نشریه علوم کاربردی و محاسباتی در مکانیک
سال بیست و دوم شماره 2 (بهار و تابستان 1390)

Acoustic emission (AE) can be use to discriminate the different types of damage occurring in a constrained composite. However, the main problem associated with data analysis is the discrimination between the different acoustic emission sources. The objective of the cluster analysis is to separate a set of data into several classes that reflect the internal structure of the data. Indeed, cluster analysis is an important tool for investigating and interpreting data. In this paper, we intend to use two kinds of classifier techniques: a mathematical procedure that is called principal component analysis (PCA) and an unsupervised one fuzzy clustering means (FCM). Glass/polyester composites specimens used for the validation of the proposed methodologies. We worked on glass/polyester unidirectional specimens, subjected to duration of Mode I delamination within different configurations, awaiting preferential damage modes in the material.

In the present study, temperature fields and residual stress states in the weld joint of the stainless steel pipe are computed numerically using a thermo-elastic-plastic analysis. This weld joint is a circumferential butt-weld one and is done in two passes using gas tungsten arc welding. Welding procedure is simulated in two and three dimensions using a subroutine developed in finite element ABAQUS software for applying heat flux. Furthermore, the element birth and death technique is used to simulate weld passes and filler metal deposition into the weld pool. When the simulation is done, the effect of welding sequence on welding residual stresses are considered using four different welding sequences proposed. Finally, the effect of hydrotest process in decreasing the residual stresses is evaluated. The results of the simulation show that selecting a suitable welding sequence can substantially decrease the amount of tensile residual stresses and can change them to compressive stresses in certain situations. Also the results of this research show that applying a hydrotest pressure after welding process can reduce welding residual stresses up to 65%.

In this paper, the continuous model for vibration analysis of a cracked beam developed by Shen and Pierre is modified. For this end, by some realistic assumptions, new functions for displacement and stress fields are proposed. Then, the equation of motion of the cracked beam with breathing crack is obtained via the Hu-Washizu variational principle. The new obtained equation of motion is self-adjoint. Moreovers, by employing the Galerkin method, the modes shape of beam with a breathing crack are obtained. Then, in order to obtain the time response of the cracked beam, a new bilinear model is introdused for the stiffness corresponding to each mode. Using this model, the governing equation of motion is converted into the standard form which can be analyzed by Lindstedt-Poincar’ method. The results show that response obtained throught the perturbation metod (Lindstedt-Poincar method) is composed of two parts. The main part is the response of a system with the equivalent stiffness, whith is equal to the main value of the stiffness corresponding to the fully open and fully close crack cases. The remaining part of the response consistence of the correction terms, which reflects the effect of opening and closing the crack during vibration. The results show that for a given crack parameters, redaction in natural frequencies for a fatigue-breathing crack are smaller than the ones caused by open cracks. Also, the results have been validated by the experimental and theoretical data reported in the literature. There is a good agreement between the results obtained through the proposed method and those obtained from the reported experimental data.

In this research, modal analysis of a second stage blade in a gas turbine is investigated and vibration modes of blade are found in various conditions. The numerical calculation by finite element method using NASTRAN-PATRAN software based on experimental test conditions is utilized and there is a good agreement between the experimental and FEM results. Finally by calculating excitation frequency in gas turbine work conditions and using a Campbell diagram, it is concluded that there is not any resonant problems in the blade and there is a good agreement between the experimental and FEM results as well

In this paper using one, two and three dimensional simulations, the accelerating flows in developing inlet pipe region are considered numerically. The developing length calculated based on different turbulence parameters is studied thoroughly. The SST turbulence model in comparison with recent reported experimental data have been used to achieve reliable predictions. The predictions obtained using all 1, 2, and 3 dimensional cases are generally the same and follow the experiments well. Thus, it seems that one dimensional simulation in fully developed region is sufficient. However, for the inlet region in developing state a two dimensional axisymmetric analysis is required. This research shows that calculating the developing length based on only mean averaged velocity is not adequate, but also turbulence kinetic energy and viscosity must be paid enough attention. Further, the comparison between steady and unsteady flows for the developing length shows that they are very different. This length also depends on both the value of flow acceleration and turbulence delay time in addition to Reynolds number and pipe diameter.

Shimmy is a self-excited oscillatory, combined lateral-yaw motion of the landing gear caused by the interaction between dynamic tyre behavior and landing gear structural dynamics. Initially, the models for evaluating shimmy stability remaineds linear. Although linear model will usually reveal basic characteristics, it will generally fail to accurately predict the behavior of a landing gear system. This study treats the torsional dynamics of the lower parts of the landing gear as a multi-degree of freedom mechanical system and tire elasticity according to the elastic string theory. First they are linearized. Then eigenvalues are computed, solving analytically the stability boundaries with a parameter space method and numerical solution by simulation of the nonlinear system for time histories. It can play the role of linear analysis verification. It seems that linearized methods are well suited to obtain extensive insight, respecting limitations of these methods. Numerical simulation, on the other hand, is a valuable tool for pointing out specific effects of a nonlinear system in large amplitude regions.

In this paper, the Levy-type solution is developed to study the static behavior of the cross-ply piezoelectric laminated composite plates whose all or some of their laminae possess the piezoelectric properties. By the use of the proposed method the behavior of the rectangular cross-ply piezoelectric laminates with at least two simply supported opposite edges can be analytically examined. The governing equations of equilibrium are derived in the framework of the first-order shear deformation plate theory and classified according to the crystallography type of piezoelectric layers. Moreover, limitations of the method for the analysis of this kind of the structures are discussed. Finally, the governing equations of equilibrium are solved analytically with the aid of the state-space approach. The numerical results are compared with those obtained by the Navier method and those obtained by other investigators. It is found that the present results have very good agreements with those obtained by other methods.

In this paper governing aeroelastic equations of the two degree of freedom airfoil containing cubic nonlinearity in an incompressible flow are presented in the both time and frequency domains. Numerical solution and harmonic balance methods are applied for the solution of govening aeroelastic equations in the time domain to obtain amplitude and frequency of limit cycle oscillations (LCO). Also, describing function method is modified and presented for the pediction of the LCO amplitude and frequency in the frequency domain. LCO frequency and amplitude are obtained via applying three methods as harmonic balance, describing function and numerical solution methods for two different cases. Comparision between the results of these three methods shows a very good agreement.