Nilesh D. Mankame

Shape memory alloy cables

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.

General Motors and the University of Michigan smart materials and structures collaborative research laboratory

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

Analysis of the Hex Cell Discretization for Topology Synthesis of Compliant Mechanisms

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 GMs Global Research Network such as HRL Laboratories in California and GMs India Science Lab in Bangalore, India. This paper provides an overview of this new CRL and gives examples of several of the projects underway.

Design for manufacture of optimal compliant topologies with honeycomb continuum representation

We use non-linear finite element simulations to study the convergence behavior of the honeycomb or hex cell design discretization for optimization-based synthesis of compliant mechanisms in this paper. Adjacent elements share exactly one common edge in the hex cell discretization, unlike the square cell discretization in which adjacent elements can be connected by a single node. As the single node connections in bilinear quadrilateral plane stress elements allow strain-free relative rotations, compliant mechanism designs obtained from square cell discretizations with these elements often contain elements with single node connections or point flexures. Point flexures are sites of lumped compliance, and as such, are undesirable as they lead to compliant mechanisms designs which deviate from the ideal of distributed compliance. The hex cell design discretization circumvents the problem of point flexures without any additional computational expense (e.g. filtering, extra constraints, etc.) by exploiting the geometry of the discretization. In this work we compare the elastic response of a group of four cells in which two adjacent cells have the least connectivity in both: the square and the hex discretizations. Simulations show that the hex cell discretization leads to a more accurate modeling of the displacement, stress and strain energy fields in the vicinity of the least connectivity regions than the square cell discretization. Therefore, the hex cell discretization does not suffer from stress singularities that plague the square cell discretization. These properties ensure that continuous optimization-based compliant mechanism synthesis procedures that use the hex cell discretization, exhibit a faster and more stable convergence to designs that can be readily manufactured than those that use the square cell discretization.Copyright

A Compliant Transmission Mechanism With Intermittent Contacts for Cycle-Doubling

We present and compare two approaches to obtain optimal binary designs of compliant mechanisms that can be manufactured as is. Honeycomb representation is used to avoid singularities due to checkerboard and point flexure pathologies that appear in square cell based discretization of the domain. A Hex cell accurately models displacement and stress fields especially when approaching its non-existing state which is contrary to its square cell counterpart. In the first approach, sigmoid material interpolation function is employed with a gradient based strategy that poses topology design as a limiting case of size optimization. Though this technique shows promise in obtaining binary designs, the latter cannot always be guaranteed. The other approach based on stochastic search decouples topology optimization from size optimization and uses a novel material mask overlay strategy that ensures optimal binary design and can be manufactured without any post processing.

On Optimal Design of Compliant Mechanisms for Specified Nonlinear Path Using Curved Frame Elements and Genetic Algorithm

A novel compliant transmission mechanism that doubles the frequency of a cyclic input is presented in this paper. The compliant cycle-doubler is a contact-aided compliant mechanism that uses intermittent contact between itself and a rigid surface. The conceptual design for the cycle-doubler was obtained using topology optimization in our earlier work. In this paper, a detailed design procedure is presented for developing the topology solution into a functional prototype. The conceptual design obtained from the topology solution did not account for the effects of large displacements, friction, and manufacturing-induced features such as fillet radii. Detailed nonlinear finite element analyses and experimental results from quasi-static tests on a macro-scale prototype are used in this paper to understand the influence of the above factors and to guide the design of the functional prototype. Although the conceptual design is based on the assumption of quasi-static operation, the modified design is shown to work well in a dynamic setting for low operating frequencies via finite element simulations. The cycle-doubler design is a monolithic elastic body that can be manufactured from a variety of materials and over a range of length scales. This makes the design scalable and thus adaptable to a wide range of operating frequencies. Explicit dynamic nonlinear finite element simulations are used to verify the functionality of the design at two different length scales: macro (device footprint of a square of 170 mm side) at an input frequency of 7.8 Hz; and meso (device footprint of a square of 3.78 mm side) at an input frequency of 1 kHz.

Shape Memory Polymer based Cellular Materials

This paper discusses topology, shape and size optimization of fully compliant mechanisms for path generation applications using curved frame elements and genetic algorithm. The topology optimization problem is treated as a discrete ‘0-1’ problem wherein the elastic modulus is chosen as 0 or some pre-specified value, and no intermediate value in between. As the Young’s moduli are discrete topology design variables, function based genetic algorithm is employed for optimization. The size optimization variables are the lengths, in-plane widths and out-of-plane thicknesses of frame elements. Shape optimization is performed using the end slopes. Kirchhoff’s shallow arch beam theory is employed along with co-rotational geometrically nonlinear formulation. Synthesis examples are presented to demonstrate the applicability of min-max criterion proposed to achieve a curved path specified using precision points.Copyright

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

We propose the concept of periodic cellular materials with programmable effective properties and present initial results from a computational study of a prototypical material that exhibits this behavior. Nonlinear FEA shows that programmed geometric imperfections at the cell level can be used to modify the effective compressive storage modulus of shape memory polymer (SMP) based periodic cellular materials after they have been manufactured. The ability of SMPs to freeze a temporary deformation for an extended period of time and the low modulus of these materials in the rubbery regime allow us to freeze controlled and reversible imperfections at the cell level following the typical temporary shape programming process for SMPs. Small geometric imperfections (2% global strain) are observed to produce variations of up to 40% in the effective initial compressive storage modulus in the prototypical material.

Shape Memory Polymer Based Reconfigurable Compliant Mechanisms: An Exploration

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.

Reconfigurable fixture device and methods of use

This paper explores the concept of reconfigurable compliant mechanisms. We define these to be fully or partially compliant mechanisms whose performance can be modified after they have been fabricated. Specifically, we are interested in the nature and extent of in situ reconfigurability in compliant mechanisms. In other words, we seek to understand the range of performance that can be achieved by these mechanisms without requiring significant reassembly. The material properties such as the storage modulus of a newly studied class of materials — shape memory polymers — vary by over an order of magnitude over a temperature range of 20 – 50 C. These polymers also allow the fixing of moderate to large strains (20 – 75%) experienced at high temperatures for extended periods of time, while retaining the ability to remember their original shape when reheated to the same high temperatures. These two properties make shape memory polymers a natural candidate for the fabrication of reconfigurable compliant mechanisms. We explore various means for introducing reconfigurability in compliant mechanisms, and from these, select a subset that is suitable for in situ reconfiguration. Quasi-static nonlinear finite element simulations are used to study the change in performance due to reconfiguration of four fully compliant mechanisms made of a shape memory polymer. Preliminary results indicate that noticeable qualitative and quantitative changes in performance can be achieved by these mechanisms.Copyright