A physical prototype of a human esophagus has been developed for reproducing the human swallowing process with the aim of studying various disorders that impair its function as well as for the development of new foods and technologies for their treatment. Several studies related to the peristalsis phenomena have been conducted in recent years by studying the effect of different parameters defining the peristaltic wave. Mathematical models have been developed to investigate the impact of an integral and a non-integral number of waves during the swallowing of food stuff such as jelly, tomato puree, among others. Swallowing through the esophagus has not only been studied numerically but also reported by using a pneumatic soft actuators. In the present work, the development of a soft actuator mechanism to reproduce the peristaltic wave as the one reported by F.J. Chen et. al. 2014 is described. Such a mechanism consists of a rubber structure that contains an array of chambers actuated by pressurized air to generate the peristaltic wave. The final chamber shape was determined after an iterative process, which involves the elastomer properties, different chamber shapes, finite element analysis and image processing. The characterization of the developed peristaltic mechanism was made by correlating a theoretical study of swallowing peristaltic model and the waveform obtained from the X-ray radiography analysis as the mechanism is actuated. As result, the soft actuator mechanism can reproduce a peristaltic waveform with a correlation coefficient near to 0.9 with respect to the mathematical model reported in literature. In addition, the manufacturing process based on additive manufacturing technologies is also presented.

In this study, a micro-mechanical model for constitutive behavior of elastomers subjected to thermo-oxidative aging is proposed. The model is based on the network decomposition concept and lies within the framework of continuum mechanics. It is assumed that the aging process leads to the formation of a new network with tighter chains. Accordingly, the strain energy of the system is constituted of two independent sources, the energy of the original soft network and the one of the reformed network. These strain energies were computed by integration of entropic energy of polymer chains in each direction of a micro-sphere. The model demonstrates good agreement with different experimental data on relaxation and intermittent tests.

The advent of additive manufacturing allows for the design of complex 3D geometries that would otherwise be difficult to manufacture using traditional processes. Stereolithographic printing of geometrically reinforced structures gives promise for tunable energy-absorbing composite materials for impact applications. These materials may be suitable for applications in personal sport protection equipment such as knee-pads or helmets. The flexible nature of additive manufacturing can be easily scaled and modified to serve a variety of impact loading applications. In the present study, a three-dimensional nested array of ridged polymeric mesh with tiered high-temperature UV-cured polymer were embedded in a polyurethane matrix to form a new class of functional composite materials designed for multi-use low velocity impact events, and a single-use high velocity or high force impact event. The reinforcements were designed to absorb impact energy by the sequential bending, bucking, and failure of the layers of nested reinforcing members. The energy absorption capacity is further enhanced by the connective elastomer matrix which serves to retain the fractured mesh structure after initial breakage. The peak load is maintained at a relatively modest level while maximizing absorbed energy. Quasi-static loading tests were conducted to measure the peak load, total energy absorbing capability of the material. The energy absorption capability is measured using force-displacement plots and multiple interactions of material combination of reinforcement ring arrays. Tests with and without elastomer matrix, were conducted to understand peak load minimization and energy absorption character of the material.

This paper presents the results of theoretical study designed to measure the suitability of a dielectric elastomer as an actuator, from a group of acrylic and silicone elastomers. The micro molecular parameters of elastomers such as monomer type, monomer chain length and density were taken into consideration for analysis of actuation properties. Identification of monomer type of selected elastomers was done by using modified dielectric equations of polarization and stress induced due to applied electric field. The effect and the importance of electric field on the selected elastomers were also considered. To further understand the actuation potential of polymers, a comparison was done on the basis of their stress-electric field analysis. A conclusion was drawn on the basis of these comparisons and recommendations are given for an effective selection of polymers for use in actuator applications.

Head protection from impact events for participants in contact sports or other activities is of current interest. Custom designed and synthesized material systems which effectively reduce the peak force transmitted to the individual and absorb a substantial portion of the impact energy are highly desirable. In applications in which recovery time of the material is not a primary concern, multiple means of energy absorption, including deformation and fracturing of the reinforcement material, may be suitable. In the present study, new composite material systems using a urethane elastomer matrix reinforced with glass and unidirectional carbon fiber rods were synthesized and subjected to compression loading along the fiber direction. The constituent fiber rods absorbed energy either through plastic deformation, buckling, or fracturing during compression loading. The effect of different reinforcement material, configuration, and length were physically tested. A cascading failure mode was achieved by embedding fibers of varying length in the matrix. This triggered successive failure events as the primary load was transferred from the longest rod to each successive shorter rod. The elastomer matrix was also experimentally characterized quasi-statically. A thin outer polymer layer further constrained the elastomer composite and allowed for fine adjustment and optimization of its crushing characteristics.

Oil seals or radial lip seals are widely used in reciprocating, oscillating and rotating shaft applications. The sealability and durability of a lip seal greatly depends on the contact load and contact pressure distribution. It is challenging to find these contact parameters of the seal due to non-linear material behavior and small contact width, therefore numerical simulation can prove to be a viable method. In this paper, to address these challenges and to develop a robust numerical methodology, a Finite Element Model of a lip seal is created in ANSYS APDL. This model includes contact elements to model the lip seal’s contact-fit with certain interference, nonlinear material properties of elastomer and effect of the finger spring molded in the rubber body of the seal. The parameters for two term Mooney Rivlin Model for elastomer are obtained from simple uniaxial tension test. The numerical results demonstrate that the contact load exerted by the composite seal (with spring) is higher than the contact load exerted by elastomer portion of seal alone. It can be implied that the spring augments the radial load and increases the stiffness of the lip, which improves the lip’s sealability and durability. Experimental study is carried to validate the numerical results. The experimental results correspond well with the numerical results.

In fluid-structure interactions (FSI), one or more structures interact with incompressible or compressible fluids. These problems are non-linear multi-physics phenomena. Most FSI problems require a numerical solution rather than an analytical solution (Gene, 2012). This is particularly true for problems involving large deformations of elastomers and for problems related to plugging of flow. The classical two way coupling of fluid and structural solver does not work well and at times fail to reach a solution. Using a monolithic solver and non-conforming mesh techniques like Coupled Eulerian Lagrange (CEL) or Arbitrary Lagrange Eulerian (ALE) formulations we show solutions for such large deformation and plugging type FSI problems. The technique has been applied to help develop an industrial flow regulator. The regulator uses a single piece elastomeric material that undergoes deformation due to pressure drop in the system. The deformation in turn restricts flow in the system and potentially developing flow related noise in the system. Rigorous FSI analyses techniques were used to design and develop the concepts minimizing prototype development time. The FSI analysis not only helped in designing the mechanical function of the regulator but also the helped evaluate the acoustic noise level in the system. The resulting design performance was validated by controlled performance tests. The design was shown to work reliably and meet performance requirements.

Rapid deployment of marine structures is of growing importance to U.S. naval forces. Surface-based inflatable structures including Rigid Inflatable Boats (RIBs), inflatable causeways and bridging, and launch and recovery systems provide unique solutions for temporary structures during sea-based missions. When performance specifications demand minimal weight and stowage, rapid deployability and temporary rigidity, solutions are limited to inflatable structures constructed of flexible materials. Driven by air pressure, today’s inflatables provide significant load-carrying capacities per unit weight (or stowed volume) utilizing technical textiles, elastomers or “soft” composites. Overloading of inflatable structures produces unique fail-safe behaviors (reversible wrinkling) that allow the structures to assume rigidity and load-carrying capacity upon load removal. Design standards are virtually nonexistent for inflatable structures involving shapes constructed of spheres, beams, arches and most recently flat panels using 3D woven drop-stitch panels. Predictive performance tools (analytical and numerical) for static applications lag significantly behind those for conventional structures. Nonlinear system behaviors (material and geometric), thermo-mechanical coupling and fluid-structure interactions (FSI’s) pose significant challenges when applying existing design tools to inflatable structures. This gap is further exacerbated for dynamic applications as inflatable structures exhibit pressure-dependent natural frequencies and mode shapes. Surface-based structures must be designed with consideration given to operational sea state frequencies and wave periods so that the onset of structural instabilities (wrinkling, buckling) and loss of load-carrying capacities can be prevented. The present research establishes the validity of physics based models using the Ideal Gas Law as an equation of state (EOS) to predict the natural frequencies and corresponding mode shapes of air-inflated drop-stitch fabric panels as functions of inflation pressure. Particular concern is given to the breathing modes for inflation pressures ranging from 5.0 to 30.0 psig. The presence of breathing modes can negatively impact the riding performance of RIBs vessels constructed with drop-stitch fabric hulls by amplification of the panel’s skin separation displacements and vertical accelerations, and are not seen in this material system for the pressures considered. Both numerical and experimental methods are pursued; the results of laboratory modal experiments are used to validate the numerical models. Predicted and experimental natural frequencies and mode shapes are compared and excellent correlation was observed. Increasing inflation pressures produced increasing in-plane and through-thickness normal stresses and modal frequencies of the drop-stitch fabric panels.

We have developed a new high strain rate experiment in biaxial tension that allows for constitutive model validation at engineering strain rates from 50/s to over 1000/s. In the experiment, a flat disk of the material is clamped at a fixed radial distance. A rail-guided impact sled with a hemispherical impact head is released from the desired height and impacts the disk at the center, potentially deforming the sample to failure. Drop height and impact mass can be varied to modify peak strain rate and impact energy, and the wide range of test conditions allow for testing to be performed on many classes of materials, including thermoplastics and elastomers. The stress and strain fields are calculated using finite element simulations with the proposed constitutive model, and the constitutive model is validated by matching the force versus displacement data of the impact head recorded during experiment to the simulation. In this paper, we discuss results from the experiment and finite element simulations of the experiment on PA (polyamide, nylon) and PEEK (polyether ether ketone). The new experiment allows for validation and refinement of constitutive models, including failure, at high strain rates and in a multiaxial stress state.

Elastomers are finding a wide variety of dynamic applications in aerospace, automobile and biomedical industries. The response of these complex material is based on the loading conditions and the strain rate at which the loading is applied. To suit the designer’s requirement, there is an ever increasing need to characterize this application specific, dynamic behavior under high strain rates. The Kolsky bar apparatus, also known as the Split Hopkinson Bar, is the most common apparatus used to test engineering materials at strain rates between 100/s and 10000/s. In this paper a modified Kolsky bar to characterize soft material is numerically modeled using Finite Element Method. The focus of the study is to numerically analyze the modifications made to a conventional Kolsky bar to specifically test nonlinear hyperelastic, soft materials. The challenge for testing low strength materials is the impedance mismatch between the bar and specimen interfaces, which results in a very weak distorted signal. One of the solution is to use a hollow transmission bar instead of solid one. With the use of FEM it can be numerically verified that using a hollow bar increases the amplitude of the transmitted signal up to several times. It is known that the rise time of the elastic wave can be increased by using a copper pulse shaper. Different dimensions of pulse shaper are modeled and the effect on the incident pulse is analyzed. The main aim of this study is to provide a detailed numerical analysis on the testing parameters, and to model one way wave propagation in Kolsky bar experiment for hyperelastic materials. The constitutive equations used to model the parts of the apparatus are also discussed.

Cellular materials have two important properties: structures and mechanisms. This property enables one to design structures with proper stiffness and flexibility. Recent advance in 3D printing technologies enable engineers to manufacture complex cellular structures. In addition, use of smart materials, e.g., shape memory polymers (SMPs), for 3D printing enables us to construct mesostructures actively responsive to environmental stimuli with a programmable function, which may be termed ‘4D Printing’ referring to additional dimension on time-dependent shape change after 3D printing. The objective of this study is to design and synthesize active reconfigurable cellular materials, which enables the advance of technology on intelligent reconfigurable cellular structures with 4D printing. A two-layer hinge of a CPS functions through a programmed thermal expansion mismatch between two layers and shape memory effect of an SMP. Starting with thermo-mechanical constitutive modeling of a compliant porous hinge consisting of laminated elastomer composites, macroscopic behaviors of a reconfigurable compliant porous structure ( CPS ) will be constructed using the strain energy method. A finite element (FE) based simulation equipped with a user subroutine will be implemented with ABAQUS/Standard to simulate time-dependent thermo mechanical behaviors of a CPS. The designed CPS with polymers shows an extremely high negative Poisson’s ratio (∼ −120) and negative thermal expansion coefficient (−2,530 × 10 −6 /C). When programmed with an appropriate thermo-mechanical procedure, the hinge of the CPS bends either in positive and negative sign, which enables to tailor the CPS into desired intermediate and final configurations, ending up with achieving a reconfigurable CPS. This paper demonstrates that actively reconfigurable compliant cellular materials ( CCMs ) with CPSes can be used for next-generation materials design in terms of tailoring mechanical properties such as modulus, strength, yield strain, Poisson’s ratios and thermal expansion coefficient together with programmable characteristics.

In an effort to explore a structure-property relationship of an elastomer, we develop a multi-scale model of a thermoplastic polyurethane (PU) elastomer. A hyperelastic nonlinear constitutive equation of PU is constructed from a molecular simulation and is implemented for obtaining macroscopic mechanical properties. A full-atomic model of the PU, whose polymeric chains consist of hard and soft segments, is built using a commercial molecular modeling code — Materials Studio. The long hard segments are decomposed again into two beads for a better coarse grained ( CG ) model. The probability distribution functions for bond lengths, angles, and pairs are found among the beads to derive the effective potential functions using Inverse Boltzmann Method (IBM). Using the effective potential functions, a CG Molecular Mechanics ( MM ) model of the PU is built and run in LAMMPS. Initial IBM-derived effective potentials are iterated through CGMD simulations along with pressure correction to keep the distribution functions, density, and thermodynamic states the same for both models at two different scales. The final model is run with iterated force-fields using isobaric-isochoric ensemble (constant pressure, constant temperature) and validated with six full-atomic radial distribution functions ( RDF s) and density from experiment. After building a validated CG model, hyperelastic strain energy functions are constructed from the CGMM simulation results of the PU under volumetric and isochoric deformation at the molecular level in order to develop an equivalent continuum model. Considering energy equivalence between the molecular and continuum models, the material constants for a hyperelastic strain energy function of the PU are determined. The derived parameters are used to predict macroscopic properties; bulk, shear, and Young’s moduli, and Poisson’s ratio from volumetric and three-dimensional shear loadings using the multi-scale approach. A coupon test of the PU at a macroscopic level is followed for comparison with the CGMM simulation. By the end of the study, we may answer the following research question: how sensitive are the iterative processes and pressure correction while building the effective CG potentials through inverse Boltzmann method.

The dielectric elastomer (DE) is an insulating membrane with extra-ordinary properties which can meet various electromechanical failures during the actuation. In the current work, we measured breakdown voltage in DE membrane with special focus on the varying boundary stress during the actuation process. The boundary stress tuned deformation state, causing the membrane to deform out-of-plane before breakdown. A theoretical model is presented, involving the strain-stiffening effect in material and boundary stress effect in geometry, to estimate the dielectric breakdown voltage. The results agree with the experiments. Then, another set of experimental investigation is conducted to study the voltage-induced wrinkling of DE membrane. Steady wrinkles, without an accompany of electrical breakdown are attained and three different failure modes of DE membrane are classified into a phase chart. Finally, a qualitative theoretical explanation on wrinkling mechanism of DE membrane is presented and verified by experimental observations.

Constant mechanical load offers stabilization in the electromechanical actuation of dielectric elastomer. Due to the viscoelasticity, the electromechanical coupling performance of dielectric elastomer varies with time as its stability evolves. The parameters of viscoelasticity are calibrated experimentally. By establishing a thermodynamics model, the evolutionary paths of electromechanical deformation are classified and investigated, where a transition, from unstable to stable, is identified. The critical time and critical load on the transition is determined, providing a path forward for improving performance of dielectric elastomer.

Recent experimental studies have shown that supported tubes with internal fluid flow lose stability by dynamic divergence which is well modelled by the sloshing mechanism described in the work by Heil & Boyle and Heil & Hazel. In this study thin tubes supported at both ends subjected to axial internal fluid flow are investigated experimentally and numerically. Experiments for different L/R ratios were conducted for elastomer thin tubes clamped at both ends and subjected to internal air-flow. The numerical analysis is based on geometrically nonlinear structural equations coupled with potential flow theory to describe the fluid-structure interaction. The theoretical model employs the Donnell nonlinear shallow shell equations to describe the geometrically nonlinear structure. The clamped beam eigenfunctions are used to describe the axial variations of the shell deformation, automatically satisfying the boundary conditions and the circumferential continuity condition exactly. The fluid is assumed to be incompressible and inviscid. The partial differential equation of motion is discretized using the Galerkin method and the final set of ordinary differential equations is integrated numerically using pseudo-arclength continuation and collocation techniques and the Gear backward differentiation formula. The current study investigates experimentally the dynamics of tubes with L/R ∼1, L/R ∼2 and L/R ∼3 in axial internal flow. The experimental results indicated that the tubes lost stability by dynamic divergence resembling the Heil & Boyle Type B oscillatory scenario discussed in Païdoussis (2014). For the L/R ∼2 case the amplitude of the maximum shell deformation observed in the experiments was extracted using digitized images from the experiments. The numerical results were analyzed using bifurcation theory predicting that the tubes lose stability by static divergence exhibiting large hysteresis. The numerical results produced for all cases presented in this paper do not predict tube oscillatory solutions for the range of flow velocities considered in this analysis. For the L/R ∼2 case, which was also modeled in this paper, the numerical results are in good agreement with experiments for the maximum tube amplitude.

A kind of novel smart buffer material, magnetic-sensitive rubber (MSR) with embedded thin-walled iron balls, is provided, and the energy absorption and energy dissipation properties are investigated numerically. Two kinds of representative volume elements (RVEs) of MSR specimens with solid or hollow iron balls are studied, respectively. Under uniaxial compression alone or combined with magnetic field, the relation between effective stress and effective strain and that between strain energy desity and total compression deformation for these two specimens are established, respectively. With the increase of strain, it is demonstrated numerically that the MSRs with hollow iron balls have better energy absorption/dissipation capability than previous MSRs with solid balls. And the applied external magnetic-field increases this capability more. Under compression, the stress concentrations in elastomer matrix around the hollow balls decrease, that could increase the ultimate compression loads before the matrix failures. This is beneficial for us to design lightweight and efficient buffers according to different application cases.

This paper examines the implementation of Nitinol wire as a complex-shape actuation source specifically targeted for low-power muscle biomimetics. Nitinol is a type of shape memory alloy (SMA) which recovers its original shape after experiencing large deformation when heated above an austenite finish temperature. Previous preliminary work by the authors demonstrated successful closed-loop force control (i.e., recovery stress) using a simple proportional controller. The work presented in this paper builds upon the previous work by demonstrating closed-loop position control of various wire arrangements in the presence of inertial loads. A predeformed NiTi (4% pre-strain) wire is energized via Joule heating (martensite to austenite) and de-energized by conductive cooling (austenite to martensite). The experimental setup consists of a horizontally arranged NiTi wire (or wire bundle) fixed at one end and connected to a hanging weight through a pulley on the opposite end. The angular displacement of the pulley is measured with a non-contact magnetostrictive angle sensor, thereby providing the control feedback signal for the wire displacement. Successful closed-loop position control is demonstrated, and the relative ease of control is assessed for increasing weights. Given the dynamic loading of the moving wire, a proportional controller alone is insufficient to obtain stabilized responses. Therefore, PID with anti-windup method is employed. Although PID requires some trial and error and is quite sensitive to varying conditions, it appears to be stable and sufficiently precise when properly tuned. The effect of bundling wires on the speed of response is experimentally characterized, and different bundling arrangements are designed and examined in order to increase the geometric rate of convective heat transfer. Increasing the rate of heat transfer is particularly important during the forward (austenite-to-martensite) transformation, since its speed relies solely on passive cooling of the wires. Limitations in controlled load capabilities are discussed in the context of wire diameter, bundle size and controller tuning. A repeatability study of a properly tuned PID controller is also carried out by comparing the first few and last few samples of a 50,000-cycle test. In addition, it is shown that identical wires, when swapped, do not require re-tuning of the PID gains. Finally, this paper shows some preliminary actuator designs that can mimic complex muscle movement. Various geometric arrangements of Nitinol wires are embedded into a curable elastomer with skin-like flexibility and durometer. The potential facial muscle movements from these arrangements are shown and discussed.

As the extraordinary thermal, electrical, and mechanical properties of carbon nanotubes (CNTs) have become better understood, they have found their way into a wide range of engineering applications. Used in conjunction with fiber-reinforced composite materials, CNTs provide enhanced thermal conductivity, interlaminar strength, and ballistic resistance of laminar composite materials. However, the direct application of the macro form of CNT sheet as a heating element for use in a thermal actuator has not been reported. In the present study, CNT sheets are used as a flexible, efficient, and fast-responding heating element that induces transverse motion in a multilayered functional polymer composite based on thermal expansion mismatch between layers. The CNT heating element is designed to have a specific cross-sectional area to length aspect ratio, giving it a specific resistance and power consumption characteristic. The heating element is bonded to a compliant silicone elastomer substrate and a stiff constraining polyimide thin film, forming a flap-like actuator. The robust design and simple operation of the actuator makes it a potential candidate for control surfaces on micro air vehicles and actuating elements in microscale fluid pumps. The heating response rate of the actuator is measured experimentally using an infrared thermal imager. The temperature change in the thermal actuator is measured as a function of input voltage. The edge deflection of the actuator is also measured as function of the applied voltage. Finally, finite element modeling of the thermal actuator, a parametric study of material selection, and deflection analysis are conducted to better understand the result of these experiments.

Dielectric elastomers hold much promise as smart materials that could rapidly adapt to changes in environmental conditions due to their mechanical response to an electrical input. They belong to the group of electroactive polymers which have unique mechanical properties such as flexibility, light-weight, and electrical field-induced deformation. These characteristics make dielectric elastomers suitable candidates as actuators, sensors, or energy converter media. The objective of this study is to characterize the structural dynamic response of a dielectric elastomer membrane exposed to stagnant air environment and steady airflow at different angles of attack. A simulation of the fluid-structure interaction of the membrane is performed by coupling an electromechanical finite element model of the membrane with a computational fluid dynamics model representing the external flow. From the fluid-structure interaction simulation, the vibration frequencies and mode shapes, the time-varying out-of-plane deformation, and the coefficients of lift and drag are determined. Furthermore, a convergence study and mesh refinement are performed to guarantee mesh independence of the calculations from the fluid-structure interaction simulation. Results indicate that the stiffness of the electroactive membrane decreases nonlinearly with an increase of the applied voltage. The electrostatic force from the applied voltage adds compressive stress to the membrane, effectively softening the membrane, increasing the out-of-plane deformation, and reducing the resonance frequency.

The large strain behaviour of a short fibre-reinforced composite is studied through numerical simulations. The reinforcing fibres yield the macroscopic response transversely isotropic which is indeed the case of many reinforcements currently used in composites: short carbon fibres, cellulose whiskers, carbon nanotubes. As a result of the analysis, it is shown that the reorientation of the fibres that takes place at large strain has a significant effect on the overall material response by changing the axis of isotropy. This behaviour can be adequately described by using a transversely isotropic model whose strain energy function depends on three invariants: two isotropic and one representing the stretch along the direction of the fibres. To assess its capabilities, the model is compared to the results of experiments carried out by the authors on nickel-coated chopped carbon fibres in a vulcanised natural rubber matrix for which the fibre orientation is achieved by controlling an external magnetic field prior to curing. Possible applications include micro-sized propulsion devices and actuators.