فصلنامه مکانیک هوافضا
سال نوزدهم شماره 1 (پیاپی 71، بهار 1402)

An elaborate numerical study with a validated LS-DYNA® immersed boundary method fluid-solid interaction code is used to characterize the influence of pre-detonation pressure and time duration on plastic deformation of thin steel triangular plates subjected to gaseous detonation. Other objectives of this numerical simulation such as estimation of deflection and stress contour of material at high strain rate are derived based on a strain-rate dependent Johnson-Cook material model. Simulation relies on the modeling of detonation by chemical reaction kinetic and its propagation by Conservative Element Solution Element (CESE) solver. Immersed boundary method is used to simulate the interface motion between the detonating gas and the deforming plate to facilitate the assessment of fluid pressure distribution on the plate surface. The numerical tool relates the pressure distribution and gaseous detonation parameters to the plate macroscopic deformation by employing multi-species reactive Euler’s equations for the gas and Lagrangian equation for plate. Numerical method as an appropriate tool in the evaluation of the deflection profile of the triangular plate shows that deflection decreases by the smaller size of the exposed area of the plate.

The purpose of this study is to provide a method to prevent unwanted vibrations in environmental tests of random vibrations of spacecraft. Unwanted vibrations in many environmental random vibration tests will lead to the hidden fault (hiding failure) that would be happened in operational conditions in a space mission. Hence, in this research, first, a method called the power spectrum density notching method was presented to prevent unwanted vibrations, and then, by using the obtained analytical relationships, environmental random vibration tests of a sub-orbital spacecraft were performed. After the test, the recorded results by the fixture and payload's accelerometer sensors have been analyzed. In this analysis, the performance of the power spectrum density notching method was examined. The obtained results indicate, although the power spectrum density notching method is a simple, practical, and operational method but can be an efficient method to prevent unwanted vibrations to spacecraft and also prevent damage to spacecraft during testing.

The use of common adhesive methods in metal-composite bonding leads to weak joints that are separated with the least load. The purpose of this research is to investigate the increase in the tensile strength of aluminum metal to glass fibers reinforced with epoxy resin using a new combined method that includes the advantages of both adhesive and mechanical (pinning) methods. In this method, the pins embedded in the structure are used as an interface for load transfer, and the aim is to investigate the amount of strength changes with different types of interface pins. Therefore, the pins are passed through them during the layering of composite fibers and then the curing process is completed. Therefore, a part of the metal structure has penetrated between the warp and weft of the composite and it is expected that the strength will increase. Four connection models without pin (pure adhesive), pin with a diameter of 1.2 mm, pin with a diameter of 1.6 mm and pin with a claw with a diameter of 1.2 mm have been evaluated and the results of the strength of the connections have been compared. Tensile test results have shown that this type of connection has a significant increase in strength compared to the sample without pins, and in three types of connections with different characteristics, 335%, 609%, and 421% increase in strength has been observed, respectively. Also, failure occurred gradually in samples with pins and suddenly in samples without pins.

This paper presents an investigation into the process parameters and conditions of friction stir processing for aluminum alloy 6061. The effect of process parameters including speed, feed, tool penetration depth, and reinforcement particles on the strength, hardness, and corrosion behavior of the samples are investigated. Tensile test, fracture mapping, and surface hardness measurement, as important mechanical properties, are performed using broken samples. The obtained results show that adding alumina particles in micrometer size with friction stir processing improves the final tensile strength, hardness, and corrosion resistance. The results show that the corrosion behavior of the processed samples is improved by reducing the corrosion rate, corrosion current, and corrosion potential compared to the base metal. The microstructure of the processed samples after the corrosion test shows that samples with micrometer-sized alumina reinforcement particles have fewer holes than others and lead to a more polished and smoother surface.

This paper presents distributed training approach for a nonlinear and heterogeneous multi-UAV system to solve a safe and optimal formation control problem. The objective of control is to ensure safety while achieving optimal performance. In this regard, the position and attitude controllers are considered in series. First, the optimal formation control design is defined as the optimal performance in position control and is modeled by the cost function. In this article, with the integration of cost functions and local control barrier functions (CBFs), a novel distributed optimization problems are introduced. Existing the local CBF in the augmented cost function ensures the safety of the position control, and as a result, collisions do not occur along the path of UAVs. The proposed method considers the safe and optimal position controllers by solving unconstrained optimization problems instead of constrained ones. In the next stage, the reference attitudes are driven by virtual position control. The attitude tracking optimal control is considered the optimal performance in the attitude control, and the related cost function models it. Finally, the stability and safety of the proposed controllers are proven. These optimal and safe policies are obtained sequentially using off-policy multi-agent reinforcement learning (MARL) algorithms which do not require knowledge of UAVs' dynamics. The proposed algorithms are validated by simulating the formation control problem of 6 UAVs with collision avoidance constraints.

Lamb waves are mechanically guided waves that propagate through plates and shells, and their speed depends on the frequency. Nowadays, researchers use these waves to detect defects in structures. This is due to the properties of Lamb waves that can propagate in the whole structure, and are quickly affected if there is a defect. Using this method instead of traditional methods due to their complexity, cost and time are desired. As an innovation in this research, the effects of lamb wave propagations in symmetrically variable stiffness fiber-metal laminated plates are investigated and for this purpose, the finite element method is used. In this analysis, curvilinear fibers are used in the composite layers instead of the straight fibers, and the effect of the number of layers, plate dimensions, and boundary conditions on the propagation of the mentioned waves are investigated. Also, to check the validity, the first three frequencies of the structure are compared with several different references. The obtained results show that the Lamb waves are propagated along the length of the plate and are reflected by hitting the boundaries of the plate or defect in the structure and lose their uniform propagation state. Based on this investigation, this method can be used to detect the defect in variable stiffness fiber-metal laminated structures.

Having been developed in accordance with the Montreal Protocol, R134a has been proposed as a potential replacement for R12. In accordance with the Kyoto Protocol, the use of R134a, which has a significant global warming potential, must be restricted. It has been stated that there is no one refrigerant or combination available that can address both the ozone depletion potential (ODP) and the global warming potential (GWP) concerns at the same time. According to the research, the goal of this effort was to produce an environmentally friendly refrigerant combination with minimal ODP and GWP values that perform virtually identically to R12 in terms of performance. With extremely low ODP and GWP values, R406a has the potential to be a useful refrigerant. Experimental investigations conducted on R406a refrigerant have shown that it may be a viable replacement for R12. In this research, the pressure drop created by the torsion in the steam condensation of R-406a inside the horizontal tubes and also changes in the heat transfer coefficient are investigated experimentally. In each experiment, parameters such as mass flow, refrigerant temperature and pressure, and water at the inlet and outlet of the condensers were measured. Examination of the results for annular and torsional fins pipes, it was determined that heat transfer and pressure drop increase with the reduction of the pitch. The use of spirals increases the average heat transfer coefficient and pressure drop by about 47% and 220%, respectively, compared to the pipe without fins. Finally, using the results from the twisted tubes and based on the relationships that best match the experimental results related to the pressure drop, a new relationship for the pressure drop in the twisted tubes was obtained. The values calculated by it are in the range of approximately 15% of the experimental values.

In this paper, the influence of the geometric parameters of a thrust measurement stand in mN level and for electrical thrusters on its performance is investigated. To measure the thrust of the electrical thrusters, a system must be used that separates the thrust force from the weight force. Thus, thrust measurement systems are designed based on a pendulum. The geometric parameters of a thrust measurement stand, such as the length of arms, the mass of different parts, and the stiffness of pivots affect the overall performance of the system, including its dynamic and static response (for example, displacement, natural frequency and stabilization time of oscillations). For this purpose, primarily, the analytical model for the behavior of a thrust measurement stand with an inverted pendulum configuration is obtained. Then, using the obtained results, the optimal design of several stands is carried out based on the specified physical requirements and using the Genetic Algorithm method. Finally, an experimental thrust measurement stand with a maximum measuring thrust of 100 mN is developed based on obtained specifications from the optimization and by validating the results of the analytical model, the error of the model is estimated to be less than 6 percent.

In this paper, the inverse dynamics solution for feedforward control of cooperative flexible manipulators is investigated. The internal dynamics of flexible manipulators are unstable, and to obtain a bounded solution to the inverse dynamics problem, the constrained nonlinear optimization method is used. In the optimization method, the aim is to minimize the elastic energy of the manipulators despite several constraints. These constraints include: 1) dynamic equations; 2) Spatial and force trajectory; 3) kinematic constraints limiting the movement of manipulators; 4) constraints related to superfluous variables and 5) constraints of the generalized α method for the stability of the solution. The method used for dynamic modeling is based on the Lagrange equation and finite element discretization. Lagrange multipliers have been used to control the internal forces applied to the payload, and to prevent the change of direction in force control, an inequality constraint has been added to the optimization constraints. This method is implemented on flexible cooperative manipulators and has the ability to control the path of the payload and the force applied to it.

An active vibration control algorithm and robust integral sliding mode control (SMC) are discussed to stabilize the attitude of the flexible spacecraft under external disturbances and actuator faults. As a coupled rigid-flexible dynamical system, the flexible spacecraft is modeled as a rigid hub with two solar panels equipped with piezoelectric (PZT) sensors and actuators. A passive fault-tolerant integral sliding mode control algorithm using a nominal proportional-derivative control algorithm and an improved fault-tolerant algorithm with time-varying additive fault is developed to increase system’s performance, prevent the system's flexible modes excitations in the phase of reaching the sliding surface. Therefore, when the system enters the sliding mode, the closed-loop dynamic behavior, including actuator faults, will be identical to that of the system without faults. It is possible to reduce the residual vibrations caused by the attitude dynamics and actuator faults by simultaneously activating the strain rate feedback (SRF) vibration control algorithm during the maneuver. The performance of the proposed integral fault-tolerant control in terms of the flexible modes excitation, the control effort, and achieving the desired attitude parameters in a comparative study demonstrated its advantage and superiority over the conventional integral sliding mode algorithms.