We demonstrate a new approach to the design of a synthetic jellyfish that imitates the morphology and kinematics of the actual animal. Since jellyfish usually move at low speeds, the locomotion can be mimicked using shape memory alloy (SMA) springs as artificial muscles. Compared to previous attempts at biomimetic underwater robots, the current research aims to simplify the design, generate larger stroke, and lower the actuation cycle for propulsion. The robot consists of a soft silicone rubber disk with an embedded pre-stretched SMA spring along its circumference, which when heated contracts to initiate large shape changes in the structure. Our approach harnesses the buckling instability of the main body to create a relatively quick motion that produces a pulsed jet of water to generate thrust. The rubber disk is also equipped with several flaps that contribute to the swimming motion by displacing the surrounding water through a rowing-like mechanism. The influence of different operation parameters, including the amplitude of the input power and the actuation frequency, are investigated on the swimming motion and propulsive thrust.
Wire rope (or cables) are a fundamental structural element in many engineering applications. Recently, there has been growing interest in stranding NiTi wires into cables to scale up the adaptive properties of NiTi tension elements and to make use of the desirable properties of wire rope. Exploratory experiments were performed to study the actuation behavior of two NiTi shape memory alloy cables and straight monofilament wire of the same material. The specimens were held under various dead loads ranging from 50 MPa to 400 MPa and thermally cycled 25 times from 140°C to 5°C at a rate of 12°C/min. Performance metrics of actuation stroke, residual strain, and work output were measured and compared between specimen types. The 7x7 cable exhibited similar actuation to the single straight wire, but with slightly longer stroke and marginally more shakedown, while maintaining equivalent specific work output. This leads to the conclusion that the 7x7 cable effectively scaled up the adaptive properties the straight wire. Under loads below 150 MPa, the 1x27 cable had up to double the actuation stroke and work output, but exhibited larger shakedown and poorer performance when loaded higher.
In this paper, experimental results are reported to study the influence of high-temperature aging on the thermo-mechanical behavior of a commercially-available, thermo-responsive shape memory polymer (SMP), Veri ex-E™ (glass transition temperature, Tg = 90−105 °C). To achieve a shape memory effect in high Tg SMPs such as this, high temperature cycles are required that can result in macromolecular scission and/or crosslinking, which we term thermo-mechanical aging (or chemo-rheological degradation). This process results in mechanical property changes and possible permanent set of the material that can limit the useful life of SMPs in practice. We compare experimental results of shape memory recovery with and without aging. Similar to the approach originated by Tobolsky in the 1950's, a combination of uniaxial constant stress and intermittent stretch experiments are also used in high temperature creep-recovery experiments to deduce the kinetics of scission of the original macromolecular network and the generation of newly formed networks having different reference configurations. The macroscopic effects of thermo-mechanical aging, in terms of the evolution of residual strains and change in elastic response, are quantified.
An experimental characterization is presented of the thermo-mechanical response of honeycombs and corrugations made of a NiTi shape memory alloy (SMA). Of particular interest are the shape memory cycle, the superelastic response, the shape memory thermal lag and the superelastic rate sensitivity. A series of in-plane compression experiments are presented on fabricated honeycombs and their responses are compared to typical monolithic SMAs, such as NiTi wire. Given local material strain limits, NiTi honeycombs exhibit an order of magnitude increase in recoverable deformation, both in the shape memory effect and superelastic effect. This comes at the cost of a reduced load carrying capacity by two orders of magnitude and a reduced (homogenized) compressive stiffness by four orders of magnitude. Due to their sparse structure and enhanced heat transfer characteristics, SMA honeycombs exhibit less superelastic rate sensitivity by two orders of magnitude while having similar thermal lag to SMA wire. The implications of these scaling results are discussed, including possible new regimes of application of SMAs for reusable energy absorption devices and high stroke actuators.
Over 60% of energy that is generated is lost as waste heat with close to 90% of this waste heat being classified as
low grade being at temperatures less than 200°C. Many technologies such as thermoelectrics have been proposed as
means for harvesting this lost thermal energy. Among them, that of SMA (shape memory alloy) heat engines appears
to be a strong candidate for converting this low grade thermal output to useful mechanical work. Unfortunately,
though proposed initially in the late 60's and the subject of significant development work in the 70's, significant
technical roadblocks have existed preventing this technology from moving from a scientific curiosity to a practical
reality. This paper/presentation provides an overview of the work performed on SMA heat engines under the US DOE
(Department of Energy) ARPA-E (Advanced Research Projects Agency - Energy) initiative. It begins with a review
of the previous art, covers the identified technical roadblocks to past advancement, presents the solution path taken to
remove these roadblocks, and describes significant breakthroughs during the project. The presentation concludes with
details of the functioning prototypes developed, which, being able to operate in air as well as fluids, dramatically
expand the operational envelop and make significant strides towards the ultimate goal of commercial viability.
A series of experiments is presented examining the thermo-electro-mechanical response of commercially-available, conditioned,
shape memory alloy (SMA) wires (Flexinol, from Dynalloy, Corp.) during cyclic thermomechanical loading. A
specialized experimental setup enables temperature control via a thermoelectric/heatsink in thermal contact with the wire
specimen during various modes of testing. It allows simultaneous measurement of elongation, load, strain and resistivity in
a selected gage length. It also allows full-field optical and infrared imaging to be performed during testing. A moderately
high transition temperature NiTi-based shape memory wire (90C Flexinol) is characterized first by differential scanning
calorimetry and a series of isothermal experiments over a range of temperatures. Subsequent experiments examine the
shakedown behavior over a range of dead loading temperature cycles. Results show a significant two-way shape memory
effect, suggesting that both residual stresses and locked-in oriented Martensite are considerable in this commercial alloy.
Repeatable behavior (little shakedown) is confirmed at relatively low stress levels, but significant evolution in the response
(shakedown behavior) exists at higher stress levels during the first several temperature cycles.
A commonly noted disadvantage of shape memory alloys is their frequency response which is limited by how fast
the material can be cooled. This paper presents a feasibility study of using vertically aligned carbon nanotubes (CNT) as
microscopic cooling fins to improve convective heat transfer. Using DC plasma enhanced chemical vapor deposition
(PECVD), aligned CNT's were successfully grown directly on ½ of the surface of a 0.38 mm diameter SMA wire,
achieving desirable thermal contact. Cooling speeds were measured with a thermal imaging camera, and the effective
convective coefficient was extracted from the temperature profiles using a basic cooling model of the wire. From this
model, the effective convective coefficient was estimated to have increased by 24% (from 50 W/m2K for untreated SMA
wire to 62 W/m2K for the nanotube treated wire), indicating that the deposition of CNT's indeed increased performance.
By extrapolating these results to full wire coverage, up to a 46% improvement in frequency response with zero weight or
volumetric penalties is predicted. Further improvements in cooling performance are likely to occur with higher CNT
densities and longer nanotube lengths, allowing further developments of this technology to benefit many future
applications utilizing high-speed miniature/micro-scale SMA actuators.
Conventional structural cables (or wire ropes) are composed of steel wires helically wound into strands, which, in turn,
are wound around a core. Cables made from shape memory alloy (SMA) wires are a new structural element with promising
properties for a broad range of new applications. Among the many potential advantages of this form are increased
bending flexibility for spooling/packaging, better fatigue performance, energy absorption and damping, reduced thermal
lag, redundancy, and signicant design flexibility. Currently there are no known studies of SMA cables in the literature,
so exploratory thermo-mechanical experiments were performed on two commercially available cable designs as part of an
ongoing research program to systematically characterize their thermomechanical behavior and demonstrate their potential
utility as adaptive or resilient tension elements.
The field of Smart Materials and Structures is evolving from high-end, one-of-a-kind products for medical, military and
aerospace applications to the point of viability for mainstream affordable high volume products for automotive
applications. For the automotive industry, there are significant potential benefits to be realized including reduction in
vehicle mass, added functionality and design flexibility and decrease in component size and cost. To further accelerate
the path from basic research and development to launched competitive products, General Motors (GM) has teamed with
the College of Engineering at the University of Michigan (UM) to establish a $2.9 Million Collaborative Research
Laboratory (CRL) in Smart Materials and Structures. Researchers at both GM and UM are working closely together to
create leap-frog technologies which start at conceptualization and proceed all the way through demonstration and
handoff to product teams, thereby bridging the traditional technology gap between industry and academia. In addition to
Smart Device Technology Innovation, other thrust areas in the CRL include Smart Material Maturity with a basic
research focus on overcoming material issues that form roadblocks to commercialism and Mechamatronic System
Design Methodology with an applied focus on development tools (synthesis and analysis) to aid the engineer in
application of smart materials to system engineering. This CRL is a global effort with partners across the nation and
world from GM's Global Research Network such as HRL Laboratories in California and GM's India Science Lab in
Bangalore, India. This paper provides an overview of this new CRL and gives examples of several of the projects underway.
Understanding thermoelastic martensitic transformations is a fundamental component in the study of shape memory alloys. These transformations involve a hysteretic change in stability of the crystal lattice between an austenite (high symmetry) phase and a martensite (low symmetry) phase within a small temperature range. In previous work, a continuum energy density W(U;θ) (as a function of the right stretch tensor U and temperature θ) for a perfect bi-atomic crystal was derived based on temperature-dependent atomic pair-potentials. For this model, only high symmetry cubic configurations were found to be stable (local energy minimizers).
The present work derives an energy density W(U,P(1),P(2),...;θ) that explicitly accounts for a set of internal atomic shifts P(i). In addition, the model permits the calculation of the crystal's dispersion relations which determine the stability of the crystal with respect to bounded perturbations of all wavelengths (Bloch-waves). Using a specific model of a bi-atomic crystal with the temperature serving as the loading parameter, a stress-free bifurcation diagram is generated. Stable equilibrium branches corresponding to the B2 (cubic) and B19 (orthorhombic) crystal structures are found to exist and overlap for certain temperatures. The group-subgroup relationship between these two crystal structures is necessary for the shape memory effect. Thus, our results are consistent with the transformations that occur in shape memory alloys such as AuCd and NiTi.
A thermomechanical model for a shape memory alloy (SMA) wire under uniaxial loading is implemented in a finite element framework, and its results are compared with new experimental data. The constitutive model is a one-dimensional continuum model of an SMA element, including two internal field variables, strain gradient effects, possible unstable mechanical behavior, and the relevant thermomechanical couplings resulting from latent heat effects. The model is calibrated to recent experiments of typical commercially available polycrystalline NiTi wire. The shape memory effect and pseudoelastic behaviors are demonstrated numerically as a function of applied loading rate and environmental parameters, and the results are found to be quite similar to experimental data. The model is then used to simulate a simple SMA actuator device, and the model proves to be a useful tool to assess the performance.
The evolution of inhomogeneous deformation in a NiTi shape memory alloy under uniaxial tension is studied both experimentally and analytically. Interesting features of stress-induced phase transformation fronts are observed in experiments of NiTi strips. The nucleation of a new phase occurs as a sharp band of localized strain inclined at 55° to the axis of loading. This angle agrees with a rather well-known hand calculation based on continuum plasticity. Under prescribed end displacement the new phase then spreads either by steady-state propagation of angled transition fronts or by an alternating pattern of finger-like features. These features are successfully captured by finite element analyses with a special trilinear stress-strain model having an intermediate unstable branch. It is a relatively simple continuum-based approach which captures the interplay of the material instability and more global structural constraints.
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