High cycle fatigue in jet engines is a current military concern. The vibratory stresses that cause fatigue can be reduced by adding damping. However, the high temperatures that occur in the gas turbine greatly hinder the application of mature damping technologies. One technology which may perform in the harsh environment is particle damping. Particle damping involves placing metallic or ceramic particles inside structural cavities. As the cavity vibrates, energy is dissipated through particle collisions. Performance is influenced by many parameters including the type, shape, and size of the particles; the amount of free volume for the particles to move in; density of the particles; and the level of vibration. This paper presents results from a series of experiments designed to gain an appreciation of the important parameters. The experimental setup consists of a cantilever beam with drilled holes. These holes are partially filled with particles. The types of particles, location of the particles, fill level, and other parameters are varied. Damping is estimated for each configuration. Trends in the results are studied to determine the influence of the varied parameter.

The main goal of this paper is to describe and understand the behavior of an absorption system with Coulomb damping with two degrees of freedom. In order to achieve this goal, a theoretical-computational model is developed and adjusted to the experimental results. The experimental model utilized consists of a cantilever beam with an absorption system installed on it. The absorption system is a spring-mass system with a friction element acting on its axis. The dynamic forces used in the experiments are: impacts, people jumping and harmonic excitation. A parametric study is performed with the adjusted theoretical-computational model, which indicates that the system is efficient, but some precautions should be considered in the project concept. These precautions are associated to nonlinear behavior of the response with this kind of damping. Finally, some relevant recommendations for the design of an absorption system are described.

We investigate collective dissipative properties of vibrated granular materials by means of molecular dynamics simulations. The rate of energy loss indicates three different phases in the amplitude-frequency plane of the external forcing, namely solid, convective and gas-like regimes. The behavior of the effective damping decrement is consistent with the glassy nature of granular solids. The gas-like regime is most promising for practical applications.

Damping is important to structures and can be achieved through the addition of viscoelastic materials (VEM). The damping of the VEM is enhanced if a constraining layer is attached to the VEM. If this constraining layer is active, the treatment is called active constrained layer damping (ACLD). In the last few years, ACLD has proven to be superior in vibration control to active or passive damping. The active element makes ACLD more effective than passive constrained layer damping. It also provides a fail-safe in case of breakdown of the active element that is not present for purely active control. It is shown that the control effort needed to damp vibration using ACLD can be significantly higher than purely active control. In order to combine the inherent damping of passive control with the effectiveness of the active element, this paper will explore different variations of active, passive and hybrid damping. Some of the variations include: passive constrained layer damping (PCLD) separate from active element but on the same side of beam, PCLD separate from active on the opposite side of the beam, and active element underneath PCLD. The discretized system equations will be obtained using assumed modes method and Lagrange's equation. The damping will be modeled using the Golla-Hughes-McTavish (GHM) method. The optimal placement and size of the active, passive, ACLD and hybrid treatments will be found using different schemes. The issue of overshoot and settling time of the output and control force using LQR will be addressed, as well as the control effort, passive and active vibration suppression, and LQR cost function. It will be shown that the hybrid treatments are capable of greater vibration control for lower control effort for different optimization schemes. 31

We present and review methods of frequency response analysis for cantilevered sandwich beams consisting of aluminum face plates and passively constrained damping layers at the mid- plane. These analysis methods include: (1) progressive wave method, (2) assumed modes method, and (3) finite element method. The progressive wave method accounts for the frequency dependent complex modulus in analyzing the frequency response function. In the assumed mode and FEM analyses, the frequency dependent complex modulus is assumed to be a constant over the frequency range of interest (i.e. the first three modes). By selecting the value of complex modulus to nominally correspond to the second modal frequency, the error in predicted natural frequency can be minimized. However, this comes at the cost of increased error in the damping ratio predictions which are minimized by selecting the value of complex modulus corresponding to the first mode.

This paper is concern with the effects of edge element symmetry on the Enhanced Active Constrained Layer (EACL) treatment. The research characterizes the relationship among the edge element stiffness distribution, the strain field of the base structure, and the system active, passive, and hybrid damping abilities. The results could provide good insights and guidelines for EACL designers.

This paper investigates the feasibility of employing Enhanced Active Constrained Layer (EACL) damping treatments on the flex beams of soft in plane bearingless main rotors (BMR) for lag mode damping and aeromechanical stability augmentation. A finite element based mathematical model of the EACL damping treatment of flex beam has been developed. The flex beam is modeled using beam-rod elements and the blade is modeled as a lumped inertia. A simple derivative controller based on the flexbeam tip transverse velocity is used in this investigation. A thorough parametric study is conducted to understand the influence of various design parameters such as viscoelastic layer thickness, PZT actuator thickness, and edge element stiffness, on actuator electrical field levels and axial stress induced in PZT actuator. The results of this study shows that the EACL treatments on the flex beams has good potential for rotor stability augmentation.

This paper presents the results of the first phase of an effort to determine the effects of centrifugal forces on viscoelastic damping concepts applicable to rotating components. The design of the experimental test specimen will be discussed along with the analytical methods used to design and evaluate damping concepts for the test specimen. A blend of classical analysis, 6th order beam theory, and finite element analysis was used. The analytical effort was divided into two tasks. The first task was to define the design of the test specimen such that: (1) a meaningful test could be conducted in the spin test facility; (2) damping concepts could be designed into the test specimen; and (3) manufacturing time and cost were reasonable. The second task of the effort was to complete a trade study evaluating various damping concepts. The trade study evaluated damping effectiveness, survivability, manufacturability, and damping material availability. The design parameters evaluated included: (1) pocket size, orientation, and number; (2) pockets with and without floating constraining layers; (3) damping concept creep potential; and (4) stresses in the test specimen due to centrifugal loading. This paper will detail the analysis techniques used, the trends found in the design parameters, and the final designs chosen for the test effort.

Cocuring layers of viscoelastic damping materials with composite material systems offers the possibility of manufacturing light-weight, stiff, highly damped structural components. The objective of this work was to design two cocured damped composite torsion shafts. The first shaft uses the extension-twist coupling mechanism of off-angle composite materials to enhance the damping performance of a damping material. The inner shell contains plies oriented at a positive ply angle in the first half of the shaft length and plies oriented at a negative ply angle in the second half. Due to the extension-twist coupling, the shell center section moves axially when the shaft undergoes torsion deformation. The outer shell plies are oriented in the opposite manner so that the outer shell center moves in the opposite direction. The relative axial deformation between the two shells places the damping material into shear, providing damping. The second shaft uses a constraining layer embedded inside the shaft that floats between two layers of damping material. The constraining layer resists torsion deformations applied to the shaft. Load transferred to the constraining layer through the damping material places the damping material into shear, again providing damping. Finite element analysis was used to determine optimal damping material shear modulus and ply orientation to maximize shaft imaginary stiffness. Four shafts total (two of each type) were built and modal tests were performed. Torsion damping increased by factors from 5.8 to 20.0 and 6.1 to 10.9 over the undamped case for the extension- twist and floating constraining layer dampers, respectively. While both damping concepts provide significant levels of damping, the performance of each was hindered due to the increase in shear modulus of the damping material as it was cocured with the composite material.

Cocuring viscoelastic damping materials in composites has been shown to be successful in greatly increasing the damping of composite structures. The damping performance, however, is often not as high in cocured composites as in secondarily bonded composites, where the damping material does not undergo the cure process. The reason for the discrepancy in damping between the cocured and secondarily bonded samples was found to be resin penetration into the damping material. Samples with a barrier layer between the damping material and the epoxy resin had a 15.7% to 92.3% higher loss factor (depending on the frequency) than cocured FasTape^TM 1125 samples without the barrier and at least 168% higher loss factor than cocured ISD 112 samples without the barrier. These higher damping values are very close to the values achieved by secondarily bonding. Viscoelastic damping materials typically have maximum recommended temperatures below that of the composite cure cycles. The effect of cure temperature on viscoelastic damping materials was also studied and it was determined that most damping materials are marginally affected by cure cycle temperature.

The experimental behavior of spinning laminated composite pretwisted plates (turbo-fan blade-like) with small (less than 10% by volume) integral viscoelastic damping patches is investigated. Two different plate sets were examined. The first set investigated tailoring patch locations and definitions to damp specific modes on spinning flat graphite/epoxy plates as a function of rotational speed. The second set investigated damping patch size and location on specific modes of pretwisted (30 degrees) graphite/epoxy plates. The results reveal that: (1) significant amount of damping can be added using a small amount of damping material, (2) the damped plates experienced no failures up to the tested 28,000 g's and 750,000 cycles, (3) centrifugal loads caused an increase in bending frequencies and corresponding reductions in bending damping levels that are proportional to the bending stiffness increase, and (4) the centrifugal loads caused a decrease in torsion natural frequency and increase in damping levels of pretwisted composite plates.

The concept of using biologically inspired methods for the placement of surface damping treatments [i.e. viscoelastic material (VEM) free-layer damping] has recently been explored by the authors and has been shown to provide an interesting approach to the distribution of damping on the surface of a vibrating structure. Previous work considered the evolution of a VEM layer; 'damping cells' (elements of viscoelastic material) were 'born' on the surface of the structure in zones of high modal strain energy. More specifically, the viscoelastic material grew in locations where the energy dissipation potential of the viscoelastic material was highest. This work extends the previous study by exploring a significantly faster procedure for evaluating the damping layer distribution which utilises the modal strain energy (MSE) method. An experimental study, involving the actual construction of the predicted VEM layer distributions on a model cantilever, was also carried out to evaluate the accuracy of the predictions.

A new class of surface treatment is proposed to provide effective means for attenuating undesirable structural vibrations. The proposed treatment relies in its operation on the use of smart damping treatments which consists of integrated arrays of constrained visco-elastic damping layers that are controlled passively by a specially arranged network of permanent magnets. The interaction between the magnets and the visco-elastic layers aims at enhancing the energy dissipation characteristics of the damping treatment. In this manner, it would be possible to manufacture structures that are light in weight which are capable of meeting strict constraints on structural vibration when subjected to unavoidable disturbances. This new treatment will be called Magnetic Constrained Layer Damping (MCLD) treatment. A finite element modeling of a plate treated with MCLD treatments is developed. This model describes the dynamics and the damping characteristics of this structure. The numerical results are verified experimentally using a cantilever plate fully treated with MCLD with the magnets placed at the root of the plate. Close agreement is obtained between theory and experiments. Also the performance characteristics of the MCLD is compared with the corresponding performance of the conventional Passive Constrained Layer Damping (PCLD). The effectiveness of the MCLD in attenuating structural vibration of the plate has been clearly demonstrated in the frequency domain. The developed theoretical and experimental techniques present invaluable tools for designing and predicting the performance of plates treated with MCLD.

We report a method for passive piezoelectric shunt-damping of multiple vibration modes using a single piezoelectric transducer (PZT). The method, which is different from other multiple-mode shunting, is to employ a 'blocking' circuit in series with each parallel resistor-inductor shunt circuit designed for one structural mode. The 'blocking' circuit consists of one parallel capacitor-inductor antiresonant circuit or a series of them. The number of antiresonant circuits in each branch circuit depends on the number of the structural modes to be shunt-damped simultaneously. These antiresonant circuits are designed to produce infinite electrical impedance, or antiresonance, at the natural frequencies of all other resistor-inductor shunt circuits. Each branch circuit is functional only at its own mode frequency but is open-circuited at all other mode frequencies. Therefore, when all branches are connected to the PZT terminals, they do not interfere with each other. The method is also easier and more reliable for fine-tuning and optimization operation. We analyze first the multiple-mode shunt-damping circuit with both the generalized circuit and the modified circuit derived from it which is to reduce the number of antiresonant circuits and to make the circuit operation simpler. Experimentally we have employed modified shunt circuits and successfully demonstrated vibration reduction of two and three structural modes using a single PZT transducer bonded on a two-wing aluminum cantilever beam. For three-mode shunt damping, the resonant amplitudes at 206, 348.5, and 484.5 Hz are reduced for 13.28, 7.30, and 9.45 dB, respectively.

A tunable solid state piezoelectric vibration absorber and an active tuning method were developed and demonstrated. A passive vibration absorber generally acts to minimize structural vibration at a specific frequency associated with either a tonal disturbance or the response of a lightly damped structural vibratos mode. Because this frequency is rarely stationary in real applications, damping is usually added to ensure some level of effectiveness over a range of frequencies. Maximum response reductions, however, are achieved only if the absorber is lightly damped and accurately tuned to the frequency of concern. Thus, an actively-tuned vibration absorber should perform better than a passive one and, furthermore, could be made lighter. In its simplest form, a vibration absorber consists of a spring-mass combination. A key feature of the tunable vibration absorber described herein is the use of piezoelectric ceramic elements as part of the device stiffness. The effective stiffnesses of these elements was adjusted electrically, using a passive capacitive shunt circuit, to tune the resonance frequency of the device. The tuning range of the absorber is thus bounded by its short- circuit and open-circuit resonance frequencies. An alternative tuning approach might employ resistive shunting, but this would introduce undesirable damping. Another feature of the device is the ability to use the piezoelectric elements as sensors. A control scheme was developed to estimate the desired tuning frequency from the sensor signals, to determine the appropriate shunt capacitance, and then to provide it. The shunt circuit itself was implemented in ten discrete steps over the tuning range, using a relay-driven parallel capacitor ladder circuit. Experimental results showed a maximum 20 dB, and a 10 dB average improvement in vibration reduction across the tuning range, as compared to a pure passive absorber tuned to the center frequency, with additional benefit extending beyond the tuning range.

Vibration dampers and dynamic absorbers are widely used in various engineering problems. This paper presents a new concept of vibration control via a distributed damper composed of an elastic bar and a dissipative boundary conditioner in its configuration. The distributed inertia and stiffness of the elastic bar enables the damper to provide wide-band damping augmentation. Preliminary results show several advantages of the distributed damper over its lumped counterparts.

In this paper, an innovative vibration control method based on a combination of Vibration Control by Confinement and conventional isolation, absorption, and damping techniques is presented. First, the vibration energy is confined to selected parts of a structure resulting in low- and high-energy regions. Second, isolation, absorption, and damping techniques are utilized to control or dissipate the trapped energy within the high-energy areas. The effectiveness of the combined approach is examined by considering the vibration suppression of a system composed of a critical component resting on its supporting structure. The commonly used vibration control methods are combined with the confinement approach and compared to a set of baseline results. Our preliminary results show that the combination of the confinement technique and conventional vibration suppression methods is effective over the frequency range of interest. It is pointed out that the proposed approach is a practical method to overcome some of the challenges of the current passive and active vibration control techniques.

A fluid surface damping (FSD) element has been designed, produced and applied for the vibration suppression of a cantilever aluminum beam. The experimental steady-state frequency response of the treated beam, measured at the free end and subjected to a burst white noise excitation at the base, is determined and compared with the corresponding analytical results. The comparison shows a disagreement between the predicted and experimental results. Discussion of the potential sources of disagreement and of possible remedial measures is presented.

Electrorheological (ER) fluids exhibit damping and stiffness properties which can be modulated by several orders of magnitude when subjected to strong electric fields. In the case of quasi-steady flow an increase in yield stress (from 0 to 3 kPa) is characteristically observed when the field is applied. Newtonian stresses are relatively unaffected by the electric field. A potential application of ER materials is the damping and isolation of dynamically excited structures. In order to capitalize on the unique properties of ER materials in this application it is desirable that the material be configured in a device in such a way that when the device undergoes characteristic motions, the device forces can be modulated to a significant degree (a factor of 10 or more). Because the range of adjustable forces is closely linked to the ratio of finite field-controllable, and flow-independent yield stresses to uncontrollable, and flow-dependent viscous stresses, it is desirable for controllable ER dampers to operate at low flow rates. In addition to the range of available forces, ER dampers should also perform well in other regards. They should have a short characteristic time, low stored electrical energy, and forces high enough to be effective for the intended application. Because of their low yield stresses, ER materials must flow over large surfaces to generate high forces and maintain a large degree of adaptability. In a cylindrical configuration, this can be accomplished by forming multiple flow paths with a series of concentric annular ducts. These ducts can be connected in parallel to maximize the range of adjustable forces or in series to maximize the absolute force levels. A synergistic result is obtained when groups of ducts are interconnected in parallel and in series within a single device. This paper presents the details of a realization of a new design paradigm for dampers which incorporates these issues. The ER damper features multiple concentric electrodes which are electrically in parallel, but may be hydraulically inter-connected through multiple paths. The number of possible interconnections of N concentric ducts is 2(^N-1). Each configuration has distinctly different properties. The design of three ER dampers that span a range of performance criteria is presented in this paper. An analysis of these device configurations is completed in closed form by virtue of a linear approximation to the non-Newtonian ER Poiseuille flow equation. These analyses show that high-force ER devices which require low energy, and respond quickly, are feasible.

We experimentally validate nonlinear quasi-steady electrorheological (ER) and magnetorheological (MR) damper models, using an idealized Bingham plastic shear flow mechanism, for the flow mode of damper operation. An electrorheological valve or bypass damper was designed, and fabricated using predominantly commercial off-the-shelf hydraulic components. Both the hydraulic cylinder and the bypass duct have cylindrical geometry, and damping forces are developed in the annular bypass via Poiseuille (flow mode) flow. Damper models assume parallel plate geometry. Three nondimensional groups are used for damper analysis, namely, the Bingham number, Bi, the nondimensional plug thickness, (delta) , and the area coefficient defined as the ratio of the piston head area, A_(rho ), to the cross-sectional area of the annular bypass, A_d. In the flow mode case, the damping coefficient, which is defined as the ratio of equivalent viscous damping of the Bingham plastic material, C_eq, to the Newtonian viscous damping, C, is a function of the nondimensional plug thickness only. The damper was tested using a mechanical damper dynamometer for sinusoidal stroke of 2 inches, over a range of frequencies below 0.63 Hz. The damping coefficient vs. nondimensional plug thickness diagram was experimentally validated using these data over a range of damper shaft velocities and applied electric field. Because the behaviors of ER and MR fluid are qualitatively similar, these ER damper modeling results may be extended to analysis of flow mode MR dampers.

A fluid model for the evaluation of electrorheological grease (ERG) is presented. This model considers the influence of a Non-Newtonian fluid using a Herschel-Bulkley fluid model. Fluid compressibility was included using a lumped parameter dynamic system model. The optimal model parameters were determined by matching theoretical and experimental data for an existing ERG damper. Results indicate that a good match between experimental and theoretical data was achieved using the Herschel-Bulkley fluid model. A sensitivity analysis indicated that the model was insensitive to the mass of the fluid, but sensitive to the bulk modulus of the fluid.

The increased requirements on the accuracy of measurements with the aid of large-weight electronic-optical devices have entailed heavy demands for their efficient protection against low-vibration vibrations produced by an occasional external load or operator's pushes. In these cases, the means of active protection against vibrations used, e.g., in adjustment stands are considered to be the most efficient ones. Among actuating mechanisms of such systems, i.e. hydro- and pneumohydraulic, electromechanical and friction devices, the viscous-friction dampers show much promise since use of electrorheological fluids in the latter allows their functional capabilities to be extended owing to control of the elastoviscous parameters of a working fluid. An analysis is made of vibrations of a single-mass system (an electronic microscope column) whose vibroprotection scheme includes four ER-dampers. Their control is accomplished by a voltage unit in discrete as well as in follow-up linear and logarithmic regimes. The results obtained are compared with experimental data for the systems without dampers, with passive dampers and controllable ER-dampers. The latter are shown to be advantageous, in particular, a three- fold decrease of the Q-factor of the system under forced vibration is reached in the optimum control.

The rheological and magnetic properties of several commercial magnetorheological (MR) fluids are presented and discussed. These fluids are compared using appropriate figures of merit based on conventional design paradigms. Some contemporary applications of MR fluids are discussed. These applications illustrate how various material properties may be balanced to provide optimal performance.

This paper presents the modeling and experimental testing of a magnetorheological (MR) fluid damper. The damper consists of a main cylinder and piston rod that pushes MR fluid through a very small clearance between the piston and the sidewalls of the cylinder. Magnetic coils are wrapped outside the cylinder to create the magnetic field. The damper model is developed based on parallel plate analysis by using both Newtonian and Bingham shear flow mechanisms. Empirical data was used to find the required parameter for a Bingham model that is the dynamic yield stress. These empirical data show the shear stress vs. shear strain at different values of magnetic flux density (B). The dynamic yield stress of MR fluid is a function of magnetic field. Finally, the model is validated through experimental testing of the damper.

The hysteresis behavior of a linear stroke magnetorheological damper is characterized for sinusoidal displacement excitation at 2.0 Hz (nominal). First, we characterize the linearized MR damper behavior using equivalent viscous damping and complex stiffness. Four different nonlinear modeling perspectives are then discussed for purposes of system identification procedures, including: (1) nonlinear Bingham plastic model, (2) nonlinear biviscous model, (3) nonlinear hysteretic biviscous model, and (4) nonlinear viscoelastic-plastic model. The first three nonlinear models are piecewise continuous in velocity. The fourth model is piecewise smooth in velocity. By adding progressively more model parameters with which to better represent preyield damper behavior, the force vs. velocity hysteresis model is substantially improved. Of the three nonlinear piecewise continuous models, the nonlinear hysteretic biviscous model provides the best representation of force vs. velocity hysteresis. The nonlinear viscoelastic plastic model is superior for purposes of simulation to the hysteretic biviscous model because it is piecewise smooth in velocity, with a smooth transition from preyield to postyield behaviors. The nonlinear models represent the force vs. displacement hysteresis behavior nearly equally well, although the nonlinear viscoelastic-plastic is quantifiably superior. Thus, any of the nonlinear damper models could be used equally successfully if only a prediction of energy dissipation or damping were of interest.

Magnetorheological (MR) fluids can be used to construct electrically controllable dampers for a wide variety of applications. Long-stroke dampers for high vibration amplitudes usually work in the flow mode. They contain MR fluid valves to modify the flow resistance of the fluid and thus the damping force of the actuator. Due to the high vibration amplitudes the MR fluid works in the post-yield area and its viscous behavior dominates. At high piston speeds extremely high flow velocities and shear rates can occur, which reduce the MR effect. At high shear rates the damping under the influence of a magnetic field is nearly independent of the piston speed, whereas the zero field damping force still increases. Low-stroke dampers can be implemented in the squeeze mode. By increasing the magnetic control field a transition from a viscous behavior to a viscoelastic behavior can be observed, which has a strong influence on the energy dissipated by the damper and on the damping force.

Much of the work done on active and passive constrained layer beams is done with models using kinematic assumptions proposed by Kerwin, Mead and Markus, and others. Typically these analyses use low-order Euler-Bernoulli beams and assume the base and constraining layers undergo identical transverse displacements. These assumptions are reasonable for cases where the middle layer (normally a relatively soft viscoelastic material) is thin and the constraining layer is relatively weak in bending, but many practical cases arise where these assumptions are violated. A few authors over the years have done studies with less restrictive kinematic assumptions, but none have specifically studied the effects of doing so in the context of passive or active damping design. The field of composite structures is rich with techniques for analyzing sandwich structures with and without simplifying assumptions, and it is on this body of work that this paper is based. The percentage of modal strain energy in the viscoelastic core is used as the primary measure of the accuracy of different sets of assumptions. Elasticity solutions are available for selected sets of assumptions and boundary conditions, and these solutions provide a basis for some of the preliminary studies. A zig-zag method is used to construct a piecewise continuous displacement field (C¹ continuity) that satisfies the appropriate stress continuity between layers in a consistent manner. Finite element analysis provides a versatile way to simulate complicated combinations of boundary conditions, degree of coverage, and kinematic assumptions.

In this paper a novel class of core material for sandwich structures is analyzed from an analytical and numerical point of view. Honeycombs with re-entrant cell geometry present negative in-plane Poisson's ratio coefficients, with an increase of bending stiffness compared to the one of hexagonal honeycomb cores. Due to the orthotropic mechanical properties of this kind of core material it is possible to employ the formulations of laminated orthotropic plates in order to describe the vibroacoustic behavior of simply supported sandwich plates. The natural frequencies of these laminates are sensitive to the geometrical parameters of the core cells. As an example application, an infinite cylindrical sandwich shell in contact with exterior and interior fluid flow is examined. An improvement of transmission loss factors is observed, suggesting a possible use of these cores in acoustic insulation. Damping properties are modeled taking into account the complex modulus of the core material. Two models are examined, one with full viscoelastic core and the other with a free-layer symmetrical beam geometry in order to describe the walls of the core cells. Both cases show a significant increase of the storage moduli of the cores compared to the ones of a regular honeycomb.

Passive constrained layer (PCL) damping treatments have been shown to be a very effective and reliable method for the damping of structures and have been implemented successfully in many commercial and defense designs for the aerospace and automotive industries. A conventional passive constrained layer damping treatment consists of a viscoelastic layer sandwiched between the vibrating structure and a cover layer. In a passive stand-off layer (PSOL) damping treatment, a stand-off or spacer layer is added to a conventional passive constrained layer damping treatment between the vibrating structure and the viscoelastic layer. 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 damping by increasing the shear angle of the viscoelastic layer. To investigate how the bending and shearing rigidities of the stand-off layer (SOL) affect the damping performance, an analytical model has been developed for a PSOL damping treatment applied to an Euler-Bernoulli beam. In this paper, the equations of motion are derived and solved. The resulting simulations of the frequency response are then discussed.

The normal stable operation speed of the disk is limited by its critical speed. Maximizing the speed of rotation of the stable disk requires effective damping mechanism to damp and stabilize disk vibrations at super-critical speeds. This paper investigate analytically the stability of a rotating disk under a non-conservative point force, which is fixed in space, composed of a viscous damping component and a circulatory force proportional to the circumferential slope of the disk surface. Approximate solutions are obtained through the KBM method when the viscous and circulatory force components are small. For arbitrary force, points possibly residing on the stability boundary are located exactly in parameter space through an energy analysis. A perturbation technique and the Galerkin method are used to predict whether these points reside on the stability boundary, and to identify the region of stable response. A propagating wave mode in the disk is stable unless the difference between the disk rotation speed and the virtual speed (ratio of the circulatory stiffness constant to the viscous damping coefficient) of the point force exceeds the wave speed observed on the disk. By properly tuning the virtual speed of the point force, disk vibrations can be damped and stabilized at super-critical speeds.

The paper presents an innovative technical solution which provides a combined damping and isolation interface with the appropriate transmissibility characteristics between a vibrating base and a sensitive payload, typically an optical terminal/telescope. The novelty of the solution is primarily found in the implementation of uncoupling and magnification of the incurred vibrations by means of flexures combined with the implementation of energy dissipation by means of a linear electro-magnetic actuator to constitute a passive integrated resistor-damped electromechanic lever block. By means of frictionless flexible lever systems, the amplitude of the payload vibrations is adapted to the optimal range of the actuator with a magnification by a factor ranging typically between 10 and 30. Passive viscous damping is obtained by simply short-circuiting the electro-magnetic motor and can be adapted by setting the impedance of the shorting connection. The desired stiffness is provided by the passive springs of the elastic motor suspension and by the stiffness of the lever flexure blades. The mobile mass of the motors also provide a reaction mass which, like damping and stiffness, is amplified by the square of the lever factor. A theoretical model of resistor-damped electromechanical lever blocks has been established. A particular property is it the good attenuation of excited vibrations only over a set frequency range. Above this range the interface properties rejoin the ones of a rigid connection. This performance makes this type of isolators particularly suitable for integration into multi-layer vibration control systems where sensitive equipment is protected by a mix of passive and active damping/isolation devices acting optimally at different frequency ranges. Experiments performed with a dummy load (80 Kg) representative of a satellite based optical terminal demonstrated the efficiency of the system in protecting the payload by passive damping for vibration excitations of amplitude up to 0.15 mm and frequency up to about 120 Hz achieving an attenuation of the eigenmodes of the load structure by more than 20 dB.

Current literature has not fully explored the choice of isolator mount frequency or isolator placement for flexible systems. It is commonly suggested that isolators should be designed with a low-mount frequency. It is shown that these isolators tend to perform best in an overall sense; however, mount frequencies designed between system modes tend to have a coupling effect. That is, the lower frequencies have such a strong interaction between each other that when isolator damping is present, multiple system modes are attenuated. Also, for low-mount frequency designs, the first natural frequency can shift as much as 15.6%. For a mid-mount frequency design, the shift of the first three modes can be as high as 34.9%, 26.6%, and 11.3%, respectively. Also, when the base and system are flexible, isolator placement becomes a critical issue. There can be as much as 16% difference in the first mode for low-frequency mount design and as high as 25% for a mid-frequency mount design.

This paper describes a benchmark to assess performance of six- axis vibration isolation systems. The targeted application, spaceborne interferometers, require isolation of the reaction wheel disturbances in order to stabilize the precision optical elements to the required levels. The problem is to isolate this vibrating payload from the quiet structure (spacecraft). The unique feature of this procedure is that isolator performance is measured in terms of the stability of the interferometer optical elements. Central to the procedure is the Micro-Precision Interferometer (MPI) testbed which is a hardware model of a future spaceborne optical interferometer. The isolation system under evaluation is mounted on the testbed and disturbance transfer functions are then measured from the isolator payload to the optical sensor output that must be stabilized. Off-line, the procedure combines these measured testbed transfer functions with an empirical model of the reaction wheel disturbance, in order predict isolator performance over the entire range of wheel speeds. The paper applies the procedure to four different disturbance interface conditions: hard mounted, passive hexapod isolator, active hexapod isolator and a passive elastomeric isolator. The paper contains all the necessary information to allow industry, academia or other organizations to evaluate custom designs in this testbed facility.

Passive isolation design is often done in a single axis fashion, while most applications have performance requirements in multiple degrees of freedom. Optimization of six by six transmissibility requires a thorough understanding of base disturbances, payload performance needs, envelope constraints, and mass/cost weighting. Singular value analysis of the disturbance to performance transfer function matrix in the frequency domain is shown to be effective in creating a single curve for multi-axis isolation performance. The various limitations to passive isolation are enumerated as constraints on the optimization process. Analysis and design optimization results are presented for the kinematic hexapod (six strut) mounting of a hypothetical commercial laser communication terminal payload.

A U.S. Air Force-sponsored team consisting of Boeing (formerly McDonnell Douglas), Honeywell Satellite Systems, and CSA Engineering has developed technology to reduce the vibration felt by an isolated payload during launch. Spacecraft designers indicate that a launch vibration isolation system (LVIS) could provide significant cost benefits in payload design, testing, launch, and lifetime. This paper contains developments occurring since those reported previously. Simulations, which included models of a 6,500 pound spacecraft, an isolating payload attach fitting (PAF) to replace an existing PAF, and the Boeing Delta II launch vehicle, were used to generate PAF performance requirements for the desired levels of attenuation. Hardware was designed to meet the requirements. The isolating PAF concept replaces portions of a conventional metallic fitting with hydraulic- pneumatic struts featuring a unique hydraulic cross-link feature that stiffens under rotation to meet rocking restrictions. The pneumatics provide low-stiffness longitudinal support. Two demonstration isolating PAF struts were designed, fabricated and tested to determine their stiffness and damping characteristics and to verify the performance of the hydraulic crosslink concept. Measurements matched analytical predictions closely. An active closed-loop control system was simulated to assess its potential isolation performance. A factor of 100 performance increase over the passive case was achieved with minor weight addition and minimal power consumption.

Servicing the Hubble Space Telescope (HST) requires the safe transportation of electronic Orbital Replacement Units (ORUs) on the Space Transportation System (STS) to replace or enhance the capability of existing units. The delicate design of these electronic ORUs makes it imperative to provide isolation from the STS launch random vibration, while maintaining fundamental modes above the transient load environment. Two methods were developed and used exclusively, on Servicing Mission 2 (SM2), to isolate the ORUs from the environmental launch loads imposed by the STS. The first load isolation system utilizes a refined open/closed cell foam design to provide the required damping and corner frequency, while the second method uses an innovative Viscoelastic Material (VEM) design. This paper addresses both systems as initially designed including finite element (FE) model analysis of the VEM system. Vibration testing of prototype systems and modifications to the design resulting from test will be discussed. The final design as flown on HST SM2 with recommendations for future applications of these technologies in transporting electronic black boxes to orbit will conclude the paper.

This paper summarizes the design, optimization, development, fabrication, and testing of a vacuum compatible coil spring with embedded constrained layer visco-elastic damping. The spring is developed as part of the NSF funded LIGO (Laser Interferometer Gravity Wave Observatory) project. Large numbers of those springs are the primary components of multi- stage, in-vacuum, passive seismic isolation stacks that provide high attenuation (-160 dB/decade above 15 Hz) of floor vibrations for ultra-sensitive (better than 10^-18 m/(root)Hz noise floor between 40 and 1000 Hz) laser interferometers. The spring design addresses both requirements for passive isolation within a single, self- contained, vacuum tight envelope: low stiffness for maximum attenuation and non-viscous damping to limit resonant amplitudes in the stack. This is achieved with a tubular coil spring design with an internal torsional constrained layer damping structure. The paper presents the analysis of this spring using closed-form analytical expressions, trend studies showing the strong dependence of spring performance on key design parameters, and explicit numerical design optimization. Manufacturing issues are briefly discussed. Finally, experimental results from static and dynamic tests performed on prototype units are presented. Results show loss factors of the order of 1.5% in the transverse direction to 3% in the axial direction, at frequencies from 1 to 2 Hz.

Multifunctional concretes capable of both structural and non- structural functions are made possible by appropriate admixtures. The use of acid treated silica fume (15% by weight of cement), latex (20 - 30% by weight of cement) or methylcellulose (0.4 - 0.8% by weight of cement) as an admixture gave the vibration damping function (with loss tangent up to 0.18, i.e., up to 390% increase, and loss modulus up to 2.2 GPa, i.e., up to 2200% increase, at 0.2 - 2 Hz loading).

The definition of the mechanical properties of viscoelastic materials, i.e. the elastic modulus and the loss factor, is carried out, according to many national and international standards, with many different techniques, both of the resonant and non-resonant type. In this paper we focus our attention on the pros and cons of the resonant technique based on the classical Oberst beam method. When the damping material to be tested is not self-supporting, its properties are determined taking start from the measured modal frequencies and loss factors of a laminated beam, constituted by one or two metallic strips, ideally undamped, and one or two viscoelastic layers. The formulae specified on the standards hold valid under the assumptions of the theory developed by Kerwin, Ungar and Ross and we try in this paper to quantify witch deviation of the results should be expected when moving away from their ideal hypotheses.

The theory of an electrically tunable Terfenol-D vibration absorber is developed in this paper. An overview of mangetostriction including discussion of the (Delta) E effect is presented. Experimental results showing agreement with prior art are included that demonstrate electrical control of a magnetostrictive actuator resonant frequency by varying the resonance between 1275 Hz and 1725 Hz. The tunability of the transducer resonant frequency is then implemented to achieve high bandwidth tunability in the performance of a Terfenol-D vibration absorber.

Tuned liquid column damper (TLCD) was developed mainly for the purpose of suppressing horizontal motion of structures. No relevant research has been found on the suppression of structural pitching vibration by using TLCD. This paper thus conducts a series of TLCD experiments with different configurations and parameters to investigate the possibility and effectiveness of applying TLCD to reduce pitching motion of structures. A theoretical model of TLCD-structure interaction under pitching motion is also developed to guide the experiments and verified by the experimental results on the other hand. In this study, the influence of variable TLCD parameters on control effectiveness are determined. It is concluded that TLCD can efficiently reduce structural pitching motion.

A study was undertaken to investigate low frequency vibration suppression of a large composite panel under acoustic excitation. Both passive and active techniques are examined. Passive damping is achieved with a peel and stick constrained layer damping treatment. The treatment uses a constraining layer with a stand-off spacer to increase damping performance. Active damping is achieved with surface bonded piezoceramic wafer actuators and a closed-loop controller. This paper will discuss the analytical modeling of both damping techniques and show comparisons with test data. The reduced vibration resulted in noise attenuation at panel resonance. This phenomenon will also be discussed with the presentation of accelerometer and microphone test data.