JOURNAL OF MECHANICAL ENGINEERING AMIRKABIR (AMIRKABIR)
Year:2016 | Volume:48 | Issue:2|Papers Count:15

In this paper, a continuous stable tracking control algorithm is proposed for spacecraft in the presence of unknown actuator failure, control input saturation and external disturbances. The design method is based on variable structure control and has the following properties: 1) fast and accurate response in the presence of bounded disturbances; 2) robust to the partial loss of actuator effectiveness; 3) explicit consideration of control input saturation. In contrast to traditional fault-tolerant control methods, the proposed controller does not require knowledge of the actuator faults and is implemented without explicit fault detection and isolation processes. In the proposed controller, a single parameter is adjusted dynamically in such a way that it is possible to prove the ultimate boundedness of both attitude and angular velocity errors. The stability proof is based on a Lyapunov direct method and the properties of the singularity free quaternion representation of spacecraft error dynamics. Results of numerical simulations state that the proposed controller is successful in achieving high attitude performance in the presence of external disturbances, actuator multiplicative faults, and control input saturation.

Understanding the effects of thermoelastic damping on the vibration parameters such as natural frequency and frequency-sensitive is essential for the design of micro-nano-electromechanical systems. In this paper, the effects of thermoelastic damping in micro- and nanomechanical resonators beam with a rectangular cross section will be analyzed. The governing equations in present system are coupled of heat conduction equation and equation of motion where this methodology has been applied to three- dimensional analyses. To solve these governing equations analytically with considering suitable assumption, first the coupled heat conduction equation is solved for the thermoelastic temperature field by considering three-dimensional (3-D) heat conduction along the length, width and thickness of the beam Next; thermoelastic coupling is modeled into the equation of motion for flexural vibrations through a temperature-dependent moment of temperature distribution. Frequency shifts and quality factor due to thermoelastic damping are analyzed. For special cases,the obtained frequency shift is compared with the result of the frequency shifts computed using 2-D heat conduction; also the obtained quality factor is compared with exact 1-D heat conduction. The results obtained showed that the model presented in this paper were in good agreement with the other models and it predicted the effects of thermoelastic damping on the behavior of micro-nano resonators accurately.

In this paper, the resonant frequency and sensitivity of vibration modes of an atomic force microscope (AFM) with an assembled cantilever probe (ACP) are analyzed utilizing the modified couple stress theory. The proposed ACP comprises a horizontal microcantilever, a vertical extension and two tips located at the free ends of the cantilever and the extension, which make AFM capable of scanning the top surface and the sidewall of the sample, simultaneously. To derive the exact solution of the proposed ACP vibration behaviour, the horizontal cantilever is modeled as two beam. Having obtained the equation of motion, boundary and continuity conditions, the resonant frequency and sensitivity are studied. The results predicted by the current theory are compared to those obtained by the numerical methods presented by Kahrobaiyan et al. and the comparison shows that there is some errors in the numerical method results. The results of the couple stress theory are also compare with those of the classical beam theory. The evaluation indicates that the vibration behaviour of the proposed ACP is completely size-dependent.

This study deals with simulation of low-cyclefatigue (LCF), followed by evaluation of fatigue parameters, which would be suitable for estimating fatigue livesunder uniaxial loading. The cyclic elastic–plastic stress–strain responses were analyzed using the incrementalplasticity procedures. Finite-element (FE) simulation inelastic–plastic regime was carried out in FE packageABAQUS. Emphasis has been laid on calibration of A356Aluminum Alloy for LCF behavior. For experimental verifications, a series of low-cycle fatigue tests were conductedunder strain-controlled, fully reversed condition in MTS 810 with MTS Flex Test GT controller at 120 and 280oC temperature. The comparisons between numerical simulations and experimental observations reveal the matching tobe satisfactory in engineering sense. Based on the cyclicelastic–plastic stress–strain response, both from experiments and simulation, loop areas, computed for variousstrain amplitude, have been identified as fatigue damageparameter. The surface microstructure of the samples were examined and images showing the microstructure of certain dendritic structure with secondary dendrite arms (SDAS) they are approximately 25 micrometers. The results of isothermal experiments at temperatures 280 and 120o C and the fixed cycle can be seen that the alloy hardening behavior of those cycles of temperature 120oC and shows the cyclic softening behavior in 280 oC temperature.

In this paper, a new analytical model has been presented for energy absorption of aluminum-foam sandwich panels under ballistic impact. The panels consist of hexagonal honeycomb core sandwiched between two aluminum skins. In analytical model cylindrical rigid projectile with flat ended has been considered. In the quasi-static loading, by using the springs-mass model, energy absorption of Aluminum skins with considering difference energy absorption mechanisms calculated. Energy absorption of honeycomb has been determined by wierzbicki model. Energy balancing equation has been employed for determination the ballistic limit and residual velocity of striker. The results of ballistic limit and residual velocity of striker computed by new model have good agreement with experimental results. Also the effects of projectile mass and diameter and cell diameter of honeycomb in energy absorption of sandwich panel has been investigated.

The relife valve is a critical part of a device that has great importance to ensure security of related persons life and financial. Use this piece is necessary in the system with possibility of increasing the internal pressure of the system. One of these systems is gas tanks that is such as a mobile bomb and in case of malfunctioning of relife valve, in which place there is a possibility of explosion. In this study, evaluation of valve failure a portable LPG gas cylinder is performed. For this purpose, chemical analysis, metallography, mechanical properties, and in particular the fractography was done on The broken sample. The surface quality of the samples were examined by stereomicroscope and scanning electron microscopy. The results show the fatigue failure. in effect of loss of surface quality due to pitting of the of inappropriate machining. The main factors contributing to this phenomenon was detected reduction of surface quality due to pitting and improper machining.

The vinous wings of insects are complex biological structures with remarkable mechanical behavior. The wings mainly consist of veins and membranes. The membranes of the wing are not mechanically tough. But, the whole wing structure reveals a significant resistance to crack propagation. In this paper, a combination of scanning electron microscopy technique, experimental tensile tests and numerical simulation is employed to investigate the effect of the veins on the “toughening mechanism” of the wings. The numerical simulation of crack propagation in the vein is based on the extended finite element method. Linear elastic material properties and linear traction-separation law are used to simulate the constitutive behavior of the vein materials. The microscopic images show that the veins have a tubular microstructure that consists of layers made of chitin and protein. The results from numerical simulations demonstrate that each vein layer effectively cope with the stresses due to external loading. But, the presence of protein plays an important role in arresting the crack growth. Comparison of the results reveals a very good agreement between numerical simulations and experimental data.

In this research side ratio effect of ultrasonic guided waves in a bar with rectangular cross section is investigated and wave structure is calculated and plotted. The motion of ultrasonic guided waves in the mentioned bar is considered as a three dimensional problem. In the homogenous equation system that is obtained to satisfy stress free surface boundary conditions, the ratios determinant of this system should equal zero. So, the frequency equation is obtained. The real roots of this equation are extracted and frequency spectrum, phase and group velocity diagrams and wave structure for longitudinal, torsional and bending waves are plotted by a written computer program by Matlab. Increasing cross section bar side ratio shows when the frequency increases the longitudinal and torsional waves phase velocity decreases and bending waves phase velocity increases. Also, for some modes replacing particles on the bar surface is considerable and for some others is negligible.

In this research, 13 passes ARB process performed on AA1050 sheets. The microstructural improvement to achieving nanometer-sized grains during ARB process was studied. In this process, the sheets were rotated 180o around the normal direction (ND) axis after each rolling passes and preheated before each passes. Moreover, the changing of strength and elongation of the ARBed sheets during the process was investigated by uniaxial tensile test in three directions (roll direction (RD), transverse direction (TD) and angle of 45o toward RD); and then the inhomogeneity of mechanical properties in these directions was measured. The least of inhomogeneity occurred in three, four and thirteenth passes of process. Finally, micro-Vickers hardness was investigated throughout thickness of the sheets in this process. The strength and hardness of sheets increased at the early passes and then did not changed apparently at the middle passes of process; and subsequently they decreased gently. The elongation of sheets decreased swiftly at the first pass of process; and then it increased slowly.

Aluminum tailor welded blanks consist of two or more aluminum sheet which welded together by one of the welding processes. Aluminum TWBs are used in different industries like automotive industry because they have high ratio of strength to weight. There are many different welding methods for aluminum welding, but melting methods cause to form brittle phases and decrease weld strength. Therefore, friction stir welding is a replacement method. Friction stir welding’s parameters have high influence on the weld quality. In the present study, some of the main parameters of friction stir welding are investigated. These parameters are type of welding tool, rotation velocity of welding tool and traveling speed of tool. Whereas tailor welded blanks are used in the out of plane forming processes, Erichsen formability test is used for investigation of weld quality. Therefore, a design of experiment is done using Tauguchi method and some aluminum tailor welded blank samples are welded. Aluminum 6061 and 5182 are base metal of tailor welded blank. Results of study show that type of welding tool has a much influence on the weld quality. Weld quality and formability increase by increasing of tool’s rotational velocity.

The coupling of the electromagnetic field and the mechanical structure field is one of the main problems in the theoretical study of Electromagnetic Forming (EMF). In this study, two possible approaches for the simulation of the electromagnetic tube compression forming process were implemented and compared: A loose-coupled and a sequential-coupled algorithm. In the loose-coupled the electromagnetic field and mechanical structure field were solved separately, but in the sequential-coupled algorithm, the electromagnetic simulation and the mechanical structure simulation were iteratively performed by using Maxwell equations and Finite Difference Method (FDM) as subroutine VDLOAD in ABAQUS software. A deformation of the tube and consequently a change in inductance of tube during the process in the sequential-coupled algorithm was considered. The depth of bead in loose-coupled algorithm compared to experimental result had a 35% error, but in a sequential-coupled algorithm this error has been reduced to 5%. To predict tearing in this process Johnson-Cook damage criterion used. Increasing of discharge voltage and tube thickness respectively, had maximum effect on Johnson-Cook damage. Amount of damage less than 0.8 is conservatively suitable for the safe area without fracture.