مجله مهندسی مکانیک امیرکبیر
سال پنجاه و پنجم شماره 10 (دی 1402)

The maneuverability of a quadrotor or octorotor UAV is limited in the standard configuration ‎because the force vectors of the propellers are parallel and only have four active degrees of ‎freedom. Therefore, they lack the controllability of six independent degrees of freedom. This ‎study designs a novel configuration for an octorotor capable of hovering with roll or pitch angles in ‎a specific position, contrary to UAVs with a standard configuration that can only hover in a ‎horizontal position. In other words, in this octorotor, orientation tracking is also added to the ‎octorotor's targets in addition to position tracking. The proposed model can be controlled by ‎altering the velocity of the eight rotors and the tilt angle of the four arms. Such alterations in ‎velocity and tilt angle are such that they can provide the aerial vehicle with most optimum ‎maneuverability. After deriving the proposed dynamic octorotor model, a controller is proposed ‎using neural networks (NNs) and reinforcement learning (RL), capable of controlling the proposed ‎octorotor with six independent degrees of freedom. Finally, trajectory tracking, octorotor position, ‎and controller robustness to possible motor malfunctions are examined, and numerical simulation ‎results are provided.‎

In this article, the piezoelectric energy harvester of cantilever type with the ability to be angled relative to the vertical direction, and also with rectangular fins attached to the end of the harvester has been investigated. The purpose of this study is to investigate the effect of design parameters such as the angle of inclination of the energy harvester, its distance from the wave source, the depth of the beam and the presence or absence of fins at the end of the beam, on the effective voltage of the energy harvester output. Low efficiency is the most important limitation for the advancement of piezoelectric energy harvesters, and for this reason, it is necessary to use methods based on tests and optimization with the aim of increasing the efficiency of piezoelectric energy harvesters. In order to carry out the experimental study, experiments have been designed using the central composite design (CCD) and experimental model analysis and design parameters have been optimized using the response surface methodology (RSM). Also, the effect of equipping the energy harvester with a fin on the final voltage has been investigated. It was observed that the optimal conditions for both finned and non-finned harvesters occurs when distance ratio of the beam from the wavemaker is minimum and the depth of penetration ratio is maximum.

In this research, two new superhard metallic carbon allotropes αC28 and βC32 are predicted using density functional theory (DFT). These stable tetragonal structures belong to the P4/MMM space group. Molecular dynamics simulation performed under canonical ensemble (NVT) in order to investigate the thermal stability of new αC28 and βC32 carbon crystals at temperatures of 300 and 1000 K, confirms their thermal stability. In addition, we calculated mechanical coefficients and band gap energy of these two structures to examine their mechanical and electronic stability. These new carbon allotropes are composed of sp2 and sp3 bond hybridization, which shows excellent mechanical properties with Vickers hardness of 45.7 and 47.9 GPa. Other mechanical properties of these crystals such as bulk modulus (265.8, 284.9), shear modulus (254.7, 273.5) and Young's modulus (579.1, 621.6) also confirm the superhardness of these structures. The results related to the electronic band structures indicate that both structures have metallic properties. The width of both conduction and valance bands for both structures is about 20 eV. The results of calculations show that αC28 and βC32 can be synthesized in laboratory in the future and will have potential applications in mechanical and electronic devices.

Many parameters,including scanning strategy,speed,power,etc,have an impact on the mechanical and physical properties of the parts made by the selective laser melting process.Correct and optimal selection of these parameters prevents defects such as porosity,incomplete fusion gaps and cracks in the parts of the manufacturing process.Despite all the advantages of the selective laser melting process,the creation of defects directly affects the mechanical properties and fatigue resistance of the parts negatively. In this research,the impact of zero,45,and90degree build orientations,the effect of changing the size of the parts on mechanical properties such as tensile and shear strength,fracture strain,and the number of defects investigated.The built parts were subjected to tensile and shear tests,and the fracture area was examined using SEM and the existing defects identified.The tensile test results showed that larger samples have higher tensile strength than smaller samples.Moreover,higher tensile strength and lower fracture strain were observed in samples made in zerodegree orientation.The results of the shear test also showed that the strength of the shear stress for the small and large samples made in all orientations is almost the same,and the highest shear strain of failure is related to the samples made in the orientation of 45degrees independent of the dimensions.The SEM results also showed that the amount and distribution of spherical holes and fusion gaps in large samples made at90degrees is more than in samples made at 0and45degrees

Nowadays, the utilization of smart materials is experiencing rapid growth across a wide range of industrial systems. Magnetorheological elastomers are a class of smart materials and possess two unique features, i.e. adjustable hardness and damping capabilities. These characteristics make them widely used in various industrial applications. Hence, understanding their behavior in different systems is necessary. The focus of this work is to study the dynamic behavior of magnetorheological elastomers under different static pre-strains in tension-compression mode. In this study, three isotropic samples were fabricated and the force-deflection features of them were acquired under harmonic excitation with various strain amplitudes, static pre-strains, frequencies, and magnetic flux densities. Assess the effect of static pre-strain on the dynamic response of the magnetorheological elastomers, studying the effects of other parameters like strains, frequencies, and magnetic flux densities on the dynamic modulus of magnetorheological elastomers, and proposing a novel phenomenological-based model to predict the viscoelastic behavior of magnetorheological elastomers are the innovative aspects of this study. The results showed that the dynamic modulus of magnetorheological elastomers will increase by superimposing the static pre-strain. Furthermore, the relative MR effect decreases when the static pre-strain is applied. The maximum relative MR effect of 288.3158% have been achieved at a strain of 4%, a frequency of 7 Hz, and a static pre-strain of 0%.

In the current research, finite element simulation and experimental tests have been used to design, manufacture and evaluate the performance of a high-power ultrasonic radiator. The two main goals in the design are to achieve a nominal resonance frequency of 20 kHz in longitudinal mode shape of the transducer and booster assembly and the flexural mode shape of the radiator plate and to remove the disturbing modes from the frequency range of the main mode shape. After designing and manufacturing the sample based on the simulation results, experimental tests of modal analysis, impact test and impedance analysis were performed to evaluate the performance characteristics of the ultrasonic radiator. In order to verify the accuracy, the simulation results, including the resonance frequency and position of the node and anti-node, were compared with the experimental test results. The results of the experimental tests to determine the resonance frequency and compare it with the simulation results (error less than 0.5%), indicate the accuracy of the prediction of the results and the matching of the resonance frequency with the designed nominal value. Also, the disturbing mode shapes were at an acceptable distance from the main flexural mode shape of the radiator. Finally, the vibration amplitude measurement test results show that the position of the node and anti-node points in the experimental test match with the finite element simulation results.