Development and Characterization of Embedded Sensory Particles Using Multi-Scale 3D Digital Image Correlation

A method for detecting fatigue cracks has been explored at NASA Langley Research Center. Microscopic NiTi shape memory alloy (sensory) particles were embedded in a 7050 aluminum alloy matrix to detect the presence of fatigue cracks. Cracks exhibit an elevated stress field near their tip inducing a martensitic phase transformation in nearby sensory particles. Detectable levels of acoustic energy are emitted upon particle phase transformation such that the existence and location of fatigue cracks can be detected. To test this concept, a fatigue crack was grown in a mode-I single-edge notch fatigue crack growth specimen containing sensory particles. As the crack approached the sensory particles, measurements of particle strain, matrix-particle debonding, and phase transformation behavior of the sensory particles were performed. Full-field deformation measurements were performed using a novel multi-scale optical 3D digital image correlation (DIC) system. This information will be used in a finite element-based study to determine optimal sensory material behavior and density.

Micro-Encapsulation and Tuning of Biomolecular Unit Cell Networks

The goal of our research is to fabricate an autonomic material system that provides compartmentalization and multi-bilayer networks for enabling collective biomolecular functionality, as is found in living cells and tissues. The material system is based on biomolecular unit cells, which consist of synthetic lipid bilayers formed at the interfaces of lipid-coated aqueous droplets submerged in oil and contained in a solid material. This paper focuses on microfluidic encapsulation of unit cells within a solid material and tuning the amount of contact between droplets, two approaches aimed at increasing the functional density of the droplet-based material system. Hydrodynamic traps within microfluidic platforms have shown to be a promising method to capture single droplets within microfluidic devices. Herein, we develop a resistive flow model to design hydrodynamic traps for collecting pairs of droplets in a direct trapping mode to form unit cells. We also compare to the model the results of droplet trapping in a prototype microfluidic device fabricated prior to model development. In addition to flow techniques for assembling unit cells in solid materials, we examine the use of mineral oil as the hydrophobic oil phase that surrounds the droplets to increase the area of the lipid membrane formed between neighboring droplets. Compared to hexadecane, mineral oil produces larger contact areas between droplets and more-tightly packed multi-bilayer networks. The total free energies of formation for droplet arrays in mineral oil and hexadecane indicate that connected droplets in mineral oil exhibit a greater decrease in free energy upon formation (i.e. they exist at a lower energy state compared to those in hexadecane) and that hexagonal packing provides the maximum amount of decrease in free energy per droplet for droplets in large arrays. Electrical measurements of unit cells formed in mineral oil initially show gigaohm resistances typical of unit cells, however these unit cells exhibit increasing values of conductance as the bilayer areas grow.

A Belleville-Spring Based Piezoelectric or Electromagnetic Energy Harvester

Energy harvesting from kinetic ambient energy requires converters able to efficiently operate in the low frequency range. A limit of the solutions proposed in the literature, both electromagnetic and piezoelectric, is their operating frequency, which generally ranges from about 50 to 300 Hz. To overcome these limitations, this work proposes an innovative energy harvester exploiting two counteracting Belleville springs. Thanks to the peculiar height to thickness ratio of the springs a highly compliant elastic system is obtained, which can be used either for electromagnetic or piezoelectric harvesting. The harvester is modelled analytically and numerically both with regard to the force-displacement and to the modal response. The experimental validation of the harvester, highlights a noticeable power output but at a higher eigenfrequency than expected.

The Influence of Thermo-Oxidative Degradation on the Behavior of Epoxy Shape Memory Polymers

Results are reported from an ongoing experimental investigation of the effects of thermo-oxidative aging on the mechanical behavior of an epoxy shape memory polymer (SMP). Chemo-rheological degradation due to macromolecular scission and cross-linking is one of the main factors contributing to the chemical aging of thermo-responsive SMPs. This aging may manifest as residual strain or irreversible material property changes, which can affect the performance and limit the useful life of a SMP. A relatively new epoxy SMP based on the diglycidyl ether of bisphenol A is synthesized, and specimens are tested under uni-axial tension using a dynamic mechanical analyzer. Fundamental viscoelastic behavior and thermal expansion coefficients are first characterized, showing a glass transition near 60 °C. Shape memory cycle experiments are performed at shape fixing temperatures of 80, 125, 150 and 175 °C, and the effect of fixing time at each temperature is examined upon subsequent strain recovery at 80 °C. Performance parameters such as recovery ratio, speed of recovery and residual strain are quantified as a function of shape fixing time and temperature. No effect of chemical aging was seen at a fixing temperature of 80 °C, although the recovery ratio decreases initially with increasing fixing time and stabilizes near 92 %. Only minor effects of chemical aging are seen in the mechanical responses for fixing temperatures of 125 and 150 °C, but specimens exhibit progressively more noticeable color changes that indicate oxidation. Significant effects are observed at the highest fixing temperature of 175 °C, where chemical aging at longer fixing times results in a reduction in recovery rate across the rubber-glass transition temperature, progressively larger residual strains, lack of complete strain recovery at 80 °C, and higher temperatures to achieve 90 % strain recovery.

Concurrent Vibration Suppression and Energy Harvesting Using Ferrofluids: An Experimental Investigation

This paper presents an experimental study which examines the design parameters affecting the performance characteristics of a Tuned Magnetic Fluid Damper (TMFD) device designed to concurrently mitigate structural vibrations and harvest vibratory energy. The device which is mounted on a vibrating structure, consists of a rectangular container carrying a magnetized ferrofluid and a pick-up coil wound around the container to enable energy harvesting. Experiments are performed to investigate the three-way interaction between the vibrations of the structure, the sloshing of the fluid, and the harvesting circuit dynamics. In particular, the tuning and optimization is examined for several design parameters including magnetic field spatial distribution and intensity, winding direction, winding location, winding density, and ferrofluid height inside the tank. The experimental response of the device is compared against the conventional TMFD at different excitation levels and frequencies. Results demonstrating the influence of the significant parameters on the relative performance are presented and discussed in terms of vibration suppression and power generation capabilities.

Experimental Validation and Numerical Simulation of Shape Memory Flat Wire Actuators

Shape memory alloys (SMA) are thermally activated smart materials. Due to their ability to change into a previously imprinted actual shape through the means of thermal activation, they are suitable as actuators for mechatronical systems. Despite of the advantages shape memory alloy actuators provide, these elements are only seldom integrated by engineers into mechatronical systems. Reasons are the complex characteristics, especially at different boundary conditions and the missing simulation- and design tools. Also the lack of knowledge and empirical data are a reason why development projects with shape memory actuators often lead to failures. This paper deals with the dynamic properties of SMA-actuators (Shape Memory Alloy) — characterized by their rate of heating and cooling procedures — that today can only be described insufficiently for different boundary conditions. Based on an analysis of energy fluxes into and out of the actuator, a numerical model of flat-wire used in a bow-like structure, implemented in MATLAB/SIMULINK, is presented. Different actuation parameters, depending on the actuator-geometry and temperature are considered in the simulation in real time. Additionally this publication sums up the needed empirical data (e.g. fatigue behavior) in order to validate the numerical two dimensional model and presents empirical data on SMA flat wire material.

A Multiple Degree-of-Freedom Velocity-Amplified Vibrational Energy Harvester: Part B — Modelling

Vibrational energy harvesting has become relevant as a power source for the reduced power requirement of electronics used in wireless sensor networks (WSNs). Vibrational energy harvesters (VEHs) are devices that can convert ambient kinetic energy into electrical energy using three principal transduction mechanisms: piezoelectric, electromagnetic and electrostatic. In this paper, a macroscopic two degree-of-freedom (2Dof) nonlinear energy harvester, which employs velocity amplification to enhance the power scavenged from ambient vibrations, is presented. Velocity amplification is achieved through sequential collisions between free-moving masses, and the final velocity is proportional to the mass ratio and the number of masses. Electromagnetic induction is chosen as the transduction mechanism because it can be readily implemented in a device which uses velocity amplification. The experimental results are presented in Part A of this paper, while in Part B three theoretical models are presented: (1) a coupled model where the two masses of the non-linear oscillator are considered as a coupled harmonic oscillators system; (2) an uncoupled model where the two masses are not linked and collisions between masses can occur; (3) a model that considers both the previous cases. The first two models act as necessary building blocks for the accurate development of the third model. This final model is essential for a better understanding of the dynamics of the 2-Dof device because it can represent the real behaviour of the system and captures the velocity amplification effect which is a key requirement of modelling device of interest in this work. Moreover, this model is essential for a future optimization of geometric and magnetic parameters in order to develop a MEMS scale multi-degree-of-freedom device.

Development of Active-Cells for Macroscopically Deformable Structures

In this paper we describe extensions and improvements upon prior work on “active cells” — small contractile electromechanical elements used in large numbers to create actuated composite structures. Each element (cell) consists of square fiberglass end-pieces encapsulating a bias spring within two telescoping tubes, actuated using two contractile shape memory coils, and occupying approx. 1cm3 when fully contracted. The end-pieces contain conductive interfaces to nearby cells, thus allowing channeling of power through a connected network of cells to provide actuation far from the source of electrical current. Prior work developed the conceptual structure of such a cell as well as preliminary prototypes. This paper describes the attachment of cells to each other and to rapid-prototyped cell interconnects — as well as improved fabrication techniques for the shape-memory coils — resulting in robust actuation for each cell, and the creation of considerably more complex chained and networked composite structures. A detailed exploration of appropriate interconnect mechanisms, powering schemes to provide network-level structural deformations, and examples of multi-cell structures are presented.

Smart Structure Integration Through Ultrasonic Additive Manufacturing

Ultrasonic additive manufacturing (UAM), a form of 3D printing based on ultrasonic metal welding, allows for room-temperature fabrication of adaptive structures with seamlessly embedded sensors and actuators. UAM combines solid-state welding of metallic foils, automated additive foil layering, and CNC machining. The most recent UAM systems utilize 9 kW of ultrasonic power for improved build strength and quality over low power systems, leading to previously unfeasible smart structures. Current UAM efforts in this area are focused on embedding smart materials, fiber optics, and cooling channels into metallic matrices. Since UAM process temperatures do not exceed one half of the melting temperature of the matrix, various alloys such as NiTi and FeGa, and polymers such as PVDF, have been successfully embedded without degradation of the smart material or the matrix. This paper aims to demonstrate the benefits of UAM, with particular emphasis on smart components for vehicle design. Example concepts include stiffness-tunable structures, thermally invariant composites, and materials with embedded cooling channels.

Analysis of the Wrist-Wearable Energy Harvester With Vibrating Piezoelectric Multiple Bimorph Cantilevers Using FEM and Topology Optimization

A small wrist-watch-like wearable electric energy harvester which can extract electricity from swinging motions of people’s arms while walking has been developed newly. The harvester consists of multiple vibrating piezoelectric cantilevered thin beams attached to a round central hub structure radially with tip masses. The cantilevers are made of a polycarbonate substrate beam, PMN-PT piezoelectric material on its both sides, and a high density tungsten tip mass. The swinging of a human arm with the harvester causes the bending deformations in each blade while walking and then produces electricity from strains in two piezoelectric layers. The swinging motion was formulated mathematically and kinematically in terms of swinging angles, angular velocities and accelerations. Finite element analysis was used to model the cantilevered beams and calculate the voltage output. The optimum shape of piezoelectric layers were calculated on the basis of the topology optimization method specialized for piezoelectric materials.