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Aeronautical and Astronautical Engineering

Structural Dynamics
^ Damping in Bolted Joints
L. A. Bergman,* A. F. Vakakis* (Mech. & Indus. Engr.), C. J. Hartwigsen, Y. Song
Sandia National Laboratories, DOE SNL BF-0162
lbergman@uiuc.edu

Mechanical joints are recognized to be responsible for much of the uncertainty in the behavior of otherwise linear structures. Two mechanisms that have been identified as both present and important are microslip in the vicinity of connectors, such as bolts, and slap between adjacent parts of a structure, particularly at high frequencies. Analysis and experiments have been used to characterize the behavior of two beams connected by a bolted lap joint, with work continuing on the development of predictive models.

^ Dynamics and Control of Highway Bridges
L. A. Bergman,* Y. Song, C. Bilello, A. Pesterev (Russian Academy of Sciences)
National Science Foundation, CMS 98-00136
lbergman@uiuc.edu

Various strategies for extending the life of highway bridges will be examined, including passive, semiactive, and active control strategies for limiting bridge response to heavily loaded vehicles. Efficient methods leading to low-order control-oriented models for the bridge-vehicle system have been developed, and various vehicle suspensions are being studied. A series of model experiments are under way to assess effects of bridge damage on response.

^ Novel Passive Control Methods for Aerostructures
L. A. Bergman,* A. F. Vakakis* (Mech. & Indus. Engr.), D. M. McFarland,* B. Ozdoganlar
Air Force Office of Scientific Research, F49620-01-1-0208
lbergman@uiuc.edu

Researchers are applying nonlinear localization and energy pumping to the vibration and shock isolation of structures representative of aircraft components. To achieve this, researchers use both analysis and experiments to gain a better understanding of the fundamental physics underlying both nonlinear localization and energy pumping. The research team is extending the energy pumping concept to flexible continuous structures.

^ Acoustic and Viscoelastic Wave Propagations with Absorbing Boundaries
H. H. Hilton,* M. J. Yedlin* (British Columbia)
University of Illinois; University of British Columbia

The previous work of Yedlin and Luo is extended and generalized to 1-D and 2-D linear viscoelastic wave propagations with absorbing boundaries. Formal analytical solutions are developed, showing that the governing relations and BCs for the 1-D and 2-D problems are of sufficient complexity that they are not amenable to analytic solutions. However, numerical formulations using finite elements and finite differences are employed, yielding excellent results. Appropriate solution methodologies are discussed and evaluated, and the influence of the various viscoelastic material parameters is examined in detail by illustrative examples. It is also shown that, as inverse problems, these formulations and their attendant solutions can be readily used for experimental material characterizations.

^ Control of Piezoviscoelastic Lifting Surfaces
H. H. Hilton,* S. Yi* (Nanyang), C. E. Beldica*
University of Illinois; Nanyang Technological University; National Center for Supercomputing Applications; DOD DAHC94-46-C-0005 (HPCMP-PET)

A systematic analytical study has been initiated to investigate static and dynamic control of lifting surfaces through material and electric damping and control. Sensitivity studies to determine significant parameters are in progress to control creep divergence, flutter, control surface effectiveness, and the impact of aerodynamic noise.

^ Finite-Element Analysis of Anisotropic Viscoelastic Composites
H. H. Hilton,* C. E. Beldica, S. Koric
University of Illinois; National Center for Supercomputing Applications; U.S. Department of Energy, DAHC94-46-C-0005 (HPCMP-PET)

Advanced composite laminates are being used in flight vehicles to improve performance by substantial structural weight savings. Present numerical analysis requires computers with large storage and lengthy real-time to complete the calculations. The method under development uses Laplace transforms and thereby requires computer real-time use comparable to elastic anisotropic analyses. Results of various loading conditions compare extremely well with exact analytical solutions. Finite-element analyses for dynamic loadings on anisotropic viscoelastic composites that save extensive computer time and storage have been developed. The numerical results compare extremely well with analytical exact solutions.

^ Generalized Viscoelastic 1-DOF Deterministic and Stochastic Nonlinear Oscillators
H. H. Hilton,* S. Yi* (Nanyang)
University of Illinois; Nanyang Technological University

In this study, the theory of deterministic and stochastic generalized viscoelastic Duffing, Roberts, and van der Pol oscillator responses are formulated and evaluated. Numerical solution protocols are developed and the results are evaluated to determine the influence of viscoelastic damping on the oscillators' performance. It has been found that generalized viscoelastic material behavior profoundly affects the displacements and phase relations of these oscillators.

^ Torsion-bending Flutter of Viscoelastic Wings
H. H. Hilton,* C. E. Beldica*
University of Illinois; National Center for Supercomputing Applications; U.S. Department of Energy, DAHC94-46-C-0005 (HPCMP-PET)

An analysis of subsonic and supersonic torsion-bending flutter (including rotary inertia, shear, and hearing effects) of a time-dependent linear viscoelastic lifting surface consisting of either a Bernoulli-Euler or a Timoshenko beam, is formulated using aerodynamic strip theory. Complex moduli models for aluminum are characterized as functions of temperature and frequency by fitting Chebyshev polynomials to actual material experimental data. The flutter analysis is carried out in the complex plane and a computerized iterative method for the determination of flutter speeds and frequencies is developed. The influence of viscoelastic material properties (storage and loss moduli), temperature, rotary inertia, and shear effects is evaluated. Finite-element protocols are formulated and evaluated.

^ Linear and Nonlinear Passive Vibration Control of Mechanical Systems
A. F. Vakakis* (Mech. & Indus. Engr.), J. Georgiadis* (Mech. & Indus. Engr.), L. A. Bergman,* X. Jiang, Y. Song, G. Raguin
U.S. Office of Naval Research, N00014-00-1-0187
avakakis@uiuc.edu

Researchers develop linear and nonlinear vibration isolation designs for naval application and passive control designs for shipboard truss-like structures. The team also examines passive approaches for ice delamination on airplane wings.

^ Nonlinear Localization for Shock Isolation of Flexible Structures
A. F. Vakakis* (Mech. & Indus. Engr.), L. A. Bergman,* D. M. McFarland,* S. Moon, Y. Wang
National Science Foundation, CMS 00-00060
avakakis@uiuc.edu

A goal of this project is to develop a new kind of shock isolation system, based on the concept of nonlinear localization, whereby induced vibrational energy is passively confined to a preassigned secondary system and away from the primary system to be isolated. Researchers show that a robust nonlinear localization phenomenon can be effected by tuning the shock isolation system so that a 1:1 resonance exists between the secondary substructure and a mode of the primary one, or by inducing a strongly localized nonlinear normal mode that is partially confined to the secondary substructure. An experiment is currently in the design stage.


Summary of Engineering Research