u = 0.85 and the second type's viscoelastic coefficients in the speed range, u = 0.62.

First row rotating blades of four axial-flow compressors were prematurely fractured. Previous investigations showed that the site atmosphere contains corrosive compounds which lead to an increase in possibility of pitting on the blades. To this end, experimental and numerical studies are considered. Replica testing, Scanning Electron Microscope (SEM) and fractography of the broken blade indicate that the pits join together and make one bigger pit under SCC mechanism which reduces the failure time. 3-D models of the pitting on the blade under existing forces are analyzed by COMSOL Multiphisics software. The simulation results show the location of maximum stress concentration inside one of the pits which is compatible with the location of initial SCC crack in the pit depth. Finite element analysis shows good similarities with fractography photos. Stress concentration and interaction of stresses around the pits are two mechanical reasons for initiation and growth of cracks. Calculations show that the occurrence of SCC at the location of the pit reduces the crack initiation time to half. The presence of pits increased the stress by approximately 130 MPa relative to the healthy blade. The part between the two pits with a stress of approximately 180 MPa showed the interaction of the two pits in the operating conditions of the compressor blade.

In this research, the fracture toughness of O-notched diagonally loaded square plate (DLSP) samples with pre-existing cracks is investigated theoretically and experimentally under pure mode II loading. These specimens are made of the stainless steel 316L, which is highly ductile and has great strain-hardening. For theoretical prediction of the fracture toughness of desired specimens, the Fictitious Material Concept (FMC) is employed. By using FMC, complex elastoplastic failure analysis can be avoided and by performing linear elastic analysis, the fracture toughness of DLSP specimens can be estimated. Based on the assumptions of FMC, the stainless steel 316L can be replaced with a virtual brittle material having linear elastic behavior. Then, by coupling FMC with mean stress, generalized mean stress, maximum tangential stress, generalized maximum tangential stress, strain energy density and generalized strain energy density criteria, fracture toughness of the cracked DLSP samples are estimated. To verify the estimated fracture toughness, several fracture tests are performed on DLSP specimens. Experimental observations reveal that these specimens experience significant plastic deformations at the onset of crack propagation. It is shown that combination of FMC with six linear elastic brittle fracture criteria is quite successful in predicting the crack growth instance in ductile DLSP specimens.

In this research article, the effect of zeolite nanoparticles on the tribological properties of high molecular weight polyethylene, which are widely used in orthopedic implants, has been studied. Improving the tribological properties of this polymer is one of the medical industry challenges which has an important effect on the life-time of orthopedic implants. Nanocomposites based on ultra-high molecular weight polyethylene (UHMWPE) blend, containing 2 to 6 phr of nano-zeolite, were prepared via melt compounding followed by injection molding. The morphology was studied using scanning electron microscopy. The wear and temperature of specimens as well as friction coefficient were characterized by employing a pin on disk test under 50 N and sliding velocity of 0.5 m/s. The wear rates, temperature and friction coefficient of nanocomposite containing 4 phr of nano-zeolite, were 56, 32 and 26%, respectively, lower than those of neat UHMWPE. In contrast, the application of 6 phr of of nano-zeolite, led to agglomeration and increased wear, temperature and coefficient of friction compared to nanocomposite samples. In addition, the morphology of nanocomposite samples, after testing, revealed a smoother surface with the mild abrasion marks than that of of pure UHMWPE sample.

Additive manufacturing (AM) technology also called rapid prototyping (RP) is a novel manufacturing process and several methods cared out this technology. One the most popular AM technique called Laser Powder Bed Fusion (LB-PBF). Several parameters involved in this method and 4 of most important factors are Laser Power (LP), Scanning Speed (SS), Infill Pattern Angle (PA) and Hatch space (HS). Change in this parameter has a direct effect on defects and fabricated parts quality. Post processing treatment such as heat treatment cared out in order to improve part property and applications. Built time and costs reduce significantly by suitable choice of process parameters and post processing treatments. In this article Genetic Algorithm (GA) cared out to highest relative density and lowest surface roughness and best value of each parameter presented. The results shown that there are number of situations corresponds best relative density and surface roughness according to the pareto front diagram, and there are several choices in a specific neighborhood. The best results could achieve by using of 102-105 wat of laser power, 623-630 mm.s-1 scan speed, 73-76 µm of hatch space and 638-640 °C of heat treatment temperature.

In this paper, the isogeometric analysis of shell structures along with the optimal method for accurate calculation of nodal vectors is proposed. The Nurbs surface with maximum regularity is used for shell mid-surface description. According to the Reisner- Mindlin hypothesis, the director vectors at control points are needed for the interpolation of the rotations. The calculated nodal direct vectors must lead to exact interpolated director vectors on the shell surface. Hence, a method has been proposed in which the components of director vectors at control points are obtained by solving a system of equations on the whole patch. The system of equations is formed using known values of direction vectors at the Greville points. The accuracy of the proposed method has been investigated by using the results of the most common problem in shell analysis. Convergence behavior for displacement at the loading points has been studied in all solved problems for different order of NURBS and net of control points. The deformation results show better convergence behavior with increasing the regularity and order of NURBS. The knot averages are in a one-to-one correspondence with control points. Thus, the system of equations on these points leads to a unique solution for the nodal direction vectors, and the time to solve equations is significantly reduced.

Several members in some structures tolerate torsional moment. Therefore, comprehending the torsional behavior of these members is essential. Investigating the torsional behavior of thin-walled sections and simple sections is possible by analytical and common numerical methods. The analysis and design of some structures with special sections (e.g., trapezoidal sections) in specific industries make it impossible to calculate these sections' responses using common methods. Therefore, the development of novel methods as alternative approaches seems very necessary. Due to the difficulty of analytical solution with asymmetric domains, semi-analytical and numerical methods are the most desirable alternatives. One of the proper methods for solving the boundary value problem is the vibrational methods. Moreover, the Kantorovich quasi-analytical method, an extension form of the Rayleigh-Ritz method, is an appropriate method for solving problems due to the lack of limitation in selecting the primary function for estimating boundary conditions. Therefore, the purpose of the present study is to develop the Kantorovich method to solve the governing equation of the torsion problem and estimate the warping and stress field of the arbitrary trapezoidal sections directly. Finally, the solution is compared with other existing methods to evaluate the accuracy of the Kantorovich method. The results indicate high precision and rapid convergence of this quasi-analytical method.

Continuous drug infusion plays an important role in drug effectiveness. However, in most cases, the size, weight and power consumption of the conventional pumps are among the most important factors that cause a lot of problems for the patient comfort. The present work aims to design and optimize a Magneto hydrodynamic micro pump for continuous drug infusion. A mathematical model of Magneto hydrodynamic micro pump is proposed and solved analytically to investigate its feasibility for drug infusion. For the patient's comfort, the micro pump is optimized using non-dominated sorting genetic algorithm II. The number of channel rows and columns, channel height and width, and driving voltage are chosen as decision variables for multi-objective optimization. The Pareto front of the optimization result is presented. Six possible cases that meet the desired specifications are selected using a fuzzy decision-making approach. A computational fluid dynamic model is adopted to predict bubble formation due to the electrolysis phenomena. With higher reliability without any mechanical part, the present design can deliver drug flow 48 times while its driving voltage is 3 times lower than a conventional micro pump. In addition, it provides potentially better reliability and a simple fabrication process without any mechanical part.

Fiber metal laminates (FMLs) are a new class of hybrid materials that consist of thin metal sheets and fiber reinforced epoxy composite layers. Due to their significant characteristics, FMLs have been widely employed in a variety of engineering applications specifically aerospace industry. In this paper, the lateral-torsional buckling behavior of thin-walled FML beams with varying I-section is perused using an innovative and accurate methodology. Considering the coupling between the bending displacements and the twist angle, the system of lateral stability equations are derived via energy method in association with Vlasov’s model for thin-walled beam and the classic lamination theory. By uncoupling the equilibrium differential equations, the system of governing equations is transformed to a fourth-order differential equation in terms of the twist angle. The differential quadrature method is then applied to solve the resulting equation and to acquire the lateral buckling loads. The accuracy of the proposed methodology has been investigated by comparing the results with the outcomes obtained using ANSYS finite element software. In the following, the effect of significant parameters such as stacking sequence, fiber angle, fiber type, web tapering ratio, load height parameter and volume fraction of metal on lateral buckling load of fixed-free FML tapered I-beam under uniformly distributed load has been investigated.

This paper presents an analytical and computational approach for investigating the behavior of a simply supported carbon nanotube-reinforced composite shell that has been exposed to temperature variations. The equations are solved using the Ritz energy system for the analytical solution and ABAQUS finite element software for numerical solution. The displacement field is the first-order shear deformation theory, and the linear equations were solved using the rule of mixture to determine the mechanical properties of carbon nanotube-reinforced composites. A uniform distribution of temperature with no heat flux in the shell and nanotubes in five distinct shapes classified as V, A, X, and O have been considered in this study. The support conditions are the same in all cases, but the temperature, thermal boundary conditions, and carbon nanotube volume function values vary. The findings are illustrated in a detailed manner in the form of diagrams, which perfectly demonstrate the differences between both of the carbon nanotube distribution material models. Validation of the results shows great compatibility in energy solution and finite element methods. The results show that increasing the volume function of the carbon nanotubes increases the stress values and thermal gradients, and on the other hand, reduces the displacement, and by increasing the temperature the number of stress increases.

Plastic injection is one of the most important processes in plastic parts production. During this process, many defects can take place that affect products quality. This research focuses on decreasing one of these defects, called Blush. For this purpose, eight parameters with the probability of affecting the occurrence of this defect have been considered to be investigated. To study the effect or lack of effect, the extent, and the manner of effect of these parameters on the Blush defect, a Fractional factorial design of experiment with eight factors and two levels was performed. Then after finite element analysis in MoldFlow software and validation of analysis with experimental tests, the area of defect that occurred in each case was calculated by geometric methods. After ANOVA of the data, it was found that the parameters of runner diameter, holding pressure, flow rate, and melt temperature respectively, have the most significant effects on this defect. Then, for the four factors mentioned, an experimental design of Composite Cube Design was performed and after performing finite element analysis and analysis of variance on the data, it was found that all four parameters have a significant impact. It was also found that by increasing melt temperature, and holding pressure and reducing the runner diameter and flow rate to the optimal level, the area of blush defect will be reduced by 82.2%.