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CONTENTS
Volume 21, Number 3, March 2018
 

Abstract
Vibrational problems in the domestic Small Horizontal Axis Wind Turbines (SHAWT) are due to flap wise vibrations caused by varying wind velocities acting perpendicular to its blade surface. It has been reported that monitoring the structural health of the turbine blades requires special attention as they are key elements of a wind power generation, and account for 15-20% of the total turbine cost. If this vibration problem is taken care, the SHAWT can be made as commercial success. In this work, Shape Memory Alloy (SMA) wires made of Nitinol (Ni-Ti) alloys are embedded into the Glass Fibre Reinforced Polymer (GFRP) wind turbine blade in order to reduce the flapwise vibrations. Experimental study of Nitinol (Ni-Ti) wire characteristics has been done and relationship between different parameters like current, displacement, time and temperature has been established. When the wind turbine blades are subjected to varying wind velocity, flapwise vibration occurs which has to be controlled continuously, otherwise the blade will be damaged due to the resonance. Therefore, in order to control these flapwise vibrations actively, a non-linear current controller unit was developed and fabricated, which provides actuation force required for active vibration control in smart blade. Experimental analysis was performed on conventional GFRP and smart blade, depicted a 20% increase in natural frequency and 20% reduction in amplitude of vibration. With addition of active vibration control unit, the smart blade showed 61% reduction in amplitude of vibration.

Key Words
active vibration control; smart blade; Nitinol [NiTi] wire; SHAWT; flapwise vibration

Address
Senthil Kumar Mouleeswaran and Keerthivasan: Department of Production Engineering, PSG college of Technology, Peelamedu, Coimbatore, Tamilnadu-641004, India
Yuvaraja Mani and Jagadeesh Veeraragu: Department of Mechanical Engineering, PSG college of Technology, Peelamedu, Coimbatore, Tamilnadu-641004, India

Abstract
Thermal Exchange Optimization (TEO) is a newly developed algorithm which mimics the thermal exchange between a solid object and its surrounding fluid. In this paper, an improved version of the TEO is developed to fix the shortcomings of the standard version. To demonstrate the viability of the new algorithm, the CEC 2016\'s single objective problems are considered along with the discrete size optimization of benchmark skeletal structures. Problem specific constraints are handled using a fly-back mechanism. The results show the validity of the improved TEO method compared to its standard version and a number of well-known algorithms.

Key Words
improved thermal exchange optimization; metaheuristic; discrete structural optimization; optimum design; steel structures

Address
A. Kaveh: Centre of Excellence for Fundamental Studies in Structural Engineering, Iran University of Science and Technology, Narmak, Tehran, P.O. Box 16846-13114, Iran
A. Dadras: Department of Civil Engineering, Iran University of Science and Technology, Narmak, Tehran, P.O. Box 16846-13114, Iran
T. Bakhshpoori: Faculty of Technology and Engineering, Department of Civil Engineering, East of Guilan,
University of Guilan, Rudsar-Vajargah, Iran



Abstract
The free longitudinal vibration of a circular truncated nanocone is investigated based on the nonlocal elasticity theory. Exact analytical formulations for tapered nanostructures are derived and the nonlinear differential governing equation of motion is developed. The nonlocal small scale effect unavailable in classical continuum theory is addressed to reveal the long-range interaction of atoms implicated in nonlocal constitutive relation. Unlike most previous studies applying the truncation method to the infinite higher-order differential equation, this paper aims to consider all higher-order terms to show the overall nonlocality. The explicit solution of nonlocal stress for longitudinal deformation is determined and it is an infinite series incorporating the classical stress derived in classical mechanics of materials and the infinite higher-order derivative of longitudinal displacement. Subsequently, the first three modes natural frequencies are calculated numerically and the significant effects of nonlocal small scale and vertex angle on natural frequencies are examined. The coupling phenomenon of natural frequency is observed and it is induced by the combined effects of nonlocal small scale and vertex angle. The critical value of nonlocal small scale is defined, and after that a new proposal for determining the range of nonlocal small scale is put forward since the principle of choosing the nonlocal small scale is still unclear at present. Additionally, two different types of nonlocal effects, namely the nonlocal stiffness weakening and strengthening, reversed phenomena existing in nanostructures are observed and verified. Hence the opposite nonlocal effects are resolved again clearly. The nano-engineers dealing with a circular truncated nanocone-based sensors and oscillators may benefit from the present work.

Key Words
nonlocal elasticity theory; nonlocal small scale; longitudinal vibration; circular truncated nanocone; nonlocal weakening; nonlocal strengthening

Address
C. Li: Department of Vehicle Engineering, School of Rail Transportation, Soochow University, Suzhou 215131, Jiangsu, China;
State Key Laboratory for Strength and Vibration of Mechanical Structures, Xi\'an Jiaotong University, Xi\'an710049, Shanxi, China
S.H. Sui and L.Q. Yao: Department of Vehicle Engineering, School of Rail Transportation, Soochow University, Suzhou 215131, Jiangsu, China
L. Chen: Department of Vehicle Technology & Railway Engineering, Suzhou Institute of Construction & Communications, Suzhou 215124, Jiangsu, China



Abstract
Applications of energy harvesting from mechanical vibrations is becoming popular but the full potential of such applications is yet to be explored. This paper addresses this issue by considering an application of energy harvesting for the dual objective of serving as an indicator of structural health monitoring (SHM) and extent of control. Variation of harvested energy from an undamaged baseline is employed for this purpose and the concept is illustrated by implementing it for active vibrations of a pipe structure. Theoretical and experimental analyses are carried out to determine the energy harvesting potential from undamaged and damaged conditions. The use of energy harvesting as indicator for control is subsequently investigated, considering the effect of the introduction of a tuned mass damper (TMD). It is found that energy harvesting can be used for the detection and monitoring of the location and magnitude of damage occurring within a pipe structure. Additionally, the harvested energy acts as an indicator of the extent of reduction of vibration of pipes when a TMD is attached. This paper extends the range of applications of energy harvesting devices for the monitoring of built infrastructure and illustrates the vast potential of energy harvesters as smart sensors.

Key Words
nergy harvesting; piezoelectric; structural health monitoring; passive control; pipes; damage; experimental analysis

Address
Paul Cahill: Centre for Marine and Renewable Energy Ireland (MaREI), Environmental Research Institute, Beaufort Building, University College Cork, Ringaskiddy, Co. Cork, Ireland
Vikram Pakrashi: Dynamical Systems and Risk Laboratory, School of Mechanical and Materials Engineering, and Centre for Marine and Renewable Energy Ireland (MaREI), University College Dublin, Belfield, Dublin 4, Ireland
Peng Sun: Department of Civil and Environmental Engineering, Rice University, Houston, TX 77005, USA
Alan Mathewson: Micro & Nano Systems Centre, Tyndall National Institute, University College Cork, Dyke Parade, Cork, Ireland
Satish Nagarajaiah: Department of Mechanical Engineering and Department of Material Science and Nano-Engineering,
Rice University, Houston, TX 77005, USA



Abstract
The second half of the 20th century was marked with a significant raise in amount of railway bridges in Austria made of reinforced concrete. Today, many of these bridges are slowly approaching the end of their envisaged service life. Current methodology of assessment and evaluation of structural condition is based on visual inspections, which, due to its subjectivity, can lead to delayed interventions, irreparable damages and additional costs. Thus, to support engineers in the process of structural evaluation and prediction of the remaining service life, the Austrian Federal Railways (oBB) commissioned the formation of a concept for an anticipatory life cycle management of engineering structures. The part concerning concrete bridges consisted of forming a bridge management system (BMS) in a form of a web-based analysis tool, known as the LeCIE_tool. Contrary to most BMSs, where prediction of a condition is based on Markovian models, in the LeCIE_tool, the time-dependent deterioration mechanisms of chloride- and carbonation-induced corrosion are used as the most common deterioration processes in transportation infrastructure. Hence, the main aim of this article is to describe the background of the introduced tool, with a discussion on exposure classes and crucial parameters of chloride ingress and carbonation models. Moreover, the article presents a verification of the generated analysis tool through service life prediction on a dozen of bridges of the Austrian railway network, as well as a case study with a more detailed description and implementation of the concept applied.

Key Words
service life; bridge management system; infrastructure management; concrete bridges; deterioration; concrete carbonation; chloride ingress

Address
Ivan Zambon, Anja Vidović and Alfred Strauss: Department of Civil Engineering and Natural Hazards, University of Natural Resources and Life Sciences, Peter-Jordan-Strabe 82, 1190 Vienna, Austria
Jose Matos: Civil Engineering Department, Campus de Azurém, Minho University, Guimaraes, Portugal
Norbert Friedl: Bridge Construction and Structural Engineering, OBB-Infrastruktur AG, Nordbahnstrabe 50, 1020 Vienna, Austria


Abstract
implified Dolphin Echolocation (SDE) algorithm is a recently developed meta-heuristic algorithm. This algorithm is an improved and simplified version of the Dolphin Echolocation Optimization (DEO) method, based on the baiting behavior of the dolphins. The main advantage of the SDE algorithm is that it needs no empirical parameter. In this paper, the SDE algorithm is applied for optimization of three well-studied frame structures. The designs are then compared with those of other meta-heuristic methods from the literature. Numerical results show the efficiency of the SDE algorithm and its competitive ability with other well-established meta-heuristics methods.

Key Words
meta-heuristic algorithms; dolphin echolocation optimization; simplified dolphin echolocation optimization; frame structures

Address
Ali Kaveh: Centre of Excellence for Fundamental Studies in Structural Engineering, School of Civil Engineering,
Iran University of Science and Technology, Tehran-16, Iran
Seyed Rohollah Hoseini Vaez and Pedram Hosseini: Department of Civil Engineering, Faculty of Engineering, University of Qom, Qom, Iran


Abstract
The fast-growing number of mobile and wearable applications has driven several innovations in small-scale electret-based energy harvesting due to the compatibility with standard microfabrication processes and the ability to generate electrical energy from ambient vibrations. However, the current modeling methods used to design these small scale transducers or microgenerators are applicable only for constant-speed rotations and small sinusoidal translations, while in practice, large amplitude sinusoidal vibrations can happen. Therefore, in this paper, we formulate an analytical model for electret-based microgenerators under general sinusoidal excitations. The proposed model is validated using finite element modeling combined with numerical simulation approaches presented in the literature. The new model demonstrates a good agreement in estimating both the output voltage and power of the microgenerator. This new model provides useful insights into the microgenerator operating mechanism and design trade-offs, and therefore, can be utilized in the design and performance optimization of these small structures.

Key Words
energy harvesting; electrostatics; electret; vibration; microgenerator

Address
Cuong C. Nguyen: Centre for Biomedical Engineering, School of Electrical and Electronic Engineering,
University of Adelaide, Adelaide, SA 5005, Australia;
Auto-ID Lab, School of Computer Science, University of Adelaide, Adelaide, SA 5005, Australia
Damith C. Ranasinghe: Auto-ID Lab, School of Computer Science, University of Adelaide, Adelaide, SA 5005, Australia
Said F. Al-Sarawi: Centre for Biomedical Engineering, School of Electrical and Electronic Engineering,
University of Adelaide, Adelaide, SA 5005, Australia



Abstract
This paper investigates the free vibration analysis of double-beam system coupled by a two-degree-of-freedom mass-spring system. In order to generalize the model, the main beams are assumed to be elastically restrained against translation and rotation at one end and free at the other. Furthermore, the mass-spring system is elastically connected to the beams at adjustable positions by means of four translational and rotational springs. The governing differential equations of the beams and the mass-spring system are derived and analytically solved by using the Fourier transform method. Moreover, as a second way, a finite element solution is derived. The frequency parameters and mode shapes of some diverse cases are obtained using both methods. Comparison of obtained results by two methods shows the accuracy of both solutions. The influence of system parameters on the free vibration response of the studied mechanical system is examined.

Key Words
double-beam; two-degree-of-freedom mass-spring system; vibration suppression; elastic supports; Fourier transform; free vibration; exact solution; finite element method

Address
Mohammad Rezaiee-Pajand, Ahmad Aftabi Sani and Seyed Mojtaba Hozhabrossadati: Department of Civil Engineering, Ferdowsi University of Mashhad, Mashhad, Iran

Abstract
Strong seismic events commonly cause large drift and deformation, and functionality failures in the superstructures. One way to prevent functionality failures is to design structures which are ductile and flexible through yielding when subjected to strong ground excitations. By developing forces that assist motion as \"negative stiffness forces\", yielding can be achieved. In this paper, we adopt the weakening and damping method to achieve a new approach to reduce all of the structural responses by further adjusting damping phase. A semi-active control system is adopted to perform the experiments. In this adaptation, negative stiffness forces through certain devices are used in weakening phase to reduce structural strength. Magneto-rheological (MR) dampers are then added to preserve stability of the structure. To adjust the voltage in MR dampers, an inverse model is employed in the control system to command MR dampers and generate the desired control forces, where a velocity control algorithm produces initial required control force. An extensive numerical study is conducted to evaluate proposed methodology by using the smart base-isolated benchmark building. Totally, nine control systems are examined to study proposed strategy. Based on the numerical results of seven earthquakes, the use of proposed strategy not only reduces base displacements, base accelerations and base shear but also leads to reduction of accelerations and inter story drifts of the superstructure. Numerical results shows that the usage of inverse model produces the desired regulated damping, thus improving the stability of the structure.

Key Words
weakening and damping; negative stiffness device; MR damper; inverse model; velocity control algorithm; smart base-isolated

Address
Arash Bahar, Mohsen Salavati-Khoshghalb and Seyed Mehdi Ejabati: Department of Civil Engineering, Faculty of Engineering, University of Guilan, Rasht, Iran

Abstract
Semi-active isolation systems based on leverage-type stiffness control strategies have been widely studied. The main concept behind this type of system is to adjust the stiffness in the isolator to match the fundamental period of the isolated system by using a simple leverage mechanism. Although this system achieves high performance under far-field earthquakes, it is unsuitable for near-fault strong ground motion. To overcome this problem, this study considers the potential energy effect in the control law of the semi-active isolation system. The minimal energy weighting (MEW) between the potential energy and kinetic energy was first optimized through a series of numerical simulations. Two MEW algorithms, namely generic and near-fault MEW control, were then developed to efficiently reduce the structural displacement responses. To demonstrate the performance of the proposed method, a two-degree-of-freedom structure was employed as a benchmark. Numerical results indicate that the dynamic response of the structure can be effectively dampened by the proposed MEW control under both far-field and near-fault earthquakes, whereas the structural responses resulting from conventional control methods may be greater than those for the purely passive control method. Moreover, according to experimental verifications, both the generic and near-fault MEW control modes yielded promising results under impulse-like earthquakes. The practicability of the proposed control algorithm was verified.

Key Words
semi-active control; near-fault earthquake; potential energy

Address
Tzu-Kang Lin and Chi-Jen Chen: Department of Civil Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 300, Taiwan
Lyan-Ywan Lu: Department of Civil Engineering, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan


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