فهرست مطالب احسان رئیسی استبرق

In the present study, the large deflection of nanobeams with surface effects has been investigated. The surface effects are modeled by using the generalized Young–Laplace equation. The finite element method has been used to investigate the mechanical behavior of nanobeams. This method provides the possibility of solving the problem with the least assumptions and with the arbitrary boundary conditions and geometry. With the use of the proposed model, nanobeams small to large deflection with different loading and support conditions can be analyzed. In this study, the large deflection of circular cross-sectional nanobeams under sinusoidal distributed load with surface effects has been investigated. The effect of various parameters such as aspect ratio, surface elastic modulus and residual surface stress on the large deflection of nanobeams has been investigated. The result obtained show the importance of considering the surface effects in the study of large deflection of nanobeams.

Ideally, bladed disk systems are tuned and all blades are identical but, in practice there always exist small, random differences among the blades. In designing bladed disks, all the blades on each stage are generally assumed to be identical, and this is called a tuned bladed disk. However, due to manufacturing tolerances, variations in material properties and wear in service, each blade on a disk is almost always slightly different to the others. These small differences, collectively called mistuning. It is well known that even a small amount of mistuning can induce a large forced response known as mode localization. In this study a representative model for the bladed disk system is presented and equation of motion derived. Then the response of tuned system was obtained. Afterward, frequency response of mistuned system with genetic algorithm for mistuned blade in mass was studied. Also maximum frequency response of the system for mistuned blade in stiffness was calculated. The result indicated the slightly mistuning in system can increase the response of the system. The worst response of mistuned system is 5.563 times greater than the response of the tuned system. Increase in frequency response of the system is undesirable and should be considered in the initial design of these systems.

The objective of this paper is to develop a technique using genetic algorithms (GA) for predicting the worst response of mistuned bladed disk. A simple but representative model for the bladed disk system is presented. The response of the system under harmonic excitation was obtained. The worst response of the mistuned system was then formulated as an optimization problem. Next, mistuning in blades stiffness, blades damping and blades stiffness and damping was studied and the results were compared. The results indicate that the system mistuned with both stiffness and damping has higher response amplitude in comparison with other mistuned system.

Bladed disks may be used in several engineering systems such as fans، impeller pumps، turbine generators and jet engines. Ideally، these systems are tuned and all blades are identical but، in practice there always exist small، random differences among the blades. Mistuning، imperfections in cyclical symmetry of bladed disks is an inevitable and perilous occurrence due to many factors including manufacturing tolerances، variations in material properties and wear in service. Each blade on a disk is almost always slightly different to the others. It can cause some unpredictable phenomena such as dramatic difference in forced vibration response. In this paper، effects of mistuning on nonlinear behavior of mistuned cyclical symmetric systems like bladed disk are investigated. A nonlinear، mistuned model based on the method of multiple scales is proposed and formulated in which nonlinearity and mistuning parameter is assumed to be small. Several mistuned system were considered and solved by the proposed technique. Numerical results show that only one jump will happen for the tuned system during the excitation and demonstrate that mistuning can lead to repeating and scattering of jump phenomena during the excitation frequency whereas in tuned cyclic system it occurs simultaneously.

Ideally, bladed disk systems are tuned and all blades are identical but, in practice there always exist small, random differences among the blades. Mistuning, imperfections in cyclical symmetry of bladed disks is an inevitable and perilous occurrence due to many factors including manufacturing tolerances and wear in service. It can cause some unpredictable phenomena such as dramatic difference in forced vibration response. In this paper first a finite element model of bladed disk system with 24 blades were created in ANSYS. The model is then used to calculate the frequency response of the blades for the tuned system. Next, two hundred experiments, with different density for each blade, were selected in a specified range. For each test case calculations were performed and the maximum response was obtained. Then, by integrating neural networks and genetic algorithm the worst frequency response of the mistuned bladed-disk system was calculated. The problem of finding the worst specification is formulated as an optimization problem subjected to constraints such as the manufacturing tolerances. Based on the calculated parameters, a new model was created and the maximum response of the mistuned system was calculated. The results indicate that the responses obtained from the neural network and genetic algorithm have reasonable accuracy and are in good agreement with responses obtained from the ANSYS and shows the efficiency of the method