INTERNATIONAL JOURNAL OF ENGINEERING
Year:2011 | Volume:24 | Issue:1 (TRANSACTIONS A: BASICS)|Papers Count:9

Strains are applied to the integration procedure in nonlinear increments to decrease the errors arising from the linearization of plastic equations. Two deformation vectors are used to achieve this. The first vector is based on the deformations obtained by the first iteration of the equilibrium step, and the second is acquired from the sum of the succeeding iterations. By applying these vectors and using sub-increments, the total strain increment can vary nonlinearly during the integration of the flow rule. Four individual variation schemes are presented for this purpose. In this paper, the strain space formulation is investigated. Numerical examples are analyzed using the traditional linear method and the suggested schemes. The examples are solved using the von Mises yield criterion and Prager's linear hardening rule. Results indicate that all nonlinear techniques increase the convergence rate of plastic analysis. In addition, such integration methods are shown to increase the stability of incremental-iterative analyses.

September 1978 Tabas earthquake incorporating the seismological/geological source-path and site model parameters by using finite-fault simulation approach. The method generalizes the stochastic ground-motion simulation technique, developed for point sources, to the case of finite faults. It subdivides the fault plane into subfaults and assumes each subfault to be a point source with a  w-2 spectrum. The length of the fault is taken as 85km and its width as 30km, and the fault plane is divided into 17×6 elements. Geometric spreading, regional anelastic attenuation and local site effect are included in the model. Satisfactory agreements between simulated and observed results validate capability of the method in prediction of ground motion in the study region.

An improved pixon-based method is proposed in this paper for image segmentation. In this approach, a wavelet thresholding technique is initially applied on the image to reduce noise and to slightly smooth the image. This technique causes an image not to be oversegmented when the pixonbased method is used. Indeed, the wavelet thresholding, as a pre-processing step, eliminates the unnecessary details of the image and results in a fewer pixon number, faster performance and more robustness against unwanted environmental noises. The image is then considered as a pixonal model with a new structure. The obtained image is segmented using the hierarchical clustering method (Fuzzy C-Means algorithm). The experimental results in this paper indicate that the proposed pixon-based approach has a reduced computational load and a better accuracy compared to the other existing image segmentation techniques.

The goal of theory of constraints (TOC) is to maximize output, which is achieved by identifying and managing the critically constrained resources. To manage the constraints, Goldratt proposed five focusing steps (5FS). If we increase constrained output, the output of system will be increased. In this paper, we focus on step four of the 5FS and use the remained capacity of nonconstraint to elevate the system’s constraint.

The ASTM F-75 (Co-28%Cr-6%Mo) is widely used as a biocompatible alloy in medicine for manufacturing implants. In this study, effect of mold preheating on the as-cast microstructure of the alloy was investigated using the solid investment casting process. Several mold preheating temperatures of 550, 700, 850 and 1000 °C were selected at the same melt superheat. The samples were characterized by optical microscopy, electron microscopy and macro-hardness test. The results showed that the size of grains and secondary carbides of the matrix was increased by increasing the mold preheating temperature. In addition, morphology of the M23C6 carbides was changed from the eutectic carbides precipitated in grain boundaries to the blocky shape precipitated in both carbide interface and dendritic matrix. The appropriate microstructure with nearly fine grains with homogeneous distribution of secondary phases was obtained at mold preheating temperature of about 850 oC.

Potential flow over rotating cylinder is usually solved by the singularity method. However, in this paper a mathematical solution is presented for this problem by direct solution of the Laplace' s equation. Flow over the cylinder was considered non-viscous. Neumann and Dirichlet boundary conditions were used on the solid surfaces and in the infinity, respectively. Because of non-viscous flow, the Laplace equation is the governing equation of the flow field. The entire flow field was divided into two parts including tree stream over a stationary cylinder and flow over a rotating cylinder with no free stream. Because of linearity of the governing equation, solutions of these flows were superposed to obtain velocity potential function from which velocity and pressure distribution was obtained. Pressure forces acting on the cylinder were obtained by integrating pressure distribution over the cylinder surface that was exactly the same as the results of the singularity method. Present work achieved the famous Kutta-Joukowski theorem in the aerodynamics and fluid mechanics. In addition, the proposed analytical model was validated by numerical solution.

Many engineering design problems involve a combination of both continuous and discrete variables. However, the number of studies scarcely exceeds a few on mixed-variable problems. In this research Particle Swarm Optimization (PSO) algorithm is employed to solve mixedvariable nonlinear problems. PSO is an efficient method of dealing with nonlinear and non-convex optimization problems. In this paper, it will be shown that PSO is one of the best optimization algorithms for solving mixed-variable nonlinear problems. Some changes are performed in the convergence criterion of PSO to reduce computational costs. Two different types of PSO methods are employed in order to find the one which is more suitable for using in this approach. Then, several practical mechanical design problems are solved by this method. Numerical results show noticeable improvements in the results in different aspects.

In this paper, an elastic shell model is presented for postbuckling prediction of a long thinwalled cylindrical shell under axial compression. The Ritz method is applied to solve the governing equilibrium equation of a cylindrical shell model based on the von-Karman type nonlinear differential equations. The postbuckling equilibrium path is obtained using the energy method for a long thin-walled cylindrical shell. Furthermore, the postbuckling relationship between the axial stress and end-shortening is investigated with different geometric parameters. Also, this theory is used for postbuckling analysis of a single-walled carbon nanotube without considering the small scale effects. Numerical results reveal that the single-walled carbon nanotube under axial compression has an unstable postbuckling behavior.

It is quite usual to encounter a beam with different types of cross section or even structural discontinuities such as a crack along its length. Furthermore, in many occasions such a beam may happen to be exposed to the oscillatory fluctuations. Therefore, any information about its natural frequencies may be worthwhile. Amongst the problems of discontinues beam analysis, in this paper a special kind of frequency analysis for a cracked and stepped beam located on an elastic foundation is considered. Accordingly, following a look out at the definition of Timoshenko beams, a special modeling trend known as the wave method is introduced. Based on the d' Alembert’s approach for the solution of wave differential equations, the technique of wave method is mainly depended on the study of transmission and reflection of waves colliding to a barrier. The method results in a global frequency matrix, which its determinant gives out the natural frequencies. The wave method is employed for the frequency analysis in some kinds of cracked and stepped beams with different types of boundary conditions. In some typical cases, the results are compared to other similar works and confirmed to be convincing.