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CONTENTS
Volume 6, Number 2, March 2010
 

Abstract
An experimental study was carried out to investigate the shape control of plates via embedded shape memory alloy (SMA) wires. An extensive body of literature proposes the use of SMA wires to actively modify the shape or stiffness of a structure; in most cases, however, the study focuses on modeling and little experimental data is available. In this work, a simple proof of concept specimen was built by attaching four prestrained SMA wires to one side of a carbon fiber laminate plate strip. The specimen was clamped at one end and tested in an environmental chamber, measuring the tip displacement and the SMA temperature. At heating, actuation of the SMA wires bends the plate; at cooling deformation is partially recovered. The specimen was actuated a few times between two fixed temperatures Tc and Th, whereas in the last actuation a temperature Tf > Th was reached. Contrary to most model predictions, in the first actuation the transformation temperatures are significantly higher than in the following cycles, which are stable. Moreover, if the temperature Th is exceeded, two separate actuations occur during heating: the first follows the path of the stable cycles; the second, starting at Th, is similar to the first cycle. An interpretation of the phenomenon is given using some differential scanning calorimeter (DSC) measurements. The observed behavior emphasizes the need to build a more comprehensive constitutive model able to include these effects.

Key Words
shape memory alloy; composite plates; active shape change.

Address
Federica Daghia; DISTART, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy
Daniel J. Inman; CIMSS, Virginia Polytechnic Institute and State University, 310 Durham Hall, Mail Code 0261, Blacksburg, VA 24061, U.S.A.
Francesco Ubertini; DISTART, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy
Erasmo Viola; DISTART, University of Bologna, Viale Risorgimento 2, 40136 Bologna, Italy

Abstract
This paper presents the results of a series of experiments conducted to model a magnetorheological damper operated in shear mode. The prototype MR damper consists of two parallel steel plates; a paddle covered with an MR fluid coated foam is placed between the plates. The force is generated when the paddle is in motion and the MR fluid is reached by the magnetic field of the coil in one end of the device. Two approaches were considered in this experiment: a parametric approach based on the Bingham, Bouc-Wen and Hyperbolic Tangent models and a non parametric approach based on a Neural Network model. The accuracy to reproduce the MR damper behavior is compared as well as some aspects related to performance are discussed.

Key Words
smart fluid; magnetorheological; MR damper; Bingham; Bouc-Wen; Hyperbolic tangent; neural network.

Address
Mauricio Zapateiro; Institute of Informatics and Applications, University of Girona. Campus de Montilivi, Edifici P4. E17071, Girona, Spain
Ningsu Luo; Institute of Informatics and Applications, University of Girona. Campus de Montilivi, Edifici P4. E17071, Girona, Spain
Ellen Taylor; Former Graduate Student, Washington University in St. Louis, St. Louis, MO, U.S.A.
Shirley J. Dyke; School of Mechanical Engineering, College of Engineering, Purdue University, 585 Purdue Mall, West Lafayette, IN 47907 - 2088, U.S.A.

Abstract
A modal filter is a tool used to extract the modal coordinates of each individual mode from a system\'s output. This is achieved by mapping the response vector from the physical space to the modal space. It decomposes the system\'s responses into modal coordinates, and thus, on the output of the filter, the frequency response with only one peak corresponding to the natural frequency to which the filter was tuned can be obtained. As was shown in the paper (Deraemecker and Preumont 2006), structural modification (e.g. a drop in stiffness or mass due to damage) causes the appearance of spurious peaks on the output of the modal filter. A modal filter is, therefore, a great indicator of damage detection, with such advantages as low computational effort due to data reduction, ease of automation and lack of sensitivity to environmental changes. This paper presents the application of modal filters for the detection of stiffness changes. Two experiments were conducted: the first one using the simulation data obtained from the numerical 7DOF model, and the second one on the experimental data from a laboratory stand in 4 states of damage.

Key Words
modal filter; structural health monitoring; damage detection; FRF synthesis.

Address
Krzysztof Mendrok and Tadeusz Uhl; AGH University of Science and Technology, Department of Robotics and Mechatronics, al. Mickiewicza 30, 30-059 Krakow, Poland

Abstract
This paper presents the perspective of the Structural Mechanics program of the Air Force Office of Scientific Research (AFOSR) on the damage assessment of structures for the period 2006-2009 when the author was serving as Program Manager at AFOSR. It is found that damage assessment of structures plays a very important role in assuring the safety and operational readiness of US Air Force fleet. The current fleet has many aging aircraft, which poses a considerable challenge for the operators and maintainers. The nondestructive evaluation technology is rather mature and able to detect damage with considerable reliability during the periodic maintenance inspections. The emerging structural health monitoring methodology has great potential, because it will use on-board damage detection sensors and systems, will be able to offer on-demand structural health bulletins. Considerable fundamental and applied research is still needed to enable the development, implementation, and dissemination of structural health monitoring technology.

Key Words
damage assessment; aging aircraft; nondestructive evaluation; structural health monitoring.

Address
Victor Giurgiutiu; Mechanical Engineering, University of South Carolina, 300 Main St., Columbia, SC 29208, U.S.A. (Formerly, Structural Mechanics Program Manager at Air Force Office of Scientific Research, 875 Randolph St., Arlington, VA 22203, U.S.A.)

Abstract
Abstract. Impact damage detection in composite structures has gained a considerable interest in many engineering areas. The capability to detect damage at the early stages reduces any risk of catastrophic failure. This paper compares two advanced signal processing methods for impact location in composite aircraft structures. The first method is based on a modified triangulation procedure and Genetic Algorithms whereas the second technique applies Artificial Neural Networks. A series of impacts is performed experimentally on a composite aircraft wing-box structure instrumented with low-profile, bonded piezoceramic sensors. The strain data are used for learning in the Neural Network approach. The triangulation procedure utilises the same data to establish impact velocities for various angles of strain wave propagation. The study demonstrates that both approaches are capable of good impact location estimates in this complex structure.

Key Words
composites; aerospace structures; impact damage detection; genetic algorithms; neural networks.

Address
Shahruddin Mahzan, Wieslaw J. Staszewski and Keith Worden; Department of Mechanical Engineering, Sheffield University, Mappin Street, Sheffield S1 3JD, United Kingdom

Abstract
An adaptronic strut for machine tools with parallel kinematics for compensation of the influence of geometric errors is introduced. Implemented within the strut is a piezoelectric sensor-actuator unit separated in function. In the first part of this contribution, the functional principle of the strut is presented. For use of one piezoelectric transducer as both, sensor and actuator as so-called self-sensing actuator, the acquisition of the sensing signal while actuating simultaneously using electrical bridge circuits as well as filter properties are examined. In the second part the control concept developed for the adaptronic strut is presented. A co-simulation model of the strut for simulating the controlled multi-body behavior of the strut is set-up. The control design for the strut as a stand-alone system is tested under various external loads. Finally, the strut is implemented into a model of the complete machine tool and the influence of the controlled strut onto the behavior of the machine tool is examined.

Key Words
parallel kinematics; self-sensing piezo actuator; control; vibrating string.

Address
Christian Rudolf; Institut fur Technische Mechanik (ITM), University Karlsruhe (TH), Kaiserstr. 10, 76131 Karlsruhe, Germany
Thomas Martin; Forschungszentrum Karlsruhe GmbH, Institut fur Angewandte Informatik (IAI) Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein Leopoldshafen, Germany
Jog Wauer; Institut fur Technische Mechanik (ITM), University Karlsruhe (TH), Kaiserstr. 10, 76131 Karlsruhe, Germany



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