Techno Press
Tp_Editing System.E (TES.E)
Login Search


scs
 
CONTENTS
Volume 36, Number 1, July10 2020
 

Abstract
This paper presents the influence of carbon nanotubes (CNTs) waviness, aspect ratio, internal pores and graphene platelets (GPLs) on the vibrational behavior of functionally graded nanocomposite sandwich beams resting on two-parameter elastic foundations. The distributions of CNTs are considered functionally graded (FG) or uniform along the thickness of upper and bottom layers of the sandwich beam and their mechanical properties are estimated by an extended rule of mixture. In this study, the classical theory concerning the mechanical efficiency of a matrix embedding finite length fibers has been modified by introducing the tube-to-tube random contact, which explicitly accounts for the progressive reduction of the tubes\' effective aspect ratio as the filler content increases. The core of structure is porous and the internal pores and graphene platelets (GPLs) are distributed in the matrix of core either uniformly or non-uniformly according to three different patterns. The elastic properties of the nanocomposite are obtained by employing Halpin-Tsai micromechanics model. The equations of motion are derived based on Timoshenko beam theory and employing Hamilton\'s principle. The problem is modeled using a semi-analytical approach composed of generalized differential quadrature method (GDQM) and series solution adopted to solve the equations of motion. Detailed parametric studies are carried out to investigate carbon nanotubes (CNTs) waviness, CNT aspect ratio, porosity coefficient, porosity distribution, graphene platelets (GPLs) distribution, Winkler foundation modulus, shear elastic foundation modulus and geometrical conditions on the vibrational behavior of the sandwich structure.

Key Words
CNTs waviness and aspect ratio; sandwich beams; vibration; rule of mixture; two-parameter elastic foundations; functionally graded materials; porous core; Halpin-Tsai micromechanics model

Address
Hua Si, Daoming Shen and Jinhong Xia: School of Civil Engineering & Architecture, Xinxiang university, Xinxiang, 453000, China
Vahid Tahouneh: Young Researchers and Elite Club, Islamshahr Branch, Islamic Azad University, Islamshahr, Iran

Abstract
This paper presents experimental and numerical studies on the seismic responses of the steel cabinet facility considering the nonlinear behavior of connections and site-response effects. Three finite element (FE) models with differences of type and number of connections between steel plates and frame members have been developed to demonstrate adequately dynamic responses of structures. The screw connections with the bilinear force-deformation relationship are proposed to represent the inelastic behavior of the cabinet. The experiment is carried out to provide a verification with improved FE models. It shows that the natural frequencies of the cabinet are sensitive to the plate and frame connectors. The screw connections reduce the free vibration compared to the weld one, with decreased values of 2.82% and 4.87% corresponding to front-to-back and side-to-side directions. Additionally, the seismic responses are investigated for various geological configurations. Input time histories are generated so that their response spectrums are compatible with a required response spectrum via the time-domain spectral matching. The results indicate that both site effects and nonlinear behavior of connections affect greatly on the seismic response of structures.

Key Words
cabinet facility; site-response effect; nonlinear connection; steel structures; spectral matching; required response spectrum

Address
Thanh-Tuan Tran: Institute of Offshore Wind Energy, Kunsan National University, 558 Daehak-ro, Gunsan-si 54150, Republic of Korea; Faculty of Technology and Technique, Quy Nhon University, 170 An Duong Vuong, Quy Nhon city, Binh Dinh, Vietnam;
Phu-Cuong Nguyen: Faculty of Civil Engineering, Ho Chi Minh City Open University, 97 Vo Van Tan, Ho Chi;
Gihwan So: Innose Tech Company, 30 Mirae-ro, Incheon, Republic of Korea;
Dookie Kim: Department of Civil and Environmental Engineering, Kongju National University, 1223-24 Cheonan-daero, Seobuk-gu,Cheonan-si, Chungcheongnam-do 31080, Republic of Korea

Abstract
The purpose of this study is to develop an effective iterative two-stage method (ITSM) for structural damage identification of offshore platform structures. In each iteration, a new damage index, Modal Energy-Based Damage Index (MEBI), is proposed to help effectively locate the potential damage elements in the first stage. Then, in the second stage, the beetle antenna search (BAS) algorithm is used to estimate the damage severity of these elements. Compared with the well-known particle swarm optimization (PSO) algorithm and genetic algorithm (GA), this algorithm has lower computational cost. A modal energy based objective function for the optimization process is proposed. Using numerical and experimental data, the efficiency and accuracy of the ITSM are studied. The effects of measurement noise and spatial incompleteness of mode shape are both considered. All the obtained results show that under these influences, the ITSM can accurately identify the true location and severity of damage. The results also show that the objective function based on modal energy is most suitable for the ITSM compared with that based on flexibility and weighted natural frequency-mode shape.

Key Words
damage identification; modal energy; beetle antennae search; iterative two-stage method; objective function; noise robustness

Address
Shuqing Wang, Yufeng Jiang, Mingqiang Xu and Zhixiong Li: Shandong Provincial Key Laboratory of Ocean Engineering, Ocean University of China, 238 Songling Road, Qingdao, Shangdong, China
Yingchao Li: College of Civil Engineering, Ludong University, 186 Hongqi Middle Road, Yantai, Shangdong, China

Abstract
In this study, free vibration behavior of trapezoidal sandwich plates with porous core and two graphene platelets (GPLs) reinforced nanocomposite outer layers are presented. The distribution of pores and GPLs are supposed to be functionally graded (FG) along the thickness of core and nanocomposite layers, respectively. The effective Young modulus of the GPL-reinforced (GPLR) nanocomposite layers is determined using the modified Halpin-Tsai micromechanics model, while the Poisson ratio and density are computed by the rule of mixtures. The FSDT plate theory is utilized to establish governing partial differential equations and boundary conditions (B.C.s) for trapezoidal plate. The governing equations together with related B.C.s are discretized using a mapping- generalized differential quadrature (GDQ) method in the spatial domain. Then natural frequencies of the trapezoidal sandwich plates are obtained by GDQ method. Validity of current study is evaluated by comparing its numerical results with those available in the literature. A special attention is drawn to the role of GPLs weight fraction, GPLs patterns of two faces through the thickness, porosity coefficient and distribution of porosity on natural frequencies characteristics. New results show the importance of this permeates on vibrational characteristics of porous/GPLR nanocomposite plates. Finally, the influences of B.C.s and dimension as well as the plate geometry such as face to core thickness ratio on the vibration behaviors of the trapezoidal plates are discussed.

Key Words
trapezoidal sandwich plate; porosity; Generalized Differential Quadrature (GDQ); vibration; graphene platelets weight fraction

Address
Di Liang: College of Mechanical Engineering, Saitama Institute of Technology, Saitama 369-0293, Japan;
Qiong Wu: College of Mechatronic Engineering, Nanjing Forestry University, Nanjing 210037, China;
Xuemei Lu: School of International Education, Nanning Normal University, Nanning 530001, China;
Vahid Tahouneh: Young Researchers and Elite Club, Islamshahr Branch, Islamic Azad University, Islamshahr, Iran

Abstract
The present research investigates post-buckling behavior of geometrically imperfect tapered curved micro-panels made of graphene oxide powder (GOP) reinforced composite. Micro-scale effects on the panel structure have been included based on strain gradient elasticity. Micro-panel is considered to be tapered based on thickness variation along longitudinal direction. Weight fractions of uniformly and linearly distributed GOPs are included in material properties based on Halpin-Tsai homogenization scheme considering. Post-buckling curves have been determined based on both perfect and imperfect micro-panel assumptions. It is found that post-buckling curves are varying with the changes of GOPs weight fraction, geometric imperfection, GOP distribution type, variable thickness parameters, panel curvature radius and strain gradient.

Key Words
Post-buckling; cylindrical shell; strain gradient theory; nano-composite material; curved panel

Address
Seyed Sajad Mirjavadi,Masoud Forsat and AMS Hamouda: Department of Mechanical and Industrial Engineering, Qatar University, P.O. Box 2713, Doha, Qatar;
Mohammad Reza Barati: Fidar project Qaem Company, Darvazeh Dolat, Tehran, Iran

Abstract
The grid structures with welded hollow spherical joint (WHSJ) have gained increasing popularity for use in industrial buildings with suspended cranes, and usually welded with steel tube (ST). The fatigue performance of steel tube-welded hollow spherical joint (ST-WHSJ) is however not yet well characterized, and there is little research on fatigue life prediction methods of ST-WHSJ. In this study, based on previous fatigue tests, three series of specimen fatigue data with different design parameters and stress ratios were compared, and two fatigue failure modes were revealed: failure at the weld toe of the ST and the WHSJ respectively. Then, S–N curves of nominal stress were uniformed. Furthermore, a finite element model (FEM) was validated by static test, and was introduced to assess fatigue behavior with the hot spot stress method (HSSM) and the effective notch stress method (ENSM). Both methods could provide conservative predictions, and these two methods had similar results. However, ENSM, especially when using von Mises stress, had a better fit for the series with a non- positive stress ratio. After including the welding residual stress and mean stress, analyses with the local stress method (LSM) and the critical distance method (CDM, including point method and line method) were carried out. It could be seen that the point method of CDM led to more accurate predictions than LSM, and was recommended for series with positive stress ratios.

Key Words
ST-WHSJ; fatigue; life prediction method; S-N curves; LSM; CDM

Address
Qi Guo and Honggang Lei: College of Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China;
Ying Xing: College of Civil Engineering, Taiyuan University of Technology, Taiyuan 030024, China; College of Civil Engineering, Hunan University, Changsha 410082, China;
Qingwei Chen: Economic & Technology Research Institute of State Grid Shandong Electric Power Company, Jinan 250021, China

Abstract
A multi-scale epoxy/CNT/fiberglass annular sector plate is studied in this paper in the view of determining nonlinear forced vibration characteristics. A 3D Mori-Tanaka model is employed for evaluating multi-scale material properties. Thus, all of glass fibers are assumed to have uni-direction alignment and CNTs have random diffusion. The geometry of annular sector plate can be described based on the open angle and the value of inner/outer radius. In order to solve governing equations and derive exact forced vibration curves for the multi-scale annular sector, Jacobi elliptic functions are used. Obtained results demonstrate the significance of CNT distribution, geometric nonlinearity, applied force, fiberglass volume, open angle and fiber directions on forced vibration characteristics of multi-scale annular sector plates.

Key Words
forced vibration; annular sector; annular plate; nano-composite material; multi-scale composite

Address
Seyed Sajad Mirjavadi and A.M.S. Hamouda: Department of Mechanical and Industrial Engineering, Qatar University, P.O. Box 2713, Doha, Qatar;
Masoud Forsat: Department of Civil and Architectural Engineering, Qatar University, Doha, Qatar;
Mohammad Reza Barati: Fidar project Qaem Company, Darvazeh Dolat, Tehran, Iran

Abstract
The static behavior of grouped large-headed studs (d = 30 mm) embedded in ultra-high performance concrete (UHPC) was investigated by conducting push-out tests and numerical analysis. In the push-out test, no splitting cracks were found in the UHPC slab, and the shank failure control the shear capacity, indicating the large-headed stud matches well with the mechanical properties of UHPC. Besides, it is found that the shear resistance of the stud embedded in UHPC is 11.4% higher than that embedded in normal strength concrete, indicating that the shear resistance was improved. Regarding the numerical analysis, the parametric study was conducted to investigate the influence of the concrete strength, aspect ratio of stud, stud diameter, and the spacing of stud in the direction of shear force on the shear performance of the large-headed stud. It is found that the stud diameter and stud spacing have an obvious influence on the shear resistance. Based on the test and numerical analysis results, a formula was established to predict the load-slip relationship. The comparison indicates that the predicted results agree well with the test results. To accurately predict the shear resistance of the stud embedded in UHPC, a design equation for shear strength is proposed. The ratio of the calculation results to the test results is 0.99.

Key Words
grouped large-headed stud; UHPC slab; UHPC shear pocket; push-out test; numerical analysis; static behavior

Address
Yuqing Hu, Zhiqi He, Jianan Qi and Jingquan Wang: Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing 210096, China
Guotang Zhao: China State Railway Group Co., Ltd. Beijing 100844, China


Techno-Press: Publishers of international journals and conference proceedings.       Copyright © 2020 Techno-Press
P.O. Box 33, Yuseong, Daejeon 34186 Korea, Tel: +82-42-828-7996, Fax : +82-2-736-6801, Email: info@techno-press.com