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  • 1
    Electronic Resource
    Electronic Resource
    Springer
    Journal of elasticity 6 (1976), S. 1-38 
    ISSN: 1573-2681
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Description / Table of Contents: Zusammenfassung Die Wechselwirkung eines identischen parallelen elastischen Fadenpaares in einem faserverstärkten Verbundstoff wird in Bezug auf die Lastaufnahme untersucht. Man nimmt an, dass die Fäden einen gleichmässigen runden Querschnitt haben, halbunendlich sind, und dass sie ohne Unterbrechung an einem überall unendlichen Grundstoff eindeutiger elastischer Eigenschaften anhaften. Im Unendlichen wird der Grundstoff einem einachsigen Zug parallel zu den Fäden ausgezetzt. Es werden zwei verschiedene, aber in Verbindung stehende, Fragenstellungen behandelt. In der Ersten erstrecken sich beide Fäden unendlich lang in derselben Richtung und ihre Endquerschnitte sind koplanar. In der Zweiten erstrecken sie sich unendlich lang in entgegengesetzten Richtungen und ihre Endquerschnitte müssen nicht koplanar sein, obwohl die Fäden sich teilweise überlagern dürfen. Die Analyse wird mit einem von Muki und Sternberg entwickelten Näherungsverfahren durchgeführt, welches zum Teil auf einer dreidimensionalen linearen Elastizitätstheorie beruht. Letzten Endes nehmen diese Probleme die Form Fredholmscher Integralgleichungen an, welche die Verteilung der Fadenlängskraft kenzeichnen. Die Integralgleichungen werden asymptotisch und zahlenmässig analysiert.
    Notes: Abstract This investigation is concerned with the interaction—as far as load-absorption is concerned—of a pair of identical parallel elastic filaments in a fiber-reinforced composite material. The filaments are assumed to have uniform circular cross-sections, are taken to be semi-infinite, and are supposed to be continuously bonded to an all-around infinite matrix of distinct elastic properties. At infinity the matrix is subjected to uniaxial tension parallel to the filaments. Two separate but related problems are treated. In the first both filaments extend to infinity in the same direction and their terminal cross-sections are coplanar. In the second problem the filaments extend to infinity in opposite directions and their terminal cross-sections need no longer be coplanar, the two filaments being permitted to overlap partly. An approximate scheme based in part on three-dimensional linear elasticity and developed originally by Muki and Sternberg is employed in the analysis. The problems are ultimately reduced to Fredholm integral equations which characterize the distribution of the axial filament force. The integral equations are analyzed asymptotically and numerically. Results are presented which show the variation of filament force with position and the effect on this variation of various relevant geometrical and material parameters. One result is of particular interest. In the second problem, involving the overlapping filaments, for certain cases the filament force exceeds its far-field asymptotic value for a portion of the filament length. Stated another way, this means that the filament is loaded in excess of that which one would calculate by equating axial strains.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 1997-01-01
    Description: Two extensions of the force reconstruction method, the sum of weighted accelerations technique (SWAT), are presented in this article. SWAT requires the use of the structure’s elastic mode shapes for reconstruction of the applied force. Although based on the same theory, the two new techniques do not rely on mode shapes to reconstruct the applied force and may be applied to structures whose mode shapes are not available. One technique uses the measured force and acceleration responses with the rigid body mode shapes to calculate the scalar weighting vector, so the technique is called SWAT-CAL (SWAT using a calibrated force input). The second technique uses the free-decay time response of the structure with the rigid body mode shapes to calculate the scalar weighting vector and is called SWAT-TEEM (SWAT using time eliminated elastic modes). All three methods are used to reconstruct forces for a simple structure.
    Print ISSN: 1070-9622
    Electronic ISSN: 1875-9203
    Topics: Mathematics
    Published by Hindawi
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  • 3
    Publication Date: 2011-08-24
    Description: One of the basic requirements in engineering analysis is the development of a mathematical model describing the system. Frequently comparisons with test data are used as a measurement of the adequacy of the model. An attempt is typically made to update or improve the model to provide a test verified analysis tool. System identification provides a systematic procedure for accomplishing this task. The terms system identification, parameter estimation, and model correlation all refer to techniques that use test information to update or verify mathematical models. The goal of system identification is to improve the correlation of model predictions with measured test data, and produce accurate, predictive models. For nonmetallic structures the modeling task is often difficult due to uncertainties in the elastic constants. A finite element model of the shell was created, which included uncertain orthotropic elastic constants. A modal survey test was then performed on the shell. The resulting modal data, along with the finite element model of the shell, were used in a Bayes estimation algorithm. This permitted the use of covariance matrices to weight the confidence in the initial parameter values as well as confidence in the measured test data. The estimation procedure also employed the concept of successive linearization to obtain an approximate solution to the original nonlinear estimation problem.
    Keywords: SPACECRAFT DESIGN, TESTING AND PERFORMANCE
    Type: JPL, Model Determination for Large Space Systems Workshop, Volume 1; p 331-372
    Format: text
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  • 4
    Publication Date: 2019-07-19
    Description: Wind turbines are very large, flexible structures, with aerodynamic forces on the rotating blades producing periodic forces with frequencies at the harmonics of the rotation frequency. Due to design consideration, these rotational frequencies are comparable to the modal frequencies; thus avoiding resonant conditions is a critical consideration. Consequently, predicting and experimentally validating the modal frequencies of wind turbines has been important to their successful design and operation. Performing modal tests on flexible structures over 120 meters tall is a substantial challenge, which has inspired innovative developments in modal test technology. A further trial to the analyst and experimentalist is that the modal frequencies are dependent on the turbine rotation speed, so testing a parked turbine does not fully validate the analytical predictions. The history and development of this modal testing technology will be reviewed, showing historical tests and techniques, ranging from two-meter to 100-meter turbines for both parked and rotating tests. The NExT (Natural Excitation Technique) was developed in the 1990's, as a predecessor to OMA to overcome these challenges. We will trace the difficulties and successes of wind turbine modal testing over the past twenty-five years from 1982 to the present.
    Keywords: Aerodynamics
    Type: International Operational Modal Analysis Conference; Apr 30, 2007 - May 02, 2007; Copenhagen; Denmark
    Format: text
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  • 5
    Publication Date: 2019-07-12
    Description: Wind turbines are immense, flexible structures with aerodynamic forces acting on the rotating blades at harmonics of the turbine rotational frequency, which are comparable to the modal frequencies of the structure. Predicting and experimentally measuring the modal frequencies of wind turbines has been important to their successful design and operation. Performing modal tests on wind turbine structures over 100 meters tall is a substantial challenge, which has inspired innovative developments in modal test technology. For wind turbines, a further complication is that the modal frequencies are dependent on the turbine rotation speed. The history and development of a new technique for acquiring the modal parameters using output-only response data, called the Natural Excitation Technique (NExT), will be reviewed, showing historical tests and techniques. The initial attempts at output-only modal testing began in the late 1980's with the development of NExT in the 1990's. NExT was a predecessor to OMA, developed to overcome these challenges of testing immense structures excited with environmental inputs. We will trace the difficulties and successes of wind turbine modal testing from 1982 to the present. Keywords: OMA, Modal Analysis, NExT, Wind Turbines, Wind Excitation
    Keywords: Aerodynamics
    Type: JSC-CN-16440 , To be published in Mechanical Systems and Signal Processing, August 2008
    Format: application/pdf
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  • 6
    Publication Date: 2019-07-12
    Description: The structural dynamicist s areas of responsibility require interaction with most other members of the wind turbine project team. These responsibilities are to predict structural loads and deflections that will occur over the lifetime of the machine, ensure favorable dynamic responses through appropriate design and operational procedures, evaluate potential design improvements for their impact on dynamic loads and stability, and correlate load and control test data with design predictions. Load prediction has been a major concern in wind turbine designs to date, and it is perhaps the single most important task faced by the structural dynamics engineer. However, even if we were able to predict all loads perfectly, this in itself would not lead to an economic system. Reduction of dynamic loads, not merely a "design to loads" policy, is required to achieve a cost-effective design. The two processes of load prediction and structural design are highly interactive: loads and deflections must be known before designers and stress analysts can perform structural sizing, which in turn influences the loads through changes in stiffness and mass. Structural design identifies "hot spots" (local areas of high stress) that would benefit most from dynamic load alleviation. Convergence of this cycle leads to a turbine structure that is neither under-designed (which may result in structural failure), nor over-designed (which will lead to excessive weight and cost).
    Keywords: Structural Mechanics
    Type: JSC-CN-18049
    Format: application/pdf
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  • 7
    Publication Date: 2019-07-13
    Description: Field testing large structures creates unique challenges such as limited choices for boundary conditions and the fact that natural excitation sources cannot be removed. Several critical developments in field testing of large structures are reviewed, including: step relaxation testing which has been developed into a useful technique to apply large forces to operational systems by careful windowing; the capability of large structures testing with free support conditions which has been expanded by implementing modeling of the support structure; natural excitation which has been developed as a viable approach to field testing; and the hybrid approach which has been developed to allow forces to be estimated in operating structures. These developments have increased the ability to extract information from large structures and are highlighted in this presentation.
    Keywords: Structural Mechanics
    Type: JSC-17731 , International Modal Analysis Conference 27th; Feb 09, 2009 - Feb 12, 2009; Orlando, FL; United States
    Format: application/pdf
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