فصلنامه مکانیک هوافضا
سال نوزدهم شماره 4 (پیاپی 74، زمستان 1402)

In this research, the buckling behavior of covalently functionalized graphene with nylon 6,6 in vacuum and aqueous environments is investigated employing the molecular dynamics (MD) simulations. The critical buckling force and strain of functionalized graphene are computed and the effects of weight percentage, different distribution patterns and attachment configurations on these values are investigated. Graphene is demonstrated to have very small critical strain and force. By covalent functionalization, the critical force of functionalized graphene increases which is more considerable in the presence of water molecules. Moreover, it is found out that critical strain is not as sensitive as critical force to the presence of water molecules. Also, by increasing the weight percentage of functional groups, the critical force increases. By contrast, the critical strain reduces by functionalization and the critical strain of functionalized graphene reduces as the weight percentage increases.  The results of this study can be used as the benchmark for the graphene-based nanocomposites.

the missile guidance and control system consists of three subsystems: navigation, guidance, and control. These sub-systems are responsible in the calculation of the deviation of the guided vehicle from the desired path so as to determine the appropriate acceleration command to compensate for the deviation and tracking the acceleration and direction of the missile towards the target. In the usual methods of designing the guidance and control system, each of the guidance and control subsystems is designed separately, assuming ideal subsystem. In the integrated guidance and control approach, the guidance law is developed separately and tested with the assumption of an ideal autopilot. The autopilot is also designed independently and is tested with the assumption of ideal guidance law. This article describes the process of designing and simulating the function of proportional, integral, derivative and optimal fuzzy controller, which is created in order to guide the missile in a two-dimensional problem of minimizing the impact time and the distance to the target. In the optimal fuzzy hybrid controller, Mamdani type fuzzy controller parameters (including input and output membership functions, fuzzy reasoning rules, and input and output gains) are set by solving an optimization problem. Next, the parameters of the proportional-integral-derivative controller of the parallel type are also determined by solving the non-convex optimization problem and it is shown that this type of controller with optimal parameters will provide an optimal guide to the missile.

In this paper, the effect of multi-walled carbon nanotubes (MWCNTs) on the ballistic limit of fibers metal laminates (FML) Exprimentaly investigated. For this purpose, the MWCNTs were added with weight percentages of 0.2, 0.4, and 0.6 to pure epoxy resin and homogenized by mechanical and ultrasonic homogenizers. Then the FMLs were fabricated by fiber glass, 2024-T3 aluminum alloy sheets, pure epoxy resin, and modified resin with MWCNTs using a hand lay-up process. In the end, Ballistic tests on the samples were conducted by using a conical nose projectile. The experimental results show that the ballistic limit of FMLs is increased by adding MWCNTs. Also highest in this increase was observed in samples containing 0.4 weight percentages of MWCNTs, but in the 0.6 weight percentage, agglomeration of nanoparticles caused a reduction in the mechanical properties. The microstructural investigations using Electron microscopy show that the addition of MWCNTs improves the interfacial adhesion between the epoxy matrix and the reinforcing fibers.

In this paper, the attitude control design and analysis of a spacecraft as a rigid body based on three controllers of linear, nonlinear based on feedback linearization and backstepping is presented. According to the global presentation of the attitude based on quaternion parameters, these parameters have been used to derive the dynamic equations. Global asymptotic stability of linear and backstepping controllers is proved based on the Lyapunov method. The closed-loop stability of the feedback linearized controller is also proved by showing there are no internal dynamics. The controller gains are determined in linear and backstepping controllers based on linearized dynamics, derived from the local linearization around the equilibrium point. While, in the feedback linearized controller, gains are determined based on the global linearized dynamic equation. The performance of these three controllers in different scenarios is compared to each other. The results show that the feedback linearized controller can satisfy accurately the desired rise time. Whereas, the maximum error in achieving the desired rise time is 17% and 22% for backstepping and linear controllers, respectively. Of course, the control effort for the feedback linearized and backstepping is 100% and 46% more than the linear controller, respectively.

In this research, the performance deterioration caused by the degradation of the main gas path components is simulated for the IGT25 industrial gas turbine. To this aim, after developing a thermodynamic model of IGT25 and verifying the model with real data it is applied to simulate the performance deterioration of the gas turbine. Considering the fouling and erosion as two common gas path faults, these two faults are simulated by deviating the flow capacity and isentropic efficiency and their effects on the performance parameters in the power turbine speed control mode are investigated. The results reveal that the occurrence of fouling on the compressor blade in the investigated control mode leads to a decrease in the gas generator turbine speed, fuel flow, compressor exhaust temperature, gas generator turbine inlet temperature, power turbine inlet temperature and exhaust gas temperature contrarily an increase occurs in air mass flow, compressor exit pressure and power turbine inlet pressure. According to the results, the maximum deviation from the normal condition due to fouling and erosion of the compressor blades observed in the exhaust gas temperature, which is 3.8% and 2.2%, respectively. It is found that the operating conditions of the gas turbine affect the amount of deviation of the performance parameters when a gas path fault occurs.

This study deals with the design and experimental implementation of an adaptive feedback linearization controller for a flexible joint lever-arm (FJLA) in the presence of uncertainties and external disturbances. An extended state observer is proposed to upgrade the reduced-order model to an accurate and reliable model with the same order. In the proposed method, the uncertainties and external disturbances are assumed as an extended state and the link position information is used to estimate this state in combination with the angular velocity of the link states. The control system designed based on the proposed observer can adapt itself to real conditions, and use enough information about uncertainties and disturbances. The proposed controller with and without using the observer is examined on a fabricated flexible-joint lever arm. The results indicate that the uncertainties and disturbances are well estimated online for being used in the controller. Therefore, the proposed adaptive controller using just one sensor has a higher accuracy in controlling the position of the FJLA under different trajectories. Also, the proposed control method is fast and suitable for online implementation.

One of the main challenges of using robotic arms in various industrial applications such as: production and assembly line, medical and surgical centers, space industries and military instruments is the lack of accurate modeling and control of the systems. In this paper, the problem of robust control based on the suboptimal estimator for highly nonlinear dynamic systems affected by systemic and environmental uncertainties is addressed. Considering the coupled electrical-driven and mechanical subsystems in modeling leads to a completer and more realistic model known as the electrical flexible joint robots (EFJR). The state-dependent Riccati equation estimator is used to determine unknown state variables that cannot be measured by sensors. By applying the proposed approach in simulating a two degree-of-freedom (DOF) arm with electrically flexible joints as a practical case study, both robustness and optimality are obtained for the system. Then, the proposed method is compared to the sliding mode control and the Kalman filter estimator. The obtained results indicate that the proposed method has improved the system robustness against uncertainty and disturbance. The norm of final error of the robot End-effector has been obtained as 4.13 mm and 37.02 mm in the proposed algorithm and Kalman filter method, respectively. Also, the norm of control input (energy consumption) has been obtained as 7.5 and 16.8 by the two methods, respectively. Therefore, the proposed method provides the possibility of achieving to the goal with a higher accuracy and less control effort.

In this article, the thermal buckling and post-buckling of functionally graded plates with a matrix of isotropic polymer reinforced with graphene platelets (GPLs) have been investigated. In these multilayer plates, the thickness of all layers is the same, and by changing the weight fraction of reinforcing graphene platelets in each layer, the type of layer arrangement of nanocomposite plates changes. The plates are reinforced with three functional graded distributions: X and O and a uniform U distribution. The governing equations of the plate are obtained with the help of the first-order shear deformation theory (FSDT). The thermal buckling behavior of plates has been studied by the differential quadrature method. To find the effective Young's modulus, the Halpin-Tsai modified micromechanical model is used, and the law of mixing is used to obtain the equivalent properties of composites. The results show that the distribution of graphene platelets with an X arrangement improves the resistance of the plate against buckling. Also, the effect of parameters such as the weight fraction of graphene platelets, aspect ratio, and length-to-thickness ratio has been investigated.

This paper deals with the robust-adaptive backstepping control of heterogeneous self-driving vehicle groups in the presence of actuator fault, model uncertainty, and external disturbance concerning group speed constraints. The actuator fault is a combination of descending control law and the actuator disturbance. A third-order dynamical model is utilized to describe the longitudinal motion of each vehicle in which the engine time constant is unknown and the external disturbance is considered. The communication structure is assumed to be bi-directional leader-following. The control design is performed in three levels: speed level, acceleration level, and the final level. At the first level, the error is defined as the difference between the actual position and the desired position of each following vehicle. After that, by employing the Lyapunov theorem, a virtual control law is introduced to make the tracking error bounded. In the second and third levels, the error respectively is defined as the difference between speed and acceleration and the virtual control law of the previous level. Finally, a Lyapunov function involving the state errors of all levels and the estimation errors of the third level is defined and an adaptive control is introduced such that the tracking error and the estimation errors will be bounded. Numerical results are provided to show the merits of this method.

In this paper, oscillatory behavior of carbon nano-onions inside single-walled carbon nanotubes is investigated. To this end, using the continuum approximation along with the Lennard-Jones potential function, analytical expressions are derived for the evaluation of van der Waals interaction force and potential energy of system. Based on the Newton’s second law and ignoring the frictional forces, the motion equation is solved numerically and the time histories of displacement and velocity of oscillator are obtained. Moreover, in order to determine the oscillation frequencies of system, a semi-analytical expression is derived based on the conservation of mechanical energy principle. The proposed expression of frequency is dependent on both geometrical parameters and initial conditions. Using this expression, a comprehensive study is performed on the oscillatory behavior of carbon nano-onions inside single-walled carbon nanotubes by varying system parameters. Numerical results indicate that the generated frequency of this type of nano-oscillators is in the gigahertz range. It is further observed that the escape velocity as well as the maximum oscillation frequency decrease as the number of layers of carbon nano-onion increases.