فصلنامه مکانیک هوافضا
سال هجدهم شماره 2 (پیاپی 68، تابستان 1401)

In this paper, governing equilibrium equations for thin and thick-walled cylindrical shells based on the classic membrane and bending theories of shells using the first order shear deformation theory (FSDT) and the third order shear deformation theory (TSDT) in the three dimensional theory of elasticity for simply supported boundary conditions according to the buckling mode shapes for axial, circumferential and radial displacement field and subjected to axial and uniform external pressure buckling load have been derived. Then, the set of governing buckling equations of the shell for the axial, circumferential and radial displacements are numerically solved to obtain axial and circumferential critical buckling load. Moreover, to verify obtained results the shell subjected to axial buckling load has been modeled in ANSYS finite element software. The accuracy of obtained result to predict the critical axial load and external pressure buckling loads of the thin and thick shells have been compared with each other and the ones reported in the literature.

In the present study, the dynamic response of rectangular steel plates under repeated impulsive loading was investigated. In this regard, experiments were performed on three different structures: unstiffened, stiffened with one and two welding lines. To investigate the deformation modes and the failure mechanism of the experimental specimens, dynamic loads in a wide range were applied up to 3 loads by 25, 35, 45, and 50 g charge masses. Experimental observations demonstrate that with increasing charge mass, the Mode Ia and Mode Ib are observed in a higher charge mass and loading repetitions in a higher number. For the unstiffened plate, at the 3rd load with a mass of 25g, a change Mode Ia is observed, however, the same deformation mode occurs for the stiffened plate with a single and two weld lines at the 3rd blast load by 35g and 45 g charge mass, respectively. These observations indicate the effect of the weld line and its numbers on the variation of failure modes. Furthermore, in the numerical modeling section, the Group Method of Data Handling (GMDH) neural network was used to present a mathematical model based on dimensionless numbers to predict the maximum permanent deflection of tested specimens. Good agreement between the proposed model and the corresponding experimental results is obtained and all data points are within the ±10% error range for every two patterns.

Rollover which causes a large number of deaths and pecuniary damages is one of the basic challenges in vehicle safety. If we can decrease the possibility of rollover in some conditions, probably other important indices are decreased like vehicle stability. It means that some indices are improved through the change of parameters but some of them are not. For this reason, to improve rollover and stability indices simultaneously, combined Topsis-Taguchi and Shannon Entropy methods were used in this paper. First, a whole vehicle as a dynamic model was chosen and to verify the most important parameters, 13 parameters in 3 levels including geometric and volumetric parameters were verified. 27 series of tests through the Taguchi method in Minitab software were prepared and evaluated in Carsim software for obtaining rolling angles and yaw rate around the Z axle. The six most important parameters in the previous level were distinguished and again 25 tests were conducted through the Taguchi method in 5 stages in Carsim Software and multi-objective optimization was conducted. Results showed the suggested method in 80km/h made rollover index, rolling angle, lateral acceleration, and yaw rate around Z axle be decreased as %28.9, %26.2, %0.3, %0.2, respectively. Again, the tests were conducted at different speeds and it was found that with increasing speed, the improvement percentage of all indicators decreases.

Proportional Navigation is commonly used in the terminal phase of homing missiles. To implement this guidance law, it is necessary to measure or calculate Line of Sight (LOS) rate. It is usually necessary to use gimballed seekers to measure the LOS rate. However, if the system is equipped with a strapdown seeker, the LOS rate must be calculated from derivation or estimation methods. Because the signal measured by seekers usually contains noise, so deriving this signal requires a low pass filter that will cause the behavior changes in the measured variable. The issue in this paper is the design of a discrete time sliding mode extended state observer to estimate the LOS rate and evaluate it in the guidance loop. This is done by performing computer simulations. Implementing continuous time observers in processors has challenges such as sampling time selection and it is better to design the observer in discrete time form from the beginning so that its implementation issues can be considered from the design level and in computer simulations.

Polymer-based adhesives undergo creep deformation under constant loading due to their viscoelastic nature. The aim of this study was Investigating the effect of 2-D defects on tensile and creep behavior of single-lap ceramic-metal joints (SLJs) manufactured with adhesive Aqua-Flex. Static tensile test was performed at three ambient temperatures, 40 and 60 ° C for flawless and defective adhesive joints in ceramics as well as defects in ceramics and aluminum, and then by applying final stress-to-strength ratios equal to 0.40 and 0.60, tensile creep test has been performed. By increasing the final stress-to-strength ratio from 0.40 to 0.60 at ambient temperature, the creep displacement for faultless connection is 36%, for defective connection in ceramic 33% and for connection defective in ceramic and aluminum 40%. Find. At 40 and 60 ° C, this increase is 35% and 18% for defective connection, 54% and 5% for defective connection in ceramic and 39% and 42% for connection with defective connection in ceramic and aluminum, respectively. Also, with increasing temperature from ambient temperature to 60 ° C, the breaking force for the three types of connections is reduced by 46%, 25% and 80%, respectively.

Physical systems often contain constraints and limitations. Actuator rate saturation is one of these limitations that has adverse effects on the flight control system which increases the oscillating response and thus the instability of the system. Actuator rate saturation is one of the main reasons for pilot-induced oscillations (PIOs). This paper proposes a model predictive control algorithm with two degrees of freedom structure to prevent the destructive effects of actuator rate saturation on the flight control system. Also, by obtaining starting point of the opening loop position in the flight system and the frequency at which the actuator rate saturation is activated for the first time, the saturation effects are well predicted. Then, by introducing an analytical method similar to one of the conventional methods of robust control known as loop shaping method, proposed model predictive controller parameters are adjusted and the stability and robustness of the closed-loop system against the uncertainty of the controller parameters are investigated. In this method, by obtaining robust patterns, an attempt has been made to shape it in such a way that disturbances and noise are suppressed in a frequency range. It also shows how the controller and pilot work by using a switch. Finally, the designed controller is compared with the proportional-integral-derivative controller (PID) that is common in the industry and its advantages are shown.

Shape memory alloys (SMAs) are a special kind of smart materials with an intrinsic shape memory effect. This characteristic brings up SMAs as an actuator in dynamical mechanisms. These materials can recover their initial length under special thermal operations. This paper deals with fuzzy-PID control of contrary mechanisms actuating with a pair of SMA wires. The Brinson model combining with the Elahinia’s conditions are employed to describe the phase transformation models. By combining the Newton second law, transformation models, heat transfer and strain relations, the governing equations of the contrary mechanism is derived. Due to nonlinear behavior of SMA actuators, the non-model-based controllers are preferred compared with the model-based ones. Therefore, the fuzzy-PID control method is employed to control the position of the closed-loop system. The fuzzy rules are defined such that the coefficients of the PID controller are tuned according to the system response. Both numerical and experimental results are provided to illustrate the effectiveness of the proposed approach. It will be shown that under the proposed method, the tracking error will converge to zero asymptotically.

In this paper, an adaptive fuzzy extended state observer is proposed to estimate the states and external disturbances simultaneously for Single-Input-Single-Output nonlinear affine systems. The observer gains are time-varying and adjusted using an adaptive law. The Takagi-Sugeno fuzzy system is used for modeling that, unlike Mamdani methods, provides a more precise and comprehensive analysis. The proposed adaptive fuzzy observer is designed to relax the limitations of the extended state observers and improve system performance as compared to the classical methods in presence of time-varying disturbances. Moreover, the stability of the proposed method and the convergence of the estimation error are analyzed using the Lyapunov stability Theory. The performance of the proposed method is shown in simulations of control of the inverted pendulum. The simulation results, as compared to the non-adaptive fuzzy observer, show better performance in terms of transient and steady-state responses, control input amplitude, and robustness in presence of measurement noise and external disturbances.

In this study, the effect of the ratio of different nanoparticle compositions on the viscosity performance of the base oil is investigated. The aim of this study is to achieve the characteristics of optimal nanolubricants. The experiments are performed at temperatures of 5-55°C, volume fractions of 0.05-1% and shear rates of 665.5-10664s-1. Experimental results showed that nanofluids have a non-Newtonian and quasi-plastic behavior. The maximum viscosity reductions are obtained for MWCNT / Al2O3 (10%: 90%) - 10W40 and MWCNT / Al2O3 (40%: 960%) - 10W40 at -8.13% and -10.85%, respectively. The results show that MWCNT / Al2O3 (10%: 90%)/10W40 nanofluids have better lubrication performance at engine start (lower oil viscosity) and engine movement (more controlled behavior) than competing nanofliud. Using the response level method, to predict the target response data, a normalized three-variable-three-degree model with the characteristics of the power transfer function, λ = -0.15 and a constant value equal to zero is presented. Margin of deviation is in the range of -"2.72" %<MOD<"2/66" %. More viscosity sensitivity also occurred at higher volume fractions.

In this research, the free vibration of rotating tapered Timoshenko beam with piezoelectric layer has been studied. It is assumed that the beam is made of Functionally Graded Materials (FGM) through the thickness direction and the boundary condition is a cantilever attached to the hub. The first-order shear deformation theory has been used to drive governing equations. At first, the total energy of the system such as potential and kinetic energies for the beam and piezoelectric layer has been derived, and then the natural frequencies of the beam have been determined by the Ritz approach based on minimizing the total system energy. After verifying the results by comparing them with other research, the effects of some parameters such as hub radius, rotational speed, taper ratios, rotary inertia, material gradient, piezoelectric voltage, and beam thickness on the natural frequencies of the tapered Timoshenko beam have been studied in detail. The results showed that with increasing the angular velocity of the beam, the natural frequency increases so that as the increasing velocity increases, the natural frequency increases with a steeper slope.

In the analysis of many offshore structures such as oil columns and structures, oil rig abutments and towers surrounded by water, the single-beam model is usually used. These models usually bear the weight of a concentrated mass, and the response amplitude of these structures is of particular importance during design.In this paper, a nonlinear beam immersed in a fluid with a concentrated mass under the effects of the harmonic flow of water has been studied. Using the harmonic balancing method, the response of a nonlinear beam with three nonlinear terms is determined in the first four modes. Examination of the frequency response by analytical solution and numerical simulation shows that the jump phenomenon occurs in the triple response zone between the bifurcation points. The jump phenomenon is hardening in the first mode and softening in the second to fourth modes. Each of the nonlinear sentences has different effects on the vibrational behavior of the system and the jump phenomenon. The behavior of the beam in the state space along with the time response and Poincaré mapping shows that the path of the phase curve has different stability points. Finally, the phenomenon of chaos in the nonlinear beam was studied. At the bifurcation points, the behavior of the system is chaos, and the geometric nonlinear sentence has the most effective effect on the response disorder.

Additive manufacturing is one of the advanced manufacturing processes that is being developed nowadays due to the capability of manufacturing parts with complex structures, speed, and low production cost. 3D printing of shaped memory parts in the form of 4D printing is an emerging phenomenon, which is referred to as the future of additive manufacturing. In this study, the effect of three-dimensional printing fusion layer parameters including layer thickness, print speed, and nozzle diameter on the rate of shape recovery of Polylactic acid was investigated. To reduce the cost and time as well as to increase the validity and accuracy, the Central Composite Design method, one of the subsets of the Response Surface Methodology, was used. Seventeen experiments were performed and a model was obtained to determine the effect of 3D printing geometrical parameters on shape recovery. R 2 and Adj R2 of the model were obtained above of 0.99 and 0.98, respectively, which indicate the high accuracy of the model and the similarity of laboratory data and experimental model. This model showed that the layer thickness, nozzle diameter, and printing speed have a greater effect on the amount of torsional shape recovery, respectively. This process was simulated to investigate the shape recovery behavior. As a result, the relationship between the recovered shape torsion angle and the applied pre-stress was obtained. To validate the model, the effect of 3D printing parameters on the shape recovery, and the relationship between the pre-stress and the rate of shape recovery,...