Elastomer Layer Research Articles

Low-velocity impact behavior and damage mechanisms of honeycomb sandwich structures with elastomeric interlayers in CFRP skins

Elastomers help improve the toughness of lightweight high-strength materials, offering significant potential for enhancing the mechanical properties. However, introducing elastomers into CFRP interlayers as skin for composite sandwich structures has not yet been explored regarding the impact responses of such novel structures. This paper, for the first time in literature studies the low-velocity impact behavior and damage mechanisms of this novel sandwich structure using a combined experimental and numerical approach. The experimental results of sandwich structures with and without elastomer layers under different impact energies are presented. Finite element models of the two sandwich structures are built and impact behaviors were compared. The differences in internal damage and energy distribution during the impact are investigated to explain the reasons for the differing impact responses of the two sandwich structures. The results reveal that elastomeric interlayers have a significant advantage in enhancing the damage resistance of composite sandwich structures, especially under high impact energy conditions. The key contributions of this paper include the experimental characterization of the impact behavior of composite sandwich structures with elastomeric interlayers, and the explanation of the reasons for the changes in impact responses caused by the elastomers from the perspectives of damage mechanisms and energy distribution.

Topology optimization of adaptive sandwich plates with magnetorheological core layer for improved vibration attenuation.

In this study the optimum topology distribution of the magnetorheological elastomer (MRE) layer in an adaptive sandwich plate is investigated. The adaptive sandwich plate consists of an MR elastomer layer embedded between two thin elastic plates. A finite element model has been first formulated to derive the governing equations of motion. A design optimization methodology incorporating the developed finite element model has been subsequently developed to identify the optimum topology treatment of the MR layer to enhance the vibration control in wide-band frequency range. For this purpose, the dynamic compliance and density of each element are defined as the objective function and design variables in the optimization problem, respectively. The method of the solid isotropic material with penalization (SIMP), is extended for material properties interpolation leading to a new MRE-based penalization (MREP) model. Method of moving asymptotes (MMA) has been subsequently utilized to solve the optimization problem. The developed finite element model and design optimization method are first validated using benchmark problems. The proposed design optimization methodology is then effectively utilized to investigate the optimal topologies of the magnetorheological elastomer (MRE) core layer in MRE-based sandwich plates under various boundary and loading conditions. Results show the effectiveness of the proposed design optimization methodology for topology optimization of MRE-based sandwich panels to mitigate the vibration in wide range of frequencies.

Development of novel low-cost isolator UMELI (unbonded mesh elastomeric layered isolator): experimental investigations

Seismic isolation is a highly efficacious method for reducing the seismic load on structures. This technique is widely adopted to safeguard structures from earthquakes. Despite the promising results demonstrated by numerous isolation techniques, their implementation remains a significant challenge, particularly in developing countries, due to the high costs associated with manufacturing. Therefore, a novel and affordable base isolation technique has been proposed, namely unbonded mesh elastomeric layered isolator (UMELI). UMELI consists of steel mesh sandwiched between the unbonded layers of elastomers, resulting in an affordable isolator to be used for lightweight, low-rise structures. One of the crucial characteristics of this novel isolator is that it does not require a specialised manufacturing process unlike other elastomeric isolators. In the present study, experimental investigations are conducted to evaluate the dynamic characteristics such as dynamic vertical stiffness, equivalent lateral stiffness, and equivalent viscous damping ratio of UMELI. Its characteristics were studied for different layered isolators and are compared with the unreinforced elastomeric isolator. The investigations have demonstrated the effectiveness of the UMELI by increasing its vertical stiffness and reducing lateral stiffness, thereby enhancing the isolation period with the addition of a steel mesh layer.

Soft Crawling Microrobot Based on Flexible Optoelectronics Enabling Autonomous Phototaxis in Terrestrial and Aquatic Environments.

Many organisms move directly toward light for prey hunting or navigation, which is called phototaxis. Mimicking this behavior in robots is crucially important in the energy industry and environmental exploration. However, the phototaxis robots with rigid bodies and sensors still face challenges in adapting to unstructured environments, and the soft phototaxis robots often have high requirements for light sources with limited locomotion performance. Here, we report a 3.5 g soft microrobot that can perceive the azimuth angle of light sources and exhibit rapid phototaxis locomotion autonomously enabled by three-dimensional flexible optoelectronics and compliant shape memory alloy (SMA) actuators. The optoelectronics is assembled from a planar patterned flexible circuit with miniature photodetectors, introducing the self-occlusion to light, resulting in high sensing ability (error < 3.5°) compared with the planar counterpart. The actuator produces a straightening motion driven by an SMA wire and is then returned to a curled shape by a prestretched elastomer layer. The actuator exhibits rapid actuation within 0.1 s, a significant degree of deformation (curvature change of ∼87 m-1) and a blocking force of ∼0.4 N, which is 68 times its own weight. Finally, we demonstrated the robot is capable of autonomously crawling toward a moving light source in a hybrid aquatic-terrestrial environment without human intervention. We envision that our microrobot could be widely used in autonomous light tracking applications.

Bending Dielectric Elastomer Actuators

Dielectric Elastomer Actuators (DEAs) are a type of electroactive polymers that transform electric energy into mechanical work and are capable of large strains and high energy densities. DEAs are often described as “artificial muscles” because they exhibit strains, energy density, and responsiveness similar to natural human muscles. Their applications as soft robotic grippers, artificial arms, underwater robots, and aerial robots have previously been demonstrated. Single-layer DEAs have a simple design, with a dielectric layer sandwiched between compliant electrodes. However, by alternating positive and negative electrodes, multilayer DEAs can exhibit larger forces and strains than single-layer DEAs. DEAs are actuated by applying voltage and forming an electric field, generating a Maxwell stress between the electrodes. The process of fabricating a multilayer DEA consists of spin coating and curing layers of elastomer, stamping carbon nanotubes (CNT), and making connections to the electrodes. By alternating positive and negative electrodes, multilayer DEAs are constructed, which can exhibit larger forces and strains than single-layer DEAs. When electrically activated, DEAs expand laterally; however, by fabricating a bimorph DEA actuator, we demonstrate bending DEAs. A bimorph DEA consists of a multilayer DEA layer attached to a thicker inactive elastomer layer. When the bimorph is actuated, only the DEA layer expands, causing the actuator to bend. We study the relationship between the bending and stiffness of the inactive layer. By changing the shape of the DEA to a triangle or wing-like shape, we also emulate the high-frequency flapping of birds. Work begun on characterizing the voltage-controlled deflection of bimorph DEAs and understanding the mechanics of the bending of a simple beam in terms of forces at Harvard University will be continued on the Navajo Reservation at Navajo Technical University.

A porous elastomer with a cavity array for three-dimensional plantar force sensing

Human gait is an important indicator for diagnosis of various movement-related disorders. Three-dimensional (3D) plantar force monitoring enables real-time recording of contact forces in both the normal and tangential directions in each step, providing useful clues for identification of irregular gait patterns or gait-related health issues. Most flexible sensors for gait monitoring focus on contact forces in the normal direction, being limited to detect forces in the tangential direction. Herein we have developed a porous elastomer with a cavity array (PECA) to measure 3D contact forces for gait analysis. The sensor consists of two fabric electrodes and a dielectric layer of porous Ecoflex elastomer containing a 5 × 5 cavity array, forming four parallel-plate capacitor units in a soft construction. Therefore, a 3D force in any arbitrary direction generates four unique capacitive electrical signals. Importantly, the PECA includes a well-designed combination of micrometer-scale pores and millimeter-sized cavities, dramatically improving the detection sensitivity and linear elasticity range. We have demonstrated high sensitivities of 0.41 N−1 and 0.23 N−1 in the normal and tangential directions, respectively. The flexible PECA-based sensors have been tested to sensitively distinguish various standing postures and different stride lengths for gait analysis, promising reliable translation into areas such as healthcare monitoring, sports training, and rehabilitation engineering.

PMUT-Based System for Continuous Monitoring of Bolted Joints Preload.

In this paper, we present a bolt preload monitoring system, including the system architecture and algorithms. We show how Finite Element Method (FEM) simulations aided the design and how we processed signals to achieve experimental validation. The preload is measured using a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) in pulse-echo mode, by detecting the Change in Time-of-Flight (CTOF) of the acoustic wave generated by the PMUT, between no-load and load conditions. We performed FEM simulations to analyze the wave propagation inside the bolt and understand the effect of different configurations and parameters, such as transducer bandwidth, transducer position (head/tip), presence or absence of threads, as well as the frequency of the acoustic waves. In order to couple the PMUT to the bolt, a novel assembly process involving the deposition of an elastomeric acoustic impedance matching layer was developed. We achieved, for the first time with PMUTs, an experimental measure of bolt preload from the CTOF, with a good signal-to-noise ratio. Due to its low cost and small size, this system has great potential for use in the field for continuous monitoring throughout the operative life of the bolt.

Fouling Release Mechanism of an Octopus‐Inspired Smart Skin

AbstractThis work proposes an octopus‐inspired smart skin for fouling release. The skin consists of twisted spiral artificial muscles (TSAMs) embedded between two layers of elastomer. Upon electro‐thermal actuation, TSAMs undergo vertical displacement (similarly to octopus’ papillae dermal muscles) and mechanically deform the top layer of the skin where the biofilm is attached. The release is achieved by generating a critical strain on the skin's top layer. In this paper, a physics‐based mechanical model is proposed to describe the fouling release mechanism of the smart skin. The model relates the critical strain to the mechanical properties of the biofilm, as well as the mechanical and electro‐thermal properties of the TSAMs. The model is experimentally validated, and a robust controller is implemented to achieve the desired critical strain and then perform release for two different types of biofilms: bacterial‐based Escherichia coli and diatom‐based Amphora coffeaeformis.

Fully 3D printed dielectric elastomer actuators based on silicone and its composites

AbstractDielectric elastomer actuator (DEA) is one of the most promising types of soft actuation technology, which has great potential in the fields of wearable devices and soft robotics. It consists of a dielectric elastomer layer, which is an electroactive polymer that can produce large deformation, and compliant electrodes to bring charges to certain locations. In this article, direct ink writing (DIW) technology, an emerging 3D printing method, was used to realize the preparation of the electrode‐elastomer‐electrode stack of the DEA. The dielectric and electrode materials were designed with suitable rheological properties to fulfill the need for the extrusion process. The formulated silicone material not only presented excellent dielectric and mechanical properties, but also good printability. Extrudable electrodes were prepared based on silicone composites with the characteristics of mechanical compliance and high conductivity. The fully printed DEA achieved a maximum actuation strain of 11.11%, a fast response time of 0.76 s and excellent electromechanical repeatability. DEA arrays were also achieved, possessing the ability to carry out on‐demand actuation, allowing each actuator to be activated singly or work in groups. Thanks to the design freedom of the DIW technology, this strategy is able to manufacture fine and complex structures with precise active zones, paving a way for the fabrication of next‐generation smart devices.Highlights Printable silicone ink was formulated with good dielectric property and softness. Carbon black/silicone composites were obtained with high conductivity and compliant nature. The silicone composites were printed into thin films to act as electrodes. Fully 3D printed dielectric elastomer actuators (DEA) were achieved by direct ink writing. DEA arrays with on‐demand actuation were realized by well‐defined printing.

Modular multi-degree-of-freedom soft origami robots with reprogrammable electrothermal actuation

Soft robots often draw inspiration from nature to navigate different environments. Although the inching motion and crawling motion of caterpillars have been widely studied in the design of soft robots, the steering motion with local bending control remains challenging. To address this challenge, we explore modular origami units which constitute building blocks for mimicking the segmented caterpillar body. Based on this concept, we report a modular soft Kresling origami crawling robot enabled by electrothermal actuation. A compact and lightweight Kresling structure is designed, fabricated, and characterized with integrated thermal bimorph actuators consisting of liquid crystal elastomer and polyimide layers. With the modular design and reprogrammable actuation, a multiunit caterpillar-inspired soft robot composed of both active units and passive units is developed for bidirectional locomotion and steering locomotion with precise curvature control. We demonstrate the modular design of the Kresling origami robot with an active robotic module picking up cargo and assembling with another robotic module to achieve a steering function. The concept of modular soft robots can provide insight into future soft robots that can grow, repair, and enhance functionality.

Microstructured Porous Capacitive Bio-pressure Sensor Using Droplet-based Microfluidics.

Devices that mimic the functions of human skin are known as "electronic skin," and they must have characteristics like high sensitivity, a wide dynamic range, high spatial homogeneity, cheap cost, wide area easy processing, and the ability to distinguish between diverse external inputs. This study introduces a novel approach, termed microfluidic droplet-based emulsion self-assembly (DMESA), for fabricating 3D microstructured elastomer layers using polydimethylsiloxane (PDMS). The method aims to produce accurate capacitive pressure sensors suitable for electronic skin (e-skin) applications. The DMESA method facilitates the creation of uniform-sized spherical micropores dispersed across a significant area without requiring a template, ensuring excellent spatial homogeneity. Micropore size adjustment, ranging from 100 to 600 μm, allows for customization of pressure sensor sensitivity. The active layer of the capacitive pressure sensor is formed by the three-dimensional elastomer itself. Experimental results demonstrate the outstanding performance of the DMESA approach. It offers simplicity in processing, the ability to adjust performance parameters, excellent spatial homogeneity, and the capability to differentiate varied inputs. Capacitive pressure sensors fabricated using this method exhibit high sensitivity and dynamic amplitude, making them promising candidates for various e-skin applications. The DMESA method presents a highly promising solution for fabricating 3D microstructured elastomer layers for capacitive pressure sensors in e-skin technology. Its simplicity, performance adjustability, spatial homogeneity, and sensitivity to different inputs make it suitable for a wide range of electronic skin applications.

Optimization of multi-layered composites against ballistic impact: A mesoscale approach

There has been an urgent need to develop and analyse multi-layered composite structures with varying material properties to withstand projectile impact. The proposed study focuses on the optimization of the multi-layer composite to achieve maximum resistance/energy dissipation. This study investigates the mechanical performance of the proposed multi-layered composite configuration under high strain rate loading through a computational approach. The proposed multi-layered structure incorporates layers of reinforced concrete, boulders, an elastomer layer, an ultra-high-performance concrete panel, and a layer of steel plate. A mesoscale-based approach has been developed for the layer comprising boulders and mortar. A total of six different configurations have been considered to arrive at the most efficient one against projectile impact. Optimization of the proposed configurations has been done by utilizing the concepts of specific energy absorption and shock impedance. Additionally, the fracture and damage characteristics of each configuration are also studied. Ductile hole enlargement in the sandy soil layer, fragmentation failure in the boulders, petaling failure in the steel plate, and spalling failure in the concrete layer have been observed. Based on the specific energy absorption and shock impedance approaches, the optimum laying sequence for the ballistic impact of each material is suggested.

Low-Frequency Resonant Magnetoelectric Effect in a Piezopolymer-Magnetoactive Elastomer Layered Structure at Different Magnetization Geometries.

The search for novel materials with enhanced characteristics for the advancement of flexible electronic devices and energy harvesting devices is currently a significant concern. Multiferroics are a prominent example of energy conversion materials. The magnetoelectric conversion in a flexible composite based on a piezopolymer layer and a magnetic elastomer layer was investigated. The study focused on investigating the dynamic magnetoelectric effect in various configurations of external alternating and constant homogeneous magnetic fields (L-T and T-T configurations). The T-T geometry exhibited a two orders of magnitude higher coefficient of the magnetoelectric effect compared to the L-T geometry. Mechanisms of structure bending in both geometries were proposed and discussed. A theory was put forward to explain the change in the resonance frequency in a uniform external field. A giant value of frequency tuning in a magnetic field of up to 362% was demonstrated; one of the highest values of the magnetoelectric effect yet recorded in polymer multiferroics was observed, reaching up to 134.3 V/(Oe∙cm).

Vibration analysis of a rotor-bearing system using magneto-rheological elastomers

Magneto rheological materials are considered as a promising adaptive means of vibration control. This paper proposes a new configuration of smart bearings comprising magneto-rheological elastomer (MRE) layer inserted between the bearing pedestal and the foundation. This configuration provides a directional vibration controllability in both horizontal and vertical directions of a rotor-bearing system. The effect of using MRE layer on the vibration response and the resonant speeds of a rotor-bearing system is investigated. The finite element method is used to derive a dynamic model for the rotor-bearing system using shaft elements based on the Rayleigh beam theory and accounting for the gyroscopic effect and internal damping of the shaft. Simulation results reveal that the use of MRE materials in passive mode leads to downshifting of the first two resonant speeds and the attenuation of the steady state vibration response of the rotor bearing system in the horizontal and vertical directions at the resonance frequencies. When the MRE layer is subjected to a magnetic field generated by an electric current produced in the magnetic coil, the horizontal and vertical vibration responses are increased at the resonant speeds but decreased at other rotational speed ranges. Furthermore, the first two resonant speeds in the horizontal and vertical directions decrease further with the increase of the electric current.

Investigation of seismic fragility curves of unbonded FREIs: Adaptive characteristics and modeling sensitivity

AbstractFiber‐reinforced elastomeric isolators (FREIs) are composed of layers of elastomer reinforced with either steel or fibers, and can be placed in a bonded or unbonded configuration between the upper and lower supports. The use of fiber reinforcement in FREIs was intended to reduce the production and installation costs compared to common steel‐reinforced elastomeric isolators (SREIs) and to develop an isolator suitable for widespread application, particularly in developing countries. The unique rollover deformation exhibited by unbonded FREIs (UFREIs), due to their flexible fiber reinforcement, enables them to adapt to multiple performance objectives at different hazard levels. In this study, the impact of different numerical models of UFREIs on the seismic response and failure probability of structures is investigated. The research employs incremental dynamic analysis and the development of fragility curves for different limit states, considering three sets of ground motions (including far‐field and pulse‐like). Moreover, the effect of full rollover was investigated by comparing the fragility curves of UFREIs when supported on modified support geometries, which involved three types of support: unmodified, accelerated, and delayed full rollover. The effectiveness of the adaptive characteristics to behave as a displacement restrain is demonstrated. The results emphasize the importance of employing an accurate model to simulate the behavior of UFREIs as an adaptive device for effectively utilizing their potential capacity, particularly at larger displacements where there is more dissipated energy due to full rollover.

Highly sensitive flexible capacitive pressure sensor with structured elastomeric dielectric layers

Sensitive yet stable, robust yet flexible and accurate yet energy efficient pressure sensors are required for variety of purposes. While a large variety of designs and dielectric materials have been explored for this purpose, there is still need of a flexible pressure sensor that will allow easy scale up and inexpensive fabrication. To this end, we have presented here the design of a flexible capacitive pressure sensor using copper coated paper as flexible electrodes and soft Ecoflex layers decorated with cylindrical micro-pillars as the dielectric. While microscopic construct of the sensor allows its easy manufacturability, softness of the layer imparts sensitivity to it. In contrast to many conventional sensors, this design yields sensitivity as high as ∼5 kPa−1 at pressure <1 kPa and somewhat smaller sensitivity as pressure exceeds 1 kPa. We have varied systematically pillar diameter, skin thickness of dielectric layer and pitch of the pillar array to optimise the design and demonstrate its easy tunability. We have presented a model based on buckling of the pillars to predict the response of the sensor. We have explored also a specific design that minimises the hysteresis.

Toward the Assembly of 2D Tunable Crystal Patterns of Spherical Colloids on a Wafer-Scale.

Entering an era of miniaturization prompted scientists to explore strategies to assemble colloidal crystals for numerous applications, including photonics. However, wet methods are intrinsically less versatile than dry methods, whereas the manual rubbing method of dry powders has been demonstrated only on sticky elastomeric layers, hindering particle transfer in printing applications and applicability in analytical screening. To address this clear impetus of broad applicability, we explore here the assembly on nonelastomeric, rigid substrates by utilizing the manual rubbing method to rapidly (≈20 s) attain monolayers comprising hexagonal closely packed (HCP) crystals of monodisperse dry powder spherical particles with a diameter ranging from 500 nm to 10 μm using a PDMS stamp. Our findings elucidate that the tribocharging-induced electrostatic attraction, particularly on relatively stiff substrates, and contact mechanics force between particles and substrates are critical contributors to attain large-scale HCP structures on conductive and insulating substrates. The best performance was obtained with polystyrene and PMMA powder, while silica was assembled only in HCP structures on fluorocarbon-coated substrates under zero-humidity conditions. Finally, we successfully demonstrated the assembly of tunable crystal patterns on a wafer-scale with great control on fluorocarbon-coated wafers, which is promising in microelectronics, bead-based assays, sensing, and anticounterfeiting applications.

Vibration analysis of double elastomer layered superstructures with extended locally continuous support model

In this study, a distributed rail pad (RP) model of rail supported by double elastomer layers for rigid superstructures like light metro is presented, and a semi-analytical solution is derived via the Galerkin's method. Apart from existing distributed RP models that contain multiple individual spring-dampers, the proposed method offers locally continuous rail bedding. The new model assumes that the rail bedding is not pointwise and connects the continuous bedding and lumped baseplate mass with a derived function, which is the first novelty of the paper. Results of the single-spring model (SSM) and extended locally continuous support models (ELCSM) are obtained and compared to indoor and outdoor measurements in the time and frequency domains. The proposed model shows similar results to the conventional model for dynamic response; however, the clear difference between the models is seen at high resonant frequencies because of the stiffer bedding of the rail. Another point is that rail stresses and displacements can be calculated more accurately compared to SSM, due to including the length of RP. The findings of the study highlight the importance of considering RP length and stiffness when designing and maintaining railway tracks and provide insights into how to optimize these parameters for better performance and durability.

A tunable acoustic absorber using reconfigurable dielectric elastomer actuated petals

Dielectric elastomer actuator (DEA)-based unimorphs that actively bend in one direction, can mimic the blooming motion of flower petals. Here we explore an application of such reconfigurable DEA to create tunable acoustic absorber capable of adapting to fluctuations in dominant noise frequency. The DEA-unimorphs consist of alternate layers of dielectric elastomers and compliant electrodes bonded to a Mylar sheet and were micro-slotted to form triangular petal-like structures that bend upon voltage activation. When arranged in an array, the micro-slotted dielectric elastomer bending actuators (MSDEBA) can open like flower petals, actively reconfiguring their open-ratio. Integrated with a base resonator comprising a micro-slotted panel (MSP) and a parallelly arranged varying-depth (VD) back-cavity, the MSDEBA forms a tunable acoustic absorber effective in the low-mid acoustic frequency range at inactive state. Meanwhile, upon voltage activation, it increased the absorber’s open-ratio and tuned the absorber to target a higher frequency. A 5 kV activation reconfigured the MSDEBA to shift its transmission loss peak by 72.74% (i.e., from 697 Hz to 1204 Hz). This acoustic spectrum tuning capability doubled the 15 dB absorption bandwidth of these absorbers from a bandwidth of ~435 Hz to 820 Hz. Such absorbers have the potential to tune the absorption spectrum to match the noise frequency in real-time to ensure optimal acoustic attenuation.

Constitutive models for confined elastomeric layers: Effects of nonlinearity and compressibility

Elastomers tend to undergo large deformations accompanied by small volumetric changes. For elastomeric layers sandwiched between rigid plates, large deformations can be significantly limited by the constraints imposed by the plates. Further, those constraints can be enhanced by material’s inability to undergo large volumetric changes. From this perspective, it is appropriate to examine the validity of constitutive models, in which an elastomer is treated as incompressible, for analysis of the confined layers. Here, this issue is addressed by considering the mechanical response of sandwiched elastomeric layers using three constitutive models. The first one, referred to as compressible neo-Hookean, is regarded as exact. The other two models are regarded as approximations. Of those two, the first one neglects nonlinearity and the second one neglects compressibility. Accordingly, the modeling errors associated with the former are treated as measures of importance of nonlinearity, and the modeling errors associated with the latter are treated as measures of importance of compressibility. The modeling errors are evaluated using the force–displacement curve and the mean stress at the layer center as the quantities of interest. Numerical results are presented for rubber and polydimethylsiloxane (PDMS), characterized by Poisson’s ratios ν=0.4999 and ν=0.49, respectively. It is shown that, even when the forces applied to the plates are large, considering nonlinearity is important for thick but not for thin layers. In contrast, considering compressibility is important for thin layers. The need for considering compressibility is further assessed by introducing a competition parameter, which reinforces the notion that compressibility is important for modeling PDMS layers and thin rubber layers.