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CONTENTS
Volume 4, Number 2, June 2017
 

Abstract
In this paper, we present a strain-sensitive composite skin-like film made up of piezoresistive zinc oxide (ZnO) nanorods embedded in a flexible poly(dimethylsiloxane) substrate, with added reduced graphene oxide (rGO) to facilitate connections between the nanorod clusters and increase strain sensitivity. Preparation of the composite is described in detail. Cyclic strain sensing tests are conducted. Experiments indicate that the resulting ZnO-PDMS/rGO composite film is strain-sensitive and thus capable of sensing cycling strain accurately. As such, it has the potential to be molded on to a structure (civil, mechanical, aerospace, or biological) in order to provide a strain sensing skin.

Key Words
strain sensing; flexible; skin; poly(dimethylsiloxane); reduced graphene oxide; zinc oxide

Address
Tejus Satish: Department of Biochemistry, Rice University, Houston, Texas, USA 77005
Kaushik Balakrishnan, Hemtej Gullapalli,
Satish Nagarajaiah, Robert Vajtai and Pulickel M. Ajayan: Department of Materials Science and Nanoengineering, Rice University, Houston, TX, USA 77005


Abstract
Classical flutter of wind turbine blades indicates a type of aeroelastic instability with fully attached boundary layer where a torsional blade mode couples to a flapwise bending mode, resulting in a mutual rapid growth of the amplitudes. In this paper the monitoring problem of onset of flutter is investigated from a detection point of view. The criterion is stated in terms of the exceeding of a defined envelope process of a specific maximum torsional vibration threshold. At a certain instant of time, a limited part of the previously measured torsional vibration signal at the tip of blade is decomposed through the Empirical Mode Decomposition (EMD) method, and the 1st Intrinsic Mode Function (IMF) is assumed to represent the response in the flutter mode. Next, an envelope time series of the indicated modal response is obtained in terms of a Hilbert transform. Finally, a flutter onset criterion is proposed, based on the indicated envelope process. The proposed online flutter monitoring method provided a practical and direct way to detect onset of flutter during operation. The algorithm has been illustrated by a 907-DOFs aeroelastic model for wind turbines, where the tower and the drive train is modelled by 7 DOFs, and each blade by means of 50 3-D Bernoulli-Euler beam elements.

Key Words
wind turbine; flutter monitoring; envelope process; Hilbert-Huang transform

Address
Bei Chen and Xu G. Hua: Key Laboratory for Wind and Bridge Engineering, Department of Civil Engineering, Hunan University, Changsha 410082, China
Zi L. Zhang: Department of Engineering, Aarhus University, Aarhus 8000, Denmark
Biswajit Basu: School of Engineering, Trinity College Dublin 2, Ireland
Soren R.K. Nielsen: Department of Civil Engineering, Aalborg University, Aalborg 9000, Denmark

Abstract
The paper describes the results of an experimental and numerical investigation into the structural and damage response of sandwich composites to low-velocity impact. Sandwich panels consisting of laminated composite skins with three different layups bonded to a PVC foam core were subjected to impact at various energy levels corresponding to barely visible impact damage (BVID) in the impacted skins. Damage assessment analyses were performed on the impacted panels to characterise the extent and the nature of the major failure mechanisms occurring in the skins. The data collected during the experimental analyses were finally used to assess the predictive capabilities of an FE tool recently developed by the authors for detailed simulation of impact damage in composite sandwich panels. Good agreement was observed between experimental results and model predictions in terms of structural response to impact, global extent of damage and typical features of individual damage mechanisms.

Key Words
sandwich composites; low-velocity impact; damage; FE simulation

Address
Dianshi Feng: Department of Civil and Environmental Engineering, National University of Singapore, No.1 Engineering Drive 2, Singapore
Francesco Aymerich: Department of Mechanical, Chemical and Materials Engineering, University of Cagliari, Via Marengo 2, Cagliari, Italy



Abstract
An improved method is presented to estimate the axial force of a bar member with vibrational measurements based on modified Timoshenko beam theory. Bending stiffness effects, rotational inertia, shear deformation, rotational inertia caused by shear deformation are all taken into account. Axial forces are estimated with certain natural frequency and corresponding mode shape, which are acquired from dynamic tests with five accelerometers. In the paper, modified Timoshenko beam theory is first presented with the inclusion of axial force and rotational inertia effects. Consistent mass and stiffness matrices for the modified Timoshenko beam theory are derived and then used in finite element simulations to investigate force identification accuracy under different boundary conditions and the influence of critical axial force ratio. The deformation coefficient which accounts for rotational inertia effects of the shearing deformation is discussed, and the relationship between the changing wave speed and the frequency is comprehensively examined to improve accuracy of the deformation coefficient. Finally, dynamic tests are conducted in our laboratory to identify progressive axial forces of a steel plate and a truss structure respectively. And the axial forces identified by the proposed method are in good agreement with the forces measured by FBG sensors and strain gauges. A significant advantage of this axial force identification method is that no assumption on boundary conditions is needed and excellent force identification accuracy can be achieved.

Key Words
axial force identification; modified Timoshenko beam theory; truss structure; dynamic test

Address
Dong-sheng Li, Yong-qiang Yuan, Kun-peng Li and Hong-nan Li: State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian, 116024, China


Abstract
Research to date has mainly focused on structural analysis and design of wind turbines considering turbulent aerodynamic loading. The combined effects of wind and seismic loading have not been studied by many researchers. With the recent expansion of wind turbines into seismically active regions research is now needed into the implications of seismic loading coupled with turbulent aerodynamic loading. This paper proposes a monitoring procedure for onshore horizontal axis wind turbines (HAWTs) subjected to this combined loading regime. The paper examines the impact of seismic loading on the 5-MW baseline HAWT developed by the National Renewable Energy Laboratory (NREL). A modified version of FAST, an open-source program developed by NREL, is used to perform the dynamic analysis.

Key Words
seismic analysis; wind turbines; FAST

Address
Breiffni Fitzgeraldand Biswajit Basu: Department of Civil, Structural & Environmental Engineering, Trinity College Dublin, Dublin, Ireland


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