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CONTENTS
Volume 49, Number 5, March10 2014
 

Abstract
In this paper, free vibration of a sandwich curved beam with a functionally graded (FG) core was investigated. Closed-form formulations of two-dimensional (2D) refined higher order beam theory (RHOBT) without neglecting the amount of z/R was derived and used. The present RHOBT analysis incorporated a trapezoidal shape factor that arose due to the fact that stresses through the beam thickness were integrated over a curved surface. The solutions presented herein were compared with the available numerical and analytical solutions in the related literature and excellent agreement was obtained. Effects of some dimensionless parameters on the structural response were investigated to show their effects on fundamental natural frequency of the curved beam. In all the cases, variations of the material constant number were calculated and presented. Effect of changing ratio of core to beam thickness on the fundamental natural frequency depended on the amount of the material constant number.

Key Words
free vibration; sandwich curved beam; FG- core; refined higher order beam

Address
K. Malekzadeh Fard : Department of Structural Analysis and Simulation, Space Research Institute, Malek Ashtar University of Technology, Tehran-Karaj Highway, PO Box. 14665-143, Tehran, Iran

Abstract
In this article, stability of composite conical shells subjected to dynamic external pressure is investigated by numerical and experimental methods. In experimental tests, cross-ply glass woven fabrics were selected for manufacturing of specimens. Hand-layup method was employed for fabricating the glassepoxy composite shells. A test-setup that includes pressure vessel and data acquisition system was designed. Also, numerical analyses are performed. In these analyses, effect of actual geometrical imperfections of experimental specimens on the numerical results is investigated. For introducing the imperfections to the numerical models, linear eigen-value buckling analyses were employed. The buckling modes are multiplied by very small numbers that are derived from measurement of actual specimens. Finally, results are compared together while a good agreement between results of imperfect numerical analyses and experimental tests is observed.

Key Words
composites shells; stability; pressure vessel; pressure rise time; critical pressure; imperfection; numerical method

Address
Sina Jalili, Jamal Zamani, M. Shariyat, M.A.B. Ajdari and M. Jafari : Mechanical Engineering Faculty, Khaje Nasir University of Technology, Tehran, Iran
N. Jalili : Mechanical Engineering Faculty, Amirkabir University of Technology, Tehran, Iran

Abstract
In this paper, a novel semi-energy finite strip method (FSM) is developed based on the concept of first order shear deformation theory (FSDT) in order to attempt the post-buckling solution for thin and relatively thick functionally graded (FG) plates under uniform end-shortening. In order to study the effects of through-the-thickness shear stresses on the post-buckling behavior of FG plates, two previously developed finite strip methods, i.e., semi-energy FSM based on the concept of classical laminated plate theory (CLPT) and a CLPT full-energy FSM, are also implemented. Moreover, the effects of aspect ratio on initial post-buckling stiffness of FG rectangular plates are studied. It has been shown that the variation of the ratio of initial post-buckling stiffness to pre-buckling stiffness (S*/S) with respect to aspects ratios is quite independent of volume fractions of constituents in thin FG plates. It has also been seen that the universal curve representing the variation of (S*/S) with aspect ratio of a FG plate demonstrate a saw shape curve. Moreover, it is revealed that for the thin FG plates in contrast to relatively thick plates, the variations of nondimensional load versus end-shortening is independent of ceramic-metal volume fraction index. This means that the post-buckling behavior of thin FG plates and the thin pure isotropic plates is similar. The results are discussed in detail and compared with those obtained from finite element method (FEM) of analysis. The study of the results may have a great influence in design of FG plates encountering post-buckling behavior.

Key Words
post-buckling; FG material; finite strip; semi-energy; first order shear deformation theory

Address
M. Hajikazemi, H.R. Ovesy and M.H. Sadr : Aerospace Engineering Department and Center of Excellence in Computational Aerospace Engineering, Amirkabir University of Technology, Tehran, Iran
H. Assaee : Mechanical & Aerospace Engineering Department, Shiraz University of Technology, Shiraz, Iran

Abstract
For the structures containing multiple discontinuities (voids, inclusions, and cracks), the simulation technologies in the framework of extended finite element method (XFEM) are discussed in details. The level set method is used for representing the location of inner discontinuous interfaces so that the mesh does not need to align with these discontinuities. Several illustrations have been given to verify that the implemented XFEM program is effective. Then, the implemented XFEM program is used to investigate the effects of the voids, inclusions, and minor cracks on the path of major crack propagation. For a plate containing cracks and voids, two possibly crack path can be observed: i) the crack propagates into the void; ii) the crack initially curves towards the void, then, the crack reorients itself and propagates along its original orientation. For a plate with a soft inclusion, the final predicted crack paths tend to close with the inclusion, and an evident difference of crack paths can be observed with different inclusion material properties. However, for a plate with a hard inclusion, the paths tend to away from the inclusion, and a slightly difference of crack paths can only be seen with different inclusion material properties. For a plate with several minor cracks, the trend of crack paths can still be described as that the crack initially curves towards these minor cracks, and then, the crack reorients itself and propagates almost horizontally along its original orientation.

Key Words
extended finite element method; crack propagation; voids; inclusions; cracks

Address
Shouyan Jiang and Chongshi Gu : College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China
Chengbin Du : College of Mechanics and Materials, Hohai University, Nanjing 210098, China

Abstract
As hyperboloidal cooling towers (HCTs) growing larger and slender, they become more sensitive to gust wind. To improve the dynamic properties of HCTs and to improve the wind resistance capability, stiffening rings have been studied and applied. Although there have been some findings, the influence mechanism of stiffening rings on the dynamic properties is still not fully understood. Based on some fundamental perceptions on the dynamic properties of HCTs and free ring structures, a concept named \"participation degree\" of stiffening rings was proposed and the influence mechanism on the dynamic properties was illustrated. The \"participation degree\" is determined by the modal deform amplitude and latitude wave number of stiffening rings. Larger modal deform amplitude and more latitude waves can both result in higher participation degree and more improvement to eigenfrequencies. Also, this concept can explain and associate the pre-existing independent findings.

Key Words
hyperboloidal cooling towers; stiffening rings; participation degree; dynamic properties; modal deform amplitude; latitude wave number

Address
Jun-Feng Zhang, Huai Chen : School of Civil Engineering, Zhengzhou University, Zhengzhou 450001, China
Yao-Jun Ge, Lin Zhao : Department of Bridge Engineering, Tongji University, Shanghai 200092, China
Shi-Tang Ke : Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China

Abstract
Seismic assessment and retrofitting of existing structure is a complicated work that typically requires more sophisticated analyses than performing a new design. Before the implementation of a Code for seismic design of buildings (GBJ 11-89), not enough attention has been paid on seismic performance of structures and a great part of the existing reinforced concrete structures built in China have been poorly designed according to the new version of the same code (GB 50011-2010). This paper presents a case study of seismic assessment of a non-seismically designed reinforced concrete building in China. The structural responses are evaluated using the nonlinear static procedure (the so-called pushover analysis), which requires its introduction within a process that allows the estimation of the demand, against which the capacity is then compared with. The capacity of all structural members can be determined following the design code. Based on the structural performance, suitable retrofitting strategies are selected and implemented to the existing system. The retrofitted structure is analyzed again to check the effectiveness of the rehabilitation. Different types of retrofitting strategy are discussed and classified according to their complexity and benefits. Finally, a proper intervention methodology is utilized to upgrade this typical lowrise non-ductile building.

Key Words
seismic assessment; nonlinear static procedure; retrofitting; capacity; demand

Address
Pengpeng Ni : ROSE Programme, UME (Understanding and Managing Extremes) Graduate School, IUSS Pavia (Institute for Advanced Study, Pavia), Via Ferrata 1, 27100 Pavia, Italy

Abstract
This paper presents the design and project of an innovative concept for a Single Point Incremental Forming (SPIF) Machine. Nowadays, equipment currently available for conducting SPIF result mostly from the adaptation of conventional CNC machine tools that results in a limited range of applications in terms of materials and geometries. There is also a limited market supply of equipment dedicated to Incremental Sheet Forming (ISF), that are costly considering low batches, making it unattractive for industry. Other factors impairing a quicker spread of SPIF are large forming times and poor geometrical accuracy of parts. The following sections will depict the development of a new equipment, designed to overcome some of the limitations of machines currently used, allowing the development of a sounding basis for further studies on the particular features of this process. The equipment here described possesses six-degrees-of freedom for the tool, for the sake of improved flexibility in terms of achievable tool-paths and an extra stiffness provided by a parallel kinematics scheme. A brief state of the art about the existing SPIF machines is provided to support the project\'s guidelines.

Key Words
parallel kinematics; single point incremental forming; machine design; Stewart platform

Address
R.J. Alves de Sousa, J.A.F. Ferreira, J.B. Sa de Farias, J.N.D. Torrao, D.G. Afonso and M.A.B.E Martins : TEMA: Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, Campus de Santiago, 3810-183 Aveiro, Portugal

Abstract
Laminated Rectangular plates embedded in elastic foundations are used in many mechanical structures. This study presents an analytical approach for transverse bending analysis of an embedded symmetric laminated rectangular plate using Mindlin plate theory. The surrounding elastic medium is simulated using Pasternak foundation. Adopting the Mindlin plate theory, the governing equations are derived based on strain-displacement relation, energy method and Hamilton\'s principle. The exact analysis is performed for this case when all four ends are simply supported. The effects of the plate length, elastic medium and applied force on the plate transverse bending are shown. Results indicate that the maximum deflection of the laminated plate decreases when considering an elastic medium. In addition, the deflection of the laminated plate increases with increasing the plate width and length.

Key Words
transverse bending; mindlin plate theory; energy method; pasternak model

Address
Mohammad Mehdi Heydari, Reza Kolahchi, Morteza Heydari and Ali Abbasi : Young Researchers and Elite Club, Kashan Branch, Islamic Azad University, Kashan, Iran


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