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CONTENTS
Volume 21, Number 6, June 2018
 

Abstract
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Key Words
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Address
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Abstract
Truss-Z (TZ) is an Extremely Modular System (EMS). Such systems allow for creation of structurally sound free-form structures, are comprised of as few types of modules as possible, and are not constrained by a regular tessellation of space. Their objective is to create spatial structures in given environments connecting given terminals without self-intersections and obstacle-intersections. TZ is a skeletal modular system for creating free-form pedestrian ramps and ramp networks. The previous research on TZ focused on global discrete geometric optimization of the spatial configuration of modules. This paper reports on the first attempts at structural optimization of the module for a single-branch TZ. The internal topology and the sizing of module beams are subject to optimization. An important challenge is that the module is to be universal: it must be designed for the worst case scenario, as defined by the module position within a TZ branch and the geometric configuration of the branch itself. There are four variations of each module, and the number of unique TZ configurations grows exponentially with the branch length. The aim is to obtain minimum-mass modules with the von Mises equivalent stress constrained under certain design load. The resulting modules are further evaluated also in terms of the typical structural criterion of compliance.

Key Words
Extremely Modular System; Truss-Z; structural optimization; modular structures; minimum mass design; frame structures

Address
Machi Zawidzkiand Lukasz Jankowski: nstitute of Fundamental Technological Research, Polish Academy of Sciences, ul. Pawińskiego 5B, 02-106 Warsaw, Poland


Abstract
Tuned mass dampers (TMDs) are passive damping devices widely employed to mitigate the pedestrian-induced vibrations on footbridges. The TMD design must ensure an adequate performance during the overall life-cycle of the structure. Although the TMD is initially adjusted to match the natural frequency of the vibration mode which needs to be controlled, its design must further take into account the change of the modal parameters of the footbridge due to the modification of the operational and environmental conditions. For this purpose, a motion-based design optimization method is proposed and implemented herein, aimed at ensuring the adequate behavior of footbridges under uncertainty conditions. The uncertainty associated with the variation of such modal parameters is simulated by a probabilistic approach based on the results of previous research reported in literature. The pedestrian action is modelled according to the recommendations of the Synpex guidelines. A comparison among the TMD parameters obtained considering different design criteria, design requirements and uncertainty levels is performed. To illustrate the proposed approach, a benchmark footbridge is considered. Results show both which is the most adequate design criterion to control the pedestrian-induced vibrations on the footbridge and the influence of the design requirements and the uncertainty level in the final TMD design.

Key Words
footbridge; passive structural control; tuned mass damper; uncertainty; probabilistic approach; constrained single-objective optimization; genetic algorithms

Address
Javier F. Jiménez-Alonso : Department of Building Structures and Geotechnical Engineering, Universidad de Sevilla, Avenida Reina Mercedes, 2, 41012 Seville, Spain
Andrés Sáez: Department of Continuum Mechanics and Structural Analysis, Universidad de Sevilla,
Camino de los Descubrimientos s/n, 41092 Seville, Spain



Abstract
Structural health monitoring (SHM) systems are necessary to achieve smart predictive maintenance and repair planning as well as they lead to a safe operation of mechanical structures. In the context of vibration-based SHM the measured structural responses are employed to draw conclusions about the structural integrity. This usually leads to a mathematically ill-posed inverse problem which needs regularization. The restriction of the solution set of this inverse problem by using prior information about the damage properties is advisable to obtain meaningful solutions. Compared to the undamaged state typically only a few local stiffness changes occur while the other areas remain unchanged. This change can be described by a sparse damage parameter vector. Such a sparse vector can be identified by employing L1-regularization techniques. This paper presents a novel framework for damage parameter identification by combining sparse solution techniques with an Extended Kalman Filter. In order to ensure sparsity of the damage parameter vector the measurement equation is expanded by an additional nonlinear L1-minimizing observation. This fictive measurement equation accomplishes stability of the Extended Kalman Filter and leads to a sparse estimation. For verification, a proof-of-concept example on a quadratic aluminum plate is presented.

Key Words
L1-minimization; sparse reconstruction; Extended Kalman Filter; damage identification

Address
Daniel Ginsberg: Department of Mechanical Engineering, University of Siegen, Paul-Bonatz-Strasse 9-11, 57076 Siegen, Germany
Claus-Peter Fritzen: 2Department of Mechanical Engineering and Center of Sensor Systems (ZESS), University of Siegen,
Paul-Bonatz-Strasse 9-11, 57076 Siegen, Germany
Otmar Loffeld: Center of Sensor Systems (ZESS), University of Siegen, Paul-Bonatz-Strasse 9-11, 57076 Siegen, Germany

Abstract
Glass fibre reinforced polymers (GFRP) are widely exploited in many industrial branches. Due to this Structural Health Monitoring systems containing embedded fibre optics sensors are applied. One of the problems that can influence on composite element durability is water contamination that can be introduced into material structure during manufacturing. Such inclusion can be a damage origin significantly decreasing mechanical properties of an element. A non-destructive method that can be applied for inspection of an internal structure of elements is THz spectroscopy. It can be used for identifications of material discontinuities that results in changes of absorption, refractive index or scattering of propagating THz waves. The limitations of THz propagation through water makes this technique a promising solution for detection of a water inclusion. The paper presents an application of THz spectroscopy for detection and localisation of a water drop inclusion embedded in a GFRP material between two fibre optics with fibre Bragg grating sensors. The proposed filtering method allowed to determine a 3D shape of the water drop.

Key Words
THz spectroscopy; glass fibre composite; fibre optics; moisture

Address
Magdalena Mieloszyk and Katarzyna Majewska: Department of Mechanics of Intelligent Structures, Institute of Fluid Flow Machinery,Polish Academy of Sciences, 14 Fiszera Str., 80-231 Gdansk, Poland
Wieslaw Ostachowicz: Department of Mechanics of Intelligent Structures, Institute of Fluid Flow Machinery,
Polish Academy of Sciences, 14 Fiszera Str., 80-231 Gdansk, Poland;
Faculty of Automotive and Construction Machinery Engineering,Warsaw University of Technology, 84 Narbutta Str., 02-524 Warsaw, Poland


Abstract
Guided wave testing is a reliable and safe method for pipeline inspection. In general, guided wave testing employs a circumferential array of piezoelectric transducers to clamp on the pipe circumference. The sensitivity of the operation depends on many factors, including transducer distribution across the circumferential array. This paper presents the sensitivity analysis of transducer array for the circumferential characteristics of guided waves in a pipe using finite element modelling and experimental studies. Various cases are investigated for the outputs of guided waves in the numerical simulations, including the number of transducers per array, transducer excitation variability and variations in transducer spacing. The effect of the dimensions of simulated notches in the pipe is also investigated for different arrangements of the transducer array. The results from the finite element numerical simulations are then compared with the related experimental results. Results show that the numerical outputs agree well with the experimental data, and the guided wave mode T(0,1) presents high sensitivity to the notch size in the circumferential direction, but low sensitivity to the notch size in the axial direction.

Key Words
guided wave; piezoelectric transducer array; finite element modelling; wave propagation; pipeline; structural integrity

Address
Xudong Niu: Department of Engineering Science, University of Greenwich, Chatham, Kent ME4 4TB, UK;
TWI Ltd. Granta Park, Cambridge, Cambridgeshire CB21 6AL, UK
Hugo R. Marques: TWI Ltd. Granta Park, Cambridge, Cambridgeshire CB21 6AL, UK
Hua-Peng Chen: Department of Engineering Science, University of Greenwich, Chatham, Kent ME4 4TB, UK


Abstract
This paper discusses the shape control of deformable mirrors for Adaptive Optics in the dynamic range. The phenomenon of control-structure interaction appears when the mirror becomes large, lowering the natural frequencies fi, and the control bandwidth fc increases to improve the performance, so that the condition fc < fi is no longer satisfied. In this case, the control system tends to amplify the response of the flexible modes and the system may become unstable. The main parameters controlling the phenomenon are the frequency ratio fc / fi and the structural damping s. Robustness tests are developed which allow to evaluate a lower bound of the stability margin. Various passive and active strategies for damping augmentation are proposed and tested in simulation.

Key Words
adaptive optics; deformable mirror; control-structure interaction; active/passive damping

Address
Kainan Wang and André Preumont: Department of Control Engineering and System Analysis, Université Libre de Bruxelles (ULB), CP. 165-55, 50 Av. F.D. Roosevelt, B-1050, Brussels, Belgium
David Alaluf: European Space Agency - ESA/ESTEC, Opto-Electronics Section, Keplerlaan 1, 2201 AZ Noordwijk ZH, The Netherlands
Bilal Mokrani: Department of Mechanical, Materials and Aerospace Engineering, University of Liverpool,
The Quadrangle, Brownlow Hill L69 3GH, United Kingdom




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