فهرست مطالب مهدی مجاهدی

In this paper, the static behavior, instability and buckling in porous micro plates under electrostatic field are investigated based on the modified couple stress theory and with regard to modeling, determining equations and solution methods. The plate is considered to be porous and the porosity distribution is considered to be non-uniform. The equations are obtained considering the distributed support load. By using the definition of dimensionless parameters such as load, voltage and length scale, the equations of motion become dimensionless. It can be seen that in the special case, by removing the dimensionless non-classical parameters, the equation of the classical plate under the electrostatic field is obtained. Galerkin mode summation and numerical methods are utilized to solve the static deformation equation and assess the pull-in instability voltages and buckling loads. Convergence analysis is done and the number of approximation functions and elements required for both methods are calculated and the compatibility of the results obtained from the two methods is examined. In the results section, the difference between classical and non-classical theories is examined and the effect of dimensionless parameters of length scale and porosity ratio on maximum displacement, pull-in instability voltages and buckling load is studied. The results show that the use of modified stress couple theory leads to a very large stiffness prediction compared to classical theory. This result highlights the necessity of using the modified couple stress couple theory for the micro-scale. It is observed that the length scale parameter plays the role of stiffening. The change of porosity ratios also shows that as this ratio increases, the displacement increases and the stable areas decrease. Variations in this ratio lead to uniform changes in buckling load and pull-in instability voltage, and linear relationships are obtained to calculate buckling load and pull-in instability voltage versus porosity ratio. Also, in small values of support load, it is shown that the relationship between the instability voltage and the compressive load of the support is linear, but in the buckling range, this relationship is not linear.

The solid state lasers consist of host mediums that are doped by an active ions. The host environment is built in the form of slab, rod, disk and fiber according to the geometric shape. In most cases, the solid lasers can be stimulated by diode pumping. During the pumping, some of the pumping energy at the crystal is converted to heat and this heat causes the temperature gradient in the crystal. The temperature gradient can produce thermal stresses and even lead to fracture in the crystals. In the fiber lasers, these phenomena disturb the laser operation and decrease the maximum input power of the laser. In this paper, the thermo-mechanical behavior of single clad Yb:YAG fiber laser is discussed under continuous end pumping. The single clad fibers have been modeled with single and four cores and they are simulated by finite element software (Ansys). Since the thermal loading and geometry of the fiber have symmetry planes, in the modeling, one quarter model is used to determine the temperature and stress distributions. In the Ansys software, the coupled thermal-structural analysis is utilized. First, the temperature distribution is calculated in the thermal environment of the software and then thermal stress fields for single core and four-core fiber lasers are obtained by applying the temperature distributions calculated through the thermal analysis. In the analysis, the mesh dependency is evaluated and for achieving convergence, the required number of element is determined. The temperature distributions in single and four core fibers, with two positions for cores, are calculated. For any case, the calculated temperatures are compared with each other and the position of core related to smaller temperature is selected. The results show that, in comparison to single-core fiber, a four core fiber has smaller maximum stress at the same power and higher pumping fracture power. Finally the other thermal effects of the pumping, including thermal lensing, for single and four core fiber are compared with each other and it is shown that the four core fiber laser has better performance in terms of thermomechanical output.

In this paper, thermal effects of Gaussian and super Gaussian pumping on the rod laser are analytically investigated. The crystal is considered as a rod, with isotropic thermomechanical characterizations, which is end-pumped. The intensity distribution of pumping spot is considered in three-types including Gaussian, second order and third order of super-Gaussian, and effects of any type on the thermal distribution and thermal lensing are compared with each other. First, the heat generations due to emission in the crystal are calculated for Gaussian and super Gaussian pumping and then the equation of temperature distribution is analytically solved and a closed form solution for temperature distribution of the rod laser is obtained. The analytical results are compared with the results of finite element method. Thereupon, the temperature distributions and the values of thermal lens for various pumping powers are calculated. The results show that calculated maximum temperature for Gaussian case is lower than for supper-Gaussian cases. In addition, the distances between focal point and the input pumping plane obtained for super-Gaussian pumping are larger than those calculated for Gaussian pumping.

In order to create a laser beam, one common method is to use beam pumping. In this method, beam is rstly emitted to the active medium of laser crystal and the electrons in the last shell of atoms are stimulated. With some internal interactions, laser beam will be created and some pumping energy is converted to heat in the crystal. In solid-state lasers, large amounts of pumping power may lead to heat generation in the active laser medium and a non-uniform temperature distribution in the crystal is generated. The temperature gradient causes thermal stress in the crystal structure and can be eective on thermomechanical behavior of the crystal and the quality of the laser beam. With increasing the pumping power, the heat generation within the active media is increased and consequently the thermal eects including the thermal lensing, polarization of output beam, mechanical stress, reducing the eciency of output beam, are intensi ed. Even in this case, thermal stress causes failure. Hence to achieve a high quality beam laser, it is necessary to investigate the thermomechanical behavior of the laser crystal, through the designing of solid state lasers. In the past works, thermomechanical behavior of the crystal was investigated in framework of Fourier heat conduction theory (Heat wave diusion speed is considered to be in nite), while this theory is not appropriate to study the heat conduction for medium under pulsed heat with appreciable relaxation time. Hence, in this research, by providing a nite element formulation based on the non-Fourier thermoelasticity (with considering relaxation time), distributions of temperature and stress in the Nd:YVO4 crystal are calculated. The results show that the maximum temperature and stress calculated by non-Fourier theory, are larger than those calculated by the Fourier theory and also the non-Fourier theory predicts lower failure power for crystal. In addition, if the relaxation time is lower than a certain value, the results of classical and nonclassical theories are closely equal to each other.

Microelectromechanical systems (MEMS) are used in many elds of industry like automotive, aerospace and medical instruments. Among the various ways to operate the MEMS devices, the electrostatic actuator is the common mechanism, due to simplicity and fast response. Previous experiments have shown that the mechanical behavior of devices, which their sizes are in order of micron and submicron, are dependent to size dependency. They also have illustrated that by decreasing the dimension of structures, the size dependent eect is highlighted. In this case, the classical theories are not capable to predict the size dependent eects and mechanical behavior of the microstructures properly. Therefore, nonclassical theories such as modi ed couple stress and strain gradient theories have been introduced. It was shown that the modi ed couple stress theory can accurately predict the size dependent behavior of microstructures. There are some in uences observed in the MEMS, that they have notable eects on the mechanical behavior of microswitches, such as fringing elds and large de- ection. When the air gap is larger than the electrode's width of microswitches, the impacts of fringing elds and geometric nonlinearity signi cantly aect the mechanical behavior of the system. Therefore, neglecting the abovementioned eects leads to errors in the instability prediction of microswitches. Most of microswitches consist of a microcantilever with a proof mass and a xed substrate which there is an air gap between them. By applying voltage to the system, the microcantilever starts to de ect into the xed substrate. In this paper, pull-in instability and de ection of MEMS switches are investigated based on the size dependent model. The nonlinear model is introduced by considering modi ed couple stress theory and fringing eld eects as well as geometric nonlinearity. Utilizing the minimum total potential energy principle, the static equation of motion is derived in framework of the nonclassical theory. The eects of various parameters on static pull-in instability are studied and errors of considering the linear model or classical theories is calculated. The results show that the presented model is capable to predict the displacement and pull-in instability of the microswitches.

In this research, vibration behavior of a microgyroscope including beam structure is investigated. microgyroscope consists of a microcantilever beam and distributed proof mass that is actuated by electrostatic force. A developed model and formulation based on the distributed assumption for proof mass are presented to investigate the instability and vibration behavior of beam microgyroscope. Considering distributed assumption for proof mass, electrostatic force changes from concentrated force to distributed force and produces a moment that is effective on the mechanical behavior of the gyroscope.The equations of motion are reduced by Galerkin’s method and solved via numerical and analytical techniques. An important nondimensional parameter named as nondimensional length parameter is presented which it is the ratio of the proof mass length to beam length. Effects of the nondimensional length parameter on the static, dynamic, vibration behaviors and natural frequency of the microsyetem are investigated. Results show that by increasing the nondimensional parameter, the differences between the results of concentration and distribution hypothesizes for proof mass increase.