Christopher B. Churchill

Shakedown response of conditioned shape memory alloy wire

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 reduced-order thermomechanical model and analytical solution for uniaxial shape memory alloy wire actuators

A lumped shape memory alloy (SMA) model is derived from the thermodynamic model of Chang et al (2006 Contin. Mech. Thermodyn. 18 83–118), using a set of simplifying assumptions, that reduces the system of partial differential equations for an SMA/bias spring actuator to a nonlinear, first-order, ordinary differential equation. Dimensionless state variables and parameters are defined that are useful for characterizing the actuator system and for studying its performance and scaling. A general analytical solution to the nonlinear differential equation governing phase transformation is found in terms of the Lambert function for a piecewise constant Joule heating input and a constant temperature convective environment. The analytical solution provides a useful and convenient tool for assessing the time-dependent, hysteretic response of this simple class of SMA actuators, with which design and optimization studies are performed.

Dynamically variable negative stiffness structures

A novel active structure supports loads while dynamically and continuously changing stiffness by more than 100× in less than 10 ms. Variable stiffness structures that enable a wide range of efficient load-bearing and dexterous activity are ubiquitous in mammalian musculoskeletal systems but are rare in engineered systems because of their complexity, power, and cost. We present a new negative stiffness–based load-bearing structure with dynamically tunable stiffness. Negative stiffness, traditionally used to achieve novel response from passive structures, is a powerful tool to achieve dynamic stiffness changes when configured with an active component. Using relatively simple hardware and low-power, low-frequency actuation, we show an assembly capable of fast (<10 ms) and useful (>100×) dynamic stiffness control. This approach mitigates limitations of conventional tunable stiffness structures that exhibit either small (<30%) stiffness change, high friction, poor load/torque transmission at low stiffness, or high power active control at the frequencies of interest. We experimentally demonstrate actively tunable vibration isolation and stiffness tuning independent of supported loads, enhancing applications such as humanoid robotic limbs and lightweight adaptive vibration isolators.

Development of a Shape Memory Alloy Heat Engine Through Experiment and Modeling

Few technologies can produce meaningful power from low temperature waste heat sources below 250°C, particularly on a per-mass basis. Since the 1970’s energy crisis, NiTi shape memory alloy (SMA) and associated thermal engines have been considered a viable heat-to-power transducer but were not adopted due to previously poor material quality, low supply, design complexity, and cost. Decades of subsequent material development, research, and commercialization have resulted in the availability of consistently high-quality, well-characterized, low cost alloys and a renewed interest in SMA as a waste heat energy recovery technology. The Lightweight Thermal Energy Recovery System (LighTERS) is an ongoing ARPA-E funded collaboration between General Motors Company, HRL Laboratories, Dynalloy, Inc., and the University of Michigan. In this paper we will present initial results from investigations of a closed loop SMA thermal engine (a refinement of the Dr. Johnson design) using a helical coil element and forced-air heat exchange. This engine generates mechanical power by continuously pulling itself through separate hot and cold air streams using the shape memory phase transformation to alternately expand and contract at frequencies between 0.25 and 2 Hz. This work cycle occurs continuously along the length of the coil loop and produces steady state power against an external moment. We present engine features and the thermal envelope that resulted in devices achieving between 0.1 and 0.5 W/g of shape memory alloy material using only forced air heat exchangers and room temperature cooling.Copyright

Thermo-electro-mechanical shakedown response of conditioned shape memory alloy wires

The shakedown response of conditioned shape memory alloy wires (Flexinol® ) is examined experimentally during constant tension thermal cycles, at several load levels. Strain, temperature, and electrical resistivity are measured simultaneously using a specialized experimental setup that enables a relatively rapid temperature rate (1 °C/s) while preserving the temperature uniformity along the gauge length to less than 1.5 °C. Both elongation and electrical resistance are measured from the same local gauge length, allowing strain-corrected electrical resistivity to be inferred. The most repeatable behavior (least shakedown) occurs at the intermediate load of 191 MPa (consistent with the supplier’s maximum stress recommendation), with a small amount of shakedown (and some loss of two-way shape memory) at lower loads and progressively larger shakedown (strain ratcheting and reduction in hysteresis) at higher loads.Copyright

Thermo-mechanical modeling of a shape memory alloy heat engine

Two thirds of the energy generated in the United States is currently lost as waste heat, representing a potentially vast source of green energy. Low Carnot efficiency is an inherent limitation of extracting energy from low-grade thermal sources (temperature gradients near or below 100C), and SMA heat engines could be useful for those applications where low weight and packaging are overriding considerations. Although many shape memory alloy (SMA) heat engines have been proposed to harvest this energy, and a few have been built and demonstrated in past decades, they have not been commercially successful. Some of the barriers to commercialization include their perceived low thermodynamic efficiency, high material cost, low material durability, complexities when using fluid baths, and the lack of robust constitutive models and design tools. Recent advances, however, in SMA longevity, reductions in materials costs (as production volumes have increased), and a better understanding of SMA behavior have stimulated new research on SMA heat engines. The L ightweight T hermal E nergy R ecovery S ystem (LighTERS) is an ongoing ARPA-E funded collaboration between General Motors, HRL Laboratories, Dynalloy, Inc., and the University of Michigan. In the LighTERS engine (a refinement of the Dr. Johnson engine), a closed loop SMA spring element generates mechanical power by pulling itself between alternating hot and cold air regions. The first known thermo-mechanical model for this type of heat engine was developed in three stages. First, the constitutive and heat transfer relationships of an SMA spring form were characterized experimentally. Second, those relationships were used as inputs in a steady-state model of the heat engine, including both convective heat transfer and large-deformation mechanics. Finally, the model was validated successfully against measurements of a experimental heat engine built at HRL Labs.© 2011 ASME

Shape memory alloy honeycombs: experiments & simulation

Metallic foams and honeycombs, with their light-weight, high speciflc stifiness, and well-developed energy absorption characteristics, are of obvious utility in engineering applications. However, these structures, often made of aluminum, sufier permanent deformation after crushing. Cellular structures made from shape-memory alloys (SMAs) are particularly intriguing for their potential to deliver shape memory and/or superelasticity in a light-weight material. Realization of open-celled Nitinol has recently become possible via a (newly discovered by Profs. D. Grummon at Michigan State Univ. and J. Shaw at Univ. of Michigan) transient-liquid reactive brazing system for creating robust metallurgical Nitinol-Nitinol bonds. With this technique, prototype sparse cellular honeycomb structures have been made and tested, showing up to 50% repeatedly recoverable strains. Two difierent microgeometry NiTi honeycombs have been tested, a hexagonal for isothermal conditions and a corrugated for thermal cycling. Moreover, the isothermal experiments are simulated using a model that accounts for flnite rotations and hysteretic responce of the ligaments and geometric imperfections of the flnite size samples.

Impact of Length Mismatch on the Fatigue Life of Parallel Shape Memory Alloy Wires

Shape memory alloy (SMA) actuator wires promise substantial size, weight, and potential cost advantages over their solenoid and electric motor counterparts. Designing actuators which fully realize these advantages is hampered by a limited understanding of how interactions between wires and their environment affect cyclic lifetime. For example, many devices use two SMA wires mechanically in parallel but electrically in series. This has practical advantages in simplified electrical routing and thermal lag. However, it complicates modeling in that wires interact both electrically and mechanically.Here we study the effect of a mismatch in length between two SMA wires in parallel. We perform a series of fatigue experiments to show that mismatch of up to 0.75%, at the chosen set of conditions, has no measurable effect on cycle life. We also perform a series of simulations to show how the mechanical interaction between the two parallel wires tends to suppress, rather than amplify, any impact of length mismatch.Copyright

Sensing of retained martensite during thermal cycling of shape memory alloy wires via electrical resistance

Shape memory alloys (SMAs) remain one of the most commercially viable active materials, thanks to a high specific work and the wide availability of high quality material. Still, significant challenges remain in predicting the degradation of SMA actuators during thermal cycling. One challenges in both the motivation and verification of degradation models is the measurement of retained martensite fraction during cycling. Direct measurement via diffraction is difficult to perform in situ, impossible for thin wires, (< 0.5mm) and prohibitively difficult for lengthy studies. As an alternative, the temperature coefficient of electrical resistivity (TCR) is used as an indicator of martensite phase fraction during thermal cycling of SMA wires. We investigate this technique with an example cycling experiment, using the TCR to successfully measure a 20% increase in retained martensite fraction over 80000 thermal cycles. As SMA wire temperature is difficult to measure directly during resistive heating, we also introduce a method to infer temperature to within 5 °C by integrating the lumped heat equation.

Lightweight thermal energy recovery system based on shape memory alloys: a DOE ARPA-E initiative

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 60s and the subject of significant development work in the 70s, 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.