D. Stefan Dancila

The influence of coupling on the free vibration of anisotropic thin-walled closed-section beams

Abstract A solution procedure for thin-walled laminated composite beams is presented. Two configurations are considered, producing extension-twist and bending-twist coupling, respectively. The influence of coupling on the characteristic equations for free vibration is isolated. It is shown that the characteristic determinant for extension-twist coupling can be expressed as the product of the two decoupled ones. For the case of bending-twist coupling, a simple-quasi-decoupled procedure is developed. This model is shown to provide accurate predictions for natural frequencies of practical interest for slender, laminated composite box beams.

Energy-Dissipating Composite Members with Progressive Failure: Concept Development and Analytical Modeling

A new concept for tailored, one-dimensional tensile composite members with sequential, progressive failure and yield-type response is presented. The concept is developed as a solution to a constrained arresting problem. Flexible composites, obtained by reinforcing an elastomeric matrix material with high-performance elastic fibers, are used to create a structure with redundant load paths and progressive failure. Three analytical models are developed for the prediction of response under stroke-controlled loading. The response of the tailored configuration is compared with that of conventional members of identical length, total cross-sectional area, and material properties. Analytical results confirm the predicted progressive failure sequence and yield-type response, as well as the increased energy-dissipation capability.

Modeling and Analysis of Active Flap Using Coiled Bender Piezoelectric Actuators

The applicability and effectiveness of three novel piezoelectric coiled bender actuators to deflect a plain trailing edge flap on a straight wing in the presence of incompressible flow airloads is investigated in this paper through analytical modeling. This work builds upon previous research in which the three actuator configurations, the linear spiral type, the helicoidal type, and the hybrid type, have been proposed and analytically modeled, and the first two configurations have been experimentally validated. A simple analytical model for the piezoelectrically actuated flap is developed and used for this investigation. Results show that the use of coiled piezoelectric actuators for flap deflection is an effective approach. The variation of flap deflection angles with applied actuation voltage and wing angle of attack is also shown, together with performance envelopes.

Free Vibration of Thin-Walled Closed-Section Composite Beams with Optimum and Near-Optimum Coupling

An investigation of the free vibration characteristics for a set of extensiontwist coupled thin-walled single-cell composite beams with optimal and near-optimal coupling is performed using an analytical model, a finite element simulation, and testing. It is found that, despite the relatively large beam slenderness ratio, the applicability of the analytical model is limited to the first bending modes, for which excellent agreement with the finite element and experimental results is obtained. For analytically predicted higher order modes, increasingly significant in-plane warping is shown by the corresponding finite element results, and an increasingly large variance is observed between the two sets of results. The occurrence of shell modes that cannot be captured by the analytical model is also shown by the finite element results. Test data confirm finite element calculations as well as low modes predicted by the analytical method. The sensitivity of natural frequencies to lay-up perturbation angle is established over a frequency range of interest, providing insight into the robustness of the modal characteristics for the global and the local optima, respectively.

ENERGY-DISSIPATING SNAP-RESISTANT TETHER

A 1D flexible composite material tailoring concept for increased energy dissipation through a progressive, sequential failure of redundant load paths of tailored length and strength has been previously developed, modeled and experimentally verified. In this work, the applicability and benefits of this tailoring concept to space tethers is analytically investigated. A response model for the case of two masses connected by a tailored tether is developed based upon an earlier model of response under impulsive loading. An approximate model of tether response is developed to greatly improve the computational effort and is shown to be very accurate for long tethers.

Energy-dissipating composite members with progressive failure: Impulsive response and experimental verification

An experimental investigation of response for tailored one-dimensional energy-dissipating composite members with progressive failure subject to quasi-static and impulsive tensile loading is presented. The tailoring concept relies upon a progressive failure sequence of redundant load paths of tailored strength and length to induce a yield-type response. In a prior publication the authors presented the concept development and analytical modeling of response under quasi-static loading. In this paper, an experimental verification is provided using a universal testing machine for members under quasi-static uniaxial loading. Furthermore, the model is extended to impulsive loading, and a custom-design drop test setup is developed to provide an experimental validation of analytical response. The results obtained confirm the hypothesized progressive failure sequence of redundant load paths, thereby validating the failure tailoring concept, as well as the accuracy and predictive power of the developed models of response in terms of both the number of partial failures induced for a given loading and the increased energy dissipation capability. Potential aerospace applications include snap-resistant tensile structures, for example space tethers, towing and cargo restraint lines, crashworthy helicopter troop seat stroke control straps, and aircraft emergency arrest gear.

Analytical Investigation of the Toughening Potential of a Failure Tailoring Concept

A parametric study has been performed to determine the upper bounds of toughening benefit achievable using an approximate model for the response of one-dimensional, tailored, flexible composite-material tethers with progressive failure. The tailoring concept is achieved through judicious arrangement of redundant load paths with unequal length and strength and has been proposed, modeled, and experimentally verified in previous research. The model is recast in this work using nondimensional material-independent parameters that are varied over ranges of interest in advanced composites. The effects of each parameter on the response of the tailored member are discussed. An increase of nearly two orders of magnitude in toughness over an untailored member is found to be attainable by taking complete advantage of the energy dissipated by the failure of each load path within the bounds of the study. Challenges to implementation of the tailored member as approximated by the model are also discussed.

Electromechanical modeling of a piezoelectric actuator for modulated/vectored blowing

A nonlinear analysis was accomplished in order to establish the dynamic response of a piezoelectric actuation system used to vector and modulate the intensity of a positive mass flow rate jet blowing. The steady state response of the electromechanical system to a sinusoidal applied voltage signal was analytically developed for the linearized model. Numerical solutions were performed for the nonlinear case, the influence of the frequency and amplitude of the excitation input signals being investigated.

ROTOR WAKE MODIFICATIONS IN HOVER USING UNSTEADY SPANWISE BLOWING

A Computational Fluid Dynamics (CFD) study investigating the effects of unsteady spanwise rotor blade tip blowing on the formation and evolution mechanisms of blade tip vortices shed during hover is presented in this paper. The specific blowing configuration consists of a single jet sheet with modulated mass flow rate controlled through slot area variation, ejected tangentially over a rounded blade tip cap. By using the beneficial Coanda effect of jet attachment to the curved wall of the blade tip cap a downward deflection of the jet sheet direction is obtained. As a consequence the location and formation of the tip vortices are modified in a time dependent manner with reference to a baseline, no-blowing case investigated for the same rotor and flight conditions.

Composite Star-Beams as Pitch Compliant Tension-Torsion Support Mechanism for Active Windmill Blade Tips

In this paper we provide a preliminary design level first evaluation of the applicability of modified cross section composite star beams as tension-torsion pitch actuation compliant mechanisms for large size wind turbine blade tips, and we provide estimates of stiffness and mass per unit length reachable within the geometric and material system constraints of a 25%R blade tip for a realistic large wind turbine blade.