Generalizations are drawn for passive viscoelastic damping both alone and in conjunction with active control for vibration suppression. There are many high payoff applications where passive damping has played a vital role in suppressing vibration and noise. Many studies have shown that passive damping can greatly decrease the size and power required and increase the robustness of active control systems. There is a strong synergism with passive damping and active control. Structural identification with its largely unrecognized challenges is discussed. In particular, the case of high modal density is covered. The results of several analytic studies are summarized together with selected experimental results. Passive damping has been shown to remove large amounts of vibratory energy, lower the response of the structure to excitation, stabilize any active control system, and make an active system more robust. For extremely challenging vibration suppression requirements, an optimum blend of active and passive is the only practical approach.

A general design methodology to integrate active control with passive damping has been demonstrated on the NASA LaRC CSI Evolutionary Model (CEM), a ground testbed for future large, flexible spacecraft. A limited bandwidth active controller is designed with a frequency-shaped H₂ algorithm which minimizes the response of a line-of-sight (LOS) measurement system. Using the modal strain energy (MSE) method, a passive damping treatment is designed based on damping level requirements for a set of targeted modes in the high-frequency band, beyond the active control bandwidth. The damping levels are selected to achieve robustness as well as the same level of performance in the high frequency band as in the active control bandwidth. A set of 60 viscoelastic damped struts were designed and fabricated for the CEM. An integrated active/passive controller was successfully implemented on the CEM and was evaluated against an active-only controller.

The use of piezoelectric materials simultaneously as passive modal devices and active elements to suppress structural vibrations is discussed. A simple modal model is developed which predicts the behavior. Experimental studies are then conducted to verify the model. The results show a reduction of 20 dB in the second mode of the structure is possible via passive shunting while simultaneously commanding the piezo-actuator. The results indicate that for vibration problems involving a few modes, piezoelectric devices combining active and passive techniques can significantly reduce the broadband structural response to disturbances.

This paper describes the development and experimental application of an actuator with built-in viscous damping. An existing passive damper was modified for use as a novel actuation device for isolation and structural control. The device functions by using the same fluid for viscous damping and as a hydraulic lever for a voice coil actuator. Applications for such an actuator include structural control and active isolation. Lumped parameter models capturing structural and fluid effects are presented. Component tests of free stroke, blocked force, and passive complex stiffness are used to update the assumed model parameters. The structural damping effectiveness of the new actuator is shown to be that of a regular D-strut passively and that of a piezoelectric strut with load cell feedback actively in a complex testbed structure. Open and closed loop results are presented for a force isolation application showing an 8 dB passive and 20 dB active improvement over an undamped mount. An optimized design for a future experimental testbed is developed.

This paper summarizes a development study that involved the design, fabrication, and test of a prototype adjustable viscous damper. The study, sponsored by NASA Langley Research, was performed by McDonnell Douglas and Honeywell, and addressed the need for an adaptable passive damping system for spacecraft by investigating methods of tuning the passive viscous damping device known as the D-Strut^TM. The D-Strut is a flight-qualified device used for both compliant isolation systems and rigid structural damping systems. The goal of the study was to demonstrate a specific design that would allow peak damping to be adjustable between any frequency from 0.1 to 10 Hz. Several tuning methods were investigated. The tapered annulus concept was selected because it is mechanically simple and provides a broad range of damping adjustment. Results were predicted by modeling and design analyses. Characterization testing was performed using impedance test methods. High, low, and intermediate adjustments were made to validate range capability. Success of the project is clearly illustrated by impedance amplitude and phase plots.

An adaptive passive damping system is described for the low-order modes of spacecraft large solar arrays. The work is motivated by the stringent sensor pointing requirements of several current satellite programs. Realistic system-level requirements are developed relating pointing error of a representative spacecraft to damping of its solar arrays. Performance is specified in terms of gain envelopes on open-loop frequency response functions for the structure with dampers. A family of remotely tunable, eddy-current tuned mass dampers (TMDs) is described which suppress several modes of a 430-lb solar array simulator in the frequency range of 0.1 - 1.0 Hz. The magnetic dampers are ground demonstration units, suitable for use in 1-G, but are designed around traceable, system-level dynamic requirements. The damper design is intended from the outset to be evolved into flight hardware. Tuning of TMD natural frequency and damping ratio to their optimum values is demonstrated through component-level base transmissibility test results.

This paper presents a novel concept for the isolation of vibrations using magnetically adjusted springs in conjunction with a feedback regulation system. At the heart of the system proposed is a nonlinear magnetic spring component, the constitutive properties of which enable it to passively achieve negative stiffness over a limited region of displacement. When mounted in parallel with a conventional, linear spring, the result is a nonlinear spring with a nearly flat constitutive curve for a certain displacement range. Within this range, therefore, the spring behaves as though it were extremely soft, thereby passively affording the desirable isolation characteristics associated with a soft spring, only without the inherent physical disadvantages. A simple feedback regulator is necessary in order to insure that the system operates within its linear range. The paper presents some background to the problem of vibration isolation and then discusses the fundamental design and operation of the magnetic springs. Next, we propose a simple PI controller for operating-point regulation and maintenance of linear operation. Following an analysis of predicted spring performance, we then present a numerical simulation of the entire system and compare this result to that obtained from experiments performed with a laboratory prototype.

The electromechanical surface damping (EMSD) technique, for controlling the peak vibration amplitudes of beam-like structures, is modified to extend the effective range of the approach. The technique is a combination of the constrained layer damping and the shunted piezoelectric methods, where the viscoelastic constrained layer attached to the vibrating surface is constrained by a shunted piezoelectric ceramic element. The mathematical model of the modified EMSD element is presented, implemented into a finite element algorithm and applied to demonstrate the ability of the technique to simultaneously and effectively suppress the first three resonant peaks of a generic aluminum cantilever beam.

Theoretical and experimental performance characteristics of the new class of actively controlled constrained layer damping (ACLD) treatment are presented. The ACLD under consideration consists of a visco-elastic damping layer which is sandwiched between two piezo-electric layers. The three-layer composite ACLD when bonded to a vibrating structure acts as a SMART constraining layer damping treatment with built-in sensing and actuation capabilities. Particular emphasis is placed on studying the performance of ACLD treatments that are provided with sensing layers of different spatial distributions. The effect of the modal weighting characteristics of these sensing layers on the broad band attenuation of the vibration of beams that are fully treated with the ACLD is presented theoretically and experimentally. The equations governing the operation of ACLD treatments with modally shaped sensors are presented. The theoretical predictions of the model are compared with the experimental performance of a computer-controlled beam treated with Dyad 606 visco-elastic layer sandwiched between two layers of polyvinylidene fluoride (PVDF) piezo-electric films. Comparisons with the performance of conventional passive constrained layer damping are also presented.

This paper applies intelligent constrained layer (ICL) damping treatments to control bending vibration of composite plate structures. The ICL damping treatment consists of a viscoelastic shear layer sandwiched between a piezoelectric constraining cover sheet and the structure to be dampened. According to measured vibration response of the structure, a feedback controller regulates in-plane deformation of the piezoelectric layer to perform active vibration control. In this paper, the equation of motion and boundary conditions governing the bending vibration of a composite plate possessing bending-stretching coupling stiffness with ICL damping treatments are derived. Finally, a numerical example is illustrated on an infinite composite plate consisting of a (45 degree(s)/-45 degree(s)) angle-ply laminate subjected to plane harmonic wave excitations. Compared with existing passive constrained layer treatments, numerical results show that the ICl damping treatment can produce significant damping. In addition, there are certain orientations, determined by the structures of the composite laminate, along which the vibration amplitude can be reduced to a minimum.

Constrained layer damping has been used for many years to increase the damping in engineering structures. Several researchers have suggested the concept of using viscoelastic materials with piezoceramics as the constraining layer. Since the piezoceramics are active materials, this concept can be referred to as active constrained layer damping. The paper presents a finite element model for a sandwich beam consisting of a host layer, a viscoelastic layer, and a piezoelectric layer. Lesieutre's method (Augmenting Thermodynamic Fields) for modeling damping was modified and applied to active constrained layer damping. Previous work on active constrained layer material has used the loss factor approach to modeling the viscoelastic layer. Such approaches are limited to steady state considerations, while the approach taken here is suitable for transient disturbances. Active damping and passive damping are individually of interest, however, here we propose to combine these two types of damping to produce an active constrained layer system with the best of both technologies.

The work described in this paper is concerned with controlling the strain of the constraining layer of a composite structure in such a way as to enhance the shear generated in the viscoelastic material and hence improve the overall damping of the composite structure. The results have indicated that this concept of active damping produces very effective levels of vibration suppression. In the case of cantilever beams the first two modes can be almost eliminated when velocity feedback of the beam tip is used. The results show that the addition of active control and passive damping in a single structure combines the advantages of passive damping in the higher modes and active control in the lower modes. In addition active damping as defined in this paper produces a fail safe mechanism in case of instability occurring in the feedback loop since passive damping is always present.

Neural network control of flexible structures demonstrates better settling time and energy dissipation than linear design methods. Optimal tuning of passive vibration absorbers for reduced order control is examined using linear and non-linear cases. Quasi-Newton (BFGS) and simplex optimization methods improved the Den Hartog parameters where unsupervised LMS or backpropagation techniques were unstable. Lessons on unsupervised training for dynamic system control are illustrated by examining convergence to the solution in `error space' (parameters vs. cost). Spring stiffness and passive damping of a reaction mass actuator (RMA) are `tuned' for best disturbance rejection using total energy as a cost function. A single neuron using two weights (one for damping and the other for the spring coefficient) improved beam energy over the Den Hartog parameters for the linear bi-modal case. The non-linear case demonstrates even better performance. A multiple layer network is then demonstrated for both the linear and non-linear cases. Optimization techniques improved linear system parameters when initiated at the linear solutions. Lab data for the linear single neuron case validates model fidelity.

A nonlinear viscoelastic solid model, comprising a combination of linear and nonlinear springs and dashpots, is developed to represent an elastomeric damper. The nonlinear constitutive differential equation obtained from the model completely characterizes the damper behavior. A method is presented to determine the spring-dashpot parameters (coefficients of the constitutive equation) from experimental data. A quartic softening spring, in series with linear Kelvin chain, is used to match experimental data. Nonlinear hysteresis cycles at different equilibrium positions are examined. The model is able to predict behavior of elastomeric dampers under dual-frequency excitations. A `two-level implicit-implicit' scheme is developed for the integration of the nonlinear damper model into a structural dynamic analysis. With the increase in amplitude of oscillatory force, the energy dissipation by the nonlinear viscoelastic damper is found to decrease, as compared to a linearized perturbation model. With increase in initial perturbation, transient decay is slower.

Passive tuned vibration absorbers (TVA) are widely used in reducing vibrations in machinery, buildings and many other vibrating structures. Recently, TVAs have been installed on structures for the purpose of reducing the sound radiation from the structure. As in the case of vibration control, the tuning and placement of TVAs for noise control is often based on the forced response of the structure, as opposed to the behavior of structural-acoustic interaction. This conventional approach for TVA placement on structures is not always effective for controlling sound radiation from the structure. This task requires proper control of the structural waves that couple well with the acoustic environment. This paper presents a preliminary study of acoustic radiation and radiation efficiency of panels instrumented with passive TVAs.

Harsh environmental conditions and dynamic loads to which many large civil engineering structures are commonly subjected can with time promote serious damages and material deterioration that can undermine serviceability and safety of the structure, its components, and appliances. Reduction of vibrations of these structures caused by new and not previously envisaged dynamic loads can be properly achieved by using a robust passive mechanical control system such as simple tuned vibration absorbers (TVAs) that can be easily inspected and maintained, thus being adequate for severe environment. This paper shows that if TVAs are correctly designed they also are very efficient in reducing vibrations of these structures. By taking into account the non-proportional damping introduced in the multi-degree of freedom structural system by the TVAs, and by using an optimization technique to calculate their optimum parameters, vibrations can be lowered to admissible levels. A practical example of application of light and simple mechanical absorbers to an existing bridge is presented and their performance is demonstrated by experimentally measured dynamic responses.

This paper is concerned with the dynamic response of truss structures containing linear- friction dampers. This type of passive energy dissipator shows different stiffness in loading and unloading, leading to triangular hysteresis loops under cyclic loading. The free and forced vibration responses of simple truss structures incorporating linear-friction dampers are analyzed. The harmonic linearization technique is used in combination with the modal strain energy method to estimate the response of truss structures with linear-friction dampers. The accuracy of this linearization technique is assessed in the context of truss structures in this paper. It is demonstrated that excellent accuracy is achieved using this linearization method.

Analytical and empirical expressions that relate the complex dynamic stiffness of an elastomeric or viscoelastic anti-vibration mount to the complex Young's modulus of the constituent elastomeric or viscoelastic material of the mount are presented. These expressions incorporate the geometrical dimensions of the mount, the shape factor effects, and the non- linear deflection characteristics of the mount. It is shown that the use of these expressions, in conjunction with the complex Young's modulus of the constituent viscoelastic material, yields accurate predictions of the complex dynamic stiffness characteristics of an anti-vibration mount which was subjected to static prestrain levels of between 0% and 30% and was tested over a wide temperature range of -50 degree(s)C to 100 degree(s)C. Furthermore, it is shown that the master curves of complex dynamic stiffness predicted for the anti-vibration mount at static prestrain levels of 10% to 20% correlate fairly well with the master curves of complex dynamic stiffness derived from measured data using the temperature-frequency superposition principle.

The measurement of the bulk longitudinal modulus, M*, of rubbery polymers is a notoriously difficult task. Measurements are traditionally done through wave transmission through a fluid environment in methods that provide M* over only very narrow frequency ranges. An alternative approach, using a resonant test fixture, is presented in this paper. A discussion of some physical limitations is developed. Among them are issues of repeatability and edge effects. A method of data analysis to address edge effects also is presented.

Interpenetrating polymer networks (IPNs) are materials composed of two or more crosslinked polymers permanently and intimately intertwined on a molecular level. The resulting distribution of microenvironments can result in a material with a high mechanical loss in the glass-rubber relaxation, that is shifted in temperature and broadened over that of either constituent polymer. Several series of polyurethane/epoxy IPNs have been prepared for evaluation as possible broad band damping materials. Dynamic mechanical analysis and differential scanning calorimetry revealed that the temperature of the loss peak could be varied widely with sample formulation. Flexible epoxy components and plasticizers were incorporated. This resulted in materials with relatively low Young's and shear moduli, with losses that were broadened in the temperature regime. Simply supported beam assemblies were used to measure damping of three layer constrained structures. Comparison of measured temperature and frequency dependent viscoelastic behavior in constrained layer structures is analyzed in terms of the Ross-Kerwin-Ungar model for coated beams, and correlated to polymer composition and morphology.

The dynamic properties of blends based on epoxidized natural rubber have been measured using a dynamic mechanical thermal analyzer (DMTA). The formulations studied were chosen according to a statistical design, and response equations for various properties derived on the basis of the results. The equations are used to select further formulations with specified damping properties. Comparison of the properties observed for the selected formulations and those expected show generally good agreement. For some formulations, master curves of dynamic properties as measured by forced non-resonance on a servohydraulic testing-machine and by a longitudinal vibration method are presented. The latter has the advantage of being able to gather results towards the glassy region of the transition zone, without the problem of machine or jig compliance. The type of blends investigated offer materials covering a range of damping properties from a broad peak covering a relatively wide frequency range to a narrow damping peak whose position on the frequency axis can be adjusted by simple changes in the formulation.

A new finite element is developed that reduces the modeling required to define the constrained layer damping treatment. The new element emulates the traditional method of modeling constrained layer damping with plate elements on either side of a thin hex element. Trade studies are performed in the application of constrained layer damping treatments to beams and plates. The effects of the thicknesses of the constraining layer and the viscoelastic layer are studied as well as some studies on the location of the damping treatment.

The modal strain energy (MSE) method is an approximate technique for the approximate analysis of structures with frequency-dependent stiffness and damping matrices. In this paper, the accuracy of this method is investigated by computing the exact and approximate mean square responses of structures containing viscoelastic dampers to random excitation. Closed- form expressions of the mean square error are obtained for single-degree-of-freedom (SDOF) systems under linear hysteretic damping and Maxwell-type damping. The accuracy of the MSE method applied to multi-degree-of-freedom (MDOF) structures with non-classical viscous damping, hysteretic damping and Maxwell-type damping is investigated.

Forced vibration of viscoelastically damped three-layer sandwiched beams is considered. The sandwiched beam is assumed to be fixed at one end and is excited by a harmonic force at the other end. The displacement response of the beam is computed and an approximate transfer function including three consecutive modes is extracted. Vibration of damped sandwich beam is actively controlled by employing a PPD feedback controller. Comparison of the results verified the effectiveness of the proposed method.

This paper explains the reason for the significant difference of payload responses when different damping methods are applied. In dynamic analyses of space structures, three different types of damping matrices have usually been employed: system damping, free-free component damping and constrained component damping. This paper explains the reason for the discrepancies by considering a dynamic system composed of two substructures, each has two degrees-of-freedom. The close form expressions of the damping matrices are obtained on the same level of coordinates for direct comparisons, from which the reasons for the response discrepancies between different damping approaches in treating dynamic problems are interpreted.

Experimental results of the dynamics of a cantilever beam constrained by shape memory wires are presented. It is observed that the damping increases significantly when the shape memory wires are pre-stressed such that they lie within the pseudoelastic hysteresis loop. Theoretical models of the inner hysteresis loop are considered, and modal analysis is used to obtain the dynamic response of the system. Simulations of the system using these models give theoretical values of damping which agree well with those observed experimentally. The proposed models of the pseudoelastic hysteresis loop are sufficient to obtain an estimate of the initial increase of damping due to the use of pre-stressed shape memory wires in structures. These results demonstrate that pseudoelasticity of shape memory wires can be used to significantly augment passive damping in structural systems.

Proof mass actuators have long been an accepted and reliable method of vibration control. With the advent of new piezoceramic technology, the proof mass can now be driven by a piezoceramic linear motor. A linear motor has been designed and its performance is evaluated by comparing it against a Physik Instrument piezoceramic stack translator. Both linear motors are used to actively add damping to the bending modes of a section of the NASA Phase Zero Evolutionary Model. A modal test has been done on the test structure and its mode shapes, natural frequencies and damping ratios have been recorded. In the unlikely event of a power failure on board such a structure, a backup control scheme is desired. Viscoelastic damping material is incorporated into the newly designed actuator to improve the stability of the closed loop system and to offer good `shut down' performance in case of a power failure. Actuator performance such as force output, stroke length, and control implementation are compared.

The Italian experience in the field of `stand alone' monitoring systems for viaducts shows that the design of said systems, as well as that of the single components utilized, must satisfy complex and often unusual operating conditions and, at times, yield to compromises of various types. This is necessitated by the fact that the electronic devices used are expensive and the characteristics of high resistance to the elements and long term reliability they must satisfy. Therefore, it is often preferable to limit the system performance envelope in the interest of ease of maintenance characteristics, intrinsic safety, simplicity of installation, resistance to the most aggressive atmospheric agents, imperviousness to outside disturbances, etc. One of the most onerous aspects the design engineers must take into account comprises the interaction, between those work site activities specifically connected with the installation of the monitoring system and those that merely relate to the construction of the structure. The difference in tradition in terms of the scope of monitoring strategies between the designer of measuring and telecontrol systems and the design engineers of the structure in question must not be forgotten.

The bridge over the Arno River built in the late 1980s, located at the boundary of a seismic zone, has been instrumented with a large number of transducers of different types, for a total of 354 measuring points. The variables monitored are: vertical load on the bedrock, stresses and temperatures in the concrete, inclinations of the piers, vertical load on the bearings, and displacements in the expansion joints. The installation of the transducers was carried out simultaneously with the different phases of the erection of the structure and the values of the monitored variables were measured throughout the period of construction and the following two years of service. Measurement data, analyzed and compared to design data, permitted the acquisition of important information on the actual process of concrete curing and the settling in of the structure, both under the influence of job site operations and the ensuing normal service loads.

The University of Dayton Research Institute is teamed with McDonnell Douglas Aerospace to develop and fly a NASA IN-STEP experiment entitled Jitter Suppression Experiment (JSX). The JSX will demonstrate a combined active/passive vibration control system on an actual full scale space structure. This paper presents the details of the design of the passive control system to be applied to six graphite/epoxy support tubes for a gimballed telescope assembly (GTA) and the results of concept development laboratory tests. The most promising damping concepts for tubes such as the GTA truss tubes were constrained layer dampers and tuned dampers. The advantages and disadvantages of each of these damping concepts and the reasons for choosing a constrained layer concept are discussed. To verify the effectiveness of the passive damping design and to determine the constrained layer damping system effectiveness for axial modes, a finite element analysis of a single truss tube with the proposed constraining layer design was performed. A laboratory simulation was designed, developed, and evaluated to verify the analysis.