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In space applications, the pursuit of deployable structures with high packaging efficiency and low mass is essential. Tensegrity structures, prestressed networks of struts and cables, offer a promising approach. In this work, a Class-2 cylindrical triplex tensegrity bay is explored for use in booms, where select cables are allowed to change length in order to modify the overall shape of the structure. A novel approach to characterize the range of equilibrated shapes a triplex can achieve is first presented. The results are then used in combination with consideration of bending stiffness, packaging efficiency, and mass to explore the effects of different configuration parameters, namely the ratio of bay height to circumscribing radius (h/r), the member cross-sectional radii, and the number of bays. This design analysis offers new insight into how triplex tensegrity structures can be modified to achieve specific design objectives. Increasing h/r for a single bay increases the range of achievable active tip motion as well as packaging efficiency. Increasing member radii decreases the range of achievable shapes and packaging efficiency while increasing stiffness and mass. When increasing the number of bays for a fixed overall height, a larger number of bays generally increases the range of achievable shapes and mass while reducing stiffness. Packaging efficiency was found to be discontinuous dependent upon the exact number of bays, but generally decreases with increasing number of bays. An example design study is conducted using the Canadarm2 for comparison and illustrates the potential utility of tensegrities for boom structures. [ABSTRACT FROM AUTHOR]

Compliant cellular structures with an internal contact mechanism are described in this paper. Contact during deformation reduces failure-causing bending stresses through stress relief thereby enabling such cellular structures to be stretched more than the corresponding structures without contact. Finite element analysis (FEA) is carried out to simulate the structure. An analytical model is developed to get results quicker than FEA and to develop insight into the mechanics of the deformation process. The error in prediction of the maximum stretching capacity using the analytical model is less than 7% when compared with finite element simulations. Several materials are investigated for such structures. Although the allowable strain of all these materials is small, the overall strain of the contact-aided cellular structures is at least an order of magnitude greater than that of the constitutive material. The contact mechanism and the induced stress relief increase the stretching capacity of the contact-aided cellular structures by as much as 100%. Experiments are conducted to validate the models, and good agreement is found. A high-strain morphing aircraft skin is examined as an application of these mechanisms. The results indicate that the proposed skin structure not only increases the morphing capacity but also decreases the structural mass by 13% as compared with a cellular skin without contact. [DOI: 10.1115/1.3165778]

This paper considers the transverse vibration of flexural structures subjected to preloads that are tangent to the structural midplane; the effects of such membrane loads on the damping of the structural vibration modes are of particular interest. The theory developed indicates that tensile loads increase the natural frequencies of vibration (as is well known), but decrease the modal damping; for beams, the effect on damping is stronger than the effect on frequency. Conversely, compressive loads decrease the natural frequencies and increase the modal damping. These conclusions are consistent for various models of material damping. Available experimental data show good agreement with theory.

The focus of this paper is an optimal design of morphing aircraft wings employing a wing structure composed of an internal layout of cables and struts. Cables are used to provide actuation and stiffness, and struts provide stiffness without actuation. Topology optimization is used to place cables and struts in a bay or a section of the wing. Nonlinear finite element analysis is used to capture the large deformations of the structure, and the optimization is achieved using the Nondominated Sorting Genetic Algorithm II. The optimization procedure is illustrated using a morphing-wing example. The effect of the upper limit on actuation forces is studied, and solutions are found with good agreement between the desired and obtained deflections under actuation and aerodynamic loads. The implemented parallelized optimization algorithm is successful in solving a computationally intense, multi-objective, multiconstraint problem with a large number of discrete and continuous design variables in a reasonable amount of time.

Particle impact dampers (PIDs) have been shown to be effective in vibration damping. However, our understanding of such dampers is still limited, based on the theoretical models existing today. Predicting the performance of the PID is an important problem, which needs to be investigated more thoroughly. This research seeks to understand the dynamics of a PID as well as those parameters which govern its behavior. The system investigated is a particle impact damper with a ceiling, under the influence of gravity. The base is harmonically excited in the vertical direction. A two-dimensional discrete map is obtained, wherein the variables at one impact uniquely dictate the variables at the next impact. This map is solved using a numerical continuation procedure. Periodic impact motions and 'irregular' motions are observed. The effects of various parameters such as the gap clearance, coefficient of restitution, and the base acceleration are analyzed. The dependence of the effective damping loss factor on these parameters is also studied. The loss factor results indicate peak damping for certain combinations of parameters. These combinations of parameters correspond to a region in parameter space where two-impacts-per-cycle motions are observed over a wide range of nondimensional base accelerations. The value of the nondimensional acceleration at which the onset of two-impacts-per-cycle solutions occurs depends on the nondimensional gap clearance and the coefficient of restitution. The range of nondimensional gap clearances over which two-impacts-per-cycle solutions are observed increases as the coefficient of restitution increases. In the regime of two-impacts-per-cycle solutions, the value of nondimensional base acceleration corresponding to onset of these solutions initially decreases and then increases with increasing nondimensional gap clearance. As the two-impacts-per-cycle solutions are associated with high loss factors that are relatively insensitive to changing conditions, they are of great interest to the designer. [DOI: 10.1115/1.2827364] Keywords: particle impact damping, nonlinear, gap clearance, loss factor, damping, impact damper

The feasibility of helicopter blade lag damping using embedded chordwise absorbers is investigated both analytically and experimentally. The basic feature of this approach is the use of tuned damped absorbers along the blades. This concept, utilizing a portion of the leading-edge weights that are already incorporated into the blade as a portion of the mass of the absorbers, can have the potential to replace or augment current blade lag dampers and reduce the complexity, aerodynamic drag, and weight of the rotor hub. The analysis is performed using an aeromechanical stability analysis program developed using the finite element method for the rotor system with blades incorporating absorbers. The test is conducted using a small-scale hingeless rotor with a single absorber on each blade. Parametric studies for blade lag damping augmentation with various absorber mass, location, damping, and tuned frequency are also presented. The analytical and experimental results show that blade damping augmentation using chordwise absorbers varies from 0.3 to 15% critical damping depending on the blade and the absorber parameters. The analytical results also show that the damping augmentation improves the ground-resonance stability of the rotor system.

The research described in this paper aims to develop a structural concept capable of achieving continuous stable deformations over a large range of aircraft shapes. The basic concept underlying the approach is a compliant cellular truss, with tendons used as active elements. The members of the truss are connected through compliant joints such that only modest bending moments can be transmitted from one member to another. Actuation is achieved by pulling on one set of cables while controlling the release of another, so that the stability of the structure is maintained in any intermediate position. The tendon-actuated compliant truss can be made to behave locally, and temporarily, as a compliant mechanism, by releasing appropriate cables. As a result, in the absence of aerodynamic forces, the structure can be morphed using relatively low forces. Highly distributed actuation also enables the achievement of global shape changes as the accumulation of local ones, while the use of compliant joints rather than true rotating joints eliminates binding as a significant concern. A six-noded octahedral unit cell with diagonal tendon actuation is developed for a bending-type deformation in the wing. Initial cell geometry is determined by 'strain matching' to the local morphing deformation required by the application. The cell size is dictated by the available space, the morphing strain, and discretization errors in approximating a smooth desired shape. A finite element analysis is performed on a wing made of these unit cells and sized for a representative vehicle weighing 3000 lb (1360 kg). The weight of the truss wing was comparable to a conventional stiffened skin construction, although its deflections are larger. Aeroelastic concerns can perhaps be addressed through the use of active control. Several ideas for a skin, required to transfer the aeroloads to the underlying structure, are presented.

An optimized method of harvesting vibrational energy with a piezoelectric element using a step-down dc-dc converter is presented. In this configuration, the converter regulates the power flow from the piezoelectric element to the desired electronic load. Analysis of the converter in discontinuous current conduction mode results in an expression for the duty cycle-power relationship. Using parameters of the mechanical system, the piezoelectric element, and the converter; the 'optimal' duty cycle can be determined where the harvested power is maximized for the level of mechanical excitation. It is shown that, as the magnitude of the mechanical excitation increases, the optimal duty cycle becomes essentially constant, greatly simplifying the control of the step-down converter. The expression is validated with experimental data showing that the optimal duty cycle can be accurately determined and maximum energy harvesting attained. A circuit is proposed which implements this relationship, and experimental results show that the converter increases the harvested power by approximately 325%. Index Terms--DC-DC, discontinuous conduction mode, energy harvesting, piezoelectric devices.

This paper describes an approach to harvesting electrical energy from a mechanically excited piezoelectric element. A vibrating piezoelectric device differs from a typical electrical power source in that it has a capacitive rather than inductive source impedance, and may be driven by mechanical vibrations of varying amplitude. An analytical expression for the optimal power flow from a rectified piezoelectric device is derived, and an 'energy harvesting' circuit is proposed which can achieve this optimal power flow. The harvesting circuit consists of an ac-dc rectifier with an output capacitor, an electrochemical battery, and a switch-mode dc-dc converter that controls the energy flow into the battery. An adaptive control technique for the dc-dc converter is used to continuously implement the optimal power transfer theory and maximize the power stored by the battery. Experimental results reveal that use of the adaptive dc-dc converter increases power transfer by over 400% as compared to when the dc-dc converter is not used. Index Terms--Adaptive control, dc-dc conversion, energy harvesting, low power, piezoelectric.

The effectiveness of bromine-intercalcated graphite-epoxy composite fiber structure in damping excessive vibration was investigated. The results showed that damping was frequency- and temperature-dependent. Pristine fiber composites featured a peak modal damping ratio of 0.8x10(super -3) while the bromine-intercalcated sample exhibited 1.6x10(super -3). This indicated that the damping of quasi-isotropic composites using intercalcated graphite fibers can be dominated by fiber damping.

Acoustic metamaterials offer an approach to reducing the dynamic response of sandwich panels. The key concept underlying this approach is to consider a metamaterial as a highly distributed system of continuous vibration absorbers that introduces multiple stop bands in which the response of the global structure is reduced. Multiple modal frequencies of the absorber system may be tuned to match global resonance frequencies and/or excitation frequencies. Using the assumed modes method, a metamaterial system is designed for integration into the honeycomb core of a representative sandwich panel. The metamaterial system is modeled as an effective distributed complex mass density in the global sandwich-panel model. The cores for two sandwich panels are fabricated using three-dimensional printing technology and then characterized statically and dynamically to determine effective elastic properties and natural frequencies of the cores, as well as loss factors of the vibration absorbers. The sandwich panels, constructed by bonding unidirectional carbon-fiber face sheets to both cores, are tested dynamically for two different boundary conditions: cantilevered and free-free. Experimental results confirm that the metamaterial core reduces the peak dynamic response at the natural frequencies of the sandwich panel with reasonably good agreement with model predictions. [ABSTRACT FROM AUTHOR]