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CONTENTS
Volume 75, Number 6, September25 2020
 

Abstract
This study presents experimental and numerical investigations on circular steel tube confined ultra high performance concrete (UHPC) columns under axial compression. The plain UHPC without fibers was designed to achieve a compressive strength ranged between 150 MPa and 200 MPa. Test results revealed that loading on only the UHPC core can generate a significant confinement effect for the UHPC core, thus leading to an increase in both strength and ductility of columns, and restricting the inherent brittleness of unconfined UHPC. All tested columns failed by shear plane failure of the UHPC core, this causes a softening stage in the axial load versus axial strain curves. In addition, an increase in the steel tube thickness or the confinement index was found to increase the strength and ductility enhancement and to reduce the magnitude of the loss of load capacity. Besides, steel tube with higher yield strength can improve the post-peak behavior. Based on the test results, the load contribution of the steel tube and the concrete core to the total load was examined. It was found that no significant confinement effect can be developed before the peak load, while the ductility of post-peak stage is mainly affected by the degree of the confinement effect. A finite element model (FEM) was also constructed in ABAQUS software to validate the test results. The effect of bond strength between the steel tube and the UHPC core was also investigated through the change of friction coefficient in FEM. Furthermore, the mechanism of circular steel tube confined UHPC columns was examined using the established FEM. Based on the results of FEM, the confining pressures along the height of each modeled column were shown. Furthermore, the interaction between the steel tube and the UHPC core was displayed through the slip length and shear stresses between two surfaces of two materials.

Key Words
UHPC; steel tube; axial compression; confinement effect; confinement index; FEM

Address
1Thuyloi University, 175 Tay son, Dong da, Hanoi, Vietnam
2NTT Hi-Tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City, Vietnam
3Department of Civil and Environmental Engineering, National University of Singapore

Abstract
The present paper employs nonlocal strain gradient theory (NSGT) to study buckling behavior of functionally graded magneto-electro-thermo-elastic (FG-METE) nanoshells under various physical fields. NSGT modeling of the nanoshell contains two size parameters, one related to nonlocal stress field and another related to strain gradients. It is considered that mechanical, thermal, electrical and magnetic loads are exerted to the nanoshell. Temperature field has uniform and linear variation in nanoshell thickness. According to a power-law function, piezo-magnetic, thermal and mechanical properties of the nanoshell are considered to be graded in thickness direction. Five coupled governing equations have been obtained by using Hamilton's principle and then solved implementing Galerkin's method. Influences of temperature field, electric voltage, magnetic potential, nonlocality, strain gradient parameter and FG material exponent on buckling loads of the FG-METE nanoshell have been studied in detail.

Key Words
buckling; classical shell theory; functionally graded material; magneto-electro-thermo-elastic material; nonlocal strain gradient theory

Address
Reza Asrari and Mohammad Mahdi Kheirikhah: Faculty of Industrial and Mechanical Engineering, Qazvin Branch, Islamic Azad University, Qazvin, Iran
Farzad Ebrahimi: Department of Mechanical Engineering, Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran

Abstract
An innovative retrofit method using pre-stressed steel strips and externally-bonded steel plates was presented in this paper. With the aim of exploring the seismic performance of the retrofitted RC interior joints, four 1/2-scale retrofitted joint specimens together with one control specimen were designed and subjected to constant axial compression and cyclic loading, with the main test parameters being the volume of steel strips and the existence of externally-bonded steel plates. The damage mechanism, force-displacement hysteretic response, force-displacement envelop curve, energy dissipation and displacement ductility ratio were analyzed to investigate the cyclic behavior of the retrofitted joints. The test results indicated that all the test specimens suffered a typical shear failure at the joint core, and the application of externally-bonded steel plates and that of pre-stressed steel strips could effectively increase the lateral capacity and deformability of the deficient RC interior joints, respectively. The best cyclic behavior could be found in the deficient RC interior joint retrofitted using both externally-bonded steel plates and pre-stressed steel strips due to the increased lateral capacity, displacement ductility and energy dissipation. Finally, based on the test results and the softened strut and tie model, a theoretical model for determining the shear capacity of the retrofitted specimens was proposed and validated.

Key Words
beam-column joints; pre-stressed steel strip; externally-bonded steel plate; retrofitting; pseudostatic test; shear capacity

Address
Yunlong Yu, Yong Yang: 1School of Civil Engineering, Xian University of Architecture & Technology, Shaanxi, China
2Key Lab of Structural Engineering and Earthquake Resistance, Ministry of Education, China
Yicong Xue:School of Civil Engineering, Xian University of Architecture & Technology, Shaanxi, China
Niannian Wang: Sichuan Provincial Architectural Design and Research Institute Co., LTD, SADI, Sichuan, China
Yaping Liu:China Qiyuan Engineering Corporation, Shaanxi, China

Abstract
Polygonal finite element provides a great flexibility in mesh generation of crack propagation problems where the topology of the domain changes significantly. However, the control of the discretization error in such problems is a main concern. In this paper, a polygonal-FEM is presented in modeling of crack propagation problems via an automatic adaptive mesh refinement procedure. The adaptive mesh refinement is accomplished based on the Zienkiewicz–Zhu error estimator in conjunction with a weighted SPR technique. Adaptive mesh refinement is employed in some steps for reduction of the discretization error and not for tracking the crack. In the steps that no adaptive mesh refinement is required, local modifications are applied on the mesh to prevent poor polygonal element shapes. Finally, several numerical examples are analyzed to demonstrate the efficiency, accuracy and robustness of the proposed computational algorithm in crack propagation problems.

Key Words
polygonal finite element; adaptive mesh refinement; error estimation; crack propagation; linear fracture mechanics

Address
Department of Civil Engineering, Shahed University,
Persian Gulf Highway, Tehran, Iran

Abstract
This papers studies nonlinear stability and post-buckling behaviors of geometrically imperfect metal foam doubly-curved shells with eccentrically stiffeners resting on elastic foundation. Metal foam is considered as porous material with uniform and non-uniform models. The doubly-curved porous shell is subjected to in-plane compressive loads as well as a transverse pressure leading to post-critical stability in nonlinear regime. The nonlinear governing equations are analytically solved with the help of Airy stress function to obtain the post-buckling load-deflection curves of the geometrically imperfect metal foam doubly-curved shell. Obtained results indicate the significance of porosity distribution, geometrical imperfection, foundation factors, stiffeners and geometrical parameters on post-buckling characteristics of porous doubly-curved shells.

Key Words
post-buckling; shell theory; porous material; curved shell; nonlinear stability; stiffeners

Address
Seyed Sajad Mirjavadi, Masoud Forsat, AMS Hamouda: Department of Mechanical and Industrial Engineering, Qatar University, P.O. Box 2713, Doha, Qatar
Yahya Zakariya Yahya: Auckland Bioengineering Institute, the University of Auckland, Auckland, New Zealand
Mohammad Reza Barati: Fidar project Qaem Company, Darvazeh Dolat, Tehran, Iran
Anirudh Narasimamurthy Jayasimha: Bonn-Rhein-Sieg University of Applied Science, Sankt Augustin, Germany

Abstract
This article aims to illustrate the damped dynamic responses of layered functionally graded (FG) thick 2D beam under dynamic pulse sinusoidal load by using finite element method, for the first time. To investigate the response of thick beam accurately, two-dimensional plane stress problem is assumed to describe the constitutive behavior of thick beam structure. The material is distributed gradually through the thickness of each layer by generalized power law function. The Kelvin–Voigt viscoelastic constitutive model is exploited to include the material internal damping effect. The governing equations are obtained by using Lagrange's equations and solved by using finite element method with twelve –node 2D plane element. The dynamic equation of motion is solved numerically by Newmark implicit time integration procedure. Numerical studies are presented to illustrate stacking sequence and material gradation index on the displacement-time response of cantilever beam structure. It is found that, the number of waves increases by increasing the graduation distribution parameter. The presented mathematical model is useful in analysis and design of nuclear, marine, vehicle and aerospace structures those manufactured from functionally graded materials (FGM).

Key Words
dynamic analysis; thick beam; pulse load; layered FGM; viscoelastic; finite element method

Address
Saeed A. Asiri: Mechanical Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. Box 80204, Jeddah, Saudi Arabia
Şeref D. Akbaş: Deparment of Civil Engineering, Bursa Technical University, Bursa, Turkey
Mohamed A. Eltaher: 1 Mechanical Engineering Department, Faculty of Engineering, King Abdulaziz University, P.O. Box 80204, Jeddah, Saudi Arabia 2 Department of Mechanical Design & Production, Faculty of Engineering, Zagazig University,P.O. Box 44519, Zagazig, Egypt

Abstract
In this paper, acoustic emission (AE) and pattern recognition are utilized to identify the AE signal signatures caused by propagation of stress corrosion cracking (SCC) in a 304 stainless steel plate. The surface of the plate is under almost uniform tensile stress at a notch. A corrosive environment is provided by exposing the notch to a solution of 1% Potassium Tetrathionate by weight. The Global b-value indicated an occurrence of the first visible crack and damage stages during the SCC. Furthermore, a method based on linear regression has been developed for damage identification using AE data.

Key Words
acoustic emission; damage identification; global b-value; pattern recognition; stress corrosion cracking

Address
V. Soltangharaei, J.W. Hill, Li Ai, R. Anay, M. Bayat and P. Ziehl: Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC, USA
B. Greer: Senior Technical Leader, Electrical Power Research Institute, USA

Abstract
This paper attempts to investigate the free vibration of functionally graded material beams with nonuniform width based on the nonlocal elasticity theory. The theoretical formulations are established following the Euler–Bernoulli beam theory, and the governing equations of motion of the system are derived from the minimum total potential energy principle using the nonlocal elasticity theory. In addition, the Differential Quadrature Method (DQM) is applied, along with the Chebyshev-Gauss-Lobatto polynomials, in order to determine the weighting coefficient matrices. Furthermore, the effects of the nonlocal parameter, cross-section area of the functionally graded material (FGM) beam and various boundary conditions on the natural frequencies are examined. It is observed that the nonlocal parameter and boundary conditions significantly influence the natural frequencies of the functionally graded material beam cross-section. The results obtained, using the Differential Quadrature Method (DQM) under various boundary conditions, are found in good agreement with analytical and numerical results available in the literature.

Key Words
free vibration; nonuniform width; Euler-Bernoulli beam; Nonlocal theory; Differential Quadrature Method; Functionally Graded Material

Address
Noureddine Elmeiche, Hichem Abbad: Civil Engineering and Environmental Laboratory, Sidi Bel Abbes University, BP 89, Sidi Bel Abbes 22000, Algeria
Ismail Mechab: LMPM, Department of Mechanical Engineering, University of Sidi Bel Abbes, Sidi Bel Abbes 22000, Algeria.
Fabrice Bernard: Université de Rennes, INSA de Rennes - LGCGM, 20 avenue des Buttes de Coësmes, 35708 Rennes cedex 7, France

Abstract
This paper provides a semi-analytical approach to investigate the variations of 3D displacement components, electric potential, stresses, electric displacements and transverse vibration frequencies in laminated piezoelectric composite plates based on the scaled boundary finite element method (SBFEM) and the precise integration algorithm (PIA). The proposed approach can analyze the static and dynamic responses of multilayered piezoelectric plates with any number of laminae, various geometrical shapes, boundary conditions, thickness-to-length ratios and stacking sequences. Only a longitudinal surface of the plate is discretized into 2D elements, which helps to improve the computational efficiency. Comparing with plate theories and other numerical methods, only three displacement components and the electric potential are set as the basic unknown variables and can be represented analytically through the transverse direction. The whole derivation is built upon the three dimensional key equations of elasticity for the piezoelectric materials and no assumptions on the plate kinematics have been taken. By virtue of the equilibrium equations, the constitutive relations and the introduced set of scaled boundary coordinates, three-dimensional governing partial differential equations are converted into the second order ordinary differential matrix equation. Furthermore, aided by the introduced internal nodal force, a first order ordinary differential equation is obtained with its general solution in the form of a matrix exponent. To further improve the accuracy of the matrix exponent in the SBFEM, the PIA is employed to make sure any desired accuracy of the mechanical and electric variables. By virtue of the kinetic energy technique, the global mass matrix of the composite plates constituted by piezoelectric laminae is constructed for the first time based on the SBFEM. Finally, comparisons with the exact solutions and available results are made to confirm the accuracy and effectiveness of the developed methodology. What\'s more, the effect of boundary conditions, thickness-to-length ratios and stacking sequences of laminae on the distributions of natural frequencies, mechanical and electric fields in laminated piezoelectric composite plates is evaluated.

Key Words
laminated piezoelectric composite plates; static bending responses; free vibration; the scaled boundary finite element method; the precise integration algorithm

Address
Pengchong Zhang, Chengzhi Qi: 1School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, Beijing 102616, China
2Beijing Advanced Innovation Center for Future Urban Design, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
Xu Sun: School of Civil and Transportation Engineering, Beijing University of Civil Engineering and Architecture, Beijing 102616, China
Hongyuan Fang: College of Water Conservancy & Environmental Engineering, Zhengzhou University. Zhengzhou 450001, China

Abstract
Reliability analysis techniques combining with various surrogate models have attracted increasing attention because of their accuracy and great efficiency. However, they primarily focus on the structures with continuous response, while very rare researches on the reliability analysis for structures with discontinuous response are carried out. Furthermore, existing adaptive reliability analysis methods based on importance sampling (IS) still have some intractable defects when dealing with small failure probability, and there is no related research on reliability analysis for structures involving discontinuous response and small failure probability. Therefore, this paper proposes a novel reliability analysis method called AGPC-IS for such structures, which combines adaptive Gaussian process classification (GPC) and adaptive-kernel-density-estimation-based IS. In AGPC-IS, an efficient adaptive strategy for design of experiments (DoE), taking into consideration the classification uncertainty, the sampling uniformity and the regional classification accuracy improvement, is developed with the purpose of improving the accuracy of Gaussian process classifier. The adaptive kernel density estimation is introduced for constructing the quasi-optimal density function of IS. In addition, a novel and more precise stopping criterion is also developed from the perspective of the stability of failure probability estimation.The efficiency, superiority and practicability of AGPC-IS are verified by three examples

Key Words
reliability analysis; discontinuous response; Gaussian process classification; small failure probability; importance sampling

Address
School of Mechanical Engineering and Automation, Northeastern University, Wenhua Road, Heping District,
Shenyang 110819, Liaoning, People's Republic of China

Abstract
As the growth of world trade has surged rapidly over the past years, the number is expected to continue growing over the coming years. Although the transportation costs can be reduced by using larger vessels, however, new berthing structures have to be constructed in order to cater for the larger vessels. This leads to a need for researching on designing a better berthing structure. For optimization of berthing structure design, we need to provide a better estimation of berthing energy than the previous methods in the existing guidelines. In this study, several berthing parameters were collected from previous works and researches. Moreover, the scenarios were selected efficiently by using a sampling technique. First, the berthing energy was calculated by executing 150 numerical simulations. Then, the numerical simulation results were compared with the results calculated by existing methods quantitatively to investigate the sensitivity of the berthing parameters and the accuracy of existing methods. The numerical method results have shown some deviation with respect to the existing method results in which the degree of deviation varies with the methods and the tendency of differences is dependent on certain berthing parameters. Then, one of the existing methods which has shown a small deviation was selected as a representative method and applied with several safety factors to obtain a suitable safety factor for the design.

Key Words
berthing energy, kinematic energy method, berthing velocity, berthing angle, safety factor

Address
Sang Woo Kim: 1Offshore Plant Engineering Laboratory, Division of Naval Architecture and Ocean Systems Engineering,
Korea Maritime and Ocean University, 49112 Busan, Republic of Korea
2Ocean & Ship Technology, Department of Civil and Environmental Engineering,
Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia
Seung Jae Lee: Offshore Plant Engineering Laboratory, Division of Naval Architecture and Ocean Systems Engineering,
Korea Maritime and Ocean University, 49112 Busan, Republic of Korea
Young Tae Kim: Daewoo Institute of Construction Technology, 16297 Suwon, Republic of Korea
Do Kyun Kim: 1 Ocean & Ship Technology, Department of Civil and Environmental Engineering,
Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia2 Group of Marine Offshore and Subsea Technology, School of Engineering, Newcastle University, NE1 7RU Newcastle upon Tyne, United Kingdom
3 Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, 37673 Pohang, Republic of Korea


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