Large Strain Response Research Articles

Fast Responsive and High-Strain Electro-Ionic Soft Actuator Based on the 3D-Structure MXene-EGaIn/MXene Bilayer Composite Electrode.

Electro-ionic soft actuators have garnered significant attention owing to their promising applications in flexible electronics, wearable devices, and soft robotics. However, achieving high actuation performance (large bending strain and fast response time) of these soft actuators under low voltage has been challenging due to issues related to ion diffusion and accumulation. In this study, an electro-ionic soft actuator is fabricated using Ti3C2Tx MXene and eutectic gallium-indium (EGaIn) composite material as the bilayer electrode and methylammonium formate/1-ethyl-3-methylimidazolium tetrafluoroborate/poly(vinylidene fluoride) (MAF-EMIMBF4/PVDF) as the ionic liquid-type electrolyte. The research results indicate that the prepared soft actuator exhibits excellent actuation performance with a peak-to-peak displacement of 35 mm and a bending strain of 0.69% (a peak-to-peak strain of 1.38%) under a low voltage (3 V). The electro-ionic soft actuator shows a wide frequency range (0.1-10 Hz), fast response time (0.35 s), and a rise time of 7.5 s. Furthermore, it demonstrates good cyclic durability, with a retention rate of 92.5% of its performance for 10 000 cycles. These excellent performances are attributed to the 3D structure of the Ti3C2Tx-EGaIn/Ti3C2Tx bilayer composite electrode, as well as the characteristics of the low viscosity, high conductivity, small ion volume, and larger volume difference between cations and anions in MAF ionic liquid. The high-performance electro-ionic soft actuator can be used in various fields such as artificial muscles, tactile devices, and soft robots.

Investigation of Effective Fracture Height Using Strain Responses in a Vertical Monitoring Well during a Well Interference Test

Summary Understanding and characterizing the effective hydraulic fracture height are critical for optimizing completion design and well performance in unconventional reservoirs. Rayleigh frequency shift distributed strain sensing (RFS-DSS) data provide crucial information about the fracture geometry by measuring the strain change along the fiber. Recently, the strain data from the vertical monitoring well [VMW; Boxwood 5 Pilot Hole (B5PH)] at Hydraulic Fracture Test Site 2 (HFTS-2) was collected during a well interference test conducted on two neighboring horizontal wells [Boxwood 3H (B3H) and Boxwood 4H (B4H)] in September 2020. However, this data set was overlooked and not comprehensively analyzed. Consequently, the valuable information that this data may provide remains unexplored. The objective of this study is to determine how to interpret this vertical strain data and assess its potential to provide insights into the effective fracture height generated in the horizontal wells. To understand the measured strain data, we have developed a coupled fluid flow and geomechanics model to simulate the vertical strain responses observed during production and the well interference test. The characteristics of the strain responses are thoroughly analyzed to establish guidelines for identifying the upper and lower tips of the effective fracture in different fracture geometry scenarios. In addition, field-measured data are examined, and a history-matching process is conducted by comparing the numerical matching results with the observed data to identify the effective fracture height. Furthermore, the pressure gauge data are analyzed and compared with the strain data results to further validate the findings. The study has demonstrated that vertical strain measurements obtained from a VMW can be used to determine the effective fracture height. Large strain responses, characterized by both negative and positive peaks, are observed near the tips of the effective fractures. The upper and lower tips of the effective fracture can be identified by examining the locations of zero-strain change. The magnitude of strain response is affected by the distance between the fracture and the fiber. It is worth noting that the locations where the absolute value of the derivative of fracture conductivity changes abruptly correspond to the positions of the fracture tips. Furthermore, we study and history match the field-measured vertical strain change in HFTS-2 data sets. The pressure gauge data are analyzed to further validate our findings. The effective fracture height determined from the strain measurements falls within the range of 11,360–11,616 ft, which is consistent with the values obtained from pressure gauges (11,360–11,635 ft). Specifically, for B4H, the effective fracture height was determined to be 256 ft at a distance of 330 ft away at the time of data acquisition. Overall, the strain measurements provide valuable, time-dependent insights into the effective fracture height and the active drainage interval.

Ultrahigh strain and superior temperature stability in lead-free ceramics via synergetic texture technique and phase structure engineering

Lead-free piezoelectric ceramics with large strain response and good temperature stability are highly required for actuator applications. However, the search for lead-free ceramics with both enhanced strain and reduced thermal-dependent behavior is a great challenge. In this work, by employing templated grain growth technique and elaborately modulate the phase structure, large unipolar strain of 0.30 %, low strain hysteresis Hs ∼ 12 %, along with small strain variation 11.8 % between 20 and 100 °C are achieved in (001)C-oriented (Ba0.95Ca0.05)(Sn0.04Ti0.96)O3 ceramics, being the optimal level of BaTiO3 family. It is revealed the ceramics exhibit a coexistence of orthorhombic and tetragonal phases. The inherent piezoelectric anisotropy of orthorhombic phase, the reversible a→c domain switch, along with the refined non-180° domain structures contribute to the remarkable electrostrain characteristic. This study proposed a promising strategy to develop high performance piezoelectric materials for actuator applications.

Phase transition and large strain response with ultra-low hysteresis of BaTiO3 doped with a high-entropy perovskite oxide

(1 − x)BaTiO3–xBi(Sc0.2Y0.2Al0.2Fe0.2Ga0.2)O3 (BT–BSYAFG, x = 0.02, 0.04, 0.06, 0.08, and 0.10) ceramics were successfully prepared via the solid-state reaction method. X-ray diffraction and Raman spectroscopy results reveal that a structural transition from the tetragonal (P4mm) to the pseudocubic (Pm3¯m) phase occurs with increasing x. At x = 0.04 and 0.06, the tetragonal and pseudocubic phases coexist. The relaxor behavior of the as-prepared samples is enhanced via BSYAFG doping due to the lattice distortion effect of high-entropy perovskite oxides. Additionally, the transition from the classic ferroelectric to the relaxor ferroelectric state implies that the long-range-ordered macro-domains are completely transformed into disordered polar nano-regions, which improves the strain properties. A large strain response (0.28 %) with an ultra-low hysteresis (4 %) is achieved at x = 0.06 under an electric field of 70 kV/cm. These results indicate that BT–BSYAFG ceramics are promising materials for application to the field of precision actuators.

Enhanced domain switching contributing to large strain response in (Bi0.5Na0.5)TiO3–SrTiO3 lead-free thin films

The acquisition of lead-free piezoelectric films with large strain is the key factor in promoting micro-actuator development. A lead-free compound, Bi0.5Na0.5TiO3, exhibits extensive potential in achieving a large strain response and a high inverse piezoelectric coefficient. In this work, (1-x)Bi0.5Na0.5TiO3-xSrTiO3 (BNT-100xST, x = 0.1, 0.2, 0.25) solid solution thin films have been prepared using the chemical solution deposition method. The dielectric nonlinearity and ferroelectric properties of BNT-ST thin films have been systematically studied to investigate the domain wall motion in the films. The most significant domain wall motion was obtained in BNT-20STST thin film, in which film a large strain of 0.89 % was obtained. In addition, the relation between structure and properties as a function of temperature was studied by the temperature-dependent dielectric properties and Raman spectra analysis.

Repeatedly Programmable Liquid Crystal Dielectric Elastomer with Multimodal Actuation.

Dielectric elastomers (DEs) are actuatable under an electric field, whose large strain and fast response speed compare favorably with natural muscles. However, the actuation of DE-based devices is generally limited to a single mode and cannot be reconfigured after fabrication, which pales in comparison to biological counterparts given the ability to alter actuation modes according to external conditions. To address this, liquid crystal dielectric elastomers (LC-DEs) that can alter the dielectric actuation modes based on the thermally triggered shape-changing are prepared. Specifically, the two shapes through the LC phase transition possess different bending stiffness, which leads to distinct actuation modes after an electric field is applied. Moreover, the two shapes can be individually programmed/reprogrammed, that is, the one before the transition is regulated through force-directed solvent evaporation and the one after the transition is via bond exchange-enabled stress relaxation. As such, the multimodal dielectric actuation behaviors upon temperature change can be readily diversified. Meanwhile, the space charge mechanism endows LC-DEs with the significantly reduced driving e-field (8 V µm-1) and bidirectional actuation manners. It is believed this unique adaptivity in the actuation modes under a low electric field shall offer versatile designs for practical soft robots.

3D nanofabricated soft microrobots with super-compliant picoforce springs as onboard sensors and actuators.

Microscale organisms and specialized motile cells use protein-based spring-like responsive structures to sense, grasp and move. Rendering this biomechanical transduction functionality in an artificial micromachine for applications in single-cell manipulations is challenging due to the need for a bio-applicable nanoscale spring system with a large and programmable strain response to piconewton-scale forces. Here we present three-dimensional nanofabrication and monolithic integration, based on an acrylic elastomer photoresist, of a magnetic spring system with quantifiable compliance sensitive to 0.5 pN, constructed with customized elasticity and magnetization distributions at the nanoscale. We demonstrate the effective design programmability of these 'picospring' ensembles as energy transduction mechanisms for the integrated construction of customized soft micromachines, with onboard sensing and actuation functions at the single-cell scale for microrobotic grasping and locomotion. The integration of active soft springs into three-dimensional nanofabrication offers an avenue to create biocompatible soft microrobots for non-disruptive interactions with biological entities.

Unified and accurate simulation for large elastic strain responses of rubberlike soft materials under multiple modes of loading

AbstractA new and explicit form of the multi-axial elastic potential for elastic soft materials is constructed by means of two invariants of the Hencky strain. The new elasticity model with this form can bypass coupling complexities and uncertainties usually involved in parameter identification. Namely, exact closed-form solutions of decoupled nature are obtainable for stress responses under multiple benchmark modes. Unlike usual solutions with numerous coupled parameters, such new solutions are independent of one another and, as such, data sets for multiple benchmark modes can be separately matched with mutually independent single-variable functions. A comparative study is presented between a few well-known models and the new model. Results show that predictions from the former agree well with uniaxial and biaxial data, as known in the literature, but would be at variance with data for the constrained stress response in the plane-strain extension. In contrast, predictions from the new model agree accurately with all data sets. Furthermore, exact solutions for the Poynting effect of freely twisted elastic thin-walled tube are obtained from the new model.

Enhanced performance of electro-responsive PVC gel actuators prepared by advanced rapid thermal melting technique

Polyvinyl chloride (PVC) gel actuators exhibit a contraction-relaxation movement characterized by large strain and rapid response when electrically stimulated, comparable to that of biological muscles, making them attractive for artificial muscles and other actuation purposes. However, the standard fabrication of PVC gel by so-called solution casting has been found to be time-consuming (i.e., typically taking 4–7 days), cost-ineffective, and prone to skin/eyes irritation hazards, limiting its application potential. In this study, a thermal melting method using non-toxic and low-cost stabilizer was proposed to fabricate PVC gel in approximately 10 min. Experimental results show that under the fabrication conditions optimized by Taguchi orthogonal array, the actuation performance of PVC gel is improved, with a contraction strain of 20.62%, an output force of 0.43 N, and a recovery response time of 1.416 s, all of which are superior to that prepared by traditional method. These results open a doorway for fast, affordable, and easily accessible fabrication technology of PVC gel actuators.

The Electrochemical Actuation Performances of Nanoporous Ternary AlCoCu Alloy with a Unique Nanosheet Structure.

Compared to traditional actuators (such as piezoelectric ceramics), metal actuators possess the advantages of a low energy consumption, large strain amplitude, and high strain energy density. However, most of the existing metal actuators with an excellent comprehensive performance are composed of precious metals, which are limited by high costs and have almost no possibility for large-scale production in the future. This study focuses on non-precious metal materials and exploits a one-step chemical dealloying method to prepare bulk nanoporous (NP) CoCuAl actuators (NP-CCA) from Al70Co20Cu10 alloy. The microstructure and actuation properties of the NP-CCA were analyzed in detail. The dense continuous nanoscale pores provide an excellent network connectivity for a large strain response, enabling the NP-CCA to achieve a strain amplitude of up to 1.19% (more than eight and two times that of NP-Pt and NP-Ag, respectively), comparable to precious metal actuators. In addition, the NP-CCA possesses a high strain energy density, which is prominent in many precious metal actuation materials (such as NP-Au, NP-Ag, and NP-Pt).

Exact simulation for direction-dependent large elastic strain responses of soft fibre-reinforced composites

Exact simulation for direction-dependent large elastic strain responses of soft fibre-reinforced composites

A large deformation model for quasi-static to high strain rate response of a rate-stiffening soft polymer

Polyborosiloxane (PBS) is an important rate-stiffening soft polymer with dynamic, reversible crosslinks used in applications ranging from self-healing sensing and actuation to body and structural protection. Its highly rate-dependent response, especially for impact-mitigating structures, is important. However, the large strain response of PBS has not been characterized over quasi-static to high strain rates. Currently, there are no constitutive models that can predict the strongly rate-dependent large-deformation elastic-viscoplastic response of PBS. To address this gap, we have developed a microstructural physics motivated constitutive model for PBS, similar soft polymers and polymer gels with dynamic crosslinks to predict their large strain, non-linear loading–unloading, and significantly rate-dependent response. We have conducted compression experiments on PBS up to true strains of ∼125% and over a wide strain rate range of 10−3 s−1 to 103 s−1. The proposed semi-physical model fairly accurately captures the response of PBS over six decades of strain rates. We propose boron–oxygen coordinate-bond dynamic crosslinks with macroscopic relaxation timescale τ≈3 s and temporary entanglement lockups at high strain rates acting as crosslinks with τ≈0.0005 s as the two types of crosslink network mechanisms in PBS. We have outlined a numerical update procedure to evaluate the convolution-like time integrals arising from dynamic crosslink kinetics. Experiments involving three-dimensional inhomogeneous deformations were used to verify the predictive capabilities of our model and its finite element implementation. The modeling framework can be adopted for other dynamically crosslinked rate-stiffening soft polymers and polymer gels that are microstructurally similar to PBS.

Additively-manufactured 3D truss-lattice materials for enhanced mechanical performance and tunable anisotropy: Simulations & experiments

Truss-lattices have wide application prospects in various important fields owing to their superior mechanical properties and energy absorption characteristics. In this paper, two typical truss-lattice materials (i.e. bending-dominated body-centred cubic (BCC) and stretch-dominated Octet architectures) were designed with enhanced mechanical properties and tunable anisotropy. The elastoplastic properties and large strain response were both investigated numerically and experimentally. Numerical results showed that a relative larger stiffness characteristic could be harvested when the shape parameter was chosen between 0.2 and 0.3, and the anisotropy degree could also be controlled through the shape parameter. Large strain multi-cell simulations also demonstrated the enhanced plastic properties and energy absorption characteristics of the designed architectures. The numerical findings were then confirmed through the uniaxial compression experiments on the 316L stainless steel truss-lattices specimens fabricated by the Selective Laser Melting (SLM) process. This study would broaden the design idea for the 3D truss-lattices with enhanced mechanical performance and tunable anisotropy, which may of great potential in engineering applications.

Potentiostatic Dealloying Fabrication and Electrochemical Actuation Performance of Bulk Nanoporous Palladium

Metallic actuators increasingly exhibit superiority over conventional actuators (such as piezoelectric ceramics) via low energy consumption and large strain amplitude. Large strain amplitude and high strain energy density (or work density) are required for an actuator with excellent comprehensive performance. Herein, we fabricated bulk nanoporous Pd (np-Pd) with a dense nanoporous structure by two-step potentiostatic dealloying of as-annealed Ni–Pd alloy with chemical corrosion resistance, and investigated the dealloying behaviors as well as electrochemical actuation performance. A visible current density oscillation occurred during dealloying, which is related to formation/dissolution of the passivating film. Additionally, since the dense and continuous ligaments establish a good network connectivity for large strain response, the np-Pd achieves a strain amplitude of up to 3.74% and high strain energy density, which stands out among many actuator materials (e.g., np-AuPt, np-Ni, and np-AlNiCu). Our study provides a useful guidance for fabricating metallic actuators with excellent comprehensive performance.

Optimizing strain response in lead-free (Bi0.5Na0.5)TiO3-BaTiO3-NaNbO3 solid solutions via ferroelectric / (non-)ergodic relaxor phase boundary engineering

Lead-free bismuth sodium titanate (Bi0.5Na0.5)TiO3 (BNT) and related solid solutions are potential piezoelectric materials for such applications as actuators and transducers if their excellent strain responses and piezoelectric properties can be optimized. In this work, a large strain response of 0.61% is achieved in lead-free (0.94-x%)(Bi0.5Na0.5)TiO3-0.06BaTiO3-x%NaNbO3 (x = 0 –6, BNT-6BT-xNN) ceramics with the composition of x = 3.5 in a pseudo-cubic structure. Coexistence of ferroelectric (FE) and relaxor (RE) domain structures is observed in all the unpoled ceramics and the enhanced strain response is believed to be related to the evolution of the ergodic relaxor (ER) and non-ergodic (NR) states thanks to the substitution of antiferroelectric NN. BNT-6BT-3.5NN is a critical composition near the FE/NR/ER phase boundary close to room temperature (RT) and its high strain response arises from a synergistic combination of a reversible electric-field-induced phase transition and an active domain switching in the mixed NR/ER state. This work provides new insights into the dynamic interplay between mesoscopic domains and macroscopic electrical properties in the BNT-based piezoceramics.

Large strain response in lanthanide-doped potassium sodium niobate-based piezoceramics

Developing polymorphic phase coexistence near room temperature in potassium sodium niobate-based piezoceramics is a widespread way to improve strain performance. Herein, 0.97(K0.48Na0.52)1-3xLaxNbO3-0.03SrTiO3 piezoceramics were studied by analyzing their grain images, crystal structures, dielectric properties, Raman spectra, ferroelectric properties, strain responses, and piezoelectric properties. A high strain of 0.43% was achieved at x = 0.0050 presenting a diffuse phase transition state, due to various intrinsic factors, including large electrostriction, small piezoelectric effects, and extrinsic microdomain wall reorientation. The large electrostriction was the chief contribution to the strain due to the considerable electrostrictive coefficient (Q33) of 0.039 m4/C2. Furthermore, the considerable Q33 was due to B-site shifting by distorting the slightly compressed NbO6 octahedron arising from La3+ doping. The compressed of the NbO6 octahedra was proven by XRD results and generated distortion according to the obtained Raman spectra.

Extremely large strain response under low driving electric fields in lead-based textured piezoelectric ceramics

The large strain response and low strain hysteresis of piezoelectric ceramics under low driving electric fields have been eagerly anticipated for actuators applications. However, it remains challenge to simultaneously achieve both of the above properties under low driving electric fields for piezoelectric ceramics. In this work, composition design and texture technique were integrated to develop 1%Sm-doped Pb(Mg1/3Nb2/3)O3–30PbTiO3 (Sm-PMN-PT) textured ceramics to address this challenge. We demonstrate Sm-PMN-PT textured ceramics with highly [001]c orientation exhibit ultrahigh strain response and the lower strain hysteresis under low driving electric fields. At 2 kV/mm, a large strain value of 0.27% and a low hysteresis of 15% were acquired. In addition, the microstructure and crystal structure of Sm-PMN-PT textured ceramics and its random counterpart were studied as well as their electrical properties. This research results provide a new promising candidate materials for developing high-performance piezoelectric actuators with low driving electric fields.

Enhanced actuation performance of silicone rubber via the synergistic effect of polyaniline particles and silicone oil

Due to the large strain and fast response characteristics of dielectric elastomers (DEs), they are regarded as a promising alternative for “artificial muscle”. Silicone rubber (SR) with good aging resistance, biocompatibility, and high insulation, is considered as an excellent candidate material for DEs. In this work, conductive polyaniline (PANI) particles were dispersed as dielectric filler to improve the value of dielectric constant (ε), while the silicone oil (SO) was incorporated as a plasticizer for increasing the flexibility of SR DE-based composites. At last, the actuated strain of 30 phr SO/PANI/SR DE-based composite reached the value of 9.5% under 60 kV/mm, which was equivalent to an increase of 265% for the pure SR (2.6%) at the same electric field. Moreover, good durability and mechanical properties were demonstrated for the as-prepared SR DE-based composites, which is of great importance for their long-term use.

3D printing of liquid crystal elastomers-based actuator for an inchworm-inspired crawling soft robot.

Liquid crystal elastomers (LCEs) have shown great potential as soft actuating materials in soft robots, with large actuation strain and fast response speed. However, to achieve the unique features of actuation, the liquid crystal mesogens should be well aligned and permanently fixed by polymer networks, limiting their practical applications. The recent progress in the 3D printing technologies of LCEs overcame the shortcomings in conventional processing techniques. In this study, the relationship between the 3D printing parameters and the actuation performance of LCEs is studied in detail. Furthermore, a type of inchworm-inspired crawling soft robot based on a liquid crystal elastomeric actuator is demonstrated, coupled with tilted fish-scale-like microstructures with anisotropic friction as the foot for moving forwards. In addition, the anisotropic friction of inclined scales with different angles is measured to demonstrate the performance of anisotropic friction. Lastly, the kinematic performance of the inchworm-inspired robot is tested on different surfaces.

Impact of stress triaxiality, strain rate, and temperature on the mechanical response and morphology of PVDF

Poly-vinylidene fluoride (PVDF) is a semi-crystalline thermoplastic featuring a high chemical resistance, good thermal stability, and high pressure resistance, rendering it attractive for a wide range of engineering applications. This study assesses the influence of temperature, strain rate, and stress triaxiality on the large-strain response of a commercial PVDF copolymer containing poly-ethylene (PE) particles.The thermo-viscoplastic response of the material was investigated at six temperatures ranging between −20°C and 100°C at a quasi-static strain rate ė of 0.005s−1, and three strain rates ė={0.005,0.1,1.0}s−1 at room temperature. To study the pressure sensitivity of the material, tensile tests using axisymmetric notched tensile specimens with three different notch radii as well as uniaxial compression tests were performed. The mechanical response was assessed in terms of net axial stress and volume strain vs. longitudinal strain, measured using digital image correlation (DIC). To gain insight into the interrelationship between the macroscopic volume strain and void morphology on the microscale, selected cross sections of deformed specimens were imaged employing scanning electron microscopy (SEM).The material exhibited a temperature- and strain-rate-dependent response as well as a pressure-sensitive flow stress. For ambient temperatures up to 60°C, pronounced plastic dilatancy was observed, promoted by increasing the stress triaxiality. The SEM study confirmed that the plastic dilatancy was a result of extensive void nucleation and growth on the microscale. Two sources of void nucleation were identified, namely particle–matrix interface separation as well as cavitation within the matrix material itself. In tests performed at ambient temperatures higher than 80°C, no plastic dilation was observed and no voids or PE particles were identified in the SEM images.Apart from an improved understanding of the material’s mechanical response, this study provides data and insights suitable for the development and calibration of constitutive models, aiding the design of robust and reliable structures using PVDF.