نشریه مهندسی مکانیک و ارتعاشات
سال دوم شماره 1 (پیاپی 2، بهار 1390)

Determining the sensitivity of vibratory characteristics of a dynamic system (frequencies and mode shapes) due to changes in its mass or stiffness properties is of significant importance. The original algorithms developed were on the basis of the first order sensitivity analysis. However, due to the limitations of these methods, extensive studies performed to develop more efficient methods for this purpose. The second order sensitivity analyses based methods are among the developed algorithms. Both the first and second order sensitivity analyses algorithms lead to acceptable results for only small changes in mass and stiffness of dynamic systems. In this paper, a new method for frequency updating of shear systems is presented based on shear building properties. The method is simple and needs less computation time compared to other methods. An iterative algorithm is developed by which the frequencies of the modified shear system can be determined to the desired accuracy. Since the tuning of the dynamic characteristics of the shear building systems has an important role in controlling their response against external loading, especially earthquake loading, this method can perform efficiently in frequency updating of the system. The performance of the proposed method is investigated using some numerical examples.

Optimal balancing is a new approach based on optimal control a which modes, controls and the amount of equilibrium scale are calculated simultaneously. This approach has a significant effect on increasing performance of industrial robots which causes maximum decreasing of energy consumption. Despite many applications of robotics such as welding and gluing, the path is predefined , but other unknowns such as velocity are unknown. In this thesis, first optimal balancing approach is implemented for the balancing of manipulators in point-to-point motion. Then this approach is used for the balancing of manipulators in a different path. Other approaches of balancing such as static balancing and dynamic balancing which are common in robotics, are considered for decreasing the gravitational effects and not consumption energy effect. Passive balancing is possible with elements such as masses and springs. Values of connecting  parameters (such as values of counterweights and stiffness of spring) are unknown which are determined in static balancing approach with assuming a constant potential energy and are determined for decrease of consumed energy optimal balancing. In previous studies in addition to static balancing ,dynamic and adaptive balancing for some configuration are presented too. In this thesis, using optimal trajectory planning and with the assumption that trajectory of end- effector defined to polynomial function and spline interpolation, and the sum of square input torques to the objective function optimal counterweights is computed with Genetic Algorithm optimization method. Finally it is determined that consumption energy and performance index, and quantity of counterweights in trajectory to spline interpolation are less.

The free vibration of cylindrical composite shell with internal liquid is studied in this paper. The shell composed of several layers and stiffeners which are rings and stringers. The first order shear theory was used for shell and stiffeners. Stiffeners were used in equations as discrete elements. The Raily-Ritz method was used for solving the problem. This method is based on minimum potential energy principle. The potential and kinetic energy of shell and each stiffeners and kinetic energy of liquid are subsituted in the functional of energy. The section shape of each stiffener is rectangle. The liquid is ideal and sloshing was neglected. For caching the best and the biggest natural base frequencies, shell and stiffeners were optimized by genetic algorithm. With use of genetic algorithm the best fiber angles of each layer of composite were obtained to reach the maximum base frequency, also best ratio of height to width was obtained and in last the number of rings and stringers and theirs height to width ratio were obtained to reach the optimized stiffened shell with highest base frequency.

The Adomian decomposition method is employed in this paper to investigate the free vibration of cable consisting of two uniform sections. Each section is considered a substructure which can be modeled using ADM. applying boundary and continuity conditions, the dimensionless natural frequencies and corresponding mode shapes can be easily obtained simultaneously. The computed results for different boundary conditions are presented. The computed results agree well with those analytical and numerical results given in the literature. These results indicate that the present analysis is accurate and provides an efficient, reliable and systematic procedure which is simple and more straightforward than the other modal analysis. Also the effect of amount of mass and spring stiffness at the boundary on variation of natural frequencies is obtained and an effective procedure is suggested for encountering these kinds of problems.

In this paper some mechanical amplifiers have been investigated then a new model has been introduced, designed and analysed. This new mechanism can present a large amplification ratio in a very compact size. High natural frequency and the absence of lateral motion are the advantages of this new model toward the other models introduced here. In order to present an analytical investigation for this new mechanism, the pseudo rigid model is utilized and optimized for the best working conditions. This model is also analysed and verified using finite element method. The results of FEM are utilized to specify the effect of parameters which cannot be seen in pseudo rigid model. An optimized model with its dimensions sketched finally.

This paper presents the fatigue life behavior of hub using random vibration based on road roughness. The main objectives of this study are to predict the fatigue life and to identify the critical location for evaluating fatigue damage. Asphalt road surface profiles obtained in favorable circumstances by topography cameras and in according road classification ISO 2631-1estimate road roughness such as power spectral density in a constant vehicle velocity or PSD Function by simulating motion model of a quarter cars in Matlab Software. The fatigue life is predicted, utilizing the finite element based on fatigue analysis code. The structural model of the hub was utilizing the cation by using CMM scanner. The finite element model and random vibration analysis were performed utilizing the finite element analysis code , and the critical stress with Von Misses criterion is estimated about 237MPa at the end of the wheel hub. In addition, the fatigue life was predicted using different types of approaches such as Dirlik, Maro and …… subjected to variable amplitude loading. Finally, this research described that fatigue life of component based on certain loading and conditions is estimated about 116000Km of travel length of the vehicle.