فصلنامه مکانیک هوافضا
سال هفدهم شماره 3 (پیاپی 65، پاییز 1400)

Selective laser melting (SLM) is a laser powder-bed fusion method that offers great potentials in producing components with complex shapes and geometries. Process parameters like laser power and scan speed have significant effect on the induced temperature gradient which determines the molten pool dimensions and surface integrity. Due to the transient feature and fine dimensions of the molten pool, monitoring and measuring the induced temperature gradient and the pool dimensions are extremely challenging. In this article, a finite element model has been used to analyze the process and investigate the scanning speed and laser power parameters during the SLM process on a substrate of Ti6Al4V alloy. In this study, first the theoretical equations of laser have been investigated and after modeling, the accuracy of the modeled laser has been compared with the experimental model. After verifying the laser modeling, the FEM analysis of SLM has been carried out for various laser powers and scan speeds to study the effects of the mentioned process parameters. For the modelling, various physics assumptions have been simultaneously used in the software including heat transfer, solid to liquid phase changes equations, surface tension (Marangoni effect), and laminar fluid flow (Navier-Stokes equation) along with the gravity effect. Molten pool dimensions, temperature gradient along the laser moving heat source, width, and depth of the molten pool, as well as the occurrence of the balling effect phenomenon, have been studied separately in each case. The results obtained by FEM analysis have been compared with the experimental model, showing good compatibility.

In this paper, two control methods are presented to control a class of underactuated systems with two degrees of freedom. The design method of the proposed controllers and how to prove the stability of the closed-loop system using solutions such as backstepping control, sliding mode control, fuzzy logic and their combinations are presented in details. Mathematical proof shows that the closed-loop system under the proposed scheme has a global asymptotic stability in the presence of structural and un-structural uncertainties. Then, the advantages and disadvantages of the proposed controls are examined and then some criteria such as simplicity in practical implementation, the control input calculations volume, the adjustment of the control input coefficients, and the amplitude of the control input are used to make comparisons between the suggested controllers. Finally, to evaluate the performance of the proposed controllers, simulations are implemented on the cart-position system with a single-link inverted pendulum. The simulation results confirm the performance of the proposed solutions.

The layered silicates (clay) are being extensively used to improve the thermal properties of polymer-based composites, because of their nano structure. In this study, the modified Montmorillonite-based nano-clay with 2.5 and 7.5 weight percentages were distributed by a melt-mixing method in a phenolic matrix and a poly acrylonitrile-based carbon cloth was used as the reinforcement. The composite was prepared by a manual layout method and cured in an autoclave at 160 °C for 4 hours and the effects of nano-clay loading with 2.5 and 7.5 weight percentages on the crystalline structure and the thermal and mechanical properties of the carbon-polymer-layered silicate nanocomposites were investigated. The silicate galleries spacing of the layered silicate platelets were determined from XRD patterns. The dispersion of the clay layers in the resin was analyzed by TEM studies. The cone calorimetric tests were used to evaluate the flame- retardancy properties of the nanocomposites and the effects of tensile strength and short beam strength were investigated. The results showed that the d-spacing of nanocomposites is doubled due to 2.5 wt. % nano-clay loading. Also, the heat release rate (HRR) is decreased up to 36% by adding the layered silicates. The tensile strength and shear strength of nanocomposites are not influenced considerably by adding the nano-clay.

In this paper, two particle filters are proposed to estimate the state and control the attitude of a projectile with a moving mass actuator. Since the dynamic equations of a flying object with a moving mass actuator are nonlinear, a nonlinear estimator and controller are used. The first filter takes the angle measurement and mass displacement along with noise as observations and uses them to estimate the angle, the rate of angle, the mass displacement, and the velocity of the mass. In this research, the nonlinear predictive control problem becomes a dynamic optimization problem and then, at each time step, the best control signal is calculated online using the second particle filter in the finite horizon and applied to the system. The cost function used for the first filter is the error of the values measured and calculated by each particle. The cost function in the predictive control problem for each particle is the error of the angle tracking and the control effort. The simulation results for the performed scenarios show that the algorithm proposed for solving the nonlinear problem of controlling the attitude of a moving mass actuated projectile has a good performance in the presence of Gaussian and non-Gaussian noises. Also, due to the random nature of the problem, statistical analysis of the results is performed using Monte Carlo simulation.

The use of autonomous underwater vehicles (AUV) for the exploration and oceanology science has been a field of interest of several research centers around the world in the last decade. The inertial navigation system (INS) has commonly been used as the principal means of localization for autonomous underwater vehicles. The main drawback of using only the inertia navigation system is the escalating error in the estimated position and attitude due to the error in the output of the inertia measurement unit and its integration. Traditionally external sensors such as: GPS, acoustic transponders, Doppler velocity logs (DVL), or cameras have been used to aid the solution of the inertial navigator by constraining the errors in the estimated positions. These external sensors have several practical disadvantages, basically related to their reliance on external information. One source of information that can assist in the localization of the vehicle, without the need for extra additional external sensing, is the vehicle’s dynamic model. The model of the vehicle is capable of representing the attitude of the system according to the control inputs and the external forces acting on it. In this work, we follow a model-aided inertial navigation system (MA-INS) for Remus 100 AUV based on a 6-DOF non-linear dynamic model. The proposed navigation system utilizes the knowledge of the device dynamics through an experimentally validated mathematical model. The results show that the proposed navigation system improves underwater navigation capabilities for the systems that lack conventional aiding equipment.

In this paper, a novel cellulose piezoelectric material embedded on a cantilever beam is used to harvest energy from the structure and the effect of reducing the width of the beam is investigated. In addition, in order to obtain the maximum output power, the series and parallel connections of the piezoelectric layers are considered. First, an experimental work is carried out on a fixed-width energy scavenger beam, and the current, voltage and output power are extracted. Then the target beam is divided into two and three equal parts and connected in series and parallel, and the results are analyzed for each case separately. Next, an analytical study of the effect of frequency on the width reduction of the beam is considered. For the first time in the analytical work, the effect of adding a piece of piezoelectric layer that covers a part of the cantilever beam is considered in terms of natural frequency and output power. Finally, the amount of energy harvested from the unit beam is analyzed analytically and the analytical solution output is verified with experimental results. The results show that there is a good agreement between the analytical and experimental data.

For the modelling of marine structures, such as the oil columns and structures, the terminal and base of oil platforms and the towers surrounded by water usually a cantilever beam or rod is used. These models are subjected to random excitation and the response amplitude of these structures is of particular importance during design. In this paper, a nonlinear cantilever beam immersed in a fluid with a concentrated mass under a narrow band random excitation has been studied and its response has been analyzed. With the development of the harmonic balance method, the variance of the random system response has been determined and the phenomenon of jumping and random jumping has been investigated. The analytical solution and the numerical simulations show that in comparison with the deterministic response, the random excitation increases the amplitude of the random response between the bifurcation points of the beam. As the intensity of the random excitation increases, the steady response changes from a circular to a cyclic state. The results also show that the random jump phenomenon occurs in the area where there are three states for the system. Finally, the results of analytical and numerical modeling indicate that there is a very good agreement between the data and the results of these two methods.

The most important steps in the metal injection molding (MIM) process are the design and manufacture of the injection mold. By far the most influential factors in the quality of the part are the injection site in the mold and how the part is filled during the injection molding, which directly affects the quality of the final part. In this paper, a powder containing 316L stainless steel is used as the raw material, and the Moldex 3D software is used for the simulations. The injection time, the cooling time, and the keeping time parameters are considered fixed for all tests with the values of 2.5, 10, and 2 seconds respectively. The speed, pressure, and temperature of injection are also considered as effective parameters. The boundary layer method (BLM) is used to simulate the process. The 3D model of the part is designed with three gate paths and different injection conditions. The injection mold is made for the optimized sample and produced under different conditions. Comparing the results of simulations with the experimental results shows the accuracy of the simulation results. The results also show that in the process of MIM, the injection fluid has a very high viscosity and different behavior compared to the similar fluid of the base polymer used in the feed. Besides, the optimal values for speed, pressure, and temperature of injection are found to be 60%, 60%, and 185 ° C, respectively.