ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V001T00A001. doi:10.1115/SMASIS2017-NS1.

This online compilation of papers from the ASME 2017 Conference on Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Development and Characterization of Multifunctional Materials

2017;():V001T01A001. doi:10.1115/SMASIS2017-3732.

In this study, 3D printed magnetorheological (MR) elastomer has been characterized through a force vibration testing. The 3D printed MR elastomer is a composite consisting three different materials, magnetic particles and two different elastomers. The MR elastomers were printed layer-by-layer by encapsulating MR fluid within the polymeric elastomer and then allowed to cure at room temperature. The 3D printing allowed to print various patterns of magnetic particles within the elastomeric matrix. In the presence of an external magnetic field, both elastic and damping properties of the 3D printed MR elastomers were changed. Natural frequency, stiffness, damping ratio, damping coefficient, and shear modulus were increased with increasing magnetic field. For the single degree-of-freedoms system, shear mode MR elastomers suppressed the transmitted vibration amplitude up to 31.4% when the magnetic field was 550 mT. The results showed that the 3D printed MR elastomer could be used as a tunable spring element for vibration absorption or isolation applications. However, further optimization of the magnetic particles’ configurations should be performed to obtain the higher MR effect.

Commentary by Dr. Valentin Fuster
2017;():V001T01A002. doi:10.1115/SMASIS2017-3767.

To continue to meet spacecraft systems ever increasing thermal management requirements, new control methods need to be developed. While advances in metamaterials have provided the ability to generate materials with a broad range of material properties, relatively little advancement has been made in the development of adaptive metamaterials. This paper is focused on the development of a thermal management metamaterial that enables the active and passive control of a metamaterial’s thermal conductance. This variable conductivity is achieved through the application of internally or externally applied loads that induce internal contact resulting in changes in the conductive path length and the effective conductive area. This capability enables active or passive control of a metamaterial’s effective thermal conduction through the application of mechanical and thermal strain. Passively applied thermal strains can be used to design a highly nonlinear material thermal conductivity as a function of temperature. Actively, this can be used to precisely control the temperature of an interface through dynamically changing the instantaneous heat flux through the metamaterial. This work expands on the field of thermal switches by enabling a non-binary configuration where the initial air gap is slowly closed as contact sequentially introduced into the metamaterial. As internally or externally developed loading is applied, contact is introduced with an increasing contact area until full contact is achieved. This intermediate step of partial contact enables unique design capabilities that enable highly nonlinear thermal conductivity as a function of temperature as well as stability regions that allow passive thermal control. An example metamaterial was developed and evaluated to quantify the potential of this concept. The specific metamaterial configuration assessed in this paper uses offset flat and curved copper plates that are connected at the edges of the plate using a low conductivity epoxy. To evaluate the metamaterial performance, the stiffness and thermal conductivity are calculated as a function of the resulting contact area and the required applied loading. This work is focused on determining the potential of this metamaterial concept by evaluating this initial concept confirmation to establish the magnitude of the thermal conductance change, and the design of the conductivity change a function of applied loading.

Commentary by Dr. Valentin Fuster
2017;():V001T01A003. doi:10.1115/SMASIS2017-3801.

In this paper, we present characterization results for thermal, mechanical, and electrical properties of a 3D-printed conductive polylactic acid (PLA) composite material. The material exhibits electrically controllable stiffness, allowing for the fabrication of novel robotic and biomedical devices. In particular, an applied voltage induces a Joule heating effect, which modulates the material stiffness. Dumbbell samples are 3D-printed and loaded into a universal testing machine (UTM) to measure their Young’s moduli at different temperatures. The conductive PLA composite shows 98.6% reduction of Young’s modulus, from 1 GPa at room temperature to 13.6 MPa at 80 °C, which is fully recovered when cooled down to its initial temperature. Measurements with differential scanning calorimeter (DSC) and thermal diffusivity analyzer are conducted to investigate the thermal behavior of this material. Electrical conductivity of the material is measured under different temperatures, where the resistivity increases about 60% from 30 °C to 100 °C and hysteresis between the resistivity and the temperature is observed. These tests have shown that the conductive PLA composite has a glass transition temperature (Tg) of 56.7 °C, melting point (Tm) of 153.8 °C, and thermal conductivity of 0.366 W/(mK). The obtained results can be used as design parameters in finite element models and computational tools to rapidly simulate multi-material components for several applications such as object manipulation, grasping, and flow sensing.

Commentary by Dr. Valentin Fuster
2017;():V001T01A004. doi:10.1115/SMASIS2017-3807.

The sensitivity of piezoelectric/polymer composite materials is inversely proportional to their dielectric permittivity. Introducing a cellular structure into these composites can decrease the permittivity while enhancing their mechanical flexibility. Foaming of highly filled polymer composites is however challenging. Polymers filled with high content of dense additives such as lead zirconate titanate (PZT) exhibit significantly decreased physical foaming ability. This can be attributed to difficulty in gas diffusion, decreased fraction of the matrix available, the reduced number of nucleated cells and the difficulty in cell growth. Here, both CO2 foaming and Expancel foaming were examined as potential methods to fabricate low-density thermoplastic polyurethane (TPU)/ PZT composite foams. While composites containing up to only 10vol.% PZT could be foamed using CO2, Expancel foaming could successfully yield highly-expanded composite foams containing up to 40vol.% (80wt.%) PZT. Dispersed Expancel particles in TPU/PZT composites acted as the blowing agent, activated by subjecting the samples to high temperatures using a hot press. Using Expancel, foams with expansion ratios of up to 9 were achieved. However, expansion ratios of greater than 4 were not of interest due to their poor structural integrity. The density of solid samples ranged from 1.8 to 3.3 g.cm−3 and dropped by a maximum of 80%, even for the highest PZT content, at an expansion ratio of 4. As the expansion increased, the dielectric permittivity of both CO2-foamed and Expancel-foamed TPU/PZT composites decreased significantly (up to 7.5 times), while the dielectric loss and electrical conductivity were affected only slightly. This combination of properties is suitable for high-sensitivity and flexible piezoelectric applications.

Commentary by Dr. Valentin Fuster
2017;():V001T01A005. doi:10.1115/SMASIS2017-3824.

Smart materials are unique in their ability to change properties in response to an environmental stimulus. These materials provide promising opportunities for adaptable aerospace structures, where they can be altered to suit their need. In this research, Honeycomb Polymer Composites (HPCs) were investigated as potential materials for this need. HPCs are new materials that consist of a polymer embedded in a honeycomb structure, and exhibit a significantly higher stiffness than the polymer or honeycomb alone. This stiffness amplification is due to the nearly incompressible polymer resisting the volume change within the honeycomb cells. HPC samples were fabricated using an aramid honeycomb, with either silicone or urethane rubber as the matrix materials to fill the honeycomb. Varying polymer stiffness, honeycomb geometry, and testing temperature were all tested to observe the effects on the material properties. The results indicated that the HPCs could be effectively tailored and modeled to suit the need for different effective moduli. This research provides important insight and results in the development of programmable honeycomb polymer composites (PHPCs), which rely on shape memory polymers (SMP) as the internal working polymer.

Commentary by Dr. Valentin Fuster
2017;():V001T01A006. doi:10.1115/SMASIS2017-3850.

Active origami designs, which incorporate smart materials such as electroactive polymers (EAPs) and magnetoactive elastomers (MAEs) into mechanical structures, have shown good promise in engineering applications. In this study, finite element analysis (FEA) models are developed using COMSOL Multiphysics software for two configurations that incorporate a combination of active and passive material layers, namely: 1) a single-notch unimorph folding configuration actuated using only external electric field and 2) a bimorph configuration which is actuated using both electric and magnetic (i.e. multifield) stimuli. Constitutive relations are developed for both electrostrictive and magnetoactive materials to model the coupled behaviors directly. Shell elements are adopted for their capacity of modeling thin films, reduction of computational cost and ability to model the intrinsic coupled behaviors in the active materials under consideration. A microstructure-based constitutive model for electromechanical coupling is introduced to capture the nonlinearity of the EAP’s relaxor ferroelectric response; the electrostrictive coefficients are then used as input in the constitutive modeling of the coupled behavior. The magnetization of the MAE is measured by experiment and then used to calculate magnetic torque under specified external magnetic field. The objective of the study is to verify the effectiveness of the constitutive models to simulate multi-field coupled behaviors of the active origami configurations. Through quantitative comparisons, simulation results show good agreement with experimental data, which is a good validation of the shell models. By investigating the impact of material selection, location, and geometric parameters, FEA can be used in design, reducing trial-and-error iterations in experiments.

Commentary by Dr. Valentin Fuster
2017;():V001T01A007. doi:10.1115/SMASIS2017-3867.

Origami folding patterns are finding use in novel applications where actual device response depends on current, possibly intermediate, shapes on the path toward the final target shape. This works investigates one origami pattern, developing metrics for performance that incorporate traditional shape approximation and actuator efficiency, while adding proxy measures of adherence to the target folding path. Magnetically actuated Miura-Ori structures were develop using an initially heuristic strategy involving experiment, observation, and computation before being studied using trade space optimization/visualization. Constructed from PDMS substrates, notched to promote the crease pattern, and neodymium magnets, four initial configurations were chosen based on heuristic arguments that (1) maximized the amount of magnetic torque applied to the creases and (2) reduced the number of magnets needed to affect all creases in the pattern. Experiments were conducted, and calculations performed, on prototypes from each configuration to determine their degree of closure for a fixed maximum field strength, their ability to follow the ideal Miura-Ori folding pattern, and the amount of work theoretically performed by each magnet on each crease. Each configuration was further optimized theoretically using the Army Trade Space Visualization (ATSV) software. A final prototype was constructed following the weighted sum scoring of the four now optimized configurations. Somewhat surprisingly, trade space optimization showed that the configuration with the highest number of actuators was theoretically the least effective per magnet at delivering torque to each crease. Unsurprisingly, optimization was successful at increasing the amount of work theoretically apportioned to each crease. Overall, though the winning configuration experimentally outperformed its initial, non-optimal counterparts, results showed that the choice of optimum configuration was heavily dependent on the weighting factors within the objective function. These results highlight the ability of the Miura-Ori to be actuated with external magnetic stimuli, the effectiveness of a hybrid heuristic - trade space design approach that focuses on the actuation mechanism, and the need to address path-dependent metrics in assessing performance in origami folding structures.

Commentary by Dr. Valentin Fuster
2017;():V001T01A008. doi:10.1115/SMASIS2017-3868.

Magneto-Active elastomers (MAEs) and magneto-rheological elastomers (MREs) are smart materials that consist of hard and soft magnetic particles, respectively, embedded in a flexible matrix. Their actuation capabilities are dependent on the arrangement of particles achieved during the fabrication process. Previous works have shown varying degrees of particle alignment and / or agglomeration as a function of fabrication process variable, most notably volume fraction of the particulates, their magnetic material type (hard vs soft), and the strength of the external field applied during curing. In this work, we simulated the dynamics of magnetic particles suspended in a fluid matrix to predict the evolution of microstructures resulting from these varying process conditions. The simulations accounted for the magnetic interaction of all particles using standard dipole-dipole interaction potentials along with dipole-field potentials developed from the Zeeman Energy. Additionally, the field local to each particle, on which magnetization depends, was determined by the sum of the external fields generated by each member of the ensemble and their demagnetizing fields. Fluid drag forces and short range particle-particle repulsion (non-overlapping) were also considered. These interactions determined the body forces and torques acting on each particle that drove the system of equations of motions for the ensemble of particles. The simulation was carried out over a nearest neighbor periodic unit cell using an adaptive time stepping numerical integration scheme until an equilibrium structure was reached. Structural parameters, related to the magnetic energy, spatial distribution, spatial alignment, and orientation alignments of the particle distributions were defined to characterize the simulated structures. The effect of volume fraction and intensity of the external magnetic field on the achieved particle distributions were studied. At low external field strengths, the particles formed long entangled chains that had very low alignment with the applied field. The remnant magnetic potential energy of these configurations was also significantly low. As the field is increased the length of the chains reduced and the alignment increased. The corresponding change in magnetic potential energy of the system with an increase in the applied field was found to follow a power law fit that spanned a wide range of magnetic field strengths. At low volume fractions the particles aligned rapidly with the field and formed short chains. As the volume fraction of the samples increased the chains grew longer and closer to each other, and magnetic potential of the structure became lower. Results of the simulations suggest that it is possible to tailor the microstructure and thus affect remanent magnetization and magnetization anisotropy, by judicious control of process parameters. This ability could have implications for newly emerging additive manufacturing techniques utilizing suspensions of magnetic particulates.

Commentary by Dr. Valentin Fuster
2017;():V001T01A009. doi:10.1115/SMASIS2017-3869.

Distributing a carbon nanotube sensing network throughout the binder phase of energetic composites is investigated in an effort for real time embedded sensing of localized heating in polymer bonded explosives (PBXs) through thermo-electromechanical response for in situ structural health monitoring (SHM) in energetic materials. The experimental effort herein is focused on using 70 wt% Ammonium Perchlorate (AP) (solid oxidizer used in solid rocket propellants) crystals embedded into epoxy binder having concentration of 0.1 wt% multi-walled carbon nanotubes (MWCNTs) relative to entire hybrid energetics. Electrical and dielectric properties of neat (i.e. no MWCNTs) energetics and MWCNT hybrid energetics are quantitatively and qualitatively evaluated under localized thermal loading. Electrical and dielectric properties showed variations for both neat energetics and MWCNT hybrid energetics depending on input frequency measurements. Significant thermo-electromechanical response was obtained for MWCNT AP hybrid energetics, providing a proof of concept for thermo-electromechanical sensing for realtime SHM in energetics.

Commentary by Dr. Valentin Fuster
2017;():V001T01A010. doi:10.1115/SMASIS2017-3886.

Among anode materials for lithium ion batteries, silicon (Si) is known for high theoretical capacity and low cost. Si changes volume by 300% during cycling, however, often resulting in fast capacity fade. With sufficiently small Si particles in a flexible composite matrix, the cycle life of Si anodes can be extended. Si anodes also demonstrate stress-potential coupling where the open circuit voltage depends on applied stress. In this paper, we present a NMC-Si battery design, utilizing the undesired volume change of Si for actuation and the stress-potential coupling effect for sensing. The battery consists of one Li(Ni1/3Mn1/3Co1/3)O2 (NMC) cathode in a separator pouch placed in an electrolyte-filled container with Si composite anode cantilevers. Models predict the shape of the cantilever as a function of battery state of charge (SOC) and the cell voltage as a function of distributed loading. Simulations of a copper current collector coated with Si active material show 11.05 mAh of energy storage, large displacement in a unimorph configuration (>60% of beam length) and over 100 mV of voltage change due to gravitational loading.

Commentary by Dr. Valentin Fuster
2017;():V001T01A011. doi:10.1115/SMASIS2017-3908.

In the present work, we report on structural supercapacitors which are based on NASICON-type solid electrolyte Li1.4Al0.4Ti1.6(PO4)3 (LATP). The nanostructured electrodes incorporate single-wall carbon nanotubes (SWCNTs) mixed with the LATP electrolyte. The complete energy storage devices are manufactured in a sandwich structure consisting of two nanostructured electrode layers which are separated by a pure LATP layer. The as-prepared specimens are embedded in composite materials with Airstone 880/886H epoxy resin as matrix. Their electrical properties are characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). At ambient temperature, the addition of 6.5 wt. % SWCNTs results in a distinct improvement by reducing the total resistance of the embedded devices and enhances the capacitance from 0.025 mF g−1 to 3.160 mF g−1 at a scan rate of 5 mV s−1. Electrical measurements of two types of specimens are then applied under different temperatures from ambient temperature to 80 °C. It is observed that the equivalent series resistance (ESR) of device with SWCNTs decreases greatly and capacitance increases comparing with the device without SWCNTs. As a conclusion, the structural supercapacitors acquire excellent performance through high efficient double layer effects realized by nanostructured electrode/electrolyte interphase (large surface electrode areas).

Commentary by Dr. Valentin Fuster
2017;():V001T01A012. doi:10.1115/SMASIS2017-3953.

Ionomers are a class of polymers which contain a small fraction of charged groups in the polymer backbone. These ionic groups aggregate (termed ionic aggregates) to form temporary cross-links that break apart over the ionic dissociation temperature and re-aggregate on cooling, influencing the mechanical properties of these polymers. In addition to enhanced mechanical properties, some ionomers also exhibit self-healing behavior. The self-healing behavior is a consequence of weakly bonded ionic aggregates breaking apart and re-aggregating after puncture or a ballistic impact. The structure and properties of ionomers have been studied over the last several decades; however, there is a lack of understanding of the influence of an electrostatic field on ionic aggregate morphology. Characterizing the effect of temperature and electric field on the formation and structure of these ionic aggregates will lead to preparation of ionomers with enhanced structural properties. This work focuses on Surlyn 8940 which a poly-ethylene methacryclic acid co-polymer in which a fraction of the carboxylic acid is terminated by sodium. In this work, Surlyn is thermoelectrically processed over its ionic dissociation temperature in the presence of a strong electrostatic field. Thermal studies are performed on the ionomer to study the effect of the thermoelectric processing. It is shown that the application of a thermoelectric field leads to increase in the ionic aggregate order in these materials and reduction in crystal size distribution. Thermal Analysis is performed using a Differential Scanning Calorimeter and the resulting thermogram analysis shows that thermoelectric processing increases the peak temperature and onset temperature of melting by 4 C and 20 C respectively. The peak width at half maximum of the melting endotherm is reduced by 10 C due to thermoelectric processing. This serves as a measure of the increased crystallinity. A parametric study on the effect of field duration and field strength is also performed.

Commentary by Dr. Valentin Fuster
2017;():V001T01A013. doi:10.1115/SMASIS2017-3975.

The actuation mechanisms of cnt-based materials are still controversially discussed. It is not common sense whether it is a macroscopic volume effect caused by ion intercalation or electrostatic repulsion of equally charged cnts or a nanoscopic effect of filled electron anti-bonding orbitals of the carbon atom or interactions with ions docking on the carbon surface. In the presented paper arrays of highly aligned multi-walled carbon nanotubes (mwcnts) are used which are stabilized by a polypyrrole-coating. The samples are tested along the cnt-orientation and in perpendicular mode to analyze the influence of the structure-ion interaction. The mwcnt-arrays exhibit only a total length of approximately 2.8 mm but by coating with polypyrrole larger geometries can be tested. The actuation is analyzed using an in-plane test and an actuated tensile testing. Free strain can be detected using the first set-up, the second method is carried out to evaluate the mechanical stability of the samples. As might be expected, the material shows a strong anisotropic active behavior with the actuation along the tube axis being only half of the value detected at the perpendicular oriented samples. The findings point out that an intercalation of ions into the charged CNT-architecture seems here to be the dominating mechanism.

Commentary by Dr. Valentin Fuster
2017;():V001T01A014. doi:10.1115/SMASIS2017-3979.

Relaxor ferroelectric polymers are a unique branch of electro-active polymers (EAPs) that generate high electromechanical strain with relatively low hysteresis and high nonlinearity. Polyvinylidene fluoride-based EAPs possess these qualities due to the semicrystalline nature of their microstructure. The interactions of electric dipoles within the microstructure of the material generate large strains under an external electric field, and the reduced crystalline domain sizes yield a relaxor effect by exhibiting low hysteresis and hyperelastic properties. This phenomenon has been partially modeled by previous works, but micro-electro-mechanisms for electrostriction in the microstructure have been largely ignored. This study focuses on the effects of various microstructural frameworks on the nonlinear dielectric behavior of dipole-based, semicrystalline EAPs. The Helmholtz free energy function of a microscopic representative volume element (RVE) is composed of an electrostatic energy and an elastic energy. The dipole-dipole interaction energy is prescribed for the electrostatic forces observed among the crystalline regions, and the elastic component attributed to the relaxation of the amorphous phase is modeled by the hyperelastic eight-chain model, which is microstructure-based. The RVE of the system is modeled by a central dipole surrounded by dipoles whose relative spatial locations are determined by a probability distribution function (PDF). The hyperelastic amorphous phase constitutes the volume separating the central and surrounding dipoles. The free energy of the RVE is implemented into a continuum description of the equilibrium of the system to obtain electromechanical relations. Additionally, this electromechanical response data is applied to a 1D structural mechanics model for simulating the large deformation of a multi-layered beam. The effects of microstructure on electrostrictive coupling are explored by varying the centers and deviations of dipole locations within the PDF. Discrete microstructural arrangements representing 3-chain network averaging schemes may be studied alongside more continuous ellipsoidal or random models of dipole spatial arrangements. The simulation results of the PDF-based networks are in good agreement with experimental data. The results indicate that the electrostrictive behavior of EAPs is strongly dependent on (1) the relative dipole spatial locations and (2) the extent of the regions containing dipoles, which represent crystalline domains. The model finds that adding extra crystalline domains in the network averaging schemes generates a better characteristic behavior due to a broader averaging of spatial orientations. These results offer a gateway to predicting microstructurally-dependent dipole-based behavior that can lead to the predictive theoretical tailoring of microstructures for desired electromechanical properties.

Commentary by Dr. Valentin Fuster
2017;():V001T01A015. doi:10.1115/SMASIS2017-3988.

In this study, we demonstrate the electric and magnetic manipulation of nanoscale M-type Barium Hexaferrite (nBF) in polydimethylsiloxane (PDMS) to engineer a multifunctional nanocomposite with improved dielectric and magnetic properties. First, we synthesized the single crystal nBF via the hydrothermal synthesis route. The hydrothermal temperature, duration, and surfactant conditions were optimized to improve the magnetic properties of the nBFs, with further improvement achieved by post-annealing. The annealed nBFs were aligned dielectrophoretically (DEP) in the polymer matrices by applying an AC electric field. Under the influence of this electric field, nBFs were observed to rotate, align and form chains within the polymer matrix. Optical microscopy (OM) imaging was used to determine the electrical alignment conditions (duration, magnitude, and frequency) and these parameters were used to fabricate the composites. A Teflon setup with Indium Tin Oxide (ITO) coated Polyethylene Terephthalate (PET) was used, where the ITO coatings act as electrodes for the electric field-manipulation. To simultaneously apply the magnetic field, this Teflon setup is placed between two permanent magnets capable of generating a 0.6 T external magnetic field. Along with electric and magnetic fields, concurrent heating was applied to cure the PDMS and freeze the microstructure formed due to electric and magnetic fields. Upon completion of the curing step, parallel chain formation is observed under OM. The X-Ray Diffraction (XRD) results also confirm that the particles are magnetically oriented in the direction of the magnetic field within the chain. Vibrating Sample Magnetometry (VSM) measurements and dielectric spectroscopy are used to characterize the extent of anisotropy and improvement in dielectric and magnetic properties compared to random composites. We find that simultaneous electric and magnetic field alignment improves the dielectric properties by 12% compared to just magnetic alignment. We also observe 19% improved squareness ratio when both fields are applied. The possibility of simultaneous electrical and magnetic alignment of magnetic nanoparticles will open up new doors to manipulate and design particle-modified polymers for various applications.

Commentary by Dr. Valentin Fuster

Mechanics and Behavior of Active Materials

2017;():V001T02A001. doi:10.1115/SMASIS2017-3774.

Magneto-rheological fluids (MRF) are commonly applied in MRF brakes and vibration damping. The apparent viscosity dependence with respect to the magnetic field has been addressed in detail in the state of the art. The aim of this paper is to experimentally study the vibration effects on the particle chain-like structures and, as a consequence, the shear stress variation applied to the fluid. Three vibration configurations have been applied to a ferromagnetic cylinder rotating between two magnetic poles filled with MRF a “Z-vibration” where the generated displacement is along the rotation axis of the shearing cylinder, a “θ-vibration”, tangential to the cylinder, and an “R-vibration”, normal to the cylinder surface. First we focus on the vibration mode characterisation in free air, and then when plunged in the fluid. In a second step, we measure the reactive torque generated on the clutch under different magnetic field intensities with different rotation speeds and vibration amplitudes. It appears that the “R-vibration” configuration is providing the most influence, up to 20% of torque reduction observed at moderate B field. The “Z-vibration” and the “θ-vibration” configurations respectively have less influence on the torque, nevertheless vibrations always tend to decrease the corresponding yield stress in the MRF.

Commentary by Dr. Valentin Fuster
2017;():V001T02A002. doi:10.1115/SMASIS2017-3787.

The thermal shape memory effect describes the ability of a deformed material to return to its original shape when heated. This effect is found in shape memory alloys (SMAs) such as nickel-titanium (NiTi). SMA actuator wire is known for its high energy density and allows for the construction of compact systems. An additional advantage is the so-called “self-sensing” effect, which can be used for sensor tasks within an actuator-sensor-system.

In most applications, a current is used to heat the SMA wires through joule heating. Usually a current between zero and four ampere is recommended by the SMA wire manufacturers depending on the wire diameter. Therefore, supply voltage is adjusted to the SMA wire’s electrical resistance to reach the recommended current.

The focus of this work is to use supply voltages of magnitudes higher than the recommended supply voltages on SMA actuator wires. This actuation method has the advantage of being able to use industry standard voltage supplies for SMA actuators. Additionally, depending on the application, faster actuation and higher strokes can be achieved.

The high voltage results in a high current in the SMA wire. To prevent the wire from being destroyed by the high current, short pulses in the micro- and millisecond range are used.

As part of the presented work, a test setup has been constructed to examine the effects of the crucial parameters such as supply voltage amplitude, pulse duration, wire diameter and wire pre-tension. The monitored parameters in this setup are the wire displacement, wire current and force generated by the SMA wire. All sensors in this setup and their timing is validated through several experiments. Additionally, a highspeed optical camera system is used to record qualitative videos of the SMA wire’s behavior under there extreme conditions. This optical feedback is necessary to fully understand and interpret the measured force and displacement signals.

Commentary by Dr. Valentin Fuster
2017;():V001T02A003. doi:10.1115/SMASIS2017-3789.

The macroscopic behavior of Nickel Titanium Shape Memory Alloy (SMA) wires suffers from hysteresis. This is related to the fraction of material that is in detwinned martensite crystallographic orientation. In this work, a novel physical model is proposed that describes the fraction of transformed material on a macroscopic level. The model is history-free, and hence, is ideal to implement in model-based control strategies.

Commentary by Dr. Valentin Fuster
2017;():V001T02A004. doi:10.1115/SMASIS2017-3862.

This paper presents a simulation-based study to investigate the damping properties of a novel piezocomposite, consisting of piezoelectric fiber and epoxy reinforced with randomly orientated double walled carbon nanotubes (DWCNT), termed as piezoelectric fiber nano reinforced composite (PFNRC). Authors have observed that the past research dealt with the effect of aligned single walled carbon nanotubes (CNT) on active damping of piezoelectric composite in extension mode (e13 and e33). It is known from the past research that DWCNT inclusions improve the passive damping of a composite. Therefore, the authors use DWCNT inclusions to study the active-passive damping of the piezoelectric composite, in this article. The random orientation of the DWCNT is considered to replicate the physical composite as it known that aligning CNTs in a single direction is not feasible due to fabrication constraints. A multistep homogenization method involving Method of Cells (MOC) is employed to obtain effective properties of PFNRC. A modified 3D-MOC is used to obtain the effective properties of epoxy matrix with DWCNT inclusions (DWCNT-epoxy), considering the effect of nano particle agglomeration. A 2D-MOC is then implemented with long fiber PZT as the active material and DWCNT-epoxy as the matrix. This procedure is followed for computing the effective material properties of extension (e33) as well as shear (e15) mode of PFNRC, when DWCNT inclusions are added into the epoxy matrix at different weight percentages. The constitutive equations are derived with the help of Maple and simulated in MATLAB. These results are used to compare the active-passive damping performance of the composites using a single degree of freedom damping model, employing Newmark’s numerical integration method. The active damping performance of the composites is evaluated by varying the displacement and velocity gains in a negative feedback system. The main focus of the study is to find the most efficient operating mode of the proposed composite for damping of structural vibrations.

Commentary by Dr. Valentin Fuster
2017;():V001T02A005. doi:10.1115/SMASIS2017-3906.

An optimization algorithm is proposed to determine the parameters of a discrete energy-averaged (DEA) model for Galfenol alloys. A new numerical approximation approach for partial derivative expressions is developed, which improves computational speed of the DEA model by 61% relative to existing partial derivative expressions. Initial estimation of model parameters and a two-step optimization procedure, including an-hysteresis and hysteresis steps, are performed to improve accuracy and efficiency of the algorithm. Initial estimation of certain material properties such as saturation magnetization, saturation magnetostriction, Young’s modulus, and anisotropy energies can improve the convergence and enhance efficiency by 41% compared to the case where these parameters are not estimated. The two-step optimization improves efficiency by 28% while preserving accuracy compared to one-step optimization. Proposed algorithm is employed to find the material properties of Galfenol samples with different compositions and heat treatments. The trends obtained from these optimizations can guide future Galfenol modeling studies.

Commentary by Dr. Valentin Fuster
2017;():V001T02A006. doi:10.1115/SMASIS2017-3909.

Magnetic shape memory alloys (MSMAs) exhibit recoverable strains of up to 10% due to reorientation of their martensitic tetragonal unit cell. A stress or magnetic field applied to the material will cause the short side of the unit cell (which is approximately aligned with the magnetic easy axis) to align with the input to the material, resulting in an apparent plastic strain. This strain can be fully recovered by an applied stress or magnetic field in a perpendicular direction.

When the martensitic variants reorient, twin boundaries, which separate the different variants, form and move throughout the specimen. A number of models have been proposed for MSMAs and many of these models are homogenized, i.e. the models do not account for twin boundaries, but rather account for the volume fraction of material in each variant. These types of models often assume that the MSMA is subject to a uniform field so that there is no appreciable difference in the volume fraction of variants in each location. In this work, we address the issue of how these models can be used when the field is not uniform.

In particular, we look at the experiments from Feigenbaum et al., in which a MSMA trained to accommodate three variants, was subject to 3-dimensional magneto-mechanical loading. Due to experimental constraints, the field applied to the MSMA was not uniform. In this work, to understand the actual field distribution during experiments, we performed a high-resolution 3-dimensional finite element analysis (FEA) of the magnetic field experienced by the MSMA sample. The FEA allowed us to determine how non-uniform the experimentally applied field was and the differences between the applied field and the field experienced by the MSMA. Furthermore, we use the FEA to determine the average field experienced by the MSMA, and identify an equivalent uniform applied field that could serve as input for the model. For the latter, we seek a uniform magnetic field which gives similar magnetic field within the MSMA specimen as the true experimental conditions.

Commentary by Dr. Valentin Fuster
2017;():V001T02A007. doi:10.1115/SMASIS2017-3914.

In the last years, researchers have presented concentrated and distributed parameter models of electromechanically coupled systems, leading to appropriate estimation of their electroelastic behavior. Equivalent electrical circuits have also been investigated and provide useful simulation tools to investigate the system behavior as well as to developed new energy harvesting or control circuits. In general, RLC (resistor, inductor and capacitor) circuits represent, respectively, the mass, damping and stiffness of single or multi-degree-of-freedom electromechanically coupled systems. In practice, however, the equivalent electrical representation of high-quality-factor systems demand equivalent circuits with extremely low internal resistance values. Furthermore, the assumption of an ideal transformer cannot be obtained in practice. This work presents a novel equivalent electrical circuit for linear and nonlinear electromechanically coupled systems. The effects of inductance, capacitance and electromechanical coupling are represented through operational-amplifier based sub-circuits of extremely low internal resistance. First, the linear behavior of a mass-spring-damping system is verified. Later, the behavior of a nonlinear electromechanically coupled system is investigated. In both cases, numerical results (Matlab-Simulink simulations) and experimental results (from breadboard implementations) will be verified against experimental results presented in the literature.

Topics: Circuits
Commentary by Dr. Valentin Fuster
2017;():V001T02A008. doi:10.1115/SMASIS2017-3916.

Ferroelectric materials exhibit strong electromechanical behavior which has led to the production of a wide variety of adaptive structures and intelligent systems, ranging from structural health monitoring sensors, energy harvesting circuits, and flow control actuators. Given the large number of applications, accurate prediction of ferroelectric materials constitutive behavior is critical. This presents many challenges, including the need to predict behavior from electronic structures up to macroscropic continuum. Many of the structure-property relations in these materials can be accurately calculated using density functional theory (DFT). However, DFT is not necessarily conducive to the large scale computations required to solve these problems on a continuum scale. Introducing a phase field polarization order parameter is an alternative approach, which provides a means to simulate the length scale gap between nano- and microscale domain structure evolution. The introduction of the phase field approximation results in uncertainty. Bayesian statistical analysis is an ideal tool for quantifying the uncertainty associated with the continuum phase field model parameters. Analyses of monodomain structures allows for identification of Landau energy and electrostrictive stress parameters. Identifying the exchange parameters, which are proportional to the polarization gradients, requires consideration of polydomain structures. This is a nontrivial problem as domain wall structures are fully coupled between the Landau energy, electrostrictive, and exchange parameters. Accurately quantifying the uncertainty in the phase field parameters will provide insight into the nonlinear constitutive behavior.

Commentary by Dr. Valentin Fuster
2017;():V001T02A009. doi:10.1115/SMASIS2017-3918.

Field induced phase transformations in ferroelectric crystals occur when the applied electrical or mechanical load exceeds a certain threshold. Mechanical cycling about these transformation field thresholds under varying open and closed circuit conditions has been shown to yield a near ideal mechanical to electrical energy harvesting technique. Numerical integration of experimentally measured stress – strain and electric field – electric displacement data has shown mechanical to electrical energy conversion efficiency near 60% for 0.24PIN-0.44PMN-0.32PT. In this work, the total irreversible energy is determined by the offset between the forward and reverse loading paths, equivalent to the hysteresis in the phase transformation behavior. This is equal to the available mechanical energy for conversion to electrical energy for harvesting. Following the ideal mechanical to electrical energy harvesting procedure, the total possible energy harvested is a direct function of the hysteresis area in the phase transformation and the electromechanical coupling factor. Efficiency is predicted to be equal to the electromechanical coupling factor, 0.596 (59.6%). Predicted results agree with experimental data from numerical integration. Energy densities are calculated up to 5 kJ/m3 with potential power densities of 102–103 kW/m3.

Commentary by Dr. Valentin Fuster
2017;():V001T02A010. doi:10.1115/SMASIS2017-3939.

Self-healing material structures with the inherent capability to mend damage will lead to a paradigm shift in design as fracture may no longer constitute a failure. Generally, there are two techniques of self-healing that operate at different scales, require different approaches and often are dealt with separately; geometric restoration and crack filling/bonding. Geometric restoration uses shape memory materials that can mechanically close fractures after they occur. Crack filling and bonding fills and chemically bonds fractured parts in place.

Materials capable of recovering from complete fractures, that have propagated across the entire component, have typically taken a sparse fiber composite form with a structural matrix encapsulating shape memory fibers. This form of self-healing material has demonstrated the ability recover original bulk geometry. However, lacking bonding, the healed structures have not had the ability to resist subsequent externally applied loads without re-opening the crack.

A new approach of pre-straining the shape memory fibers before curing them in a matrix in the pre-strained state is presented in this paper with basic theory and experimental results. Pre-straining the shape memory fibers before casting them in the matrix causes them to undergo constrained recovery upon activation. Thus, the samples create closing loads across the crack which are capable of withstanding external loads without re-opening.

Commentary by Dr. Valentin Fuster
2017;():V001T02A011. doi:10.1115/SMASIS2017-3942.

Smart materials can be integrated into textile structures to produce active textiles with tailored mechanical properties and large, complex actuation motions. Active textiles have the potential to enable a wide range of applications including wearable technologies, soft robots, medical devices, and aerospace structures. One type of active textile is the shape memory alloy (SMA) knitted structure. SMA knitted structures produce a range of kinematic actuation motions as a result of the bending, torsion, extension, and buckling of the SMA wire during the loop-based knitting manufacturing process. The kinematic motions of several different patterns of SMA knitted actuators have been cataloged, and the mechanical performance of basic knitted patterns have been characterized. However, the effect of shape-setting of knitted SMA structures has not been explored. This paper investigates the effect of post-manufacturing shape-setting on the kinematic and kinetic performance of basic SMA knitted structures. A design of experiment methodology was employed to isolate the impact of knitted pattern, SMA wire diameter, and shape-set curvature on mechanical performance. The introduction of a large curvature shape-set in the SMA wire resulted in a very stiff textile structure with a minimal change in length between the austenite and martensite states, thus, minimal capacity for large actuation deformations. Meanwhile, the introduction of a small curvature in the SMA wire resulted in a nearly constant force plateau and a larger change in length between the austenite and martensite state for the same applied load, and the potential for enhanced structural actuation deformations. Shape-setting is an additional design parameter that can be employed to enhance and tune the mechanical performance of knitted SMA structures.

Topics: Shapes
Commentary by Dr. Valentin Fuster
2017;():V001T02A012. doi:10.1115/SMASIS2017-3984.

Strain in solid materials under external loads cannot be visualized until they reach a high value or failure occurs; and the common measuring method of using strain sensors is effective but limited to wiring or power supply. In this study, we introduce a new concept of self-sensing solid materials by designing thin surface circular delamination regions on a material body to sense and predict the elastic global strain through controlled elastic local buckling. Delamination buckling is an undesirable failure occurrence in laminated composites under compression. However, it can translate imperceptible small global strains on the main material body to a visible large deformation in the surface of the delaminated region due to buckling. We analytically studied the buckling and post-buckling response of a clamped circular thin plate with unilateral constraint using an energy method to obtain the critical buckling loads, the buckling configurations, and the center out-of-plane displacement under uniaxial and biaxial loading conditions. The results show that for a given buckling configuration in the local region, the global strain condition of the main material body can be predicted. The study thus explores and proves a feasible way to design self-sensing materials through controlled delamination buckling.

Commentary by Dr. Valentin Fuster

Bioinspired Smart Materials and Systems

2017;():V001T06A001. doi:10.1115/SMASIS2017-3723.

Limitations on neuron firing rates restrict the frequency bandwidth of many biological sensory systems. The cochlea overcomes these limitations to hear high frequency sounds through its tonotopic structure and non-synchronous sampling. The cochlea’s tapering basilar membrane serves as a filter bank decomposing an applied sound into its frequency components. Auditory hair cells produce neural impulses at the peaks in the local basilar membrane oscillations resulting in an event-driven, sub-Nyquist rate sampling strategy. These two effects extend the human hearing range to about 20 kHz despite maximum neuron firing rates of just a few hundred hertz. Inspired by this, this paper presents a concept demonstration of an accelerometer and signal compression strategy for high-rate impact events using a similar filter bank approach. A series of clamped-clamped beams will serve as analog analysis filters much like the cochlea’s basilar membrane. This paper focuses on the design and simulation of such a beam array and how the natural frequencies and damping ratios of the beams’ first modes affect measuring a broadband impact excitation.

Topics: Accelerometers
Commentary by Dr. Valentin Fuster
2017;():V001T06A002. doi:10.1115/SMASIS2017-3729.

This paper presents a study of bioinspired wall-climbing robot (WCR) using spiny toes. The first part of the paper describes a design of a flexible spiny toe inspired by the features of a typical wall-climbing insect Serica orientalis Motschulsky’s tarsal system. A simple contact model of the spiny toe is proposed by considering the contact asperities as spheres. With the help of the finite element method (FEM), the stiffness matrices as well as the directional adhesive properties of the spiny toe are obtained. A single spiny toe and its array are fabricated via fast prototyping. The adhesive forces and pull-off positions of the single toe are measured with a homebuilt apparatus using displacement-control method under different compressive deformations. As for the spiny array, the effect of the dragging path on the adhesive forces is evaluated. The results show that, both the single toe and array exhibit directional adhesive features. The value of compressive deformation of the single toe influences the contacting angle, as a consequence the directional adhesive behavior is achieved. When forming an array with numerous spiny toes, the adhesive ability is strengthened, which is also affected by the random distribution of the surface asperity height.

In the second part of the paper, a prototype of bioinspired WCR is designed and fabricated based on a fully understanding of the spiny contact mechanism. The robot has two feet, each of which has spring-actuated gripper. An inchworm gait is generated according to the trajectory planning of the feet. Using the proposed spiny arrays, the robot archives scaling on vertical and inverted rough surfaces, and can also transition between vertical and ceiling walls. The performance of the prototype of bioinspired WCR shows promising in developing an intelligent and maneuverable WCR system in practical applications.

Topics: Robots , Biomimetics
Commentary by Dr. Valentin Fuster
2017;():V001T06A003. doi:10.1115/SMASIS2017-3730.

Newts display superior attachment and climbing abilities under wet conditions due to the distinct pattern of micro- and nanoscale structures (i.e., polygonal epidermal cells with raised boundaries, and dense nanopillar arrays) on their foot pad surfaces. Inspired by the surface features of newt foot pads, a microstructured surface consisting of structural elements of round pillars surrounded by a closed hexagonal ridge is produced on the polydimethylsiloxane (PDMS) elastomer, and its wet adhesion properties are studied experimentally. A homebuilt adhesion tester is developed to carry out the pull-off experiments, in which the adhesion force of samples can be measured directly. The PDMS sample is fixed on the substrate, and a flat glass cylinder of 10 mm diameter is adopted as indenter. Different amounts of liquid are added to the area of contact by using a micropipette. Influences of preload, retraction speed, area ratio of round pillars, amount of liquid and approach-retraction cycle on wet adhesion of the patterned surface of PDMS samples are investigated.

Results show that the pull-off forces of all samples increase with preload and retraction speed. However, the pull-off forces increase slowly when the preload is over 3 N for both dry and wet conditions. The area ratios of round pillars increase the pull-off forces for the dry condition. When a small amount of liquid (0.1 μl) is added, the effect of the area ratios of round pillars on the pull-off force is not consistent with that of dry condition. Effects of amount of liquid on pull-off force for different area ratios of round pillars are various. For a certain amount of liquid, it is observed that the pull-off forces in general show a relatively high value for the first approach-retraction cycle, then decrease to a lower level starting from the second cycle, and suddenly increase and maintain a relatively constant value after several times of approach-retraction cycles. Our results can give insights into the repeated sliding actions of newt foot pads when climbing in wet environments as well as the possible functions of dense nanopillar arrays on newt footpad surface.

Commentary by Dr. Valentin Fuster
2017;():V001T06A004. doi:10.1115/SMASIS2017-3756.

It is of interest to exploit the insight from the lateral line system of fish for flow sensing applications. The lateral line consists of arrays of flow sensors, known as neuromasts, with hair cells encased within a gel-like structure called cupula. There are two types of neuromasts, superficial neuromasts, which reside on the surface, and canal neuromasts, which are recessed within a channel with its ends open at the body’s surface. In this work we investigate the modeling of a canal-type artificial lateral line system. The canal is filled with viscous fluid to emulate its biological counterpart. The artificial neuromast consists of an ionic polymer-metal composite (IPMC) sensor embedded within a soft molded cupula structure. The displacement of the cupula structure and the resulting short-circuit current of the IPMC sensor under an oscillatory flow are modeled and solved with finite-element methods. The Poisson-Nernst-Planck (PNP) model is used to describe the fundamental physics within the IPMC, where the bending stimulus due to the cupula displacement is coupled to the PNP model through the cation convective flux term. Comparison of the numerically computed cupula displacement with an analytical approximation is conducted. The effects of material stiffness and and device size on the device sensitivity are further explored.

Commentary by Dr. Valentin Fuster
2017;():V001T06A005. doi:10.1115/SMASIS2017-3770.

This work is based on the original concept of coupling two resonant vibration modes to reproduce insect wing kinematics and generate lift. The key issue is designing the geometry and the elastic properties of the artificial wings to achieve quadrature coupling of the bending and twisting motions using only one actuator. Qualitatively, this implies bringing the frequency of the two resonant modes closer. In the light of this challenge, an optimal wing configuration was determined for a micromachined polymer prototype three centimeters wide and validated through experimental modal analyses to illustrate the proximity of the frequencies of the bending and twisting modes. Then, a dedicated lift force measurement bench was developed and used to demonstrate a lift force equivalent to 110% of the prototype weight. For the first time, high-speed camera measurements of the wing motion confirmed that maximum lift was obtained as expected for bending and twisting motions in phase quadrature with a fully resonant motion of the wings using a single actuator.

Topics: Resonance , Wings
Commentary by Dr. Valentin Fuster
2017;():V001T06A006. doi:10.1115/SMASIS2017-3803.

Artificial muscle systems have the potential to impact many technologies ranging from advanced prosthesis to miniature robotics. Recently, it has been shown that twisting drawn polymer monofilaments, such as nylon fishing line or sewing thread, can result in a biomimetic thermally activated torsional actuator. The actuation phenomenon in these twisted polymer actuators (TPAs) is thought to be a result of an untwisting that occurs about the fiber’s axis due to an anisotropic thermal expansion. Before being twisted, the precursor fibers are comprised of polymer chains that are aligned axially. During fabrication of TPAs, the polymer chains reorient as the precursor fiber is twisted about the central axis of the monofilament. At the end of the fabrication process, the TPA is annealed in order to relieve internal stresses and to keep the fiber in the twisted configuration. The mechanism of untwisting actuation is generally thought to be a result of radial expansion and axial contraction. After being twisted, these radial and axial expansion relationships remain relatively unchanged, but the polymer chain direction is no longer axially aligned. Thus, upon heating the twisted fibers of the TPA, the fibers untwist and torsional actuation occurs. This actuation phenomenon has been used in the past to create linear actuators, but can also be use directly as a torsional actuator. Compared to other torsional actuators TPAs are low cost, lightweight, and can actuate reasonably high torques per unit volume. However, because TPAs are thermally activated, they may not be suitable for all applications. In this work, we present a novel TPA design for use as a torsional actuator for miniature actuation and artificial muscle applications. Our design bundles twisted monofilaments to increase the torque. Both fabrication and testing methods of the new design are presented. Results for temperature versus torsional displacement under various loads give insights as to how these actuators may be used and the reversibility of the actuation process under different fabrication loads. Additionally, comparisons are made between these bundled actuators and similarly loaded single TPA monofilament actuation.

Commentary by Dr. Valentin Fuster
2017;():V001T06A007. doi:10.1115/SMASIS2017-3819.

This paper focuses on the design and development of a bio-inspired mobile robot using piezoelectric transducers as drives. The design of the device aimed to imitate the trajectory movement of a crawl-like animal. Design constraints as producing controlled movement with piezoelectric transducer, as well as the combination of multiple piezoelectric patches into one mobile robot are presented in their practical aspects. The robot uses 2 piezoelectric transducers as main drives, but also as main structural components of the device. The patches are connected with a thin light rod, and the kinematic of movement is achieved with 4 tiny wooden legs connected on each of the patches.

The project investigates the possibility and effectiveness of the piezoelectric transducers for movement of the bio-inspired mobile robot. From conceptual development, to the mechanical design and control, the mobile robot is used to test different trajectories of movement. Ni RIO Evaluation kit has been incorporated as a real-time and FPGA control platform for the mobile robot while using Labview programing environment. To accomplish complex trajectories of movement the velocity of the robot was measured for straight line and rotation of the robot.

Commentary by Dr. Valentin Fuster
2017;():V001T06A008. doi:10.1115/SMASIS2017-3836.

Metamerism, in biology, occurs when a creature has multiple segments, which are similar in structure and configured in series. True metamerism means that these connected segments include a repetition of all organs and muscle tissues. Earthworms are examples of true metameric creatures. Animals use metameric structures to increase maneuverability and enable multiple modes (gaits) of locomotion along with other functions. This work presents the design of a crawling robot that is inspired by the crawling gait and true metamerism of earthworms. The building block of each segment is a bistable origami structure that extends and contracts its length. The robot moves forward by using directional friction on its feet to enable forward motion and turning. [1]. Using a series of connected origami building blocks provides the robot with a modular metameric structure. This paper presents a true metameric robot design where different segments can be detached and reattached to one another but remain fully functional in each state. The docking system uses shape memory alloy (SMA) wire coils as actuators for a clutching mechanism to disengage the different segments. A directional magnetic arrangement is used to reattach the segments. The actuation architecture exploits the bistability of the origami building blocks to improve the power efficiency of the robot. Future work includes implementing a control algorithm to plan the paths of the different segments and allow for autonomous segmentation and docking in various operational environments.

Commentary by Dr. Valentin Fuster
2017;():V001T06A009. doi:10.1115/SMASIS2017-3841.

A recent achievement in the droplet interface bilayer (DIB) technique is the ability to link multiple lipid-encased aqueous droplets in an oil medium to construct a membrane-based network. Highly flexible, efficient and durable compared to other lipid bilayer modeling techniques, these systems establish a framework for the creation of biocompatible and stimuli-responsive smart materials with applications ranging from biosensing to reliable micro-actuation. Incorporating ferrofluids droplets into this platform has proven to accelerate the networks’ building mechanism through remote magnetic-control of the droplets movement and has reduced the likelihood of failure during the pre-network-completion phase. Additionally, ferrofluid drops may be placed in the final network structure as they are macroscopically homogenous and behave as single phased liquids. Due to their paramagnetic characteristics, no residual magnetization is observed in the ferrofluid upon removal of the external magnetic field, allowing for simple control of the magnetically responsive droplets. Aside from the ferrofluids reliability in contact-free manipulation of bilayer networks, this work shows a different feature of having such hybrid ferrofluid-water DIB networks: magnetic-sensibility and actuation. Once pre-structured mixed networks are formed, a magnetic source is used to generate various magnetic fields in the vicinity of the DIB webs; changes in structural responses are then observed and used to induce protein channel gating in DIB networks channeling the functionality of a switch. Tailored architectures are accordingly evaluated and their suitability for the creation of microfluidic-magneto sensors and actuators is assessed.

Commentary by Dr. Valentin Fuster
2017;():V001T06A010. doi:10.1115/SMASIS2017-3842.

The optimal stiffnesses and spatial locations of contact-aided compliant mechanisms in a dynamic flapping wing structure are found using a numerical dynamics model and multi-objective genetic algorithm. Using mathematical descriptions of how wing shape change impact pitch agility in a flapping wing mechanical bird, an optimization problem was formulated to find compliant joint stiffness and location parameters which induce desired shape change. Specifically, the goal of the shape change was to induce forward sweep at the upstroke to downstroke transition while otherwise remaining stiff in an effort to move the aerodynamic center ahead of the center of gravity. A single compliant joint in the leading edge spar of an ornithopter wing was considered. A multi-objective genetic algorithm was used to solve the optimization problem, generating 3892 unique designs over 20 generations. Machine learning visualization and regression was used to better understand the data set. The data set was narrowed using higher level decisions, and one optimal design which satisfied the design requirements was chosen based on its relative performance and design parameters. The design was able to achieve the desired forward sweep while only allowing small bending and twist motion compared to the wing structure without a compliant joint inserted.

Commentary by Dr. Valentin Fuster
2017;():V001T06A011. doi:10.1115/SMASIS2017-3843.

A coupled 3 degree-of-freedom contact-aided compliant mechanism called the Forward Swept Compliant Mechanism (FSCM) is designed optimized for coupling orthogonal translational motion. The purpose of this mechanism is to allow desirable wing morphing passively in an ornithopter wing structure to improve free flight pitch agility via sweeping the wing tip forward during downstroke. This new contact-aided compliant mechanism design, based on the coupled three degree of freedom Bend-Twist-and-Sweep Compliant Mechanism, was developed to couple motion in bending to forward sweep during downstroke to destabilize the downstroke, and thereby increasing pitch agility. This is made possible due to an axial rotation of the mechanism, positioning the angled compliant joint such that the axis of deformation is skewed from the lifting direction. A multi-objective optimization problem was formulated and solved using a multi-objective genetic algorithm. The objectives of this optimization were to maximize forward sweep while minimizing bending, twist, peak stress, and mass. During the optimization, 3084 designs were simulated throughout 37 generations. The complete data set from the optimization was used to understand the relationship between each design variable and each objective, as well as in a random forest of regression trees to determine each variable’s importance to each objective. Two designs were chosen and compared for performance tradeoffs, where additional shape change is achieved at the expense of higher peak stress. The first design achieved the desired 2 degrees of forward sweep, and the second design achieved 5 degrees of forward sweep at the expense of larger bending and a higher peak stress.

Commentary by Dr. Valentin Fuster
2017;():V001T06A012. doi:10.1115/SMASIS2017-3851.

Needles are among the most common used instruments in surgery by medical professionals either for diagnosing the disease such as biopsy or for medical intervention such as drug delivery. Generally, needles are assumed to be minimally invasive, however it is desirable to decrease the insertion and pulling out force in order to prevent tissue damages. The hypothesis is that reducing the resistance forces caused by needle-tissue interaction leads to less tissue damage and less pain. Bioinspired needles mimicking insect stingers have been designed to reduce this resistance force and this design could provide to a more sophisticated steering of needle. Although our earlier study on honeybee-mimicking needle has shown the reduction of insertion force by having barbs on the needle body, the pull-out force is a big concern in particular during the extraction of the needle. A special mechanism to control the barbs at the end of the insertion procedure is designed. In this study, we investigated the use of SMA to control the barb functions so that it will reduce the pull-out force of the bioinspired needles. In this work, smart barb design is proposed. Circular barbs are divided to two symmetric parts connected by a ring around the central axis of the needle and the rings are connected to form the base part of its structure. Barbs are designed to have parallel faces with a desired angle through the insertion mechanism and are connected with a SMA wire at their bottom that is connected to the rear and front part of the needle. After insertion, actuating the SMA wires force the barbs to rotate around the rings due to the torque provided by wire shrinkage. As a result, barbs have now the same angle along the movement of needle for pulling out as they have for insertion mechanism.

Topics: Design , Surgery , needles
Commentary by Dr. Valentin Fuster
2017;():V001T06A013. doi:10.1115/SMASIS2017-3863.

Materials capable of exhibiting inherent morphing are rare and typically reliant on nanometric chemical properties. The resulting shape adaptability is thus slow and limited to specific environmental conditions. In contrast, natural composites, such as those found in carnivorous plants, have evolved hierarchical architectures displaying remarkably fast adaptation upon environmental stimuli. These biological materials have inspired the fabrication of snapping composite shells through the careful design of the internal microstructure of synthetic materials by magnetic alignment of reinforcements. The ability to accurately model such programmable materials using finite element analysis (FEA) is necessary to facilitate the design optimization of resulting structures. Using similar material parameters as explored in previous experimental studies, we employ nonlinear FEA to investigate the effects of introducing curvilinear spatially distributed micro-reinforcements on the deformation of a shell with bioinspired geometry. The FEA model is subject to a preliminary experimental validation. Comparison to a conventional [0/90] composite layup and simplified models demonstrates the advantages of magnetically aligned reinforcements to achieve complex, snapping morphing structures with tailored characteristics.

Commentary by Dr. Valentin Fuster
2017;():V001T06A014. doi:10.1115/SMASIS2017-3878.

In this study, we utilize a novel microfluidic device to perform simultaneous electrical interrogation of an array of droplet interface bilayers (DIB) that feature asymmetric phospholipid leaflet compositions. While asymmetry is vital to many cellular functions, it is has received very little attention in membrane-based engineered material systems for sensing, energy conversion, or actuation. This gap is due to challenges in constructing and interrogating networks of asymmetric membranes, limiting our understanding of how lipid asymmetry affects membrane active peptides, and vice versa. Our system overcomes these difficulties by enabling asymmetric membrane formation between many pairs of lipid-coated droplets in oil. We demonstrate its use in probing the interactions between alamethicin, a membrane-active peptide that forms voltage-induced ion channels, and asymmetric DPhPC:DOPhPC membranes, a choice that creates an intrinsic intramembrane potential of |137 mV| due to differences in their respective dipole potentials. Our experiments show that adding alamethicin peptides to one side of the membrane causes this inherent membrane potential to decrease over time, and it alters the value of external voltage that must be applied to drive alamethicin insertion for ion channel formation. These effects take place over the course of 1 to 5 hours after membrane formation, and both results are consistent with translocation and mixing of lipids across the leaflets of the membrane.

Commentary by Dr. Valentin Fuster
2017;():V001T06A015. doi:10.1115/SMASIS2017-3879.

Total Knee Replacement (TKR) is a very common procedure in the United States, especially with the aging population. However, despite high numbers of procedures and advancing technology, about 20% of patients with TKR are unsatisfied with the level of discomfort they experience with their replacement. Prevailing theories suggest that this is due to gradual misalignment of the knee. Multiple methods have been attempted to detect the cause of mechanical failure in replacements. One possible method for performing state detection in knees is the embedding of piezoelectric transducers (PZTs) into the bearing component. Preliminary testing of PZT’s embedded in simplified plastic components has shown that this method contains promise. With this said, further testing on realistic knee implant components is still needed to solidify the method’s validity. Commercial knee implant bearings utilize medical grade Ultra-High Molecular Weight Polyethylene (UHMW) and manufacturers utilize proprietary processing technology to develop the final components. This work focuses on the development of surrogate knee implant prototypes that replicate the material and geometric properties of actual knee implants to provide a convenient and economical solution to evaluate the performance of embedded PZTs. In this work, scans of an original knee bearing are taken and used to create a 3D model. From there, a variety of processes including 3D printing and Computer Numerical Controlled (CNC) machining are used to develop surrogate prototypes that are compared for accuracy to a benchmark. This benchmark is taken as a polished CNC machined non-medical grade UHMW prototype. Standards that the prototypes must meet include cost and time effectiveness as well as similarity in geometry and material property to the benchmark. The performance of the prototypes is experimentally compared through mechanical load testing by using pressure sensitive films placed between the femoral and bearing components of the implant as well as measuring piezoelectric output. In addition, the measured voltage output is compared to predictions from an analytical model for validation of the piezoelectric performance. These two experiments help to derive information about the applied load distribution and location, allowing comparisons to be made to the benchmark. This study shows that, while some types of 3D printing, such as fused deposition modeling, provide fast and cheap prototypes, other options such as stereolithography printing produce higher quality and more replicative components. Results of this study can be used in the development of useful surrogates for the advancement of biomedical sensors.

Commentary by Dr. Valentin Fuster
2017;():V001T06A016. doi:10.1115/SMASIS2017-3915.

Although needle-based surgeries are considered as minimally invasive surgeries, the damage caused by the needle insertion in soft tissues, namely brain needs to be reduced. Any minor damage, swelling or bleeding in the brain tissue can lead to a long-lasting traumatic brain injury. Our approach to this challenge is to search for a proper solution in nature such as honeybees. In our previous studies, some new bioinspired needles (passive/active) mimicking honeybee stingers have been proposed and tested by conducting needle insertion tests in tissue gel phantoms. The main feature of the bioinspired needles is specially-design barbs on the needle structures. It was discovered that the insertion forces of the bioinspired needles are decreased by as much as 35%, which means that there is a decrease in tissue gel phantom damages. It was also observed that the needle path deflection in the tissue was greatly affected by the reduction in needle bending stiffness and the insertion force. The reduction in the bending stiffness would require lower forces of Nitinol actuators to navigate our smart/active needle inside the tissues. This work specifically aims to investigate the mechanics of the bioinspired needles in bovine brain tissues. The needle insertion tests in real tissues are designed and performed. The insertion mechanics of the bioinspired needles in bovine brain is studied and presented.

Commentary by Dr. Valentin Fuster

Energy Harvesting

2017;():V001T07A001. doi:10.1115/SMASIS2017-3717.

This study is at attempt to explore the nonlinear behavior of bistable composite laminates for vibration energy harvesting. Asymmetric four-ply [0/90/0/90] carbon-fiber plate with two cylindrical stable equilibria supported at its center and free at all boundaries is used for the experimental testing. Macro-fiber composite (MFC) patches are attached to the plate to transform the mechanical vibration energy into electrical energy. The mechanical bistable property of the plate makes it possible to snap from one stable equilibrium state to the other. This snapthrough motion is highly nonlinear and associated with large-amplitude vibrations. The experimental tests aim at exploiting the nonlinearity due to the snapthrough motion to enhance the energy extraction. First, the resonant frequencies and damping of the plate are identified. A primary-resonance excitations of the first mode are carried out using two schemes: amplitude sweep and frequency sweep. In the first case, amplitude sweep, the excitation frequency is kept fixed at the resonant frequency and the amplitude of excitation is increased. The time history and FFT of the response as well as the output voltage are measured and reported. In the second case, frequency sweep, the excitation frequency is varied around the resonant frequency while the excitation amplitude is kept fixed. In both cases, the response shows a small-amplitude single-well vibrations at low excitation amplitudes and chaotic and periodic snapthrough motion as the amplitude and frequency of excitation are varied. The snapthrough motion has been found to greatly enhance the energy extraction capability. This study can serve as a motive for more testing and modeling efforts in order to understand the complex nonlinear behavior of bistable composite laminates and exploit it for vibration energy harvesting.

Commentary by Dr. Valentin Fuster
2017;():V001T07A002. doi:10.1115/SMASIS2017-3741.

This paper investigates an experimental approach for enhancing the output power of a piezoelectric energy harvester. The proposed method adopts inductance to reduce the effect of the piezoelectric harvester’s impedance, and boost the output power. Four electrical circuits for a piezoelectric beam harvester are investigated experimentally; Simple Resistive Load (SRL), Inductive Load (IL), Standard AC-DC, and Inductive AC-DC circuits. The results show that the adaptation of inductor in the IL and Inductive AC-DC improves the output power compared to the SRL and Standard AC-DC respectively. The Inductive AC-DC circuit is shown to increase the output power by 6.7% in comparison to the existing standard AC-DC circuits.

Commentary by Dr. Valentin Fuster
2017;():V001T07A003. doi:10.1115/SMASIS2017-3772.

This paper introduces the mechanism of a buoy-type wave energy converter (WEC) with a tuned inertial mass (TIM) mechanism. The TIM mechanism consists of a rotational mass and motor connected in series with a tuning spring. While it is common to control the current of the power take-off system, the stiffness of the spring is tuned in addition so that the inertial mass part resonates with the dominant frequency of the wave motion. The method to design the parameters to maximize the power generation capability is introduced and numerical studies for both narrowband and broadband sea states are carried out. It is shown that the proposed device demonstrates better energy harvesting performance compared to the WEC without the TIM mechanism to band-limited stationary random vibration.

Commentary by Dr. Valentin Fuster
2017;():V001T07A004. doi:10.1115/SMASIS2017-3778.

This paper investigates the complicated dynamic behavior and power generation efficiency of the cantilevered laminated composite piezoelectric beam with the unilateral layer separate. The effect of the external excitation on the voltage output, the impacts of the layered length of composite layers and the influence of the magnetic distance on the voltage output and the effective frequency bandwidth are examined. Simultaneously, the output voltage and the effective frequency bandwidth of the traditional cantilevered laminated composite piezoelectric beam are measured experimentally to verify the developed model. The amplitude of the harmonic excitation is given the certain value and is not changed. Experimental results show that the developed structure has lower natural frequency, great voltage output and great effective frequency bandwidth when the length of the separate parts between composite layers is in the range. For the different layered lengths of the developed bistable piezoelectric beam, there exist the optimal magnetic distance and an optimal layered length, respectively. The power generation efficiency of the developed bistable piezoelectric beam is better than that of the developed monostable piezoelectric beam. When the layered length of the separate parts between composite layers is optimal, the voltage output of the piezoelectric beam has four peak voltages. In addition, the power generation efficiency of the developed structure are superior to that of the traditional one. The maximum peak voltage of this structure is 6.73 times than that of the traditional piezoelectric beam, and its effective frequency bandwidth promotes 8.4 times.

Commentary by Dr. Valentin Fuster
2017;():V001T07A005. doi:10.1115/SMASIS2017-3796.

This paper proposes a self-start piezoelectric energy harvesting circuit with an undervoltage-lockout (UVLO) converter for a wireless sensor network (WSN). First, a self-start circuit with mini piezoelectric energy harvester (PEH) is designed to supply the power for operation of the oscillator without battery. The experimental results show that a batteryless self-start circuit successfully operates the oscillator with mini-PEH, and self-starting time is 0.45 s. Second, this paper proposes an adjustable UVLO converter that can supply the power even if a power consumption of a wireless sensor node is higher than generated power from PEH. The experimental result shows the adjustable UVLO converter supplies 45 mW for 0.12 s after charging the output power of an impedance matching circuit (1.7 mW) for 10 s. This paper shows that the proposed circuit successfully overcomes challenging issues — self-start and lower power generation — for powering WSN.

Commentary by Dr. Valentin Fuster
2017;():V001T07A006. doi:10.1115/SMASIS2017-3799.

We investigate the potential of harvesting vibration energy via a bi-stable beam subjected to subharmonic parametric excitations. The vibrating structure is a buckled beam with two stable equilibria separated by a potential barrier. The beam is subjected to a superposition of a static axial load beyond its critical buckling load and a harmonic axial excitation which frequency is around twice the frequency of the first vibration mode. A micro-fiber composite (MFC) is attached to one side of the beam to convert the strain energy resulting from the beams oscillation into electricity. The study considers two regimes of excitations: an amplitude sweep and a frequency sweep. In the first regime, the amplitude of excitation is varied while the excitation frequency is tuned at twice the natural frequency of the first vibration mode. In the second regime, the excitation frequency is swept forward and backward around the subharmonic resonant frequency while the amplitude of excitation is kept constant. A theoretical model which governs the electromechanical coupling of the transverse vibrations of the beam and the output voltage is used to monitor the response as the excitation parameters are changed. An experimental setup is built and a series of tests is performed. The theoretical results are in good agreement with their experimental counterparts. The experiment also shows that this type of bi-stable energy harvesters exhibits a broadband frequency response as compared to the classical linear harvesters.

Commentary by Dr. Valentin Fuster
2017;():V001T07A007. doi:10.1115/SMASIS2017-3805.

Vibration-based energy harvesting has been widely investigated to as a means to generate low levels of electrical energy for applications such as wireless sensor networks. However, due to the fact that vibration from the environment is typically random and varies with different magnitudes and frequencies, it is a challenge to implement frequency matching in order to maximize the power output of the energy harvester with a wider frequency bandwidth for applications where there is a time-dependent, varying source frequency. Possible solutions of frequency matching include widening the bandwidth of the energy harvesters themselves in order to implement frequency matching and to perform resonance-based tuning approach, the latter of which shows the most promise to implement a frequency matching design. Here three tuning strategies are discussed. First a two-dimensional resonant frequency tuning technique for the cantilever-geometry energy harvesting device which extended previous 1D tuning approaches was developed. This 2D approach could be used in applications where space constraints impact the available design space of the energy harvester. In addition, two novel resonant frequency tuning approaches (tuning via mechanical stretch and tuning via applied bias voltage, respectively) for electroactive polymer (EAP) membrane-based geometry energy harvesters was proposed, such that the resulting changes in membrane tension were used to tune the device for applications targeting variable ambient frequency environments.

Commentary by Dr. Valentin Fuster
2017;():V001T07A008. doi:10.1115/SMASIS2017-3827.

In this paper, we proposed a Bi-directional U-shaped piezoelectric energy harvester that is capable of scavenging vibration energy in two orthogonal directions. The U-shaped harvester is a three-segment beam with piezoelectric layers attached. The harvester is designed to work in its first three resonant frequencies. A theoretical model of the harvester is derived based on the Euler-Bernoulli beam theory. The model is capable of simulating the electromechanical coupling system and obtaining the frequency responses under excitations of two orthogonal directions. The resonant frequencies of the harvester can be tuned by simply altering the length-to-width ratio of the beam structure. It is shown in the simulation that the U-shaped design can effectively harvest vibration energy in two directions of excitations.

Commentary by Dr. Valentin Fuster
2017;():V001T07A009. doi:10.1115/SMASIS2017-3844.

Electret based energy scavenging devices utilize electro-static induction to convert mechanical energy into electrical energy. Uses for these devices include harvesting ambient energy in the environment and acting as sensors for a range of applications. These types of devices have been used in MEMS applications for over a decade. However, recently there is an interest in triboelectric generators/harvesters, i.e., electret based harvesters that utilize triboelectrification as well as electrostatic induction. The literature is filled with a variety of designs for the latter devices, constructed from materials ranging from paper and thin films; rendering the generators lightweight, flexible and inexpensive. However, most of the design of these devices is ad-hoc and not based on exploiting the underlying physics that govern their behavior; the few models that exist neglect the coupled electromechanical behavior of the devices. Motivated by the lack of a comprehensive dynamic model of these devices this manuscript presents a generalized framework based on a Lagrangian formulation to derive electromechanical equation for a lumped parameter dynamic model of an electret-based harvester. The framework is robust, capturing the effects of traditional MEMS devices as well as triboelectric generators. Exploiting numerical simulations the predictions are used to examine the behavior of electret based devices for a variety of loading conditions simulating real-world applications such as power scavengers under simple harmonic forcing and in pedestrian walking.

Commentary by Dr. Valentin Fuster
2017;():V001T07A010. doi:10.1115/SMASIS2017-3860.

There is a growing interest to convert ambient mechanical energy to electrical energy by vibration energy harvesters. Realistic vibrations are random and spread over a large frequency range. Most energy harvesters are linear with narrow frequency bandwidth and show low performance, which led to creation of nonlinear harvesters that have larger bandwidth. This article presents a simulation study of a nonlinear energy harvester that contains two cantilever beams coupled by magnetic force. One of the cantilever beam is covered partially by piezoelectric material, while the other beam is normal to the first one and is used to create a variable potential energy function. The variable double-well potential function enables optimum conversion of the kinetic energy and thus larger output. The system is modeled by coupled Duffing oscillator equations. To represent the ambient vibrations, the response to Gaussian random input signal (generated by Shinozuka formula) is studied using power spectral density. The effects of different parameters on the system are also investigated. The results show that the double cantilever harvester has a threshold distance, where the harvester can perform optimally regardless of the excitation level. This observation is opposite to that of the conventional fixed magnet cantilever system where the optimal distance varies with the excitation level. Results of this study can be used to enhance energy efficiency of vibration energy harvesters.

Commentary by Dr. Valentin Fuster
2017;():V001T07A011. doi:10.1115/SMASIS2017-3881.

Total knee replacement has been utilized to restore the functionality of diseased knee joints for more than four decades. Today, despite the relatively high level of patient satisfaction, still about 20% of patients are not fulfilled with their surgical outcomes in terms of function and reduction in pain. There is still an ongoing discussion on correlating the postoperative functionality of the joint to intraoperative alignment, which suffers from lack of in vivo data from the knee after surgery. However, it is necessary to mention that using computer assisted surgical techniques, the outcomes of knee replacement procedures have been remarkably improved. In order to obtain information about the knee function after the operation, the design of a self-powered instrumented knee implant is proposed in this study. The design is a total knee replacement ultra high molecular weight polyethylene insert equipped with four piezoelectric transducers distributed in the medial and lateral compartments of the bearing. The piezoelectric elements are employed to measure the axial force applied on the tibial insert through the femoral component of the joint as well as to track the movement in the center of pressure. In addition, generated voltage from the piezoelectrics is harvested and stored to power embedded electronics for further signal conditioning and data transmitting purposes. The performance of the instrumented implant is investigated via experimental testing on a fabricated prototype in terms of sensing and power harvesting capacity. Piezoelectric force and center of pressure measurements are compared to the actual quantities recorded from the load frame and pressure sensitive films in order to evaluate the performance of the sensing system. The output voltage of the piezoelectric transducers is rectified and stored in a capacitor to evaluate the energy harvesting ability of the system. The results show only a small level of error in sensing the force and the location of center of pressure. Additionally, a 4.9 V constant voltage is stored in a 3.3 mF capacitor after 3333 loading cycles. The sensing and energy harvesting results present the promising potential of this system to be used as an integrated self-powered instrumented knee implant.

Commentary by Dr. Valentin Fuster
2017;():V001T07A012. doi:10.1115/SMASIS2017-3894.

This manuscript investigates energy harvesting from arterial blood pressure via the piezoelectric effect for the purpose of powering embedded micro-sensors in the human brain. One of the major hurdles in recording and measuring electrical data in the human nervous system is the lack of implantable and long term interfaces that record neural activity for extended periods of time. Recently, some authors have proposed micro sensors implanted deep in the brain that measure local electrical and physiological data which is then communicated to an external interrogator. This paper proposes a way of powering such interfaces. The geometry of the proposed harvester consists of a piezoelectric, circular, curved bimorph that fits into the blood vessel (specifically, the Carotid artery) and undergoes bending motion because of blood pressure variation. In addition, the harvester thickness is constrained such that it does not modify arterial wall dynamics. This transforms the problem into a known strain problem and the integral form of Gauss’s law is used to obtain an equation relating arterial wall motion to the induced voltage. The theoretical model is validated by means of a Multiphysics 3D-FEA simulation comparing the harvested power at different load resistances. The peak harvested power achieved for the Carotid artery (proximal to Brain), with PZT-5H, was 11.7 μ W. The peak power for the Aorta was 203.4 μ W. Further, the variation of harvested power with variation in harvester width and thickness, arterial contractility and the pulse rate is investigated. Moreover, potential application of the harvester as a chronic, implantable and real-time Blood pressure sensor is considered. Energy harvested via this mechanism will also have applications in long-term, implantable Brain Micro-stimulation.

Commentary by Dr. Valentin Fuster
2017;():V001T07A013. doi:10.1115/SMASIS2017-3931.

We explore the modeling and analysis of nonlinear non-conservative dynamics of macro-fiber composite (MFC) piezo-electric structures, guided by rigorous experiments, for resonant vibration-based energy harvesting, as well as other applications leveraging the direct piezoelectric effect, such as resonant sensing. The MFCs employ piezoelectric fibers of rectangular cross section embedded in kapton with interdigitated electrodes to exploit the 33-mode of piezoelectricity. Existing frameworks for resonant nonlinearities have so far considered conventional piezoceramics that use the 31-mode of piezoelectricity. In the present work, we develop a framework to represent and predict nonlinear electroelastic dynamics of MFC bimorph cantilevers under resonant base excitation. The interdigitated electrodes are shunted to a set of resistive electrical loads to quantify the electrical power output. Experiments are conducted on a set of MFC bimorphs over a broad range of mechanical excitation levels to identify the types of nonlinearities present and to compare the model predictions and experiments. The experimentally observed interaction of material softening and geometric hardening effects, as well as dissipative effects, is captured and demonstrated by the model.

Commentary by Dr. Valentin Fuster
2017;():V001T07A014. doi:10.1115/SMASIS2017-3951.

Vibration-based energy harvesting is a growing field for generating low-power electricity to use in wireless electronic devices, such as the sensor networks used in structural health monitoring applications. Locally resonant metastructures, which are structures that comprise locally resonant metamaterial components, enable bandgap formation at wavelengths much longer than the lattice size, for critical applications such as low-frequency vibration attenuation in flexible structures. This work aims to bridge the domains of energy harvesting and locally resonant metamaterials to form multifunctional structures that exhibit both low-power electricity generation and vibration attenuation capabilities. A fully coupled electromechanical modeling framework is developed for two characteristic systems and their modal analysis is presented. Simulations are performed to explore the vibration and electrical power frequency response maps for varying electrical load resistance, and optimal loading conditions are presented. Case studies are presented to understand the interaction of bandgap formation and energy harvesting capabilities of this new class of multifunctional energy-harvesting locally resonant metastructures. It is shown that useful energy can be harvested from the locally resonant metastructure without significantly diminishing their dramatic vibration attenuation in the locally resonant bandgap. Thus, by integrating energy harvesters into a locally resonant metastructure, there is new potential for multifunctional self-powering or self-sensing locally resonant metastructures.

Commentary by Dr. Valentin Fuster
2017;():V001T07A015. doi:10.1115/SMASIS2017-3957.

Elastic lens and mirror concepts that have been explored to date for enhanced structure-borne wave energy harvesting are suitable for relatively high-frequency waves (e.g. tens of kHz), which are very much outside the typical ambient structural frequency energy spectrum. One direct way of reducing the design frequency of such phononic crystal-based lens and reflector/mirror designs is to increase their size, which would yield very large dimensions to operate at ambient vibration frequencies (∼hundreds of Hz). In this work, we exploit locally resonant (LR) metamaterials to enable low-frequency elastic wave focusing via LR lens and mirror concepts with practical size limitations. LR lens is designed in a similar way to its phononic crystal counterpart by tailoring the refractive index profile of the LR unit cell distribution. However, LR approach enables altering the dispersion characteristics, and thereby the phase velocity distribution, at much lower frequencies right below the local resonance frequency. Other than the local resonance frequency of the unit cells, the key factor in design is the mass ratio of the resonators to achieve a desired refractive index profile and focusing. LR mirror uses the low-frequency bandgap which is right above the resonance frequency of the unit cells. LR unit cells arranged in the form of a parabola, for instance, makes a low-frequency LR mirror that operates in the bandgap for plane wave focusing. These LR focusing concepts can be used in vibration civil, aerospace, and mechanical systems to localize and harvest structure-borne wave energy.

Commentary by Dr. Valentin Fuster
2017;():V001T07A016. doi:10.1115/SMASIS2017-3994.

In this paper, a design of an energy harvesting device which converts a translational relative motion to an oscillatory motion via stick-slip phenomenon is presented. In this design, an L-shape cantilever is used as an energy converter, the tip of which is rubbed by a linearly moving rubber pad. The induced stick-slip motion produces a relatively high frequency oscillation in the middle part of the cantilever during the stick phase, which is then converted to the electrical energy via a piezoelectric element attached on the cantilever surface. Testing of a proof-of-concept prototype reveals how the linear relative motion induces the stick-slip motion and the high frequency oscillation of the cantilever. The dependence of the stick-slip frequency on the design parameters is preliminary studied. Then, the load resistance optimization and the maximum output power are discussed, and the energy efficiency which is defined as the ratio of the output electrical energy to the input mechanical work during the rubbing motion is evaluated.

Commentary by Dr. Valentin Fuster

Emerging Technologies

2017;():V001T08A001. doi:10.1115/SMASIS2017-3727.

High performance airfoils with laminar airflow exhibit minimum drag and maximum lift, but tend to sudden stall due to flow separation at low air speed. This requires an increased approach speed of the aircraft, resulting in less steep approaches and a higher noise exposure of the surroundings. New active vortex generators, deployed only on demand at low speed, energizing the boundary layer of air flow and reducing flow separation, can help to overcome this critical situation. Active hybrid composites, combining the actuation capability of shape memory alloys (SMA) with the possibility of tailoring the compliance of fiber reinforced polymers (FRP) on the materials level, provide an active aerodynamic system with high lightweight potential and small space requirements. Being one of the first applications of active hybrid structures from SMA and FRP we will demonstrate the potential of this new technology with an integrated system of active vortex generators for a glider. In this contribution we present - the design process, based on a FE-model and careful characterization of the actuating SMA and the composite material - manufacturing relevant aspects for reliable series production - the testing of single vortex generators in lab scale under aerodynamic load - and an overview of the whole system.

Commentary by Dr. Valentin Fuster
2017;():V001T08A002. doi:10.1115/SMASIS2017-3747.

A Helmholtz resonator is a passive acoustic device that enables noise reduction at a given frequency. This frequency is directly related to the volume of the resonator and to the size of the neck that couples the resonator to the acoustic domain. In other words, controlling the volume of the cavity allows a real time tunability of the device, which means noise control at any desired frequency. To that end, we propose an Origami-based tunable Helmholtz resonator. The design is inspired from the well-known origami base, waterbomb. Such foldable structures offer a wide range of volume shifting which corresponds to a frequency shifting in the application of interest. The foldability of the structure is first investigated. Then, a series of numerical simulations and experimental tests were preformed are presented, in order to explore the capabilities of this origami structures in acoustic control. A shift in the frequency domain of up to 197 Hz (131–328 Hz) was achieved in an experimental testing using 3D printed rigid devices.

Topics: Noise control
Commentary by Dr. Valentin Fuster
2017;():V001T08A003. doi:10.1115/SMASIS2017-3773.

This paper introduces a 4D printing method to program shape memory polymers (SMPs) during fabrication process. Fused deposition modeling is employed to program SMPs during depositing the material. This approach is implemented to fabricate complicated polymeric structures by self-bending features without need of any post-programming. Experiments are conducted to demonstrate feasibility of one-dimensional (1D)-to 2D and 2D-to-3D self-bending. It is shown that 4D printed plate structures can transform into 3D curved shell structures by simply heating. A 3D macroscopic constitutive model is developed to predict thermo-mechanical behaviors of the printed SMPs. Governing equations are also established to simulate programming mechanism during printing process and shape change of self-bending structures. In this respect, a finite element formulation is developed considering von-Kármán geometric non-linearity and solved by implementing iterative Newton-Raphson scheme. The accuracy of the computational approach is checked with experimental results. It is shown that the structural-material model is capable of replicating the main features observed in the experiments.

Commentary by Dr. Valentin Fuster
2017;():V001T08A004. doi:10.1115/SMASIS2017-3808.

As the soft robotics industry continues to grow, the need for new materials and simplified manufacturing techniques are essential. Of interest is the development of highly flexible strain sensors that are easily integrated into these robotic components. Current strain sensing solutions using piezoresistive materials often involve complex fabrication techniques with multiple steps. Recent work by the authors has shown that thermoplastic polyurethane/multiwall carbon nanotubes (TPU/MWCNT) has good piezoresistive behavior and can be easily fabricated into strain sensors using Fused Deposition Modeling (FDM). This work expands upon that effort to characterize the mechanical properties of FDM-printed TPU/MWCNT as a function of the FDM processing parameters. In this study, the air gap, raster orientation, and MWCNT weight percent were varied and tensile tests performed. The stress-strain behavior, modulus of elasticity, and ultimate tensile strength (UTS) are compared to assess the influence of the processing conditions. Optical microscopy was also carried out to correlate the mechanical behavior to the printed mesostructures. The results show that with increased MWCNT content, the UTS decreased by as much at 47% for 2wt.%MWCNT, while the modulus of elasticity increased by 54%, compared to those of pure TPU. The results of this work provide an understanding of the mechanical performance in relation to the print parameters and sets the base to tune the mechanical properties of printed flexible functional nanocomposites.

Commentary by Dr. Valentin Fuster
2017;():V001T08A005. doi:10.1115/SMASIS2017-3820.

Memristors are solid-state devices that exhibit voltage-controlled conductance. This tunable functionality enables the implementation of biologically-inspired synaptic functions in solid-state neuromorphic computing systems. However, while memristors are meant to emulate an intricate signal transduction process performed by soft biomolecular structures, they are commonly constructed from silicon- or polymer-based materials. As a result, the volatility, intricate design, and high-energy resistance switching in memristive devices, usually, leads to energy consumption in memristors that is several orders of magnitude higher than in natural synapses. Additionally, solid-state memristors fail to achieve the coupled dynamics and selectivity of synaptic ion exchange that are believed to be necessary for initiating both short- and long-term potentiation (STP and LTP) in neural synapses, as well as paired-pulse facilitation (PPF) in the presynaptic terminal. LTP is a phenomenon mostly responsible for driving synaptic learning and memory, features that enable signal transduction between neurons to be history-dependent and adaptable. In contrast, current memristive devices rely on engineered external programming parameters to imitate LTP. Because of these fundamental differences, we believe a biomolecular approach offers untapped potential for constructing synapse-like systems. Here, we report on a synthetic biomembrane system with biomolecule-regulated (alamethicin) variable ion conductance that emulates vital operational principals of biological synapse. The proposed system consists of a synthetic droplet interface bilayer (DIB) assembled at the conjoining interface of two monolayer-encased aqueous droplets in oil. The droplets contain voltage-activated alamethicin (Alm) peptides, capable of creating conductive pathways for ion transport through the impermeable lipid membrane. The insertion of the peptides and formation of transmembrane ion channels is achieved at externally applied potentials higher than ∼70 m V. Just like in biological synapses, where the incorporation of additional receptors is responsible for changing the synaptic weight (i.e. conductance), we demonstrate that the weight of our synaptic mimic may be changed by controlling the number of alamethicin ion channels created in a synthetic lipid membrane. More alamethicin peptides are incorporated by increasing the post-threshold external potential, thus leading to higher conductance levels for ion transport. The current-voltage responses of the alamethicin-based synapse also exhibit significant “pinched” hysteresis — a characteristic of memristors that is fundamental to mimicking synapse plasticity. We demonstrate the system’s capability of exhibiting STP/PPF behaviors in response to high-frequency 50 ms, 150 mV voltage pulses. We also present and discuss an analytical model for an alamethicin-based memristor, classifying that later as a “generic memristor”.

Commentary by Dr. Valentin Fuster
2017;():V001T08A006. doi:10.1115/SMASIS2017-3821.

Photochemical actuation systems, those that employ coupled photo-stimuli and chemical reactions to power and control mechanical motion, have the potential to combine the benefits of precise light driven control with chemical energy storage. Furthermore, these systems are inherently soft, making them ideal for use in the emerging field of soft robotics. However, such systems have received comparatively little attention, perhaps due to the poor cycle life and limited activation time of past systems. Here we address these two challenges by switching from the technique of past systems, that of aqueous photoacid solutions and pH-responsive hydrogel actuators, to one employing organic solvents instead. While this switch of solvents successfully eliminates cycle life constraints and allows for tuning of the activation recovery time it also shifts the relative activation point of the hydrogel actuator in such a way that actuation is no longer observed. Several options for addressing this are discussed, with the prospect of using the lessons learned within to make a more informed selection of a different photoacid compound considered the most feasible. While the exploration of photochemical actuation systems is still in a nascent stage, we have great hope for such systems to form the basis of future smart machines with unique functionality.

Commentary by Dr. Valentin Fuster
2017;():V001T08A007. doi:10.1115/SMASIS2017-3829.

Self-healing materials have emerged as an alternative solution to the repair of damage in fibre-reinforced composites. Recent developments have largely focused on a vascular approach, due to the ability to transport healing agents over long distances and continually replenish from an external source. However fracture of the vascular network is required to enable the healing agents to infiltrate the crack plane, ceasing its primary function in transporting fluid and preventing the repair of any further damage events. Here we present a novel approach to vascular self-healing through the development and integration of 3D printed, porous, thermoplastic networks into a thermoset matrix. This concept exploits the inherently low surface chemistry of thermoplastic materials, which results in adhesive failure between the thermoplastic network and thermoset matrix on arrival of a propagating crack, thus exposing the radial pores of the network and allowing the healing agents to flow into the damage site. We investigate the potential of two additive manufacturing techniques, fused deposition modeling (FDM) and stereolithography, to fabricate free-standing, self-healing networks. Furthermore, we assess the interaction of a crack with branched network structures under static indentation in order to establish the feasibility of additive manufacture for multi-dimensional 3D printed self-healing networks.

Commentary by Dr. Valentin Fuster
2017;():V001T08A008. doi:10.1115/SMASIS2017-3855.

Additive manufacturing (AM) offers a new and unique method for the fabrication of functional and smart material and structures. In this method, parts are fabricated directly from a 3D computer model layer by layer. Fused deposition modeling (FDM) is the most widely adapted AM method. In this method, the feedstock is usually a thermoplastic-based material. In recent years, flexible smart materials have gained unflagging interests due to their promising applications in health monitoring, sensing, actuation, etc. However, the 3D printing of flexible materials is recent with its own challenges and limited sources of feedstock.

Conductive polymer nanocomposites (CPNs) have many promising uses within sensing filed including liquid sensing. Sensing chemical leakage is one the important capabilities of liquid sensors. There is a good number of studies on the fabrication and sensitivity characterization of CPN-based liquid sensors. However, the sensitivity and response time of CPN-based liquid sensors do not yet meet the industrial demands and should be further enhanced for their practical and widespread applications.

This study presents an attempt to integrate the tunability of CPN’s conductivity behavior and the design flexibility of 3D printing to explore the benefits that their coupling may offer toward more sensitive and/or faster liquid sensing. Thermoplastic polyurethane/multiwalled carbon nanotube (TPU/MWCNT) nanocomposites were selected as a model material system and their filaments were first fabricated using melt-mixing by twin-screw extruder at 1, 2 and 3 wt.% of MWCNT. Flexible U-shaped TPU/MWCNT specimens were designed and successfully 3D-printed as a liquid sensor. Specimens fabricated at three different raster patterns of linear, 0–90, and 45/−45 and three infill percent levels of 100, 75, and 50%. Ethanol was used as the model chemical and the resistivity change of the sensors was measured as a function of time when immersed in ethanol. The results revealed that the printed sensors greatly outperformed the pressed bulk counterparts. This enhancement in the 3D printed sensors was primarily due to the increased surface area, and thus higher surface/volume ratio, enabling faster liquid uptake. In addition, MWCNT content, raster pattern, and infill percent all affected the overall response time as well as the sensor sensitivity. This work suggests that highly sensitive liquid sensors can be developed by material and structure optimizations via FDM 3D printing.

Commentary by Dr. Valentin Fuster
2017;():V001T08A009. doi:10.1115/SMASIS2017-3856.

The flexibility offered by additive manufacturing (AM) technologies to fabricate complex geometries poses several challenges to non-destructive evaluation (NDE) and quality control (QC) techniques. Existing NDE and QC techniques are not optimized for AM processes, materials, or parts. Such lack of reliable means to verify and qualify AM parts is a significant barrier to further industrial adoption of AM technologies.

Electromechanical impedance measurements have been recently introduced as an alternative solution to detect anomalies in AM parts. With this approach, piezoelectric wafers bonded to the part under test are utilized as collocated sensors and actuators. Due to the coupled electromechanical characteristics of piezoelectric materials, the measured electrical impedance of the piezoelectric wafer depends on the mechanical impedance of the part under test, allowing build defects to be detected. This paper investigates the effectiveness of impedance-based NDE approach to detect internal porosity in AM parts. This type of build defects is uniquely challenging as voids are normally embedded within the structure and filled with unhardened model or supporting material. The impact of internal voids on the electromechanical impedance of AM parts is studied at several frequency ranges.

Topics: Porosity
Commentary by Dr. Valentin Fuster
2017;():V001T08A010. doi:10.1115/SMASIS2017-3873.

In the field of Additive Manufacturing (AM), one of the major applications of laser-based 3D metal printing is the creation of custom implants for medical purposes. However, a significant challenge in the manufacturing of implants using Selective Laser Melting (SLM) is the formation of partially melted particles on the surface of medical implants. These particles result in a multitude of issues including plurality of structurally weak points on the designed implants, obstruction of important design features, and possibility of dislodgement over the service life span, thereby posing a threat to the recipient. To address the above challenges, it is imperative to develop a simple but effective surface cleaning method to remove partially melted particles from the surface without damage to the designed medical implants.

In this work, a comparative study was conducted to investigate the effect of both chemical and electro-plasma based cleaning processes on the removal of partially melted particles from the surfaces of 3D printed Ti-6Al-4V medical screw implants. These techniques include chemically polishing implants with HF-HNO3 acid solutions and using an electro-plasma based cleaning process. With the field of additive manufacturing rapidly expanding, this work offers valuable insight on proper post-process treatment of 3D printed parts for future medical purposes in biomedical fields.

Commentary by Dr. Valentin Fuster
2017;():V001T08A011. doi:10.1115/SMASIS2017-3907.

Printing technologies are attractive methods for high-throughput additive manufacturing of nanomaterials-based thin film electronics. Roll-to-roll (R2R) compatible techniques such as gravure printing can operate at high-speed (1–10 m/s) and high-resolution (< 10 μm) to drive down manufacturing costs and produce higher quality flexible electronic devices. However, large-scale deployment of printed wireless sensors, flexible displays, and wearable electronics, will require greater understanding of the printing physics of nanomaterial-based inks in order to improve the resolution, reliability, and uniformity of printed systems.

In this study, we designed and constructed a custom sheet-fed gravure printer which features registered multilayer printing for nanomaterial exploration and thin film device development. The design allows precise, independent control of the speeds and forces of each of the subprocesses of gravure (ink filling, wiping, and transfer), enabling novel experimental controls for dissecting the printing process fluid mechanics. We use these new capabilities to investigate the primary artifacts which distort printed nanomaterial patterns, such as dragout tails, edge roughness, and pinholes. These artifacts are studied as a function of print parameters such as contact pressure, wiping speed, and transfer speed, by printing silver nanoparticle ink to form continuous features with dimensions in the range of 100 μm to 10 mm. We found that the contact mechanics of the ink transfer process have a strong influence on the formation of dragout artifacts, indicating the presence of a transfer-driven squeezing flow which distorts the trailing edges of features. By engineering the transfer contact mechanics with varying rubber substrate backing stiffness, we found it is also possible to suppress this artifact formation for a particular nanomaterial ink. The improved areal uniformity and print quality achieved using these methods highlight the potential for gravure printing to be a versatile nano-manufacturing tool for patterning a variety of thin film smart materials. We also hope that the open-source printer designs presented here can serve to accelerate the development of high-speed nanomaterial printing.

Commentary by Dr. Valentin Fuster
2017;():V001T08A012. doi:10.1115/SMASIS2017-3917.

Network theory is used to formulate an atomistic material network. Spectral sparsification is applied to the network as a method for approximating the interatomic forces. Local molecular forces and the total force balance is quantified when the internal forces are approximated. In particular, we compare spectral sparsification to conventional thresholding (radial cut-off distance) of molecular forces for a Lennard-Jones potential and a Coulomb potential. The spectral sparsification for the Lennard-Jones potential yields comparable results while spectral sparsification of the Coulomb potential outperforms the thresholding approach. The results show promising opportunities which may accelerate molecular simulations containing long-range electrical interactions which are relevant to many multifunctional materials.

Commentary by Dr. Valentin Fuster
2017;():V001T08A013. doi:10.1115/SMASIS2017-3919.

The Maximum Entropy (ME) method is shown to provide a new approach for quantifying model uncertainty in the presence of complex, heterogeneous data. This is important in model validation of a variety of multifunctional constitutive relations. For example, multifunctional materials contain field-coupled material parameters that should be self-consistent regardless of the measurement. A classical example is piezoelectricity which may be quantified from charge induced by stress or strain induced by an electric field. The proposed tools provide new statistical information to address measurement discrepancies, guide model development, and catalyze materials discovery for data fusion problems. The error between the model outputs and heterogeneous data is quantified and used to formulate a second moment constraint within the entropy functional. This leads to an augmented likelihood function that weights each individual set of data by its respective variance and covariance between each data set. As a first step, the method is evaluated on a piezoelectric ceramic to illustrate how the covariance matrix influences piezoelectric parameter estimation from heterogeneous electric displacement and strain data.

Commentary by Dr. Valentin Fuster
2017;():V001T08A014. doi:10.1115/SMASIS2017-3928.

In this article, it is proposed that a membrane with tunable ionic conductivity can be used as a separator between the electrodes of a supercapacitor to both allow normal charge/discharge operation and minimize self-discharge when not in use. It is shown that the redox active conducting polymer PPy(DBS), when polymerized on a porous substrate, will span across the pores of the membrane. PPy(DBS) is also shown to function as an ionic redox transistor, in which the transmembrane ionic conductivity of the polymer membrane is a function of its redox state. The PPy(DBS) ionic redox transistor is applied between the electrodes in a supercapacitor as a smart membrane separator. It is demonstrated that the maximum tunable ionic conductivity of the smart membrane separator is comparable in operation to an industry standard separator at maximum ionic conductivity, with a self-discharge leakage current of ∼0.12mA/cm2 at 1V. The minimum tunable ionic conductivity of the smart membrane separator is shown to decrease the supercapacitor self-discharge when not in use by a factor of 10, with a leakage current of 0.012mA/cm2 at 1V. This range of tunable ionic conductivity could lead to the emergence of redox transistor batteries with high energy density and low self-discharge for short and long-term storage applications.

Commentary by Dr. Valentin Fuster
2017;():V001T08A015. doi:10.1115/SMASIS2017-3986.

Recent work has used self-folding origami inspired composites to produce complex, scalable, affordable, and lightweight morphing structures [1]. These characteristics are of interest for engineering applications, in fields including aerospace [2] and medical devices [3]. Due to these advantages, research on self-folding smart composites has grown, with a particular focus on the use of laminate manufacturing techniques that stack layers of heterogeneous materials to generate functional composites. Previous work used this approach to manufacture self-folding origami inspired robots [1]. A simple shape memory composite design consists of a smart material (e.g. a one-way shape memory polymer, or SMP) sandwiched between patterned rigid layers. These SMPs change their shape in response to an external stimulus (e.g. temperature). Upon heating above the phase transition temperature of the polymer (Tt), the SMP contracts, causing the laminate to fold. The SMPs used in self-folding laminate composites are unidirectional and thus the laminate is unable to recover its original state without application of external force. In this work, we study the use of thermal responsive liquid crystal elastomers (LCE) for reversible self-folding and actuation of origami inspired composites using laminate manufacturing. LCEs are smart materials that exhibit reversible deformation, good strain recoverability, and tailorable properties (i.e. phase transition temperature, strain, and orientation of deformation) [4–6]. We explore two composite hinge designs using laminate manufacturing process [1, 7] with a Joule heating layer to enable self-folding: one where the LCE acts as a tensile actuator connected only on the edges of the rigid layer, which we call a tensional hinge, and a second where the LCE is attached along the patterned rigid layer hinge, which we call a flexural hinge. The angular displacements of these two hinge designs are estimated using geometric models that account for the contraction of the LCE upon heating, and compared against experimental measurements. The maximum blocked torque of the composite hinges is also measured experimentally. To demonstrate the use of LCE as an active layer for origami inspired composites, we also present a laminate crawler robot. The crawling locomotion is controlled with an electrical heating layer laminated on the LCE. These results demonstrate the possibility of using LCE to achieve rapid, reversible folding and to generate similar torques, as compared to previous work in origami inspired self-folding composite.

Commentary by Dr. Valentin Fuster
2017;():V001T08A016. doi:10.1115/SMASIS2017-3987.

The “flasher” pattern (an origami base that folds into a 3D structure that can be radially deployed into a 2D surface) has been recognized for its potential application in the deployment of large structures from relatively small volumes. Such structures can be internally deployed by smart materials or stored strain energy, or externally deployed by actuators or inertial forces. Various flasher folding patterns can be created by varying three basic geometric parameters: (1) the number of sides of the center polygon, (2) the number of rings comprising the array, and (3) the number of radially-distributed elements of each ring. In this paper, these three parameters are studied for their effect on dynamic performance, using multi-body dynamic (MBD) simulation software. As a basis for comparison, all the designs are held to the same surface area in the deployed flat state. Each MBD model is created automatically by a series of previously reported scripts that transform a crease pattern into a fully defined engineering model. The primary focus is to investigate the variation of (a) the deployment time, (b) reaction torque at the center of the flasher, (c) force and torque distribution in the entire structure, (d) bending angle of the panels, and (e) rigid foldability. An experimental test-bed is also described, with provision of preliminary physical validation results. The overall effort provides insight to force distribution within the structure, which can guide the placement of integrated smart material actuators. The results also help in flasher design parameter decisions by giving insight into their effect on future applications such as star occulter designs, solar arrays, solar reflectors, sunshields, smallsat antennas, and solar sails.

Commentary by Dr. Valentin Fuster
2017;():V001T08A017. doi:10.1115/SMASIS2017-4011.

Recently, carbon fiber-reinforced thermoplastics (CFRTPs) have become popular choices in desktop-based additive manufacturing, but there is limited information on their effective usage. In Fused Deposition Modeling (FDM), a structure is created by layers of extruded beads. The degree of bonding between beads, bead orientation, degree of interlayer bonding, type of infill and the type of material, determines overall mechanical performance. The presence of chopped fibers in thermoplastics increases melt viscosity, changes coefficients of thermal expansion, may have layer adhesion issues, and causes increased wear on nozzles, which makes FDM fabrication of thermoplastic composites somewhat different from neat thermoplastics. In the current work, best practices and the effect of annealing and infill patterns on the mechanical performance of FDM-fabricated composite parts were investigated. Materials included commercially available PLA, CF-PLA, ABS, CF-ABS, PETG, and CF-PETG. Two sets of ASTM D638 tensile and ASTM D790 flexural test specimens with 3 different infill patterns and each material were fabricated, one set annealed, and all tested. Anisotropic behavior was observed as a function of infill pattern. As expected, strength and stiffness were higher when the beads were oriented in the direction of the load, even for neat resins. All fiber-filled tensile results showed an increase in stiffness, but surprisingly, not in strength (likely due to very short fiber lengths). Tests of annealed specimens resulted in clear improvements in tensile strength, tensile stiffness and flexural strength for PLA, CF-PLA, and PETG, CF-PETG but a reduction in flexural stiffness. Also, annealing resulted in mixed improvements for ABS and CF-ABS and is only useful in certain infill patterns. This work also establishes ‘Best Practices’ of FDM-type fabrication of thermoplastic composite structures and documents the minimum critical fiber lengths and fiber fractions of several CF-filled FDM filaments.

Commentary by Dr. Valentin Fuster

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