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CONTENTS
Volume 73, Number 5, March10 2020
 

Abstract
This paper deals with the buckling and optimization of a nanocomposite beam. The agglomeration of nanoparticles was assumed by Mori-Tanaka model. The harmony search optimization algorithm is adaptively improved using two adjusted processes based on dynamic parameters. The governing equations were derived by Timoshenko beam model by energy method. The optimum conditions of the nanocomposite beam- based proposed AIHS are compared with several existing harmony search algorithms. Applying DQ and Hs methods, the optimum values of radius and FS were obtained. The effects of thickness, agglomeration, volume percent of CNTs and boundary conditions were assumed. The results show that with increasing the volume percent of CNTs, the optimum radius of the beam decreases while the FS was improved.

Key Words
harmony search; buckling; optimization nanocomposite beam; nanoparticles

Address
Mohsen Motezaker: School of Railway Engineering, Iran University of Science and Technology, Tehran, Iran
Arameh Eyvazian: Mechanical and Industrial Engineering Department, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar

Abstract
Stiffeners can be utilised to enhance the strength of thin-walled wind turbine towers in engineering practise, thus, structural performance of wind turbine towers by means of different stiffening schemes should be compared to explore the optimal structural enhancement method. In this paper two alternative stiffening methods, employing horizontal or vertical stiffeners, for steel tubular wind turbine towers have been studied. In particular, two groups of three wind turbine towers of 50m, 150m and 250m in height, stiffened by horizontal rings and vertical strips respectively, were analysed by using FEM software of ABAQUS. For each height level tower, the mass of the stiffening rings is equal to that of vertical stiffeners each other. The maximum von Mises stresses and horizontal sways of these towers with vertical stiffeners is compared with the corresponding ring-stiffened towers. A linear buckling analysis is conducted to study the buckling modes and critical buckling loads of the three height levels of tower. The buckling modes and eigenvalues of the 50m, 150m and 250m vertically stiffened towers were also compared with those of the horizontally stiffened towers. The numbers and central angles of the vertical stiffeners are considered as design variables to study the effect of vertical stiffeners on the structural performance of wind turbine towers. Following an extensive parametric study, these strengthening techniques were compared with each other and it is obtained that the use of vertical stiffeners is a more efficient approach to enhance the stability and strength of intermediate and high towers than the use of horizontal rings.

Key Words
wind turbine tower; shell structures; stiffening rings; vertical stiffeners; parametric study; finite element analysis

Address
Yu Hu, Jian Yang, Feiliang Wang :School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
Yu Hu, Jian Yang, Charalambos C. Baniotopoulos: School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom


Abstract
In the present work, the optimization of machining parameters to achieve the desired technological parameters such as surface roughness, tool radial vibration and material removal rate have been carried out using response surface methodology (RSM). The hard turning of EN19 alloy steel with coated carbide (GC3015) cutting tools was studied. The main problem faced in manufacturer of hard and high precision components is the selection of optimum combination of cutting parameters for achieving required quality of surface finish with maximum production rate. This problem can be solved by development of mathematical model and execution of experiments by RSM. A face centred central composite design (FCCD), which comes under the RSM approach, with cutting parameters (cutting speed, feed rate and depth of cut) was used for statistical analysis. A second-order regression model were developed to correlate the cutting parameters with surface roughness, tool vibration and material removal rate. Consequently, numerical and graphical optimization were performed to obtain the most appropriate cutting parameters to produce the lowest surface roughness with minimal tool vibration and maximum material removal rate using desirability function approach. Finally, confirmation experiments were performed to verify the pertinence of the developed mathematical models.

Key Words
hard turning; optimization; mathematical model; surface roughness; tool vibration

Address
Mohamed Walid Azizi, Ouahid Keblouti, Lakhdar Boulanouar: Advanced Technologies in Mechanical Production Research Laboratory (LRTAPM), Badji Mokhtar - Annaba University,
P.O Box 12, 23000 Annaba, Algeria
Mohamed Walid Azizi: Technical Science Department, Abdelhafid Boussouf-Mila University Center, 43000, Algeria
Mohamed Athmane Yallese: Mechanics and Structures Research Laboratory (LMS), May 8th 1945 University, P.O. Box 401, Guelma 24000, Algeria

Abstract
This paper presents a stress field approach for the shear capacity of stirrup-reinforced concrete beams that explicitly incorporates the contribution of principal tensile stresses in concrete. This formulation represents an extension of the variable strut inclination method adopted in the Eurocode 2. In this model, the stress fields in web concrete consist of principal compressive stresses inclined at an angle &theta combined with principal tensile stresses oriented along a direction orthogonal to the former (the latter being typically neglected in other formulations). Three different failure mechanisms are identified, from which the strut inclination angle and the corresponding shear strength are determined through equilibrium principles and the static theorem of limit analysis, similar to the EC-2 approach. It is demonstrated that incorporating the contribution of principal tensile stresses of concrete slightly increases the ultimate inclination angle of the compression struts as well as the shear capacity of reinforced concrete beams. The proposed stress field approach improves the prediction of the shear strength in comparison with the Eurocode 2 model, in terms of both accuracy (mean) and precision (CoV), as demonstrated by a broad comparison with more than 200 published experimental results from the literature.

Key Words
reinforced concrete beam; limit analysis; shear strength; principal tensile stresses; smeared truss model; concrete compression struts; transverse reinforcement; Eurocode 2; stirrups; variable strut inclination method

Address
Department of Engineering, University of Messina, Contrada Di Dio, Villaggio Sant'Agata, 98166 Messina, Italy

Abstract
As limited well-documented experimental data are available for assessing the attributes of different deformation components of flanged walls, few appropriate models have been established for predicting the inelastic responses of flanged walls, especially those of asymmetrical flanged walls. This study presents the experimental results for three large-scale T-shaped reinforced concrete walls and examines the variations in the flexural, shear, and sliding components of deformation with the total deformation over the entire loading process. Based on the observed deformation behavior, a simple model based on moment-curvature analysis is established to estimate flexural deformations, in which the changes in plastic hinge length are considered and the deformations due to strain penetration are modeled individually. Based on the similar gross shapes of the curvature and shear strain distributions over the wall height, a proportional relationship is established between shear displacement and flexural rotation. By integrating the deformations due to flexure, shear, and strain penetration, a new load-deformation analytical model is proposed for flexure-dominant flanged walls. The proposed model provides engineers with a simple, accurate modeling tool appropriate for routine design work that can be applied to flexural walls with arbitrary sections and is capable of determining displacements at any position over the wall height. By further simplifying the analytical model, a simple procedure for estimating the ultimate displacement capacity of flanged walls is proposed, which will be valuable for performance-based seismic designs and seismic capacity evaluations.

Key Words
flanged walls; cyclic test; deformation; analytical model; ultimate displacement

Address
Bin Wang, Qing-Xuan Shi,Wen-Zhe Cai and YI-Gong Peng: State Key Laboratory of Green Building in Western China, Xi'an University of Architecture and Technology, Xi'an 710055, Peoples R China
Bin Wang, Qing-Xuan Shi: School of Civil Engineering, Xi'an University of Architecture and Technology, Xi'an, 710055, Peoples R China

Abstract
This paper aims to investigate the seismic performance of RC short columns and long columns with welding stirrups. Through the low-cyclic horizontal loading test of specimens, the seismic performance indexes such as failure modes, hysteretic curve, skeleton curve, ductility, energy dissipation capacity, stiffness degradation and strength degradation were emphatically analyzed. Furthermore, the effects of shear span ratio, stirrups ratio and axial compression ratio on the performance of specimens were studied. The results showed that the seismic performance of the RC short columns with welding stirrups were basically the same as that of the RC short columns with traditional stirrups, but the seismic performance of RC long columns with welding stirrups was better than that of RC long columns with traditional stirrups. The seismic performance of RC short columns and long columns with welding stirrups could be improved by increasing stirrup ratio and shear span ratio and reducing axial pressure ratio. Moreover, the welding stirrup have the advantages of steel saving, industrialization and standardization production, convenient construction, and reducing time, which indicated that the welding stirrups could be applied in practical engineering.

Key Words
welding stirrups; reinforced concrete; short column; long column; seismic performance

Address
Yunlong Yu, Zhaohui Dang, Yong Yang, Yang Chen and Hui Li: School of Civil Engineering, Xi'an University of Architecture & Technology, Xi'an, Shaanxi 710055, China
Yunlong Yu: Key Lab of Structural Engineering and Earthquake Resistance, Ministry of Education (XAUAT)

Abstract
The objective of present paper is assessment of dynamic buckling behavior of an embedded sandwich microplates in thermal environment in which the layers are reinforced through functionally graded carbon nanotubes (FG-CNTs). Therefore, mixture rule is taken into consideration for obtaining effective material characteristics. In order to model this structure much more realistic, Kelvin–Voigt model is presumed and the sandwich structure is rested on visco-Pasternak medium. Exponential shear deformation theory (ESDT) in addition to Eringen's nonlocal theory are utilized to obtain motion equations. Further, differential cubature method (DCM) as well as Bolotin's procedure are used to solve governing equations and achieve dynamic instability region (DIR) related to sandwich structure. Different parameters focusing on volume percent of CNTs, dispersion kinds of CNTs, thermal environment, small scale effect and structural damping and their influences upon the dynamic behavior of sandwich structure are investigated. So as to indicate the accuracy of applied theories as well as methods, the results are collated with another paper. According to results, presence of CNTs and their dispersion kind can alter system's dynamic response as well.

Key Words
dynamic buckling; FG-CNT; nanocomposite sandwich micro plate; ESDT; viscoelastic

Address
Ahmad Farokhian: Mechanical Engineering group, Pardis College, Isfahan University of Technology, Isfahan 84156-83111, Iran
Reza Kolahchi: Institute of Research and Development, Duy Tan University, Da Nang 550000, Viet Nam

Abstract
The nonlinear thermo-electro-elastic buckling behavior of viscoelastic nanoplates under magnetic field is investigated based on nonlocal elasticity theory. Employing nonlinear strain-displacement relations, the geometrical nonlinearity is modeled while governing equations are derived through Hamilton\'s principle and they are solved applying semi-analytical generalized differential quadrature (GDQ) method. Eringen\'s nonlocal elasticity theory considers the effect of small size, which enables the present model to become effective in the analysis and design of nano-sensors and nano actuators. Based on Kelvin-Voigt model, the influence of the viscoelastic coefficient is also discussed. It is demonstrated that the GDQ method has high precision and computational efficiency in the buckling analysis of viscoelastic nanoplates. The good agreement between the results of this article and those available in literature validated the presented approach. The detailed mathematical derivations are presented and numerical investigations are performed while the emphasis is placed on investigating the effect of the several parameters such as electric voltage, small scale effects, elastomeric medium, magnetic field, temperature effects, the viscidity and aspect ratio of the nanoplate on its nonlinear buckling characteristics. It is explicitly shown that the thermo-electro-elastic nonlinear buckling behavior of viscoelastic nanoplates is significantly influenced by these effects. Numerical results are presented to serve as benchmarks for future analyses of viscoelastic nanoplates as fundamental elements in nanoelectromechanical systems.

Key Words
small scale effect; nonlinear buckling; double nanoplate; viscoelastic; GDQ

Address
Farzad Ebrahimi, S. Hamed S. Hosseini: Department of Mechanical Engineering, Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran
Rajendran Selvamani: Department of mathematics, Karunya Institute of Technology and Sciences, Coimbatore, India

Abstract
We develop design model within which nucleation and propagation of crack in a fibrous composite is described. It is assumed that under loading, crack initiation and fracture of material happens in the composite. The problem of equilibrium of a composite with embryonic crack is reduced to the solution of the system of nonlinear singular integral equations with the Cauchy type kernel. Normal and tangential forces in the crack nucleation zone are determined from the solution of this system of equations. The crack appearance conditions in the composite are formed with regard to criterion of ultimate stretching of the material\'s bonds. We study the case when near the fiber, the binder has several arbitrary arranged rectilinear prefracture zones and a crack with interfacial bonds. The proposed computational model allows one to obtain the size and location of the zones of damages (prefracture zones) depending on geometric and mechanical characteristics of the fibrous composite and applied external load. Based on the suggested design model that takes into account the existence of damages (the zones of weakened interparticle bonds of the material) and cracks with end zones in the composite, we worked out a method for calculating the parameters of the composite, at which crack nucleation and crack growth occurs.

Key Words
binder; fiber; prefracture zone; crack nucleation; cohesive forces; crack growth

Address
Vagif M. Mirsalimov and Shahin H. Hasanov: Department of Mechanics, Azerbaijan Technical University, Baku, H. Javid av, 25, Azerbaijan
Vagif M. Mirsalimov: Institute of Mathematics and Mechanics, National Academy of Sciences of Azerbaijan, Baku, B. Vahabzade, 9, Azerbaijan

Abstract
A combination of a gravity wall and an anchor beam is widely used to support the high soil deposit on rock mass. In this study, two groups of shaking table test were performed to investigate the responses of such combined retaining structure, where the rock masses were shaped with a flat surface and a curved surface, respectively. Meanwhile, the dynamic numerical analysis was carried out for a comparison or an extensive study. The results were studied and compared between the combined retaining structures with different shaped rock masses with regard to the acceleration response, the earth pressure response, and the axial anchor force. The acceleration response is not significantly influenced by the surface shape of rock mass. The earth pressure response on the combined retaining structure with a flat rock surface is more intensive than the one with a curved rock surface. The anchor force is significantly enlarged by seismic excitation with a main earthquake-induced increment at the first intensive pulse of Wenchuan motion. The value of anchor force in the combined retaining structure with a flat rock surface is generally larger than the one with a curved rock surface. Generally, the combined retaining structure with a curved rock surface presents a better seismic performance.

Key Words
gravity wall; anchor beam; shaped rock mass; shaking table test; numerical simulation

Address
Yu-liang Lin, Lian-heng Zhao, T.Y. Yang, Guo-lin Yang and Xiao-bin Chen
Yu-liang Lin, Lian-heng Zhao, Guo-lin Yang and Xiao-bin Chen: School of Civil Engineering, Central South University, Changsha, 410075, China
Yu-liang Lin, T.Y. Yang: Department of Civil Engineering, University of British Columbia, Vancouver, V6T 1Z4, Canada
Yu-liang Lin: State Key Laboratory for GeoMechanics and Deep Underground Engineering, China University of Mining & Technology,
Xuzhou, 221008, China
T.Y. Yang: International Joint Research Laboratory of Earthquake Engineering, Tongi University, Shanghai, 200092, China


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