To study new damping augmentation methods for helicopter rotor systems, accurate and reliable nonlinear damping identification techniques are needed. For example, current studies on applications of magnetorheological (MR) dampers for rotor stability augmentation suggest that a strong Coulomb damping characteristic will be manifested as the field applied to the MR fluid is maximized. Therefore, in this work, a single degree of freedom (SDOF) system having either nonlinear Coulomb or quadratic damping is considered. This paper evaluates three analyses for identifying damping from transient test data; an FFT-based moving block analysis, an analysis based on a periodic Fourier series decomposition, and a Hilbert transform based technique. Analytical studies are used to determine the effects of block length, noise, and error in identified modal frequency on the accuracy of the identified damping level. The FFT-based moving block has unacceptable performance for systems with nonlinear damping. These problems were remedied in the Fourier series based analysis and acceptable performance is obtained for nonlinear damping identification from both this technique and the Hilbert transform based method. To more closely simulate a helicopter rotor system test, these techniques were then applied to a signal composed of two closely spaced modes. This data was developed to simulate a response containing the first lag and 1/rev modes. The primary mode of interest (simulated lag mode) had either Coulomb or quadratic damping, and the close mode (1/rev) was either undamped or had a specified viscous damping level. A comprehensive evaluation of the effects of close mode amplitude, frequency, and damping level was performed. A classifier was also developed to identify the dominant damping mechanism in a signal of 'unknown' composition. This classifier is based on the LMS error of a fit of the analytical envelope expression to the experimentally identified envelope signal. In most instances, the classifier identifies the damping mechanism correctly, erring only when the close mode significantly affects the envelope signal.

Particle Impact Damping (PID) is a means for achieving high structural damping by the use of a particle-filled enclosure attached to the structure in a region of high displacements. The particles absorb kinetic energy of the structure and convert it into heat through inelastic collisions between the particles and the enclosure, and amongst the particles. PID is measured for a cantilevered aluminum beam with the damping enclosure attached to its free end; lead particles are used in this study. The effect of acceleration amplitude and clearance inside the enclosure on PID is studied. PID is found to be highly nonlinear. The maximum Specific Damping Capacity (SDC) is about 50%, which is more than one order of magnitude higher than the intrinsic material damping of a majority of structural metals [O(1%)]. Driven by the experimental observations, an elementary analytical model of PID is constructed. A satisfactory comparison between the theory and the experiment is observed. An encouraging result is that in spite of its simplicity, the model captures the essential physics of particle impact damping.

Simulation models in space industry may allocate a few mesh points for such fine details as granular dampers embedded in mechanical parts. As a result, collective dynamics of grains inside dampers cannot be adequately resolved. A hierarchical approach to modeling and design has been explored by practitioners to study this phenomenon. Available models fail to explain the observed spectral response. This article is an attempt to examine models where dampers are replaced by calibrated linear and non-linear visco-elastic units with stochastic forcing. Namely, the parameterization of a non- linear visco-elastic unit is described. Non-linearity facilitates mode-mixing and shifts of major resonances, while stochastic forcing contributes to the overall risk assessment. Limitations of the proposed reductions of granular hydrodynamics are discussed.

This paper presents the development and evaluation of a controllable, semi-active magneto-rheological (MR) fluid shock absorber. This is a new design that is tailored for structures and ground vehicles that undergo a wide range of dynamic loading including the capability for passive, load-specific, individual rebound and compression characteristics. The specific application for testing and proof-of-concept is the AM General HMMWV (High Mobility, Multipurpose Wheeled Vehicle). The new MR shock absorber emulates stock, original equipment manufacturer (OEM) shock absorber behavior in passive mode (i.e., zero-field) and provides a wide range of controllable damping force above (and below, if needed) zero- field damping levels.

There are a number of designs that have been proposed for MR and ER fluid actuators. While studies have shown that the fluids themselves behave similarly (in terms of constitutive behavior), the design issues are clearly different. ER fluids require electric fields to control their behavior, while MR fluids require magnetic fields. The goal in each case is to maximize the field to achieve the highest possible change in fluid properties in the working volume. Design challenges are faced in trying to maximize this field in an actuator. This paper will focus on design issues for MR fluid actuators. The focus will be on how to get the most out of the field that is produced. One critical issue is how to produce high force with a low magnetic field. Magnetic field alignment and actuator geometry are investigated as design issues. Experimental data shall be presented for two distinct actuator designs.

It is widely acknowledged that the inherent non-linearity of smart fluid dampers is inhibiting the development of effective control regimes, and mass-production devices. In an earlier publication, an innovative solution to this problem was presented -- using a simple feedback control strategy to linearize the response. The study used a quasi-steady model of a long-stroke Electrorheological damper, and showed how proportional feedback control could linearize the simulated response. However, this initial research did not consider the dynamics of the damper's behavior, and so the development of a more advanced model has been necessary. In this article, the authors present an extension to this earlier study, using a model of the damper's response that is capable of accurately predicting the dynamic response of the damper. To introduce the topic, the electrorheological long-stroke damper test rig is described, and an overview of the earlier study is given. The advanced model is then derived, and its predictions are compared to experimental data from the test rig. This model is then incorporated into the feedback control simulations, and it is shown how the control strategy is still able to linearize the response in simulations.

A novel approach for reducing mode localization of bladed-disk assemblies in turbomachinery is presented .A virtual shroud is created with piezoelectric coupling to reduce the sensitivity of the compressor disks to model localization. Both direct and resonant shunts are evaluated using a simple lumped-mass model. Resonant shunts are shown to be effective in reducing localization of certain structural modes.

Passive damping using a piezoelectric device is a well-known technique. Both resistor and inductor loads connected to the piezoceramic are commonly used to attenuate a given resonance mode on a structure equipped with piezo dampers. The main drawback of this technique is its narrow band behavior and especially in the case of an inductor tuned passive piezo damper. The proposed technique is inherently wide band and does not rely on any tuned electric load. The piezoelectric device is simply continuously switched from open-circuit to short-circuit synchronously to the mechanical strain. It is called semi-passive because of the need of a sensor giving the strain of the piezo device. There is no need for external power supply unless for the low-level circuitry of the switch device. The damping efficiency appears to be twice what is obtained with pure resistive damping and is equivalent to what is achievable with a tuned inductor damper. It can work at any frequency without the need for large inductor especially for low frequency applications. A qualitative model gives an understanding of the damping mechanism.

In an earlier paper we reported a method for multiple-mode shunt-damping of structural vibration modes. It was successfully demonstrated in experiments on a two-wing cantilever beam to reduce three structural vibration modes simultaneously using a single piezoelectric PZT transducer. This multiple-mode shunting method is particularly useful for reduction of several vibration modes on structures that are limited in surface area and must be lightweight. To obtain a better overall damping performance for some structures, we have extended this shunting technique by employing multiple PZT transducers; each is implemented with a multiple-mode shunt circuit. This paper reports the reason why multiple transducers are needed and how we design the multiple-mode shunt circuits for them. This is described with experiments performed on two structures: one, a simple cantilever beam and the other, a cut-out panel from an F-15 fighter aircraft. Both of the structures are bonded with several PZT transducers implemented with multiple-mode shunt circuits. In the cantilever beam structure, we show the advantage of using a different transducer for controlling modes that are otherwise not controllable with the first transducer alone. For the F-15 panel, five PZT transducers were bonded on the inner surface of the panel. During the shunting experiment, the panel was excited with an acoustic load. We report how we determined the locations and the number of the transducers, and how we designed and implemented them with multiple-mode shunt circuits to control the two high-structural-vibration modes. Detailed experimental results are presented.

In this paper, a semi-active control law is used to switch the electrical shunt circuit of a piezoelectric actuator for energy dissipation in a simple mechanical system. Switching is done between open-circuit (high stiffness) and short- or resistive-circuit (low stiffness) states. The actuator is held in its high stiffness state when the system is moving such that energy can be stored in the actuator. When the system's motion would cause it to receive energy back from the actuator, the actuator is switched to a low stiffness state, dissipating the energy. In this numerical study, the state- switched piezoelectric system is used to suppress motions in flexural bending problem.

To augment weakly damped lag mode stability of a hingeless helicopter rotor blade in hover, piezoelectric shunt with a resistor and an inductor circuits for passive damping has been studied. A shunted piezoceramics bonded to a flexure of rotor blade converts mechanical strain energy to electrical charge energy which is dissipated through the resistor in the R-L series shunt circuit. Because the fundamental lag mode frequency of a soft-in-plane hingeless helicopter rotor blade is generally about 0.7/rev, the design frequency of the blade system with flexure sets to be so. Experimentally, the measured lag mode frequency is 0.7227/rev under the short circuit condition. Therefore the suppression mode of this passive damping vibration absorber is adjusted to 0.7227/rev. As a result of damping enhancement using passive control, the passive damper which consists of a piezoelectric material and shunt circuits has a stabilizing effect on inherently weakly damped lag mode of the rotor blades, at the optimum tuning and resistor condition.

Conventional vibration isolation mounts are not as effective as expected since the system/structure to be isolated from is normally not dynamically rigid and has resonance frequencies within the bandwidth of interest. Besides, the low frequency enhancement is a characteristic of the conventional passive mounts. Applying inertia actuators to the attachment plate of the conventional mounts overcomes these shortcomings and enhances their performance significantly. This design concept has universal application since it is applicable to any dynamic system. It allows using the widely implemented vibration suppression algorithms such as the collocated velocity feedback and the filtered-X LMS, thereby simplifying the controller design issue normally encountered in a practical complex system. In addition, it requires very little power and force capacity, i.e., a small percentage of the disturbance force, from the actuators to be effective for frequencies higher than the resonance frequency of the mount itself. In this work, the performance of the 'Robust Passive- Active Mount' on a realistic foundation (a system/structure from which to be isolated) and the performance of the multiple 'Robust Passive-Active Mounts' on the same foundation are simulated. The passive-active mounts are modeled as independent spring-mass-dashpot systems in state space form. The dynamics and the load transmissibility of a foundation are modeled as a continuous system in modal state space form. The mounts and the foundation are then formulated as a coupled system with actuator forces as feedback. The issue of the conventional mounts on a realistic foundation is illustrated first by showing the load transmissibility comparison with a rigid foundation. Then the effectiveness of the Passive-Active mounts, designed by the two commercial-off-the-shelf controllers, for machinery is demonstrated on the load transmissibility reduction at the foundation support (fixed end) due to disturbances from the machinery. A general method for simulating the isolation performance by advanced mounts from an elastic structure has been developed. This method is valid for any combination of multiple advanced mounts at any preferred strategic locations on any structure.

The transmission of sound and vibration across a sandwich structure has been studied in this paper. The main research issues are identified and the applied concept has been explained in detail. This concept has been validated in a previous study. It has been observed that successful manipulation of certain material parameters can minimize the sound and vibration transmission through the structure significantly. The results of this work could provide guidelines for material scientists to design new composite materials with better vibration and noise transmission properties.

The objective of this paper is to show an application of structural vibration control using a magnetorheological (MR) fluid actuator for a structural system to which is applied external forces. In order to control the structural vibration two control methods are applied and compared on the basis of variation control performance and energy consumption. One method is a fully-on control where the MR fluids are energized all the time so that the actuator performs as a tuned passive damper. The other method is a semi-active control of the input voltage whereby an MR fluid actuator is activated only for part of the vibration cycle. The structure used in this paper is a simply supported, uniform, homogeneous flexible beam. The MR fluid actuator is attached to the structural system in parallel. In this system the control inputs are moments at the points of the actuator attached to the system. The results of this paper show that the MR fluid actuator with the semi- active control method provide good vibration suppression with low energy consumption.

A whole-spacecraft isolation system for the GFO/Taurus mission was designed, fabricated, tested, and subsequently flown on February 10, 1998. This isolation system was designed to reduce dynamic responses on the GFO spacecraft caused by the resonant burn dynamic load introduced by the Castor 120 solid rocket motor. Longitudinal (flight direction) response of the GFO spacecraft center of gravity, due to the resonant burn load, was reduced by a factor of seven. The isolation system design was very nonintrusive to existing hardware, lightweight, and effective. Flight data indicates that the isolation system performed as designed. The GFO spacecraft had a successful launch and is currently operational on-orbit. A second flight of this type of isolation system occurred in October 1998. Similar isolation systems are planned for other flights in 1999 and 2000. This whole-spacecraft isolation technology was highly successful for the GFO/Taurus mission.

This paper describes the design of a solar array damper that will be built into each of two new solar arrays to be installed on the Hubble Space Telescope (HST) during Servicing Mission 3. On this mission, currently scheduled for August 2000, two 'rigid' solar array wings will replace the 'flexible' wings currently providing power for HST. In addition to increased power, the new arrays will provide the capability for HST to survive re-boost to a higher orbit. The objective of the damper is to reduce the dynamic interaction of these new wings with the Telescope spacecraft. The damper, which is integral to the mast of the solar array, suppresses the fundamental bending modes of the deployed wings at 1.2 Hz (in-plane) and 1.6 Hz (out-of-plane). With the flight version of the damper, modal damping of 2.3% of critical is expected over the temperature range of -4 degrees Celsius to 23 degrees Celsius with a peak damping level of 3.9%. The unique damper design, a combination of titanium spring and viscoelastic damper, was developed using a system finite element model of the solar array wing and measured viscoelastic material properties. Direct complex stiffness (DCS) testing was performed to characterize the frequency- and temperature-dependent behavior of the damper prior to fixed- base modal testing of the wing at NASA/Goddard Space Flight Center (GSFC).

Active Constrained Layer Damping (ACLD) treatment has been used successfully for controlling the vibration of various flexible structures. The treatment provides an effective means for augmenting the simplicity and reliability of passive damping with the low weight and high efficiency of active controls to attain high damping characteristics over broad frequency bands. In this study, a self-sensing configuration of the ACLD treatment is utilized to simultaneously suppress the bending and torsional vibrations of plates. The treatment considered ensures collocation of the sensors/actuators pairs in order to guarantee stable operation. A three-layer network of the Self-sensing Active Constrained Layer Damping (SACLD) treatment is used to control multi-modes of vibration of a flexible aluminum plate (0.264 m X 0.127 m X 4.826E-4 m) which is mounted in a cantilevered arrangement. Two ACLD patches (0.264 m X 0.0635 m) with self-sensing polyvinylidine fluoride (PVDF) actuators oriented by (14 degrees/-14 degrees) configuration are treated on one side of plate. The theoretical characteristics of the multi-layer treatment are presented in this paper and compared with the experimental performance.

Experimental and analytical validations of a Galerkin analysis of sandwich plates is presented in this paper. The 3-layered sandwich plate specimen consists of isotropic face-plates with surface bonded piezo-electric patch actuators, and a viscoelastic core. The experimental validation is conducted by testing sandwiched plates that are 67.31 cm (26.5') long, 52.07 cm (20.5') wide and nominally 0.16 cm (1/16') thick. The analysis includes the membrane and transverse energies in the face plates, and shear energies in the core. The shear modulus of the dissipative core is assumed to be complex and variant with frequency and temperature. The Golla-Hughes-McTavish (GHM) method is used to account for the frequency dependent properties of the viscoelastic core. Experiments have been conducted on sandwich plates with aluminum face-plates under clamped boundary conditions to validate the model for isotropic face-plates. Symmetric and asymmetric sandwiches have been tested. The maximum error in damped natural frequency predictions obtained via the assumed modes solutions is less than 11%. Analytical studies on the influence of the number of assumed modes in the Galerkin approximation, and the temperature variation, have been conducted. Error in the first plate bending mode is 112% when only a single in-plane mode is used; error reduces to 3.95% as the number of in-plane modes is increased to 25 in each of the in-plane directions. The study on the temperature influence shows that every plate mode has a corresponding temperature, wherein the loss factor is maximized.

Passive stand-off layer (PSOL) damping treatments are presently being implemented in many aerospace and defense designs. In a PSOL damping treatment, a stand-off or spacer layer is added to a conventional passive constrained layer (PCL) damping treatment. The addition of this stand-off layer increases the distance of the viscoelastic and constraining layers from the neutral axis of the vibrating structure. This is thought to enhance the damping by increasing the shear angle of the viscoelastic layer. In this experimental study, a PSOL damping treatment was applied to an Euler-Bernoulli beam. The frequency response of the treated PSOL beam was then compared with a conventionally treated PCL beam of similar dimensions and materials. Previous theoretical studies indicated that PSOL treatments provided greater damping than similarly sized conventional PCL treatments. This study verified experimentally that the beam treated with PSOL had greater damping of the first four modes than a similarly sized beam treated with PCL.

Electromechanical Surface Damping (EMSD) is a hybrid technique that incorporates constrained layer damping (CLD) and shunted piezoelectric element methods for the suppression of vibration in light beam-like or plate-like structures. The EMSD technique enhances the damping effectiveness (peak amplitude suppression) at targeted resonant frequencies, and may therefore be used to extend the damping effectiveness of the constrained layer damping technique over a broader temperature and frequency range than CLD alone. This performance enhancement was demonstrated experimentally by comparing the steady state frequency response of partially treated cantilever beams with that of an untreated beam. The experimental results also agreed with the results of a corresponding analytical model.

Active constrained layer damping has been shown to be an effective way of controlling structural vibration. The success of the technique depends significantly on the strain imparted to the constraining layer by an actuator made of a smart material -- typically a piezoelectric ceramic. The possibility of using magnetostrictive smart patches as actuators is explored in this regard. Comparisons of performance are made between commonly used piezoelectric ceramic and proposed magnetostrictive actuators on the basis of maximum power consumption. It is shown that magnetostrictives offer benefits particularly in the lower frequencies.

The concept of enhancing energy dissipation in thin beams and panels by adding viscoelastic materials to a structure dates back at least to the early 1950s. Kerwin in 1959 was the first to present a general analysis of viscoelastic material constrained by another metal layer. He made several key simplifying assumptions in the mathematics, as did DiTaranto (1965) and Mead and Markus (1969) in follow-up studies: (1) the constraining layer bends in the transverse direction exactly as the base layer, (2) the viscoelastic layer undergoes pure shear, and (3) the viscoelastic layer does not change its thickness during deformation. While appropriate for damping problems of that time, the role of passive, and now active, damping has expanded in the decades since to the point that many problems of practical engineering interest are no longer represented well by these mathematical models. This paper explores a few pitfalls of simplified modeling through some trade studies using benign-looking sandwich beams. The Mead and Markus assumptions are implemented using finite elements and are compared to a beam comprised entirely of higher order elements. A sandwich beam is also modeled using Euler-Bernoulli beams (acting independently) as facesheets and a linear element for the viscoelastic material, similar to how a sandwich might be modeled using standard elements in a commercial code. The accuracy of damping predictions is inferred from the accuracy of strain energy distributions.

This paper presents an experimental and analytical investigation of an elastomeric damping material and assesses its potential application to stability augmentation of hingeless and bearingless helicopter rotors. Double lap shear specimens were tested on a servo-hydraulic testing machine. Single frequency sinusoidal tests were conducted over a strain amplitude range of 0 - 30% at three frequencies (lag/rev, 1/rev and a lower harmonic of the rotor). The frequencies were chosen such that the effect of the damper in mitigating instability phenomena, like ground and air resonance, could be analyzed. The effects of frequency, amplitude, pre-load and material self-heating were studied. A three-element mechanisms-based damper model was developed that accurately captures the energy dissipation and hysteresis behavior of the damper. The model incorporates a linear stiffness, viscous damping and a non-linear slip element that are placed in parallel to each other. The parameters of the model were identified using an LMS technique. The model was validated by reconstructing measured hysteresis cycles using these parameters.

In this paper a theoretical and numerical study on the viscoelastic behavior of auxetic polymers and cellular materials is presented. Negative Poisson's ratio materials ((alpha) (upsilon) (eta) (xi) (epsilon) (omicron) (sigma) in Greek) expand in all directions when pulled in only one, and contract when compressed in one direction. This behavior is due to the special geometrical layout of their unit cells. A theoretical model including viscoelastic and inertia effects on the unit cell has been prepared in order to compute the equivalent in- plane dynamic storage modulus and loss factor of the cellular material. The calculations show how inertia effects and geometric layout of the unit cell affect the viscoelastic behavior of the material over the frequency domain. The results show a very good agreement with the ones from analogous FEM models. Auxetic honeycombs are a good example of cellular materials with negative Poisson's ratio behavior. A Finite Element model has been elaborated to model also the viscoelastic response of the transverse shear modulus of this kind of honeycombs and compared with analytical results.

The conventional method for reducing structureborne vibrations is by the application of parasitic viscoelastic claddings. To be effective that cladding needs to extensive, it adds weight to the structure and is often costly to apply and maintain. An effective strain energy dissipation method is described that is applicable to fiber reinforced structures and is localized to stiffeners or joints. An example is described for composite box section stiffeners that are typical of those employed in ship construction. It is shown that the stiffeners can be a point of very high vibrational mobility and that use of a viscoelastic insert embedded at its root can provide a very effective means of dissipating vibrational energy. Thus it is possible to provide a high degree of structural damping, with minimal weight increase, that avoids extensive cladding and forms an integral part of the structure. The principle of the viscoelastic insert is described together with its physical dynamic characteristics. Experimental data is provided that compares the use of the viscoelastic insert on a stiffened composite beam structure to a conventional beam without inserts. An experiment is described where each beam had three similar stiffeners and vibrational transfer function data was measured across each stiffener. It is shown the viscoelastic insert can provide substantial strain energy dissipation for both compressional and flexural waves and that it acts as effective acoustic sink. The implications for noise reduced composite structure, damage tolerance and fatigue are also discussed.

The use of damping materials for joints in thin plate structures is seen as a cheaper and lighter alternative to surface coating technologies. This paper assesses the ability of such joints to damp thin plates over a wide frequency range. Finite element models of several joints are validated experimentally both for static strength and stiffness and for frequency response behavior. The static strength and stiffness of such joints are found to be significantly weaker than jointless plate only under extensional loads. The damping achieved by using these joints is found to depend on the joint flexibility. Typical joints involving less than 1/30th of the size of the structure were found to yield average loss factors of around 2% over a wide frequency range. The addition of simple fasteners such bolts was found to be a practical way of improving the static performance without dramatically reducing the damping.

Energy absorbing composite joints can be effective in noise and vibration suppression of marine and aerospace structures. The vibration suppression in a typical Top-Hat stiffener structural joint, with viscoelastic insert is presented in this paper. Using FEM modeling, the damping performance of this composite joint is predicted. Experiments are carried out to validate the numerical analysis.

Viscoelastic inserts have many useful applications in noise and vibration suppression of marine and aerospace structures. It is observed that the shape of such inserts play an important role in efficient absorption of vibration. Tapering of viscoelastic inserts may improve noise and vibration reduction in composite joints. Numerical simulations using FEM are carried out to find out the effect of the shape of inserts on vibration reduction and a combined FEM and Wave approach is developed to compare the inferences drawn from FEM analysis.

The minimization of unwanted vibrations is an important technical challenge. Purely passive systems often do not achieve the postulated results. Purely active systems are costly because of the required additional power and the necessary maintenance. Currently it seems that semi-active methods of vibration reduction are as competitive as any other methods. Semi-active damping control can be realized with electro- or magnetorheological fluids. These change their characteristic in the presence of an electric or magnetic field or by bypasses combined with magnetic valves. The methods known in linear control theory cannot be used for the controller design because no explicit external forces can be generated whenever they are needed. Forces can only be generated when relative velocities between the endpoints of the damper exist. It is important to investigate control methods which will reduce vibration with controlled damping. In this paper three different methods for establishing control laws are presented. The first is based on the consideration of power flow in the system. It is discussed in detail. The second method uses Bellmans dynamic optimization. The last transforms a multi degree of freedom system by modal analysis into uncoupled single degree of freedom systems. The control methods developed by these three methods all lead to the same vibration reduction strategy. The control laws are verified with simulation results.

The objective of this work is to find a robust, practical procedure to identify damping matrices for structures well modeled by linear viscous damping. The process of modeling damping matrices and experimental verification of those is challenging because damping can not be determined via static tests as can mass and stiffness. Furthermore, damping is more difficult to determine from dynamic measurements than natural frequency. Aspects of the damping identification procedure that are investigated include noise, spatial incompleteness and modal incompleteness. The procedures for damping identification presented herein are based on prior knowledge of the finite element or analytical mass matrices and measured eigendata. Alternately, a procedure is based on knowledge of the mass and stiffness matrices and the eigendata. Several examples, including experimental examples, are used to illustrate the use of these new damping matrix identification algorithms and to explore their robustness. First, an analytically modeled plate example is used for illustration. Our next example is an experimental work on a bolted beam with various boundary conditions. The effects of changes in torque on damping matrices are investigated using this new damping identification algorithm. Changes in damping matrices are analytically obtained, as the applied torque is varied.

The design, analysis, and fabrication methods of embedding small viscoelastic damping patches into scaled composite fan blades is presented, where the goal is to improve the blade fatigue characteristics by increasing the damping in the chord-wise modes. This discussion concentrates on improving the damping levels in a research composite shell/titanium spar fan blade, developed by NASA-Lewis and Pratt and Whitney. First, the geometry and material definition of the existing composite fan blade are presented. Second, methods for sizing and locating the damping patch are presented based upon the modal strain energy method. The layered damping patch is composed of outer layers of a TEDLAR (or KAPTON) barrier film, which encompasses a viscoelastic damping material and loose- weave scrim cloth (creep protection). Two different patch sizes and locations are discussed to provide maximum damping as well as optimal damping. Finally, procedures are outlined for fabricating the integrally damped composite fan blades. Fabricated blades will be tested at the NASA-Lewis vacuum facility.

Current viscoelastic-damping materials behave isotopically so that their stiffness and damping properties are the same in all directions. There is a desire to develop viscoelastic- damping materials that behave orthotropically so that the stiffness and damping properties vary with material orientation. These damping materials can be made othrotropic by embedding rows of thin wires within the viscoelastic damping material. These wires add significant directional stiffness and strength to the damping materials, where the stiffness and strength variation with wire orientation follows classical lamination theory. The presence of these wires introduce different damping mechanisms (longitudinal, transverse, and longitudinal shear damping coefficients) that depend upon mode type and orientation angle. Results from experimental studies show that the magnitude of the loss factor and shear modulus depends upon the mode type and orientation angle of these wires within the damping material. The in-plane axial mode loss factor is highly dependent upon the longitudinal coefficient for (0 degrees) wire orientation, the transverse coefficient for (90 degree) wire orientation, and the longitudinal shear-damping coefficient for all other off-angle wire orientations. The loss factor for the out-of- plane bending and torsion modes is highly dependent upon all three damping coefficients.

Embedding viscoelastic damping materials into graphite/epoxy composites can greatly increase the damping of composite structures. Cocuring the damping material with the composite, however, has been shown to increase the modulus and lower the damping in many viscoelastic materials because epoxy penetrates many damping materials (especially acrylics). In this paper, the changes in shear modulus were measured using double lap shear tests. Also presented are shear moduli comparisons of samples cured with three different barrier film layers, Kapton^R, Tedlar^R,and polyester, which are used to prevent the epoxy penetration. Lastly, samples with an embedded loosely woven scrim cloth placed between two damping material layers are tested to measure how the scrim affects the shear modulus.

The equations of motion of structures with elastic and linear viscoelastic materials in the time domain are derived. The approach leads to a system of equations where the matrices are symmetric, real, and composed of constant coefficients. A four-parameter fractional derivative model is used to model the frequency dependence of the linear viscoelastic material since experimental data can be fitted successfully over a wide frequency range. The resulting equations of motion are the fractional order elastic-viscoelastic equations of motion. The closed-form, steady state solution of a single degree of freedom system is obtained in the frequency domain and is used to compare the results obtained by using numerical procedures. The proper selection of the stiffness for viscoelastic dampers placed in elastic structural systems is discussed in order to ensure that the damper is effective in reducing dynamic amplification of the structure.