The present study focuses on a bi-stable von-Mises truss (VMT), with integrated Shape Memory Alloy (SMA) wires which are resistively heated to provide the actuation force to transition the VMT from one stable equilibrium condition to the other, and back. This coupled VMT-SMA system can potentially be used in structural morphing applications. The paper considers in detail the design of the system, equilibrium between the VMT and the SMA wires, the initial pre-stress required in the two SMA wires, explains how the active (heated) SMA wire drives the VMT beyond the unstable equilibrium state, and the VMT in moving to the second stable equilibrium state pre-stresses the passive (unheated) SMA wire. The two SMA wires switch roles in moving the VMT back from the second to the first stable equilibrium state. A prototype is designed and fabricated and the transition of the VMT from one equilibrium state to the other, and back, is experimentally demonstrated. The governing differential equation representing the VMT behavior is coupled with equations representing the SMA behavior based on the Brinson thermo-mechanical model. The numerical predictions of system displacements versus temperature and time show good correlation with experimental results.

This paper focuses on the design, fabrication, testing and analysis of a novel load-bearing element with good energy dissipation capability over a decade of variation in frequency and harmonic load amplitude. A single layer of the compact sandwiched-plate-like element is comprised of two von-Mises trusses (VMTs) between an upper and lower plate, connected to two dampers which stroke in the in-plane direction as the VMTs cycle between the two stable equilibrium states. The elements can be assembled in-plane to form a large plate-like structure or stacked with different properties in each layer for improved load-adaptability. Also introduced in the elements are pre-loaded springs (PLSs) that provide very high initial stiffness and allow the element to carry a design static load even when the VMTs lose their load carrying capability under harmonic disturbance input. Simulations of the system behavior using the Simscape environment show good overall correlation with test data. Good energy dissipation capability is observed over a frequency range from 0.1 Hz to 2 Hz. While the VMT parameters of a single layer can be optimized to a particular harmonic load amplitude, having two layers with softer and stiffer VMTs allow the system to show good energy dissipation characteristics at different harmonic load amplitude levels. The test and simulation results show that a two layer prototype can provide good energy dissipation over a decade of variation in harmonic load amplitude, while retaining the ability to carry static load on account of the PLSs. The paper discusses how system design parameter changes affect the static load capability and the hysteresis behavior.

This paper examines the potential of a bi-stable von-Mises truss (VMT) connected to viscous damping elements in absorbing impact loads, and reducing loads and accelerations transmitted to the base. A compact sandwich-panel-like element is designed, fabricated and tested in an impact testing machine which comprises of a top plate, a bottom plate, and VMTs in-between that stroke the dashpot as they snap through. The loads transmitted to the bottom plate showed reductions of over 70% for lower drop heights, while reductions approaching 50% were seen for larger drop heights. The introduction of pre-loaded springs in parallel to the VMTs resulted in the prototype not snapping through, except at the largest drop heights tested. The reductions in the transmitted loads were modest for small drop heights but improved as the drop heights increased and snap through occurred. Accelerations transmitted to the bottom plate also showed reductions of 83–88% relative to the upper plate accelerations. A Simscape model of the system showed good overall agreement with tests for loads transmitted to the bottom plate when using the measured loads on the top plate as an input.

Previous studies have shown that chord extension morphing over a spanwise section of helicopter rotor blades can reduce main rotor power requirement in stall-dominant flight conditions while at the same time being able to increase the maximum gross weight, altitude, and flight speed capability of the aircraft. This study examines a centrifugally driven, fully passive chord morphing mechanism for helicopter rotor blades. It is based on a von-Mises truss situated aft of the leading-edge spar, connected to a rigid extension plate which deploys through a slit in the trailing-edge. When the rotor RPM increases beyond a critical value the chordwise component of centrifugal (CF) force on the von-Mises truss and plate assembly results in the deployment of the plate beyond the slit in the trailing edge, effectively increasing chord length. On reducing the RPM, a retraction spring pulls the plate back within the confines of the blade. This study presents the design process, iterations and the final design solution for a configuration that undergoes 20% chord extension. A prototype was fabricated and tested on the bench-top as well as on a rotor test stand at rotational speeds simulating 70% full-scale CF loads. The test results demonstrate that the concept works. However, effects such as friction lead to higher force (or RPM) requirements for deployment than predicted by simulation, and are present during retraction as well. The effects are more pronounced in the high CF field in the rotor test.

Cellular structures with hexagonal unit cells show a high degree of flexibility in design. Based on the geometry of the unit cells, highly orthotropic structures, structures with negative Poisson’s ratios, structures with high strain capability in a particular direction, or other desirable characteristics may be designed. Much of the prior work on cellular structures is based on hexagonal honeycomb-like unit cells, without any inclusions. The present paper envisages extending conventional cellular honeycomb-like structures to have inclusions or internal features within the unit cells. Various types of internal features such as contact elements, buckling beams, bi-stable/snap-through elements or viscous dashpots can result in unit cells which display stiffening, softening, negative stiffness, or dissipative behavior. A structure can be assembled using a specific element type, or different types of elements in specific arrangements, to provide desired system level behavior. The ability to so optimally and flexibly design cellular structures could potentially lead to their replacing conventional structures made from bulk materials. As a first step, this paper presents work on hexagonal unit cells with linear springs as the simplest of inclusions. The hexagonal cell itself is comprised of rigid links and pin-joints. Since such a cell has no stiffness of its own, the behavior of any internal feature is emphasized. However, for kinematic stability purposes, three springs are required, and the significance of these constraints within the cell will be discussed. For different spring arrangements, closed-form analytical expressions are derived for the in-plane modulus and Poisson’s ratio. The analytical expressions are validated using NASTRAN finite element simulations, as well as against experimental tensile/compressive tests of fabricated unit cells with internal springs. When the spring stiffness exceeds certain values, the rigid cell wall assumption is no longer valid, and these bounds are established. The validated analysis is used to conduct design studies on how the cell modulus would vary with geometric parameters such as cell angle and cell wall length ratio.

Cellular structures with hexagonal unit cells show a high degree of flexibility in design. Based on the geometry of the unit cells, highly orthotropic structures, structures with negative Poisson’s ratios, structures with high strain capability in a particular direction, or other desirable characteristics may be designed. Much of the prior work on cellular structures is based on hexagonal honeycomb-like unit cells, without any inclusions. A companion paper to the current paper presented a vision of cellular honeycomb-like structures with diverse inclusions or internal features within the unit cells (such as contact elements resulting in stiffening behavior, buckling beams resulting in softening behavior, bi-stable elements producing negative stiffness or viscous dashpots producing dissipative behavior). That paper further went into details on linear springs as the most fundamental of inclusions. In the present paper, a buckling beam and viscous dashpots are used as inclusions in the basic pin-jointed rigid-walled hexagonal unit cell. The buckling beam provides the cell with a high initial stiffness and load carrying capability. At loads beyond the critical buckling load, the unit cell softens (while still retaining the ability to carry a “design” load), and undergoes large deformation under incremental load. The viscous dampers undergo a correspondingly large stroke resulting in high dissipative capability and loss factor under harmonic or transient disturbance beyond the design load. In the paper, an analysis and design study of the cell behavior with variation in unit cell geometric parameters, buckling beam parameters and viscous dashpot parameters is presented. The analytical results in the paper are validated against ANSYS Finite Element results. Further, a prototype unit cell with an aluminum internal buckling beam and viscous dashpots is fabricated and tested under static and dynamic loads in an Instron machine. Good correlation is observed between the tests, the FE results and the analytical simulations when accounting for the non-linear behavior of the viscous dashpot used in the tests.

Chord extension in helicopter rotors allows for expansion of the flight envelope, with the helicopter capable of flying at higher gross weights, altitudes and maximum speeds. A fixed large chord, however, results in a penalty when the helicopter is well within the envelope (for example, at low to moderate gross weight, sea level, and at moderate speed cruise). Chord morphing allows the helicopter to perform optimally in these diverse conditions. In this paper, the authors present a morphing mechanism to extend the chord of a section of the helicopter rotor blade. The region aft of the leading-edge spar contains a morphing cellular structure. In the “compact” state the edge of the cellular structure aligns with the trailing-edge of the rest of the blade. When the morphing cellular structure is in the “extended” state the chord of that section of the blade is increased by close to 30% (with the trailing-edge extending beyond that of the rest of the blade). In transitioning from compact to extended states, the cellular structure slides along ribs which define the boundaries of the morphing section in the span-wise direction. The cellular section has mini-spars running along the span-wise direction to attach the flexible skin and provide stiffness against camber-like deformations due to aerodynamic loads. The paper presents a finite element analysis and a design study of the morphing cellular structure, ensuring that the local strains in the cellular structure do not exceed maximum allowables even as the section undergoes large global strain. On the other hand, the morphing cellular structure is required to be stiff enough so that the pre-stretched skin that is attached to the surfaces does not result in deformation. Another question that is considered in detail is various methods of attachment of the flexible skin to the morphing substructure, the levels of pre-strain required, and their ramifications. A model of a blade section is fabricated and shown to undergo chord morphing, as designed.

The adaptive structures concept is of great interest in the aeronautical field because of the several benefits which can be accomplished in the design of future aircraft. Improvements in terms of aerodynamic efficiency, aero-elastic behaviour and manoeuvrability were proved by many international studies. The development of new structural architectures implementing and integrating innovative materials is mandatory for succeeding in these critical tasks. The so-called Smart Structure idea is more and more taken into account in aerospace applications Among the family of Smart Materials, Shape Memory Alloys (SMAs) certainly represents a convenient solution for many static applications. In this work, an application for a morphing wing trailing edge is presented as alternative for conventional flap devices. A compliant rib structure has been designed, based on SMA components working both as actuators, controlling wing chamber, and as structural elements, sustaining external aerodynamic loads. Achievable performance has been estimated by a FE approach; SMA behaviour has been modelled through a dedicated routine implementing the Liang & Rogers’ model for evaluating the internal stress and the minimum temperature necessary for activation. The numerical results have been presented in terms of induced displacements and morphed shape.