Michael E. Pontecorvo

Bistable arches for morphing applications

This article examines the bistable behavior of an arch for morphing applications. The arch has a cosine profile, is clamped at both ends, and is restrained axially by a spring at one end. Fabrication and testing of several Delrin and NiTiNOL arch specimens (with varying arch height, thickness, and spring stiffness) were followed by ANSYS finite element modeling, and the ANSYS simulation results showed good overall agreement with the test results. A parametric study was conducted using the ANSYS model to assess the influence of arch thickness, height, and spring stiffness on the bistable behavior. The results indicated that lower arch thickness, larger arch height, and higher spring stiffness tend to promote bistability; lower arch thickness and height reduce peak strains as the arch moves between equilibrium states, but increasing spring stiffness has a smaller effect; and higher arch thickness, height, and spring stiffness increase the snap-through force, which in turn increases the actuation force requirement as well as load carrying capability of the bistable morphing arch. If the arch slenderness ratio is unchanged, change in arch span (size) does not change the maximum stress while increasing the peak snap-through force proportionally.

Energy Dissipation of a Bi-Stable von-Mises Truss under Harmonic Excitation

This paper examines the energy absorption and dissipation properties of a von-Mises truss capable of snap-through, driving a viscous damper attached to its vertex. The proposed system exhibits almost opposite behavior if under displacement or force harmonic input, being able only to reproduce the negative stiffness or the snap-through in each loading condition, respectively. The loading method also greatly influences the system performance, and the effects of varying forcing frequency and damping coefficient on hysteresis loop shape and area are investigated. It is shown that the proposed system is able to combine stiffness, tailored to the application almost independently, and damping capabilities with performance above traditional materials and structures, and loss factors in excess of 0.6. The occurrence of snap-through is responsible for this performance, increasing the velocity in the damper and, therefore, energy dissipated.

Cellular Honeycomb-Like Structures With Internal Buckling and Viscous Elements for Simultaneous Load-Bearing and Dissipative Capability

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.Copyright

Energy Dissipation of a Bi-stable von-Mises Truss under Impulsive Excitation

This paper examines the energy absorption and dissipation properties of a bi-stable system due to large deformation capability and bi-stability. The considered bi-stable system is a von-Mises truss, driving a damper attached to the truss vertex, used as a shock absorber. Performance of the proposed system is compared against a traditional mass-spring-damper assembly. The VMT system undergoes larger displacements due to snap-through and can dissipate larger amounts of energy. In addition, snap-through in the VMT limits the max velocity at which the system oscillate.

Design studies on cellular structures with pneumatic artificial muscle inclusions for modulus variation

This article presents a variable modulus cellular structure based on a hexagonal unit cell with pneumatic artificial muscle inclusions. The cell is pin-jointed, loaded in the horizontal direction, with three pneumatic artificial muscles (one vertical pneumatic artificial muscle and two horizontal pneumatic artificial muscles) 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 pneumatic artificial muscles. Simulation is then compared to experimental measurement of the unit cell modulus in the horizontal direction over a pressure range of 682 kPa, and an increase in cell modulus of 200% is demonstrated experimentally. A design study considering parametric variation in cell angle, vertical to inclined wall length ratio, and pneumatic artificial muscle contraction ratios shows that changes in modulus of over 1000% are possible when the pneumatic artificial muscles are pressurized to 1992 kPa. This concept provides a way to create a structural unit cell whose in-plane modulus can be tuned based on the orientation of pneumatic artificial muscles within the cell and the pressure supplied to the individual muscles.

Variable modulus cellular structures using pneumatic artificial muscles

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.

A Novel Structural Element Combining Load Carrying and Energy Dissipation Capability

This paper reports on the design, fabrication, testing and simulation of a novel structural element that combines load carrying and energy dissipation capability. The principal components comprise post-buckling elements (PBEs), and von-Mises trusses (VMTs) coupled to a viscous dashpot, all integrated in a compact panel-like element. Load carrying capability of the unit comes from the PBEs which provide a high initial stiffness and very little deformation up to the critical buckling load. Energy dissipation is obtained through the deformation of VMTs at the top and bottom of a hexagonal cell, connected to the ends of the viscous dashpot. Under harmonic excitation the VMTs undergo large displacement, stroking the damper in the process. The paper explains the design procedure of the structural element in detail, and describes a prototype which is fabricated and tested under harmonic excitation. Under harmonic displacement input the energy dissipated increased over the frequency range (with loss factor increasing from 0.84 at 0.1 Hz, to 2.69 at 4 Hz). Although simulation predicted both top and bottom VMTs moving simultaneously while the tests showed one transitioning before the other, the experimental and simulation hysteresis loop areas compared well. Under harmonic force input, data was obtained only at 0.1 Hz and 0.5 Hz, as the testing machine could not move at the high-speeds associated with VMT snap-through. The energy dissipated (hysteresis loop areas) in these tests was slightly lower than that in the displacement controlled tests at the same frequencies, with loss factors of 0.61 at 0.1 Hz and 0.97 at 0.5 Hz calculated from the measured hysteresis cycles. The experimental and simulation work done to date establish the basis feasibility of developing novel structural designs which can carry high static load and also dissipate energy from undesirable dynamic inputs. Such designs have broad potential applications in aerospace, marine and ground structures.

A Three-Dimensional Multi-Stable Unit Cell for Energy Dissipation

This paper presents a three-dimensional version of a planar hexagonal cell whose top and bottom inclined members act as a three-dimensional von-Mises truss (VMT), displaying negative stiffness or snap-through behavior. A damper connected between the top and bottom vertices of the 3-D cell facilitates large energy dissipation under harmonic excitation. Tests conducted under a harmonic displacement input showed the top and bottom units transitioning sequentially through their negative stiffness regions, and Simscape simulations showed excellent correlation with tests at 0.5, 1, and 2 Hz. At the lower frequencies, the effect of the 3-D VMTs at the top and the bottom of the cell was evident in the hysteresis loops, but at the higher frequency, the damper was the dominating influence. A special test designed to capture snap-through allowed the rapid motion of the vertex of the three-dimensional cell at velocities greater than the capability of the Instron machine. The occurrence of snap-through was observed in both test and simulation, and results showed that the hysteresis loop area was larger by about 25%, compared to the case of a harmonic displacement input at the cell vertex which does not produce snap-through. Thus, snap-through increases the energy dissipation capability of the 3-D cell.

Stiffness Control with Pneumatic Artificial Muscle Inclusions in a Cellular Honeycomb Unit

Pneumatic artificial muscles are potentially useful actuators for the development of tunable modulus structures. This paper details the simulation and experimental validation of a unit Delrin hexagonal cell with three pneumatic artificial muscle inclusions for the purpose of actively controlling the modulus of the cell in the horizontal or vertical directions. The complete set of analytical equations used to simulate the cell is presented, including the PAM force equations, the cell wall modulus expressions, the force balance for the fully-assembled cell in the horizontal and vertical directions, and the secant modulus method. Then, simulation is compared to experimental measurements of the cell modulus in the horizontal and vertical directions over a range of pressures up to 1302 kPa. It is shown that simulation captures the trends evident in the experimental results and that the maximum increase in cell modulus is achieved when only the set of muscles perpendicular to the direction of loading is fully pressurized. A maximum increase in cell modulus of 227% is demonstrated in the horizontal direction with a corresponding change in equilibrium cell angle of only 3.75%.

A Load-Bearing Structural Element With Energy Dissipation Capability Under Harmonic Excitation

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.Copyright