This PDF file contains the front matter associated with SPIE Proceedings Volume 10968 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Returning samples from Mars to Earth in a future NASA mission would require protection of our planet from the potential risks of bringing uncontrolled biological materials back with the samples. Part of the planetary protection process would involve a process called “breaking the chain of contact (BTC)”, where any returned material reaching Earth for further analysis would have to be sealed inside a container at extremely high confidence. The sterilization process would require destroying any potential biological materials that may contaminate the external surface of the container. A novel process for containing returning samples has been conceived and is under development at JPL. The process consists of using induction heated brazing to synchronously sterilize, separate, seam and seal the container. The braze material is heated to the melting temperature that is higher than 500°C and, thus, it assures sterilizing exposed areas since all carbon bonds are broken above this temperature. The container consists of double walls with “Earth clean” interstitial space. The process consists of two-steps, where one is intended to be prepared on Earth and the second would be executed on orbit around Mars. The latest results of this study will be described and discussed.

Photostrictive polymers are a shape memory material that actuates between two mechanical states due to application of different wavelengths of light. Photostrictive materials often exhibit large strains during this process. In this work a novel photostrictive polymer was synthesized using a similar procedure to the synthesis of nylon threads. A monomer containing azobenzene, a photoreactive switch, was synthesized. The novel monomer was polymerized into a material with similar properties to that of nylon where the photoswitch replaces some of the alkyl units created by the adipoyl chloride monomer. This creates a new photostrictive thread alternative to flat sheets for artificial muscle applications. Additionally, the photoactive monomer and the original nylon monomer, adipoyl chloride, were used concurrently to produce a copolymer with varying degrees of the photoactive unit. Utilizing a copolymer methodology allows the innovative thread-like material to be tunable, by varying the amount of photoactive units embedded into the polymer strand. Ratios of the photoswitch unit to adipoyl chloride of 1:5, 1:10, 1:25, and 1:50 were synthesized. The synthesized polymer strands were dissolved in formic acid so that the incorporation of the photoactive units could be measured using Ultra Violet –Visible (UV-Vis) spectroscopy. The absorbance at varying wavelengths was determined and compared to the base polymer, nylon. All azobenzene samples exhibited a direct correlation to an absorption band at 345 nm and the concentration of the photoswitch unit.

This study investigates the broadband vibration attenuation mechanism of optimal cylindrical metamaterial inclusions embedded within a hollow tubular beam. The optimal metamaterial inclusions are obtained by leveraging a genetic algorithm with an analytical model of the system. The analytical model is formulated on an energy method and approximately solved by the Ritz method. Experimental efforts affirm that the model identifies the optimal metamaterial inclusions to best provide broadband vibration control capability. The results suggest that optimal inclusions often exhibit quasi-solid cross-section geometries to maximize damping behaviors similar to constrained layer dampers. Results from this research give the insight on the most influential vibration attenuation mechanisms by elastomeric metamaterials.

In the paper, we discuss the use of the homogenized energy model (HEM) to develop a dynamic mode decomposition surrogate model for a PZT bimorph actuator used for micro-air vehicles including Robobee. The HEM quantifies the nonlinear, hysteretic, and rate-dependent behavior inherent to PZT in highly dynamic operating regimes. Due to the computation complexity of the HEM, we must develop a surrogate model. The surrogate model must be parameter- and control-dependent to be able to perform inverse problems or uncertainty quantification in different driving regimes. In the literature, DMD can be adapted to address different control inputs. We will discuss using interpolation over the parameters to adapt the DMD to include parameter dependence. Finally, we will discuss the results and limitations of the new surrogate model.

In this paper, we investigate hyperelastic and viscoelastic model parameters using Global Sensitivity Analysis (GSA). These models are used to characterize the physical response of many soft-elastomers, which are used in a wide variety of smart material applications. Recent research has shown the effectiveness of using fractionalorder calculus operators in modeling the viscoelastic response. The GSA is performed using parameter subset selection (PSS), which quantifies the relative parameter contributions to the linear and nonlinear, fractionalorder viscoelastic models. Calibration has been performed to quantify the model parameter uncertainty; however, this analysis has led to questions regarding parameter sensitivity and whether or not the parameters can be uniquely identified given the available data. By performing GSA we can determine which parameters are most influential in the model, and fix non-influential parameters at a nominal value. The model calibration can then be performed to quantify the uncertainty of the influential parameters.

Plastic bonded explosives (PBXs), consisting of high energy density energetic crystals in a polymer binder, are a class of energetic materials which have been widely studied in regards to their shock and ignition response. Of increasing interest is the response of such energetic materials to non-shock mechanical insults, e.g. accidental drop and dynamic/vibration loads in transport, which have been observed to produce localized damage and thermal loading due to the formation of hot spots. In some cases, the formation of these hot spots can lead to sufficient levels of localized heating capable of sustaining chemical reactions and transitioning to detonation. While there are several proposed mechanisms which could drive the formation of hot spots, the primary driver(s) for sustaining the chemical reaction and triggering detonation are not well understood. This is in part due to the difficulty in experimentally characterizing the distribution and interaction of hot spots at the mesoscale, given the small length and time scales over which they exist. Recently, Seidel and co-workers have explored the application of the distribution of carbon nanotubes within the binder phase of energetic materials as a means of introducing a significant piezoresistive response within the energetic material. Doing this can provide a means for strain and damage sensing at the mesoscale. While initial fabrication and testing of ammonium perchlorate and sugar-mock PDMS- and epoxy-binder energetic materials have provided initial proof-of-concept demonstrations of strain and damage sensing, successful application towards locating and characterizing damage and hot spots requires greater understanding of the piezoresistive network at the mesoscale, and how it responds to localized heating. In this work, a mesoscale model corresponding to a representative volume element of an energetic material having a piezoresistive carbon nanotube nanocomposite binder is developed and subjected to localized heating. An electro-thermo-mechanical peridynamics formulation is developed which includes the generation of heat energy due to fracture and friction, and is applied to assess the differences between strain and damage sensing. Efforts are also made to assess the response of the mesoscale sensing network to localized heating and damage due to the presence of and interactions between increasing amounts of prescribed hot spots. Initial modeling results from these simulations reveal that the distribution of localized heating (leading to interactions between heat sources) and heating rate are strong indicators of whether or not such thermally induced damage will propagate beyond its local origin.

Coiled nylon actuator is an important field of actuators in soft robotics due to its negative thermal expansion, large actuation stroke and high force output. Coiled nylon actuators are easily manufactured and an inexpensive material. They are produced by attaching one end to a rotating motor and at the bottom to a suspended mass. The bottom end is constrained to prevent rotation in the axial direction. After coiling, it is annealed to reduce any internal strain. Its means of actuation is due to the torqueing behavior when heated which is induced by nylons thermal expansion, but by constraining the rotation, its response is to displace in the negative axial direction. The goal in this study is to capture the displacement and force output of the nylon actuator using COMSOL Multiphysics with the thermomechanical properties, heat and suspended mass (force) being the inputs of the model. The model will heavily rely on the classical lamination theory where the on-axis thermomechanical properties are obtained first and then the off-axis reduced stiffness matrix and effective coefficient of thermal expansion is obtained. The COMSOL Multiphysics displacement and force output results will be compared to experimentally obtained results. The experimental results will be obtained using a TA Instrument Dynamic Mechanical Analysis (DMA) which will be used to perform a strain controlled dynamic temperature scan at a frequency of 1 Hz on the nylon actuator. Temperature will be scanned from room temperature 24°C to 150°C at 1 °C/min.

Fracture behavior of a martensitic Ni_50.3Ti_29.7Hf₂₀ (at.%) high-temperature shape memory alloy (SMA) under mode-I nominally isothermal loading was examined. The material was characterized using differential scanning calorimetry to identify the transformation temperatures and uniaxial tensile testing to portray the stress–strain behavior. Pre-cracked disk-shaped compact specimen was tested at room temperature and unstable crack growth was observed. Fracture toughness was measured using the associated load versus crack-mouth opening displacement record. Digital image correlation was implemented to obtain the strain field on the surface and near the crack-tip. Moreover, fracture surface was examined using scanning electron microscopy. The results provide key insights into the fracture properties of NiTiHf high-temperature SMAs.

In the past, several studies have explored the fundamentals and applications of deforming an elastic component using a shape memory alloy (SMA) component. Previous explorations have been primarily motivated by the capability of SMA actuation against a spring biasing load and dynamic response where energy dissipation upon perturbation or thermal tunability is desired. This current work instead explores elastically biased SMA components in the context of a static system, where both stress-induced and thermally-induced phase transformations are employed to reproduce and improve upon the advantages of the shape memory effect (SME). While deformation of an SMA component utilizing stress-free SME can only be mechanically generated and thermally recovered, a system composed of an elastically biased SMA component can generate and recover deformation both mechanically and thermally. Additionally, the applied stress necessary to induce deformation is thermally tunable in both systems, but the non-zero stress state of the elastically biased SMA component enables operation at higher temperatures. This study also introduces employing the same antagonistic concept as a low power intensive two-way actuating system that utilizes “impulsive” heating and cooling to generate and recover deformation, while the balance of internal reaction forces enables deformation to be maintained. In this work, experimental and finite element analysis (FEA) results will demonstrate the capabilities of an SMA component biased against a cantilevered beam composed of elastic material. The results from this investigation will also introduce an abstraction termed the equilibrium domain, which represents the range of equilibrium points in stress-strain-temperature space.

Pneumatic actuators are a lightweight, compliant means of exerting force and are especially promising for systems that interact with the human body. These actuators typically consist of an elastomeric bladder and a surrounding sleeve. The sleeve is created using traditional textile manufacturing processes (e.g., wrapping, braiding, and knitting) where the fibers within the textile sleeves are inextensible components that limit the local expansion of the bladder. By varying the fiber angles and applied pressure, different kinematic motions (extension, contraction, bending, and twisting) are achieved. Anti-symmetric fiber angles within the textile sleeve architecture result in axial and radial motion. In this experimental investigation, shape memory alloy (SMA) wires are integrated into two types of passive sleeves (wrapped and knitted) to increase the range of motion of the actuator. Applying pressure to the system pre-strains the martensitic SMA through radial expansion of the actuator. When heated above the austenite finish temperature, the shape memory effect enables contraction of the SMA wire. This fiber contraction causes reorientation of the textile architecture, resulting in additional motion. The effect of the control variables, air pressure and electrical current, on axial displacements and rotations are experimentally characterized using discrete marker tracking. This study explores two basic textile sleeve architectures for hybrid SMA-pneumatic actuators and sets the foundation for increased kinematic tailorability through the design of complex multi-functional actuating textile sleeves.

Multiple applications of shape memory alloys (SMA) involve operation under partial transformation (PT), where reversal of the transformation direction takes place while the material is in a mixed phase state. Typical applications of SMAs include: actuators in adaptive/morphing structures which should repeatedly reach various target shapes or to follow time trajectories at higher time rates; dampers vibrating pseudo-elastically under varying amplitudes of dynamic loads. While the thermo-mechanically coupled behavior of SMAs under full transformation has been studied during the past and various models have been proposed, their response under PT has yet to receive the required attention to fully unravel the potential of these materials. In this paper, an experimental study of SMA wires under PT is presented along with a modified constitutive model. The physical constitutive model of Lagoudas et al.,¹ is combined with a new expression of the hardening function to enable the accurate and efficient prediction of PT behaviour. The predicted PT response is correlated with isobaric, thermally induced PT cycle experiments. Very good agreement is obtained with measured partial cycles, especially for PT cycles formed near the middle of the major hysteresis loop. The new constitutive equations are included into a finite element framework to investigate the effect of PT on SMA actuation function in morphing airfoils for active load alleviation in large wind turbine blades, and numerical results are correlated with experimental data. The correlations prove the importance of PT behavior in the actuator performance of SMAs, resulting in substantially more accurate predictions in deformation, stress and temperature.

In this paper, a dynamic model for an artificial finger driven by Shape Memory Alloy (SMA) wires is presented. Due to their high energy density, these alloys permit the realization of highly compact actuation solutions with potential applications in many areas of robotics, ranging from industrial to biomedical ones. Despite many advantages, SMAs exhibit a highly nonlinear and hysteretic behavior which complicates system design, modeling, and control. In case SMA wires are used to activate complex robotic systems, the further kinematic nonlinearities and contact problems make the modeling significantly more challenging. In this paper, we present a finite element model for a finger prototype actuated by a bundle of SMA wires. The commercially available software COMSOL is used to couple the finger structure with the SMA material, described via the Müller-Achenbach-Seelecke model. By means of several experiments, it is demonstrated how the model reproduces the finger response for different control inputs and actuator geometries.

In this study, we investigate the hierarchical microarchitecture formation of magnetic barium hexaferrite (BF) platelets in polydimethylsiloxane (PDMS) using electric and magnetic field assembly technique. First, external fields are applied to the colloidal solution to form the microstructure before curing the composites. After microstructure formation, the composites are thermally cured to freeze the microstructure. We investigate two different cases in this study-(1) magnetic field processed composites and (2) multi-field processed composites, which were processed under both magnetic and electric fields. We observe that macro-chains formed due to simultaneous application of electric and magnetic fields had a much higher length compared to the macro-chains formed due to just magnetic field. For both cases individual BHFs are found to be oriented in the direction of the external field. The analysis of SEM microstructures using ImageJ and MATLAB showed that at least two different levels of hierarchies are present in the microstructure for both cases, which are referred to as BHF stacks and micro-chains. From the experimental quantitative microstructure analysis, BHFs are found to be slightly better oriented (magnetic easy-axis direction in relation to the external field) at all scales for the electric and magnetic field processed composites compared to just the magnetic field processed composites. Magneto-electrohydrodynamics modeling of the polymer-particulate mixture predicts a similar behavior. Computational simulations are performed wherein particulates, subjected to both DEP forces resulting from an applied electric field, and magnetic dipole interactions in response to applied magnetic field, are allowed to form quasi-equilibrium structures before locking in a final structure to represent curing. Results from simulation confirms the finding on longer macro-chain formation similar to the experiment for the case of magnetic and electric fields compared to just magnetic field. Analysis of the microstructures from simulation also confirms that multiple levels of hierarchies are present in the composites’ microstructure for both cases. In future, quantifying the corresponding metrics at each level of hierarchy will help to better understand the microstructure and can be served as input to the model and also used to validate the model.

Emerging interests in hardware security as well as environmental concerns have given rise to the field of transient or temporary electronics, which can be decommissioned by an external stimulus with minimal impact to the surrounding environment. In this study, an all graphene based film is produced by a one-step deposition process. The conversion of graphene oxide (GO) to reduced graphene oxide (rGO) depends on an interfacial reduction reaction. Control of processing conditions such as the underlying substrate, pH of GO and the film drying environment results in an ability to tailor the internal architecture of the films and their electronic properties. Furthermore, the ability to create masks for selective reduction of GO during deposition was also demonstrated, which was used to create intricate yet well-defined patterns and connections required in electronic circuits and devices. All graphene based freestanding films with selectively reduced GO were used in transient electronics application as circuitry and RFID tag patterns. Furthermore, the reversible and controllable hygromorphic actuation ability of GO films is demonstrated along with application in humidity sensing.

Polymer nanocomposites exhibit unique effective properties that do not follow conventional effective media approaches. The nanoparticle-polymer interphase has been shown to strongly influence the nanocomposites behavior due to its significant volume when the particles are nano-sized, affording an opportunity to tune the dielectric response of the resulting nanocomposite. In this study, we investigate the effects of TiO₂ nanoparticles on the electrical properties and the charges distribution and transport in polydimethylsiloxane (PDMS) nanocomposites. Impedance spectroscopy shows suppression of interfacial Maxwell-Wagner-Sillars (MWS) polarization accompanied by a reduction in the low frequency dielectric permittivity and loss at high temperatures in the presence of the TiO₂ nanoparticles. Thermally stimulated discharge current measurements confirm that the suppression of the interfacial polarization relaxations happens by redistributing or depleting the charges through the composite and hindering their mobility, potentially resulting in lower electrical conduction and higher breakdown strength. Although the model materials investigated here are TiO₂ nanoparticles and Sylgard 184 PDMS, our findings can be extended to other nanoparticulate-filled elastomer composites to design lightweight dielectrics, actuators and sensors with improved capabilities.

Polymer composites of particulate smart materials are increasingly relevant due to emerging trends in using additive manufacturing of replacement parts in automotive and aerospace applications. The mechanical properties of the polymer matrix is sufficiently well understood from decades of research and the mechanical properties of the interface between the particulate (smart material) and matrix (polymer) phase is not very well defined. The challenges associated with understanding the interface requires a multidisciplinary approach and expertise and varies dramatically between various material combinations. In this article, we establish a robust approach to experimentally determine the linear interface modulus using cohesive zone model and demonstrate its application for mechanoluminescent elastomeric composite. The phenomenon of light emission induced by any mechanical action is termed mechanoluminescence and is classified based on the applied load regime as fracto, plastico, and elastico-mechanoluminescence (EML). EML is repeatable as the applied load is within elastic limits and induced strains are recoverable. Repetitiveness paves way for utilization in several applications including, but not limited to, stress sensing, stress visualization, fatigue monitoring, damage detection and failure prevention. Previously published work on EML materials have mostly been exploratory in nature with motivation to fabricate brighter EML materials and characterize/model their mechanism of emission. Through such work, doped zinc sulfide phosphors and doped strontium aluminate phosphors have been reported to emit the brightest emission till date. Characterization studies generally involve testing composites with EML crystals impregnated in a polymeric matrix, or EML thin films grown on various substrates. With both approaches, the focus is generally placed on correlating light output to various applied macroscopic inputs. However, there are no known efforts focusing on interfacial adhesion of EML crystals with the surrounding matrix or the substrate. Preliminary experimental and numerical research work on elastico-mechanoluminescent (EML) composites revealed that the interfacial adhesion between particle and matrix played a significant role in stress transfer and EML emission. Contemporary models for particle-reinforced composites focus on estimating macroscopic composite properties [1, 2 and 3]. No known model focusses on stresses imparted to the functional filler particles at the interface. Since EML emission depends on stress on the filler EML particles and not the macroscopic applied stress, it becomes necessary to develop models for stress transfer between the matrix and the particle at the interface. Estimation of stress transfer at the interface requires knowledge of stress distribution in the elastomer matrix. Hence, the objective of the models to be developed for elastomer composites is to capture variations in macroscopic stress of the matrix due to softening of matrix and degradation of interface with fatigue. EML-Elastomer Composites Elastomers are highly non-linear elastic materials which undergo significant softening during first few actuation cycles (Mullins effect) [4]. Mechanical behavior is rate-dependent exhibiting hysteresis during cyclic loading [5], and fatigue analysis of this complex system requires knowledge about loading history, composite composition and filler properties [6]. Debonding of filler-matrix interface is also an important non-linearity that cannot be neglected. While focusing attention on damage and fatigue of filled elastomers, micromechanics based and finite element based approaches have been adopted in combination by researchers to estimate macroscopic mechanical properties of the composite [7, 8]. The micromechanics approach to constitutive modeling of particulate composites generally builds upon the pioneering works of J.D. Eshelby in 1957 and T. Mori and K. Tanaka in 1973 [9, 10]. J.D. Eshelby derived analytical solutions to problems involving ellipsoidal elastic inclusions in infinite elastic bodies which are in general referred to as Eshelby’s solutions. Mori and Tanaka modeled composites with N number of filler particles as ellipsoidal inclusions in perfect adherence with matrix. Extension of the Mori-Tanaka model to elastomeric composites requires consideration of nonlinear hyperelastic and viscoelastic behavior of matrix as well as particle debonding. The non-linearity from hyperelastic behavior of matrix is generally accounted for by finite strain deformations and strain energy based formulations (for example, Ogden’s model), while particle debonding is modeled through non-linear cohesive laws arrived at from experimental data. Cohesive law is the relationship between traction forces and displacements at the interface [11]. Viscoelastic damage and hysteresis have to be accounted for separately, by modifying appropriate energy functions to include viscous dissipation determined from experimental data [12, 13, and 14]. In this article, we establish a robust approach to experimentally determine the linear interface modulus using cohesive zone model and demonstrate its application for mechanoluminescent elastomeric composite. The experimental design uses photoluminescence property of phosphors (Osram Sylvania GG45 - ZnS:Cu) to determine damage propagation and digital image correlation (DIC) to determine stress and strain in the polymer composite at the micron scale.A sample measuring 15mm x 5mm x 1.5mm is prepared using PDMS and GG45 and a notch is introduced to the sample. A microscope is modified to pull the sample apart at a constant speed and the process is observed using a microscope objective and a 4K video camera. The photoluminescence of dispersed GG45 particles is adjusted by mixing black ink to the composite to control halo effect and subsequent processing in image processing. Stress and strain computed from NICORR running in MATLAB is coupled with the opening displacement to obtain a measure of stiffness in Stage-I, II and III stiffness in a sample of varying extents of initial damage. The stiffness measured from linear softening CZM response is used to obtain the interface stiffness that can be subsequently applied in finite element models for modeling the overall property of composite. We demonstrate that an undamaged sample has the highest stiffness. With the formation of micron scale damage in the polymer composite at the interface, the interface stiffness decreases and reduces the maximum load that can be applied to the material in subsequent cycles. References: [1] Allan Zhong, Wolfgang G. Knauss, X. (2000). Effects of particle interaction and size variation on damage evolution in filled elastomers. Mechanics of composite materials and structures, 7(1), 35-53. [2] Yeh, J. R. (1992). The effect of interface on the transverse properties of composites. International journal of solids and structures, 29(20), 2493-2502. [3] Nakamura, Y., Yamaguchi, M., Okubo, M., & Matsumoto, T. (1992). Effect of particle size on the fracture toughness of epoxy resin filled with spherical silica. Polymer, 33(16), 3415-3426. [4] Mullins, L., and N. R. Tobin. (1957). "Theoretical model for the elastic behavior of filler-reinforced vulcanized rubbers." Rubber Chemistry and Technology 30.2: 555-571. [5] Ravichandran, G., and C. T. Liu. (1995). "Modeling constitutive behavior of particulate composites undergoing damage." International Journal of Solids and Structures 32.6-7: 979-990. [6] Dorfmann, A., and Ray W. Ogden. (2004). "A constitutive model for the Mullins effect with permanent set in particle-reinforced rubber." International Journal of Solids and Structures 41.7:1855-1878.Eshelby, John D. (1957). "The determination of the elastic field of an ellipsoidal inclusion, and related problems." Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. Vol. 241. No. 1226. The Royal Society. [7] Tan, H., Huang, Y., Liu, C., Ravichandran, G., Inglis, H. M., & Geubelle, P. H. (2007). The uniaxial tension of particulate composite materials with nonlinear interface debonding. International Journal of Solids and Structures, 44(6), 1809-1822. [8] Toulemonde, P. A., Diani, J., Gilormini, P., & Desgardin, N. (2016). On the account of a cohesive interface for modeling the behavior until break of highly filled elastomers. Mechanics of Materials, 93, 124-133. [9] Eshelby, John D. (1957). "The determination of the elastic field of an ellipsoidal inclusion, and related problems." Proceedings of the Royal Society of London A: Mathematical, Physical and Engineering Sciences. Vol. 241. No. 1226. The Royal Society. [10] Mori, Tanaka, and K. Tanaka. (1973). "Average stress in matrix and average elastic energy of materials with misfitting inclusions." Acta Metallurgica 21.5:571-574. [11] Needleman, Alan. (1987). "A continuum model for void nucleation by inclusion debonding." Journal of applied mechanics 54.3:525-531. [12] Ravichandran, G., & Liu, C. T. (1995). Modeling constitutive behavior of particulate composites undergoing damage. International Journal of Solids and Structures, 32(6-7), 979-990. [13] Yang, B. J., Kim, B. R., & Lee, H. K. (2012). Micromechanics-based viscoelastic damage model for particle-reinforced polymeric composites. Acta Mechanica, 223(6), 1307. [14] Jung, G. D., Youn, S. K., & Kim, B. K. (2000). A three-dimensional nonlinear viscoelastic constitutive model of solid propellant. International Journal of Solids and Structures, 37(34), 4715-4732.

Flexible and highly sensitive piezoresistive nanocomposites have been demonstrated to possess considerable potential for monitoring structural integrity and human physiological performance. To enhance the mechanical and strain sensing properties of these nanocomposites, different nanofillers (e.g., metal nanowires, carbon nanotube, and graphene) have been incorporated in polymeric matrices to establish electrically conductive pathways that are also sensitive to applied strains. Their piezoresistivity mainly stem from nanofillers’ intrinsic piezoresistivity, tunneling effect, and contact resistance changes of the nanofiller networks. Although many high-performance nanocomposite strain sensors have been developed and using different techniques, the empirically guided fabrication approach can be laborious, inefficient, and, most importantly, unpredictable. Therefore, this study proposes a topological design-based approach to strategically control and manipulate the strain sensing performance of the nanocomposites, simply by altering its geometric pattern design. First, polyethylene terephthalate (PET) substrates were patterned with pre-designed hierarchical inhomogeneous topologies and kirigami cuts created using a laser cutter. Second, the substrates were spray-coated using a carbon nanotube (CNT)-latex to deposit the strain-sensitive thin films. Third, the strain sensing performance of the CNT-latex nanocomposite thin films of different topologies was characterized and compared. It was found that, as the initial solid mechanics analysis predicted, the hierarchical inhomogeneous topology effectively enhanced the nanocomposites’ strain sensitivity, while the kirigami cuts significantly reduced sensitivity. The proposed methodology can help guide the development of high-performance nanocomposites with pre-programmed sensing properties for structural and human health monitoring applications.

The gaining popularity of composites and their typical applications (e.g. aerospace, energy and defence) are driving the requirements for the dynamic characterisation of these materials. Carbon fibre reinforced polymers (CFRP), which are the main concern in this work, are composed of stiff, brittle fibres encased in epoxy resin. Their microstructure results in pronounced anisotropy which makes their characterisation challenging even in basic quasi-static mechanical tests. It must be pointed out that the anisotropy and heterogeneity lead to a complexity in behaviour of these materials including a number of failure mechanisms in the material that are activated by different loading conditions. Despite extensive research in the last three decades, a widely accepted and reliable failure theory for composites does not exist [1][2]. The work in progress, presented here, is related to development of the damage part of a constitutive model intended for modelling of high velocity impact on CFRP aerospace structures. The model is based on spectral decomposition of the material stiffness tensor and strain energy. The model development was supported by extensive mesoscale modelling of the effects of physical damage on the damage parameters related to the material deformation eigenmodes. This is done as part of an integrated effort to produce tools for modelling of high velocity impact on composites in the European project EXTREME**.

With the advent of 3D printing and the increasing list of available materials, various functional devices can be printed for low-cost, rapid prototyping. In particular, 3D-printed strain gauges show promise in multiple applications such as robotics and structural health monitoring. However, characterization and compensation of the thermal dependence of such strain gauges have been limited in the literature. In this work the temperaturedependent resistive behavior is characterized for strain gauges printed with a commercially available filament, conductive PLA (Polylactic Acid), which has also shown other desirable uses such as stiffness-tuning for soft robots. The relationship between temperature and resistance is shown to be hysteretic. Several compensation methods (Temperature-based algebraic subtraction, Material-based algebraic subtraction, and a Wheatstone bridge-based method) are explored to mitigate the effect of temperature and show the material’s feasibility as a strain gauge. The compensation methods are quantitatively compared by calculating the mean squared error between the predicted and the ground truth strain values. It is shown that the Wheatstone bridge-based method provides the best compensation. This method achieves average errors of less than 10% and a maximum error less than 20% over a working range of approximately 15,000 microstrain (0.15% strain) over a range 30 to 40°C.

We envision implementing direct ink writing for 3-D printing while aligning microfibers in the resin using standing wave ultrasonics; the aligned fibers would control desired mechanical properties such as strength and ductility, and 3-D printing would match the mechanical properties to the particular part geometry. At this time we work with highviscosity fluids as a physical simulant of representative resins, and spherical polystyrene microparticles or glass microrods instead of microfibers. In this paper we show experimental results using square glass capillaries (with interior dimensions ranging from 0.4 to 1.0 mm) as our microfluidic systems, which are inherently well-suited by their geometry to act as print nozzles, sandwiched between two piezoceramic plates that generate the ultrasonic standing waves. We report experimental data for particle alignment as we change from our initial test fluid, water, to high-viscosity fluids. Similarly, we report experimental data of the fluid behavior pertinent to direct ink writing; we enforce controlled volumetric flow rates (which correspond to print speeds) for high-viscosity fluids under pressurized flow through glass capillaries of varying cross-sectional areas and varying lengths, observing and measuring the approximate ink line width and height. Our use of commercially available square glass capillaries (sandwiched between piezoceramic transducers that are driven at frequencies away from transducer resonance) is novel and distinguishes our approach from that of other research groups; the underlying physics of our devices differs from that of Lund-type acoustic resonators.

Smart materials and structures are enabling complex shape change in applications from self-folding origami to metamaterials. This presentation will focus on recent work on modeling and design optimization of self-folding origami actuated by electrostrictive terpolymer and magneto active elastomer materials. Since finite element analysis of these soft materials with large deformations due to multi-field actuation can be computationally intensive, the problem is decomposed into two stages. The first stage optimizes a computationally efficient reduced order rigid body model, while the second stage implements the full finite element analysis only to refine the features. Ongoing work in optimal design of metamaterials enabled by functionally graded superelastic NiTi will also be highlighted.

Electroactive polymer (EAPs)-based technologies have shown promise in areas such as artificial muscles, aerospace, medical devices and soft robotics because of large electromechanical actuation at relatively high speed. The promises of EAPs have led us to study EAP-based grippers. The in-plane actuation of P(VDF-TrFE-CTFE) is converted into bending actuation using unimorph configurations, where one passive substrate layer is attached to the active polymer. On-demand segmented folding is harnessed from this pure bending actuation by creating notch samples with an aim to implement them for applications like soft robotics gripper. In this paper, we studied the effect of various design parameters of notched folding actuators to establish a design reference and maximize the actuation performance of EAP based devices. Both finite element analysis (FEA) and micromechanics based analytical study is performed to investigate the effect of actuator parameters on the folding actuation of notched samples. The notched configuration has been analyzed via FEA for the non-uniform deformations and stress-fields. FEA analysis shows the importance of notch positioning to maximize the electromechanical performance. On the other hand, analytical study has proposed a design curve for the selection of proper notch parameters (e.g. notch length and Young’s Modulus) to maximize the actuation performance. Finally, based on the FEA and analytical analysis, a human finger inspired ‘finger-like’ biomimetic actuator is realized by assigning multiple notches to the structure.

Origami cores have the advantage of stiffness to weight ratio over conventional sandwich cores. The main idea of designing sandwich cores is to have low-density structure with high stiffness, that allows the sandwiches to withstand compression and impact loads. Recently, the implementation of origami cores in sandwich structures has attracted engineers to design and investigate different types of origami cores for compression and impact applications. Such cores can give the sandwich structure, high stiffness to weight ratio, and they can dissipate the incoming loads by high strain deformation. In this work, we study numerically and experimentally, two different origami configurations; the study investigates origami unit cells’ and cores’ stiffness and load resistance. Origami unit cells and cores are fabricated with fused deposition modeling. The results of the numerical simulation validated with the experimental results.

This paper presents a framework for the design, fabrication, and experimental testing of self-folding origami structures that deform from two-dimensional forms towards three-dimensional goal shapes of arbitrary local Gaussian curvature via uniform heating. Due to the general inability of the widely employed unfolding polyhedra method to generate origami designs for structures having negative Gaussian curvature, a tuck-folding method is implemented for self-folding composites driven by shape memory polymer actuation. As implementation examples, meshes of a pyramid, a saddle, and a combination of both are chosen to represent surfaces of positive and negative Gaussian curvature, and all three structures are shown to successfully fold towards their intended goal shape. The presented framework can be applied to origami design problems that consider other goal shapes and active materials.

Morphing structures have been a subject of much research recently because of their promising potentials in aerospace, wind turbine, and many other applications. There exists many different approaches to achieve shape morphing, among which the origami-inspired folding is particularly interesting in that folding is fundamentally three-dimensional, scalable, and customizable. However, activating and attaining large amplitude folding autonomously are challenging. Active materials, such as shape memory alloys, have been used to activate folding, but they are limited due to the power supply requirement to maintain the folded configurations. One possible solution is to embed bi-stability into the origami structure. Bi-stability can play two significant roles: First, it can significantly reduce the actuation requirement to induce shape morning; and second, it can maintain the shape change without demanding sustained energy supply. In this study, we demonstrate the feasibility of using dynamic excitation to induce shape morphing (or folding) between the two stable states of water-bomb base. For the first time, we derive the dynamic equation of motion for a water-bomb base origami and use it extensively to analyze its time responses under harmonic excitation. Via numerical simulations, we show that by harnessing the intra-well resonance of the water-bomb structure, we can achieve rapid bi-directional morphing using relatively low actuation magnitudes in comparison with quasi-static loading.

bstract Novel sensors and energy harvesting transducers take advantage of the significantly expanded design space made possible by recent advances in structural magnetostrictive alloys. These alloys can be machined and welded, have high fracture toughness, and can actuate, sense, and carry load while subjected to tension, compression, and bending. The talk includes an introduction to magnetostrictive materials and transduction, and a discussion on the use of low-cost rolling and annealing methods in lieu of more costly crystal growth methods for making bulk iron-gallium (Galfenol) and iron-aluminum (Alfenol) alloys. The process of using magnetostrictive materials to convert mechanical energy into magnetic energy and then into electrical energy is explained and demonstrated using sensors and energy harvesting devices as examples. Examples of magnetostrictive devices include prototypes ranging in size from nanowire-based pressure sensors to huge structures floating in the ocean that convert wave energy into electrical power for “community-scale” energy needs. The recent discovery of a particularly unique attribute of these alloys, their auxetic behavior, will also be discussed. In both Galfenol and Alfenol, both strain and magnetic fields can produce simultaneous increases in lateral and longitudinal dimensions, with measured values of the resulting Poisson ratio being not only negative, but as low as -2.0 in some cases. Mechanical, aerospace and civil engineers should find the discussion on the use of magnetic fields to control auxetic behavior quite interesting.

Work on magnetoelastic particulate composites and magnetorheological (MR) fluids has traditionally focused on frequencies that are small compared to the ferromagnetic resonance (FMR) frequency. Under these conditions the structural or fluid dynamics may be of importance, but the magnetic response is essentially quasi-static. This is in contrast to the response of magnetic materials in microwave devices where the nonlinear spin dynamics present a variety of novel phenomena. Notably, shifting the FMR frequency controls the transmission, absorption, and reflection of electromagnetic waves, with potential non-reciprocal propagation. The present work provides an analysis of a magnetoelastic inclusion in solid and fluid dielectrics, and shows how mechanical loads can control the resonant and relaxation characteristics of these composites at microwave frequencies. Both analytical and numerical analysis of the coupled spin and mechanical dynamics will be provided. In the magnetic inclusion the equations of elastodynamics are coupled to the Landau-Lifshitz-Gilbert (LLG) equation. The considered magnetic anisotropy energies include Zeeman, magnetocrystalline, and magnetoelastic interactions. Altering the magnetocrystalline and magnetoelastic energies allows the magnetic moment to either rotate independent of the lattice or strongly couple to it (i.e., modeling superparamagnetic or permanent magnetics). Non-symmetric stress tensors arise in the strongly coupled case via the Maxwell stress. It will be shown under what loading conditions the effective magnetoelastic field shifts the FMR frequency and characteristic relaxation times. The effects are found to depend strongly on the crystallinity of the magnetic inclusion and nature of the applied load (i.e., hydrostatic pressure or general 3D stress state).

We proposed a new type of phase transforming auxetic material (PTAM) by embedding magnets into cellular auxetic material with rotating cubes. To find out the effect of magnets on the mechanics of PTAM, several samples were fabricated and attractive magnets were embedded. Quasi-static uniaxial compression tests were performed and the results show that the phase of the material can be successfully changed by embedding attractive magnets,which means this material exhibits from one phase to two phases.

The fabrication of micro-structured films with dynamic porosity is useful in various applications. This article presents fabrication of microcellular films exhibiting dynamic porosity upon applying an external pressure stimulus. Interestingly, we show that the dynamic porosity is in conjunction with an opaque to transparent transition (OTT) that imparts the material with a unique optomechanical behavior. The OTT experienced foams will have a transparency almost equal of the as-cast films. The foaming process used for this study is solid-state foming technique with CO₂ gas as physical blowing agent. A styrene-ethylene-butylene-styrene (SEBS) triblock copolymer was used as the foaming material. Polystyrene (PS) blocks in SEBS play a key role in this process since CO₂ reduces the T_g of PS and enable ethylenebutylene (EB) parts to swell during saturating stage while upon depressurization their T_g increases again that prevents the forming pores from collapse. A custom-made in-situ optomechanical setup was used to characterize the optical and mechanical behavior of the produced foams. Optomechanical tests at different strain rates and loads revealed that the films undergo a gradual OTT behavior indicating their ability to be used as optical pressure-sensitive films. Quenching temperature is of great importance in this process since the foams show various OTT behavior once the quenching temperature changes. Moreover, here we show after pressing the foams, the transparent films can be re-foamed by the same process for many cycles.

Colorless shape memory polyimide (CSMPI) has potential applications in broad fields, especially in advanced optoelectronics due to the excellent optical transparency, shape memory effect and high temperature resistance. In this work, CSMPI prepared by high flexible dianhydrides and fluorine-containing diamines has excellent optical transparency, shape memory properties and high temperature resistance. High flexible dianhydrides that makes the molecule chains more easily twisted and tangled to form physical crosslinking points is favorable for possessing great shape memory property. The fluorine-containing diamines effectively destroyed the highly conjugated molecular structure and inhibited the formation of CTC, ensuring the CSMPI with excellent optical transparency. The effects of monomer ratio and imidization temperature on the molecular structure and properties were discussed. The CSMPI film possesses a higher glass transition temperature (Tg) of 234 °C, compared to the reported transparent shape memory polymers (SMPs). Most importantly, the transmittance of CSMPI film is 87~90% at 450~800nm, meeting the requirements of heat resistance and transmittance of the substrate. Both shape recovery and shape fixity are over 97%. Flexible and colorless CSMPI films has potential applications in broad fields, especially in advanced optoelectronics, such as flexible substrates for OLED and OPV devices, etc.

This paper presents a design for a shape memory alloy (SMA) based flexible manipulator on miniature underwater Remote Operated Vehicles (ROVs). Conventional manipulators using separate electrical motor actuators and sensors can lead to problems in installation and control of miniature ROV during underwater operation. The size, weight and dimensions of the conventional manipulators are the main reasons for load interactions between ROV and manipulator which affects the stability and control. As an alternative approach here, an SMA wire trained with two-way shape memory effect and the concept of segment actuation is presented as a flexible manipulator for miniature ROV. Forward kinematic expressions as a function of segment length are developed and used for understanding the workspace and manipulation capabilities of the manipulator. Actuation models for heating and cooling of the segments independently and the experimental validation of the analytical expressions for underwater application are presented. The effect of fluid interaction on the conventional manipulator and SMA manipulator and the stability and control effect on ROV are also discussed.

This research investigates the methods of fabrication for non-traditional, non-rectangular bistable structures using Carbon Fiber Reinforced Polymers. Currently, the non-rectangular shapes that have been used are rhombi (diamonds), triangles, and circles. Each shape is cut from a 12x12 inch sheet of composite laminate. The shape, when cut, must maintain a 12-inch dimension in one aspect of height, diameter, or length. As these shapes are fabricated and postprocessed, it is observed that the boundary conditions, performance, curvature and options for fixturing vary significantly. It has also been observed that much of the remaining material from post-processing cutting methods also retain much of its disability, allowing for usage in alternative capacities.

Bistable composites are attractive for morphing structures because they can hold deformed shapes without actuation and can be driven by compact, embedded smart actuators such as piezoelectric laminae and shape memory alloys. Mechanically-prestressed bistable composites exhibit weakly-coupled cylindrical shapes when their prestressed laminae are orthogonal to each other. High-order analytical models have been developed to model the stability and actuation of mechanically-prestressed composites with two sources of residual stress. Based on these models, this paper presents a study on the effect of planform shape on shape-bifurcation phenomena in bistable plates. A high-order analytical model is presented and the shapes of composites with linearly-tapered planform are calculated. Model-based parametric studies are presented to calculate the sensitivity of stable shapes and actuation forces to variations in planform taper, spatial positions of the prestressed layers, and aspect ratio. The results guide the selection of geometric parameters for the design of bistable composites.

It is well-known that electrically conductive polymer composites can be fabricated via incorporating highly conductive fillers such as carbon fibres (CFs) and carbon nanotubes (CNTs) into a polymer system through either melt blending or solvent casting method. Nevertheless, one of the greatest challenges lies in the proper particle dispersion to achieve a low percolation threshold and high conductivity performance. Recently, it was found that CNTs have phasesensitive localization property when incorporated in a composite system formed by two immersible phases, such as polylactic acid (PLA) and thermoplastic polyurethane (TPU). As a result, composites with ultra-low percolation threshold can be formed by tuning the ratio of the two polymer phases. In this study, we reported that such property can be further enhanced via the introduction of a small amount of plasticizer into the polymer system. It was observed that the incorporation of poly(ethylene glycol) (PEG) affected the immiscibility of the two polymer matrix as significant changes in morphologies and thermal behaviours were also detected. Finally, by adding 5 wt% PEG, the electrical conductivity for sample contacting 2.5 wt% CNT increased from to 6.8x10^-6 to 3.6x10^-4 S/cm. Such results suggest that plasticizer is an effective agent for improving particle distribution and conductivity enhancement.

The use of a single material as a multifunctional insulator (i.e. substrate, gate dielectric, and passivation layer) in the same device will reduce cost and improve the sustainability of flexible integrated circuits. Towards that goal, use of the oblique-angle physicochemical deposition technique to fabricate columnar microfibrous thin films of Parylene C and thereby lower the relative permittivity motivated the examination of these materials as interlayer dielectrics in flexible electronics. The static Young’s moduli, yield strengths, and relative permittivity of columnar microfibrous thin films of Parylene C were correlated to the porosity, crystallinity, and the deposition angle. The Poole–Frenkel conduction mechanism is responsible for the DC leakage current at temperatures not exceeding 100°C, and the AC leakage current is attributable to small-polaron tunneling mechanism. The leakage current after the application of a constant-voltage stress for a certain duration conforms to the Kohlrausch– Williams–Watts relaxation model, and the capacitance of a columnar microfibrous thin film will degrade by 20% in 10 years.

Molecular dynamics (MD) simulations are considered to be an effective way to analyze the thermal behavior of nanocomposites for various components. In this study, we investigate the thermal conductivities of PDMS chains as well as the GNPs/PDMS composites by non-equilibrium molecular dynamics (NEMD) simulations. Firstly, the thermal properties of PDMS chains are calculated by taking account of the effects of temperature, length and number of chains. Then, a series of GNPs/PDMS composite models which have various PDMS chains are parallel to the surface of GNPs are proposed, and their thermal conductivities are studied in terms of the numbers and position of the chains. The simulation results show that the thermal conductivity of the PDMS chains linearly increases with their lengths and numbers. However, the temperature has minimum influence on thermal conductivity. The proposed GNPs/PDMS composite model with aligned GNPs/PDMS, which could avoid disordered and phonon scattering at interfaces has a huge increase in thermal conductivity compared to the PDMS chains. A case study shows that the position of the PDMS chains has effect on the thermal conductivity of GNPs/PDMS composites, and the PDMS chains in one side of GNPs has higher thermal conductivity than the PDMS chains in two sides of GNPs.

Active membranes with switchable pores that are based on hydrogels can be used to measure the cell size distribution in blood samples. The system investigated in the present research is based on a polyethylene terephthalate (PET) membrane that is surface polymerized with poly(N-isopropyl acrylamide) (PNiPAAm) to form active pores of arbitrary geometry. The PET membrane provides the functionality of a backbone for mechanical rigidity, while the soft PNiPAAm hydrogel forms the active pores. Modeling and simulation of the active hydrogel behavior proved to adequately predict the opening and closing of the pores under application of an activating stimulus, e.g. temperature. The applied model is called Temperature-Expansion-Model and uses the analogy of thermal expansion to model the volume swelling of hydrogels. The Normalized Extended Temperature-Expansion-Model can englobe arbitrary hydrogels and large geometric displacements. Studies of pore opening – performed by using commercial finite element tools – show good agreement of the experimentally measured shape change of active pores. Based on these studies, the particulate fluid flow through the switchable pores is analyzed. Through application of a membrane process, i.e. a given variation of applied pressure and switching stimulus for the hydrogel, the size profile of the blocking particles can be measured directly using the flux difference under constant pressure. This allows the measurement of the cell size distribution in blood samples, e.g. to detect circulating tumor cells or anomalies in the distribution that hint to anemia.

Wurtzite aluminum nitride (AlN) of space group P63mc has long been recognized as a non-ferroelectric material, lacking the polarization switching ability. This paper reports the induction of ferroelectricity in a single crystalline epitaxial AlN ultrathin film with a thickness of 8−10 nm. The ferroelectric AlN epilayer was grown on a single crystalline GaN layer, forming a [0001]-oriented AlN/GaN epitaxial heterostructure with two reversible polar variants: [000-1] and [0001]. The AlN epilayer exhibited soft ferroelectricity with large switching currents and a polarization value of ~3.0 μCcm^-2 during a 180° polarization switch. The AlN epilayer was prepared by the atomic layer deposition technique at 300°C in conjunction with in-situ atomic layer annealing. The two-dimensional electron gas (2DEG) at the AlN/GaN interface could be manipulated by the ferroelectric switching in the AlN epilayer. Strain engineering via lattice mismatch at the AlN/GaN interface was the key to creating a ferroelectric AlN/GaN heterojunction. Based on the reciprocal space mapping analysis, the AlN ferroelectricity is believed to be stemming from the out-of-plane compressive strain and inplane tensile strain present in the [0001]-oriented AlN epilayer. The discovery of low-temperature prepared, CMOScompatible AlN ultrathin films with soft ferroelectric characteristics will undoubtedly spur new fundamental and applied research in low-dimensional ferroelectric systems based on the AlN/GaN heterojunction.

In this paper, a first-order system with hysteretic characteristics is used to describe the hysteresis caused by the inertia of the internal magnetic domain of magnetostrictive materials. A shape function is introduced to describe the interaction of different magnetic domains inside of magnetostrictive materials and the saturation properties of the hysteresis. Under a proposed frame of “first-order system (inertial system) + shape function” (ISSF-Duhem model), a new hysteresis model is proposed for magnetostrictive actuators. Specifically, the expression of the first-order system is constructed based on its general expression, and the Grompertz function is employed as the shape function. Feasibility and capability of the hysteresis model are verified and evaluated by describing and predicting the hysteresis of a commercial magnetostrictive actuator.

Delamination of reinforced polymer materials is one of the catastrophic and an unsolved mystery for composite structures. Very often, a composite structure under shear loading experiences an interlayer fracture. Among many of the fracture modes, crack initiation due to the failure of the matrix is very common. The polymer matrix is weaker in mechanical properties than the fibers and shows inferior shear performance under different loading conditions. In this study, epoxy layers under shear loading have been analyzed by cohesive contact generated by the finite element method by ABAQUS. The development of stress and failure features have been identified by existing cohesive zone theories. A comparative analysis has been performed for composite structures fabricated with neat epoxy and epoxy modified with electrospun carbon nanofiber. The qualitative study indicates composites structure fabricated from nanomaterial modified matrix may show different cohesive zone. Therefore, composite materials reinforced with an additional phase in the matrix may exhibit different delamination mechanics under short beam shear loading. A detailed analysis of the cohesive contact parameters has been introduced and to characterize the delamination behavior in the matrix layer in between the interlaminar region has been discussed. The current study indicates a change in the cohesive zone is necessary for better representation of nanofiber modified epoxy contact than the conventional epoxy layer analysis.