Hilary Bart-Smith

Influence of imperfections on the performance of metal foam core sandwich panels

Sandwich panels and beams are used in bending and compression dominated components. The retention of their load capacity in the presence of imperfections is a central consideration. To address this issue, sandwich beams with metallic foam cores have been tested in four-point bending following the introduction of imperfections, created by impressing the face sheets. Limit load expressions for face yielding, core shear, and indentation failure have been developed and used to construct failure mechanism maps. From these maps, specimen designs were determined. Imperfections were introduced by indenting to varying penetrations. The indents were located on both the compressive and tensile side of bending configurations. Experimental measurements of the load/deflection response are obtained and compared with finite element results.

Batoid Fishes: Inspiration for the Next Generation of Underwater Robots

For millions of years, aquatic species have utilized the principles of unsteady hydrodynamics for propulsion and maneuvering. They have evolved high-endurance swimming that can outperform current underwater vehicle technology in the areas of stealth, maneuverability and control authority. Batoid fishes, including the manta ray, Manta birostris, the cownose ray, Rhinoptera bonasus, and the Atlantic stingray, Dasyatis sabina, have been identified as a high-performing species due to their ability to migrate long distances, maneuver in spaces the size of their tip-to-tip wing span, produce enough thrust to leap out of the water, populate many underwater regions, and attain sustained swimming speeds of 2.8 m/s with low flapping/ undulating frequencies. These characteristics make batoid fishes an ideal platform to emulate in the design of a bio-inspired autonomous underwater vehicle. The enlargedpectoral finsofeachrayundergoescomplexmotionsthatcouplespanwise curvature with a chordwise traveling wave to produce thrust and to maneuver. Researchersareinvestigatingtheseamazingspeciestounderstandthebiologicalprinciples for locomotion. The continuum of swimming motions—from undulatory to oscillatory—demonstrates the range of capabilities, environments, and behaviors exhibited by these fishes. Direct comparisons between observed swimming motions and the underlying cartilage structure of the pectoral fin have been made. A simple yet powerful analytical model to describe the swimming motions of batoid fishes has been developed and is being used to quantify their hydrodynamic performance. This model is also being used as the design target for artificial pectoral fin design. Various strategies have been employed to replicate pectoral fin motion. Active tensegrity structures, electro-active polymers, and fluid muscles are three structure/actuator approaches that have successfully demonstrated pectoral-finlike motions. This paper explores these recent studies to understand the relationship between form and swimming function of batoid fishes and describes attempts to emulate their abilities in the next generation of bio-inspired underwater vehicles.

The Analysis of Tensegrity Structures for the Design of a Morphing Wing

Current attempts to build fast, efficient, and maneuverable underwater vehicles have looked to nature for inspiration. However, they have all been based on traditional propulsive techniques, i.e., rotary motors. In the current study a promising and potentially revolutionary approach is taken that overcomes the limitations of these traditional methods-morphing structure concepts with integrated actuation and sensing. Inspiration for this work comes from the manta ray (Manta birostris) and other batoid fish. These creatures are highly maneuverable but are also able to cruise at high speeds over long distances. In this paper, the structural foundation for the biomimetic morphing wing is a tensegrity structure. A preliminary procedure is presented for developing morphing tensegrity structures that include actuating elements. To do this, the virtual work method has been modified to allow for individual actuation of struts and cables. The actuation response of tensegrity beams and plates are studied and results are presented. Specifically, global deflections resulting from actuation of specific elements have been calculated with or without external loads. Finally, a shape optimization analysis of different tensegrity structures to the biological displacement field will be presented.

Mitigation of tensile failure in released nanoporous metal microstructures via thermal treatment

The authors have developed a method to fabricate released microstructures of nanoporous Au (np-Au) by dealloying a Au–Ag alloy film patterned over a sacrificial Al layer. Doubly clamped bridges fail during dealloying owing to large tensile stresses induced during the dealloying process. Thermal treatments of released microstructures prior to dealloying generate sufficient compressive stress to induce plastic buckling. This buckling compensates the tensile stresses generated during the dealloying process, thus mitigating fracture of the np-Au.

Central Pattern Generator Control of a Tensegrity Swimmer

Rhythmic motion employed in animal locomotion is ultimately controlled by neuronal circuits known as central pattern generators (CPGs). It appears that these controllers produce efficient, oscillatory command signals by entraining to an efficient or economic gait via sensory feedback. This property is of great interest in the control of autonomous vehicles. The objective of this study is to experimentally validate synthesized CPG control of a tensegrity swimmer. The prestressed cables in a tensegrity structure provide a method of simultaneous actuation and sensing, analogous to the biological motor control mechanism of regulating muscle stiffness through motoneuron activation and sensing the resulting motion by stretch receptors. A three cell, class 2 tensegrity swimmer is designed and built, and open-loop control tests characterize its swimming performance. We then determine gaits for desired entrainment, and use a graphical design method to construct and test the closed-loop system. Lastly, we perform perturbed tests of the swimmer to illustrate the robustness of CPG control.

Bio-inspired robotic manta ray powered by ionic polymer–metal composite artificial muscles

The manta ray (Manta birostris) is the largest species of rays that demonstrates excellent swimming capabilities via large-amplitude flapping of its pectoral fins. In this article, we present a bio-inspired robotic manta ray using ionic polymer–metal composite (IPMC) as artificial muscles to mimic the swimming behavior of the manta ray. The robot utilizes two artificial pectoral fins to generate undulatory flapping motions, which produce thrust for the robot. Each pectoral fin consists of an IPMC muscle in the leading edge and a passive polydimethylsiloxane membrane in the trailing edge. When the IPMC is actuated, the passive polydimethylsiloxane membrane follows the bending of the leading edge with a phase delay, which leads to an undulatory flapping motion on the fin. Characterization of the pectoral fin has shown that the fin can generate flapping motions with up to 100% tip deflection and 40° twist angle. To test the free-swimming performance of the robot, a light and compact on-board control unit with a lithium ion polymer battery has been developed. The experimental results have shown that the robot can swim at 0.067 BL/s with portable power consumption of under 2.5 W.

Surface Diffusion and Dissolution Kinetics in the Electrolyte–Metal Interface

Modeling of dealloying has often used a local bond-breaking approach to define the energy barrier to simulate dissolution and surface diffusion. The energy barriers are tacitly assumed to be independent of the local solution chemistry at the metal/solution interface. In this work, an interaction energy parameter is added to the local bond-breaking model that accounts for the species-specific physics of the actual atom-water molecule, atom-ion interactions and allows complex atomistic behavior to be abstracted in the modeling of the diffusion and dissolution processes. Variations in the interactions of the electrolyte components with the metal atoms led to the prediction of different surface morphologies on a binary alloy sample surface that mirror the behaviors experimentally observed in dealloying experiments in Au-Cu alloys including the formation of Au-enriched surface islands at applied potentials below the critical potential and three-dimensional porosity at applied potentials above the critical potential. .

Bio-Inspired Robotic Cownose Ray Propelled by Electroactive Polymer Pectoral Fin

The cownose ray (Rhinoptera bonasus) demonstrates excellent swimming capabilities; generating highly efficient thrust via flapping of dorsally flattened pectoral fins. In this paper, we present a bio-inspired and free swimming robot that mimics the swimming behavior of the cownose ray. The robot has two artificial pectoral fins to generate thrust through a twisted flapping motion. Each artificial pectoral fin consists of one ionic polymer-metal composite (IPMC) as artificial muscle in the leading edge and a passive PDMS membrane in the trailing edge. By applying voltage signal to the IPMC, the passive PDMS membrane follows the bending of IPMC with a phase delay, which leads to a twist angle on the fin. The characterization results have shown that the pectoral fin was able to generate up to 40% tip deflection and 10° twist angle with less than 1 Watt power consumption. A bio-inspired rigid body was designed using Computerized Axial Tomography (CT Scan) data of the cownose ray body and printed using a 3-dimensional printer. A light and compact on-board control unit with a lithium ion polymer battery has been developed for the free swimming robot. Experimental results have shown that the robot swam at 0.034 BL/S.Copyright

Investigating the Thrust Production of a Myliobatoid-Inspired Oscillating Wing

Myliobatidae is a family of large pelagic rays including cownose, eagle and manta rays. They are extremely efficient swimmers, can cruise at high speeds and can perform turn-on-a-dime maneuvering, making these fishes excellent inspiration for an autonomous underwater vehicle. Myliobatoids have been studied extensively from a biological perspective; however the fluid mechanisms that produce thrust for their large-amplitude oscillatory-style pectoral fin flapping are unknown. An experimental robotic flapping wing has been developed that closely matches the camber and planform shapes of myliobatoids. The wing can produce significant spanwise curvature, phase delays down the span, and oscillating frequencies of up to 1 Hz, capturing the dominant kinematic modes of flapping for myliobatoids. This paper uses dye flow visualization to qualitatively characterize the fluid mechanisms at work during steady-state oscillation. It is shown that oscillatory swimming uses fundamentally different fluid mechanisms than undulatory swimming by the generation of leading-edge vortices. Lessons are distilled from studying the fluid dynamics of myliobatoids that can be applied to the design of biomimetic underwater vehicles using morphing wing technology.

Experimental Validation of Robust Resonance Entrainment for CPG-Controlled Tensegrity Structures

Rhythmic motion employed in animal locomotion is ultimately controlled by neuronal circuits known as central pattern generators (CPGs). It appears that these controllers produce efficient oscillatory command signals by entraining to a resonant gait via sensory feedback. This property is of great interest in the control of autonomous vehicles. In this paper, we experimentally validate synthesized CPG control of tensegrity structures. The prestressed cables in a tensegrity structure provide a method of simultaneous actuation and sensing, analogous to the biological motor control mechanism of regulating muscle stiffness through motoneuron activation and sensing the resulting motion by stretch receptors. A three-cell class-two tensegrity structure is designed, built, and modeled to predict the structures dynamic response. The models are experimentally validated using open-loop control tests. Next, a simple CPG, called a reciprocal inhibition oscillator (RIO), is designed and synthesized in real time. The RIOs outputs are used as actuation commands, while sensory signals from the tensegrity are fed back to the RIO. Multiple controller configurations are tested to validate an RIO design method developed and reported in a complementary study. Finally, the tensegrity dynamics are perturbed by altering the mass of the tensegrity, and the robustness of RIO control is demonstrated through its ability to entrain to the perturbed system.