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Volume 31, Number 2, April25 2019

In this study, carbon fiber/epoxy (CF/EP) composites were interleaved with aramid nonwoven veils with an areal weight density of 8.5 g/m2 to improve their Mode-I fracture toughness. The control and aramid interleaved CF/EP composite laminates were manufactured by VARTM in a [0]4 configuration. Tensile, three-point bending, compression, interlaminar shear, Charpy impact and Mode-I (DCB) fracture toughness values were determined to evaluate the effects of aramid nonwoven fabrics on the mechanical performance of the CF/EP composites. Thermomechanical behavior of the specimens was investigated by Dynamic Mechanical Analysis (DMA). The results showed that the propagation Mode-I fracture toughness values of CF/EP composites can be significantly improved (by about 72%) using aramid nonwoven fabrics. It was found that the main extrinsic toughening mechanism is aramid microfiber bridging acting behind the crack-tip. The incorporation of these nonwovens also increased interlaminar shear and Charpy impact strength by 10 and 16.5%, respectively. Moreover, it was revealed that the damping ability of the composites increased with the incorporation of aramid nonwoven fabrics in the interlaminar region of composites. On the other hand, they caused a reduction in in-plane mechanical properties due to the reduced carbon fiber volume fraction, increased thickness and void formation in the composites.

Key Words
composite structures; crack; fiber reinforced polymers (FRPs); fracture/fracture criteria; delamination; bending and shear strength; axial compression

(1) Bertan Beylergil:
Department of Mechanical Engineering, Alanya Alaaddin Keykubat University, Alanya, Antalya, Turkey;
(2) Bertan Beylergil, Metin Tanoğlu:
Department of Mechanical Engineering, Izmir Institute of Technology, İzmir, Turkey;
(3) Engin Aktaş:
Department of Civil Engineering, Izmir Institute of Technology, Urla, İzmir, Turkey.

This paper presents the results of an experimental study to investigate the mechanical behavior of partially encased composite columns confined by CFRP under axial compression. The results show that the failure of the partially encased composite columns confined by CFRP occurred due to rupture of the CFRP followed by local buckling of the steel flanges. External wrapping of CFRP effectively delayed the local buckling of the steel flanges. The load carrying capacity of the column increased with the application of CFRP sheet. And the enhancement effect of the column was increased with the number of CFRP layer.

Key Words
column; CFRP; compression; enhancement; partially encased

(1) Jiongfeng Liang:
State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China Institute of Technology, Nanchang, P.R. China;
(2) Jiongfeng Liang, Guangwu Zhang, Jianbao Wang, Minghua Hu:
Faculty of Civil & Architecture Engineering, East China Institute of Technology, Nanchang, P.R. China.

This paper reports on the energy absorption characteristics of a lattice-web reinforced composite sandwich cylinder (LRCSC) which is composed of glass fiber reinforced polymer (GFRP) face sheets, GFRP lattice webs, polyurethane (PU) foam and ceramsite filler. Quasi-static compression experiments on the LRCSC manufactured by a vacuum assisted resin infusion process (VARIP) were performed to demonstrate the feasibility of the proposed cylinders. Compared with the cylinders without lattice webs, a maximum increase in the ultimate elastic load of the lattice-web reinforced cylinders of approximately 928% can be obtained. Moreover, due to the use of ceramsite filler, the energy absorption was increased by 662%. Several numerical simulations using ANSYS/LS-DYNA were conducted to parametrically investigate the effects of the number of longitudinal lattice webs, the number of transverse lattice webs, and the thickness of the transverse lattice web and GFRP face sheet. The effectiveness and feasibility of the numerical model were verified by a series of experimental results. The numerical results demonstrated that a larger number of thicker transverse lattice webs can significantly enhance the ultimate elastic load and initial stiffness. Moreover, the ultimate elastic load and initial stiffness were hardly affected by the number of longitudinal lattice webs.

Key Words
cylinder; ceramsite filler; quasi-static compression; energy absorption; numerical simulation

(1) Jiye Chen, Hai Fang, Lu Zhu, Ziyan Fan:
College of Civil Engineering, Nanjing Tech University, Nanjing, China;
(2) Yong Zhuang:
China Railway Major Bridge Reconnaissance & Design Institute Co., Ltd, Wuhan, China;
(3) Weiqing Liu:
Advanced Engineering Composites Research Center, Nanjing Tech University, Nanjing, China.

The force-deformation behavior, strain distribution and failure modes of a variable damping self-centering brace (VD.SCB) are theoretically analyzed, experimentally studied, and numerically simulated to guide its design. The working principle of the brace is explained by describing the working stages and the key feature points of the hysteretic curve. A large-scale brace specimen was tested under different sinusoidal excitations to analyze the recentering capability and energy dissipation. Results demonstrate that the VD.SCB exhibits a full quasi-flag-shaped hysteretic response, high ultimate bearing capacity, low activation force and residual deformation, and excellent recentering and energy dissipation capabilities. Calculation equations of the strain distribution in different parts of the brace are proposed and are compared with the experimental data and simulated results. The developments of two failure modes are compared. Under normal circumstances, the brace fails due to the yielding of the spring blocking plates, which are easily replaced to restore the normal operating conditions of the brace. A brief description of the design procedure of the brace is proposed for application.

Key Words
self-centering brace; variable damping; hysteretic response; strain distribution; failure mode

(1) Xing-Si Xie, Long-He Xu:
School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China;
(2) Zhong-Xian Li:
Key Laboratory of Coast Civil Structure Safety of China Ministry of Education, Tianjin University, Tianjin 300072, China.

Strengthening of civil infrastructure with advanced composites have recently become one of the most popular methods. The use of Fiber Reinforced Polymer (FRP) strips plates and fabric for strengthening of reinforced concrete structures has well established design guidelines and standards. Research on the application of FRP composites to steel structures compared to concrete structures is limited, especially for shear strengthening applications. Whereas, there is a need for cost-effective system that could be used to strengthen steel high-way bridge girders to cope with losses due to corrosion in addition to continuous demands for increasing traffic loads. In this study, a parametric finite element study is performed to investigate the effect of applying thick CFRP strips diagonally on webs of plate girders on the shear strength of end-web panels. The study focuses on illustrating the effect of several geometric parameters on nominal shear strength. Hence, a formula is developed to determine the enhancement of shear strength gained upon the application of CFRP strips.

Key Words
finite element; Carbon Fiber Reinforced Polymer (CFRP); plate girders; strengthening

(1) Haitham A. Shalaby, Maha M. Hassan, Sherif S. Safar:
Structural Engineering Department, Cairo University, Gamaet El Qahera Street,
Egypt;(2) Maha M. Hassan:
Civil Engineering Department, University of Prince Mugrin, Medina, Saudi Arabia
(On leave from Cairo University).

This paper presents experimental and numerical investigations of aluminum tubular members subjected to combined bending and web crippling. A series of tests was performed on square hollow sections (SHS) fabricated by extrusion using 6061-T6 heat-treated aluminum alloy. Different specimen lengths were tested to obtain the interaction relationship between moment and concentrated load. The non-linear finite element models were developed and verified against the experimental results obtained in this study and test data from existing literature for aluminum tubular sections subjected to pure bending, pure web crippling, and combined bending and web crippling. Geometric and material non-linearities were included in the finite element models. The finite element models closely predicted the strengths and failure modes of the tested specimens. Hence, the models were used for an extensive parametric study of cross-section geometries, and the web slenderness values ranged from 6.0 to 86.2. The combined bending and web crippling test results and strengths predicted from the finite element analysis were compared with the design strengths obtained using the current American Specification, Australian/New Zealand Standard and European Code for aluminum structures. The findings suggest that the current specifications are either quite conservative or unconservative for aluminum square hollow sections subjected to combined bending and web crippling. Hence, a bending and web crippling interaction equation for aluminum square hollow section specimens is proposed in this paper.

Key Words
aluminum; bending; experimental investigation; finite element analysis; square hollow section; tubular sections; web crippling

(1) Feng Zhou:
State Key Laboratory of Disaster Reduction in Civil Engineering, Tongji University, Shanghai 200092, China;
(2) Feng Zhou:
Department of Structural Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China;
(3) Ben Young:
Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China (Formerly, Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, China).

This paper presents the semi-analytical development of the dynamic instability behavior and the dynamic response of functionally graded (FG) cylindrical shallow shell panel subjected to different type of periodic axial compression. First, in prebuckling analysis, the stresses distribution within the panels are determined for respective loading type and these stresses are used to study the dynamic instability behavior and the dynamic response. The prebuckling stresses within the shell panel are the same as applied in-plane edge loading for the case of uniform and linearly varying loadings. However, this is not true for the case of parabolic loadings. The parabolic edge loading produces all the stresses (

Key Words
FG cylindrical panel; prebuckling stresses; parabolic loading; Galerkin; dynamic instability; IHB

(1) Rajesh Kumar:
Department of Civil Engineering, Birla Institute of Technology and Science, Pilani-333031, India;
(2) Tanish Dey, Sarat K. Panda:
Department of Civil Engineering, Indian Institute of Technology (ISM), Dhanbad-826004, India.

This scientific paper provides a theoretical, numerical and experimental analysis of local stability of axially compressed columns made of thin-walled rectangular concrete-filled steel tubes (CFSTs), with the consideration of initial geometric imperfections. The work presented introduces the theory of elastic critical stresses in local buckling of rectangular wall members under uniform compression. Moreover, a numerical calculation method for the determination of the critical stress coefficient is presented, using a differential equation for a slender wall with a variety of boundary conditions. For comparison of the results of the numerical analysis with those collected by experiments, a new model is created to study the behaviour of the composite members in question by means of the ABAQUS computational-graphical software whose principles are based on the finite element method (FEM). In modelling the analysed members, the actual boundary and loading conditions and real material properties are taken into account, obtained from the experiments and material tests on these members. Finally, the results of experiments on such members are analysed and then compared with the numerical values. In conclusion, several recommendations for the design of axially compressed composite columns made of rectangular concrete-filled thin-walled steel tubes are suggested as a result of this comparison.

Key Words
composite structures; local buckling; concrete-filled steel tubes (CFSTs); design codes; FEM analysis

Department of Steel and Timber Structures, Institute of Structural Engineering, Civil Engineering Faculty, Technical University of Kosice, Vysokoskolska 4, 042 00 Kosice, Slovakia.

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