This paper presents a novel variable modulus cellular structure based on a hexagonal unit cell with pneumatic artificial muscle (PAM) inclusions. The cell considered is pin-jointed, loaded in the horizontal direction, with three PAMs (one vertical PAM and two horizontal PAMs) oriented in an “H” configuration between the vertices of the cell. A method for calculation of the hexagonal cell modulus is introduced, as is an expression for the balance of tensile forces between the horizontal and vertical PAMs. An aluminum hexagonal unit cell is fabricated and simulation of the hexagonal cell with PAM inclusions is then compared to experimental measurement of the unit cell modulus in the horizontal direction with all three muscles pressurized to the same value over a pressure range up to 758 kPa. A change in cell modulus by a factor of 1.33 and a corresponding change in cell angle of 0.41° are demonstrated experimentally. A design study via simulation predicts that differential pressurization of the PAMs up to 2068 kPa can change the cell modulus in the horizontal direction by a factor of 6.83 with a change in cell angle of only 2.75°. Both experiment and simulation show that this concept provides a way to decouple the length change of a PAM from the change in modulus to create a structural unit cell whose in-plane modulus in a given direction can be tuned based on the orientation of PAMs within the cell and the pressure supplied to the individual muscles.
The concept proposed in thei work for chord extension is the use of a bistable arch and thin plate system. There are two foci of this paper: (1) Design of the arch and (2)Model validation via experiment. Results show that bistability and symmetric deformation can be achieved when there are flexible hinges at the boundary and input. In addition, the presented finite element model provides good agreement with experimental results.
KEYWORDS: Finite element methods, Systems modeling, Aerodynamics, Motion models, Performance modeling, Composites, Actuators, Smart structures, System integration, Current controlled current source
Research efforts have shown that helicopter rotor blade morphing is an effective means to improve flight performance.
Previous example of rotor blade morphing include using smart-materials for trailing deflection and rotor blade twist and
tip twist, the development of a comfortable airfoil using compliant mechanisms, the use of a Gurney flap for air-flow
deflection and centrifugal force actuated device to increase the span of the blade. In this paper we explore the use of a
bistable mechanism for rotor morphing, specifically, blade chord extension using a bistable arc. Increasing the chord of
the rotor blade is expected to generate more lift-load and improve helicopter performance. Bistable or "snap through"
mechanisms have multiple stable equilibrium states and are a novel way to achieve large actuation output stroke.
Bistable mechanisms do not require energy input to maintain a stable equilibrium state as both states do not require
locking. In this work, we introduce a methodology for the design of bistable arcs for chord morphing using the finite
element analysis and pseudo-rigid body model, to study the effect of different arc types, applied loads and rigidity on arc
performance.
In this paper we explore the use of bistable mechanisms for rotor morphing, specifically, blade tip twist. The optimal
blade twist distributions for hover and high-speed forward flight are very different, and the ability of the rotor to change
effective twist is expected to be advantageous. Bistable or "snap-through" mechanisms have multiple stable equilibrium
states and are a novel way to achieve large actuation output stroke at relatively modest effort for gross rotor morphing
applications. This is because in addition to the large actuation stroke associated with the snap-through (relative to
conventional actuator/ amplification systems) coming at relatively low actuation effort, no locking is required in either
equilibrium state (since they are both stable). In this work, the performance of a bistable twisting device is evaluated
under an aerodynamic lift load. The device is analyzed using finite element analysis to predict the device's load carrying
capability and bistable behavior.
This paper describes a method to vary the flexural bending stiffness of a multi-layered beam. The multi-layered beam
comprises of a base layer with polymer layers on the upper and lower surfaces, and stiff cover layers. Flexural stiffness
variation is based on the concept that when the polymer layer is stiff, the cover layers are strongly coupled to the base
beam and the entire multi-layered beam bends as an integral unit. In effect, we have a "thick" beam with contributions
from all layers to the flexural bending stiffness. On the other hand, if the shear modulus of the polymer layers is
reduced, the polymer layers shear as the base beam undergoes flexural bending, the cover layers are largely decoupled
from the base, and the overall flexural bending stiffness correspondingly reduces. The shear modulus of the polymer
layer is reduced by increasing its temperature through glass transition. This is accomplished by using embedded ultrathin
electric heating blankets. From experiments conducted using two different polymer materials, polymer layer
thicknesses and beam lengths the flexural stiffness of the multi-layered beam at low temperature was observed to be
between 2-4 times greater than that at high temperature.
Data from comprehensive thermomechanical tests of shape memory polymers are reported, with specimens tested up to
75% strain and between 30-120°C temperatures. The data is analyzed and key observations are drawn. The stress/strain
behavior during loading at temperatures above glass transition for the Veriflex shape memory polymer tested was
linear and did not show much variation with the actual temperature. When the polymer is cooled with end constraints,
thermally induced tensile stresses developed, but only after the temperature reduced below glass transition and the
material stiffened. When the constraints were then released, 97-98% of the original strain was locked in. Reheating the
shape memory polymer beyond the glass transition temperature resulted in shape recovery (shape memory effect). When
the polymer was reheated while constraining the strain, the full recovery stress developed was about the stress the
polymer was initially loaded to during deformation at high temperature. Examining the Young's modulus at elevated
temperature and low temperature showed that Veriflex softened by around 40-60 times when heated through glass
transition.
In active constrained layer (ACL) damping treatments there are two distinct physical mechanisms that contribute to the damping of resonant oscillations -- increased passive damping due to increased shear in the viscoelastic material (VEM) layer, and damping due to transmission of active forces to the host structure. The present study demonstrates that the first mechanism is dominant when proportional feedback is used while the second mechanism is dominant when derivative feedback is used. In the case of proportional feedback, the shear in the VEM increases considerably so that the passive damping is significantly larger than that obtained for zero-voltage (PCL case), but the active action is actually slightly detrimental. In the case of derivative feedback, the shear strain levels in the VEM are virtually unchanged, and all of the damping augmentation is due to the active action. While previous studies have suggested that a high VEM shear modulus would enhance the active damping augmentation due to improved transmissibility of active forces from the piezoelectric layer to the host structure, voltage (or electric field) limits on the piezoelectric layer were never directly considered. In the present study it is concluded that for high VEM shear modulus the low inherent damping results in large resonant response amplitudes. In such a case, the allowable control gains (so as not to exceed the piezoelectric voltage limits) would be reduced, and the damping increases predicted previously (without considering the voltage limits) are no longer available. The present results indicate that when voltage limits are considered, the maximum damping augmentation is available in the VEM shear modulus range that provides optimal passive damping, since these allow the largest control gains.
The purpose of this research is to investigate and improve constrained layer (CL) damping treatment for high damping and low interlaminar stresses (better durability). In this paper a mathematical model is developed to calculate interlaminar stresses in a CL treatment. The model is based on the Built-Up Bar (BUB) theory but includes numerous fundamental modifications to handle the behavior of various coversheet and viscoelastic materials. A parametric study is conducted. It is shown that the interlaminar peeling and shearing stresses in a CL treatment could be very high, especially at the free edges due to discontinuities in the material properties. It is also illustrated that these interlaminar stresses are of local type, i.e. the high stresses are limited to a region that is close to the free edge and is of the same order-of-magnitude in length as the layer thickness. The observation is that the designs that provide high damping usually have high interlaminar stresses. This means that the existing high performance CL designs that provide high damping usually have high interlaminar stresses.This means that the existing high performance CL designs could fail, especially under high load operations. From this research, it is shown that through some simple yet innovative modifications (e.g., slightly tapering the constraining layer at the free ends), the interlaminar stresses in the CL treatment can be significantly reduced while maintaining high levels of damping.
A series of mechanism-based phenomenological models, comprising of springs and friction elements, are presented for the pseudoelastic damping behavior of Shape Memory Alloys. The constitutive equations and a method for identifying the parameters from experimental hysteresis cycles are presented for each of the models. Comparisons are made with an existing phenomenological model. Unlike the thermodynamic-based models, the present models do not require calculation of austenite- martensite phase transformations. The mechanical analogies provide a strong physical basis to the models, and clear relationships are established between the unlocking of the friction elements and the occurrence of phase transformation.
This paper explores the feasibility of using Magnetorheological (MR) fluid-based dampers for lag damping augmentation in helicopters. A MR damper model is integrated with a rotor aeromechanical model. Two different control schemes are presented--namely the On-Off scheme and the Feedback Linearization scheme. In the On-Off scheme, two criteria are used to obtain equivalent linear damping for the nonlinear MR damper as a function of the size of perturbation and the applied field. The Feedback Linearization scheme uses a feedback controller to linearize the force output of the MR damper. The two control schemes are compared for lag transient response in ground resonance and their ability to reduce damper load in forward flight. It is shown that a MR damper of a size comparable to an elastomeric damper can provide sufficient damping for ground resonance stabilization and can significantly reduce periodic damper loads with a judicious choice of operation scheme.
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.
KEYWORDS: Differential equations, Oscillators, Silicon, Complex systems, Motion models, Structural dynamics, Data modeling, Solids, System identification, Chemical elements
A nonlinear viscoelastic solid model, comprising a combination of linear and nonlinear springs and dashpots, is developed to represent an elastomeric damper. The nonlinear constitutive differential equation obtained from the model completely characterizes the damper behavior. A method is presented to determine the spring-dashpot parameters (coefficients of the constitutive equation) from experimental data. A quartic softening spring, in series with linear Kelvin chain, is used to match experimental data. Nonlinear hysteresis cycles at different equilibrium positions are examined. The model is able to predict behavior of elastomeric dampers under dual-frequency excitations. A `two-level implicit-implicit' scheme is developed for the integration of the nonlinear damper model into a structural dynamic analysis. With the increase in amplitude of oscillatory force, the energy dissipation by the nonlinear viscoelastic damper is found to decrease, as compared to a linearized perturbation model. With increase in initial perturbation, transient decay is slower.
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