مجله مکانیک سازه ها و شاره ها
سال یازدهم شماره 6 (بهمن و اسفند 1400)

Flexible roll forming process is a novel method for producing profiles with variable cross-section. In this process, to produce the profiles with variable cross-section, the position of the rolls is controlled at any time and moved in accordance with their geometry. In this paper, using finite element simulations and experimental tests, the deformation length is investigated as one of the important parameters in the flexible roll forming process. For this purpose, a finite element model was created in Abaqus software, and then the accuracy of the model was confirmed by comparing the simulation results and experimental data for longitudinal strain in the compression and stretching zones of a channel profile with variable cross-section. The results showed that the deformation length in the flexible roll forming is changed not only in the different zones of the profile, but also in different areas of each zone. The highest deformation length was obtained at the middle of the stretching zone, which is 38% longer than it at the middle of the narrow and wide zones and 48% longer than it at the middle of the compression zone. It was also found that with increasing the forming angle and flange length of the profile, the deformation length increases in all zones of the profile with variable cross-section.

Dynamic behavior of a horizontally supported Jeffcott-rotor system vial four electromagnetics poles is investigated. The magnetic force has a highly nonlinear relation to the control current and the air gap in the active magnetic bearing. The nonlinearities due to the centrifugal force, electromagnetic force, and the rotor weight are considered in the rotor model. The lateral oscillation of the rotor is controlled using a state feedback control system. The vibration of the rotor is modeled by a second order nonlinear ordinary differential equations with quadratic and cubic nonlinearities. The oscillatory characteristics of the considered system is investigated using the multiple time scales perturbation method assuming weak nonlinearity and soft excitation. Numerical simulations are carried out to validate the accuracy of the analytical results. The results show high efficiency of the applied controller velocity gain in forcing the considered nonlinear system to respond as a linear one with a single periodic attractor.

In this article, a new method is proposed to design robust dynamic output feedback control for switched uncertain continuous-time linear parameter varying (LPV) systems. The system matrices are supposed to depend on both the time-varying scheduling and uncertain parameters. Hysteresis switching law is exploited for the switching controller synthesis. Firstly, the robust switching controllers are robustly designed so that the stability and the induced L2-gain performance of the switched closed-loop uncertain LPV system can be guaranteed. The usual switching logics can cause discontinuous chattering control signal which are not acceptable in a practical situation. Accordingly, a smooth switching strategy for control design has been tackled in the following. By utilizing an independent family of parameter-dependent Lyapunov matrices and slack variables, performance improvement is achieved when compared to the previous works. The proposed method is formulated in terms of solutions to a set of parameter-dependent LMIs using the presented iterative algorithm. Finally, an inverted pendulum on a cart is illustrated to verify the advantages of the presented approach.

In this study, non Newtonian fluid flow simulations are obtained through a tri-cone drilling bit. F-3 drill bit of size 7-7/8 inch is used for pressure drop calculations. The drilling bit contains three nozzles of equal size. Nozzle sizes of 7/32, 9/32, 10/32, and 11/32 inch are selected for simulations and each solution considered three nozzles of equal size. Eight drilling fluids of different density and rheological properties are selected and Bingham rheological model is employed in this study. Rheological data were obtained using a Fann 35 viscometer. To validate the results, calculated pressure drops from flow simulations are compared to experimental data and Eckel-Bielstein equation. The results show that flow simulations have predicted the pressure drop for all nozzle sizes with an absolute average percentage error of 9.66, whereas Eckel-Bielstein equation has an absolute average percentage error of 17.16. A new equation is proposed to calculate the pressure drop in the tri-cone bit for Bingham rheological model. The proposed equation considers effects of rheological parameters on the pressure drop. Comparison of results with experimental data revealed that the proposed equation can predict pressure drops with an absolute average percentage error of 8.31.

The explosion process is a sharp increase in volume and a sudden release of energy, usually accompanied by an increase in temperature and the release of gas. The phenomenon of underwater explosions is different from explosions that occur on the surface of the earth due to the characteristics of water. In the process of explosive forming, the choice of water as an interface due to much higher mechanical impedance of water than air and consequently higher inertia of water, is a more appropriate option. In underwater explosive forming, the wave caused by the explosion is propagated by the water environment in a fraction of a second to the target sample, and after colliding and transferring energy, it causes a very high strain rate in it, which increases and Improves sample deformation. In this study, important parameters in the underwater explosion process such as explosion depth, water depth and water vessel geometry and their impact on underwater explosive forming have been investigated. Also, the purpose of this study is to comprehensively investigate the effective parameters, analytical relationships, background researches and simulations performed in the field of underwater explosive forming.

Abrasion in many industrial machines increases the clearance between moving parts, reduces accuracy, causes vibration, fatigue, and can ultimately cause complete machine failure and impose high costs on the industry. In this paper, the dry sliding behavior of metal on the surface of epoxy-based nanocomposites and their mechanical properties have been studied. Neat epoxy, carbon nanotube/ epoxy, nanoclay/ epoxy and carbon nanotube/ nanoclay/ epoxy are the samples. To investigate the sliding behavior, the samples were made in the form of a disc and according to the standard. The abrasive metal pin moved a distance of 1000 m in a circular path on the surface of the sample. Different axial loads were applied to the steel pin. The coefficient of friction between the metal and the samples, and the weight loss of the samples were measured. Also the mechanical properties of samples were determined by simple tensile test. The results showed that the addition of nanostructured reinforcements in all three nanocomposite samples made the sample more resistant to abrasion compared to the pure epoxy sample. At the maximum axial load, the coefficient of friction between the metal pin and the epoxy was measured to be 0.27, which indicated a reduction of 40% after adding 1.5% by weight of nanoclay to the epoxy.

In this paper, effect of wet surface of cylindrical shell on vibrational frequency in contact with water in different immersion depth is investigated vertically and numerically. Three steel cylindrical shells with different D/L ratio are investigated. Numerical simulation and experimental tests were compared in eight different type of water-shell contact. Effects of immersion depth, effect of difference of water-shell contact between the inner and outer surface with water and wet surface geometry for semi-immersed in vertical and horizontal direction are evaluated. The results show that the natural frequencies are suddenly reduced at the beginning of immersion and its end (full immersion). The reduction of natural frequencies in two modes of contact between the inner surface of the cylindrical shell and water as well as its outer surface contact with water is equal. For the equal wet area, more natural frequency reduction was observed at a horizontal position compared with the vertical position. Also, in the equal wetted surface, more frequency reduction is observed in horizontal than vertical directions.

In this paper, effects of initial geometric imperfection on free vibration analysis of multi-directional functionally graded nanoplates are studied using an isogeometric approach together with the quasi-3D shear deformation theory. Geometric imperfections may exist inherently in nanoplates or purposely created by researchers. These imperfections strongly affect the plates’ natural frequency. The initial geometric imperfection is considered as an initial curvature and modeled by an analytical function in the governing equations of the nanoplate. The effective material properties distribution is stated based on the Mori-Tanaka scheme, which in addition to the thickness of the plate, also changes in the in-plane directions. A four-variable quasi-3D theory with new distribution function is proposed. Based on Hamilton’s principle, a weak form of free vibration problem for nonlocal plates is derived. These discrete systems of equations are solved using an isogeometric approach. The accuracy of the present study was verified by comparing the results with those given in published papers. Present results indicate the importance of various parameters, especially imperfection amplitude, and material indexes in all directions, on the free vibration behavior of FG nanoplates.

In this study, a composite sandwich panel with M-type lattice core made of carbon fiber reinforced by nano-SiO2 has been manufactured in order to achieve a laminate without any defect. Afterward, polyurethane foam has been injected into the core of the sandwich panel. The vacuum assisted resin transfer molding (VARTM) has been used in the present research. Eventually, The effects of parameters such as the amount of nano-SiO2 on the tensile strength of the laminate and high-velocity impact on the carbon fiber sandwich panel resistance were investigated. It was figured out that adding 1 to 3 wt% of nano-SiO2 into the carbon fiber had the most desirable effects on the enhancement of tensile strength. Also, in the high-velocity impact test, the results showed that by increasing 1 to 3 wt% of nano-SiO2 in the carbon fiber, the output velocity of the projectile decreases. On the other hand, the results showed when the projectile collides with the M-type core of the sandwich panel, the output speed will be zero, but when the projectile does not hit the core, output velocity will have a value. The output velocity of the projectile from the sandwich panel with the polyurethane foam is more less compared to sandwich panel without foam. The scanning electron microscope (SEM) images showed nano-SiO2 has been well distributed between the resin and the fibers.

Ratcheting is known as the accumulation of plastic strains resulting from cyclic loadings. In this research، the simulation of a S275 carbon steel sheet's behavior subjected to cyclic thermal loading and axial load has been investigated. This analysis was performed using the finite element method and employing Ansys Parametric Design Language (APDL). This study investigates the Bree diagram and simulates the behaviors specified in this diagram concerning temperature and pressure parameters. According to the results، it turned out that the ratchet strain is always larger in the first cycles than in subsequent cycles. Nevertheless، the plastic strain during cyclic thermal load is equal to the plastic strain while removing the cyclic thermal load in the plastic area. In the shakedown area، after the first plastic strain، elastic behavior occurs in the sheet. Finally، it was found that Bree's diagram has a unique application in predicting the behavior of different types of sheets، and sheets with any dimensions and material can be used to study the behavior of shakedown، ratcheting، elasticity، and plasticity.

For the first time in the present study, the vibration of embedded rotating macro/microbeam in the Winkler-Pasternak foundation with the variable cross-sectional area and various boundary conditions in hygro-thermal-magnetic environments under axial and follower forces based on the Rayleigh beam model is studied. In this investigation, a parametric study is performed to clarify the impacts of various parameters such as boundary conditions, cross-sectional profile, foundation coefficients, rotary inertia factor, environmental conditions and size-dependent effects on the natural vibrational frequencies of the system. Couple stress theory is utilized to model the system. First, the dynamical equations of the system are derived using the Hamilton principle. To discretize the dynamical equation, the Galerkin method has been used, and the natural frequencies of the system are obtained by solving the eigenvalue problem. The results of the present study are compared and validated with the results presented in the technical literature. The results show that in high-temperature and low-temperature environments, the system has different dynamical behavior. It is observed that compared to the Euler-Bernoulli beam theory, the Rayleigh beam has lower frequencies. It is also concluded that in contrast to the effects of axial and follower forces, increasing the elastic and shear coefficients of the substrate improves the dynamical behavior of the system. Meanwhile, the vibrational frequencies of the structure increase by increasing the width ratio parameter. The results of the present study can be useful in the design of advanced structures such as microturbines and micromotors.

This article identifies crack in beams under moving forces (a moving load, a moving mass, a moving oscillator and a four degree of freedom moving system) using the Hilbert Huang transform. Timely identification and repair of damage in structures (such as cracks in bridge decks), especially in those structures that are always under dynamic loads, is very important. In recent years, new methods have been proposed based on the recorded dynamic responses of the structure. most of these methods are based on developed model updating methods which are usually very costly computationally. To overcome this problem, this study presents a method based on the Hilbert-Huang transform. In this method, the bridge is modeled in the form of an Euler-Bernoulli beam and the vehicle is also modeled in different conditions as a moving load, a moving mass, a moving oscillator and a moving system. The results show that the proposed method is able to detect cracks in bridge decks with an acceptable accuracy in all cases.

GLARE is a kind of fiber-metal composites in which aluminum alloy sheets are used, along with composite laminates with glass reinforcements. In this paper, Glare FMLs were subjected to repeated low-velocity impact loading with an energy of 150 J. Repeated low-velocity impacts were applied using a drop-weight testing machine. To fabricate testing specimens, 2024-T3 aluminum alloy plates with a thickness of 1 mm as the face-sheets as well as the core made of the glass-epoxy composite were used. The epoxy glass laminate used also consisted of five layers of E-type single-sided glass fibers in the epoxy resin. Besides, 99.9% purity silica nanoparticles were added to the base material. To investigate the effect of adding nanoparticles on the behavior of FMLs under repeated low-velocity impacts, silica nanoparticles with a weight percentage of 0 to 4% were added to the matrix of the fabricated samples. The experimental results showed that the sample with 1% nanoparticles had better performance in terms of impact resistance so that after five loads, it was not seriously damaged; however, the perforation phenomenon occurred for samples with 3% and 4% nanoparticles in the second impact load.

In this paper, the dynamic instability of swept wings by using the geometrically exact fully intrinsic beam equations and with considering the static stall effects is investigated. Study of variations of the limit cycle amplitudes by using the fully intrinsic beam equations and ONERA unsteady aerodynamic model with static stall effects in swept wings is the achievement of this article. the geometrically exact fully intrinsic beam equations involve only moments, forces, velocity and angular velocity, and in these equations, the displacements and rotations will not appear explicitly. In this study, the aerodynamic loads on the wing in an incompressible flow regime are determined by using the ONERA unsteady aerodynamic model. In order to check the instability behavior of the system, first the resulting non-linear partial differential equations are discretized by using the central finite difference method, and then time responses are obtained. The obtained results are compared with those available in the literature. Furthermore, the effects of sweep angle are studied. Finally, it is observed that by using the geometrically exact fully intrinsic beam equations, the instability of the swept wings can be determined accurately and selection of suitable sweep angle can postpone the occurrence of limit cycle oscillation.