Shear Stiffening Research Articles

Bioinspired Fiber Networks With Tunable Mechanical Properties by Additive Manufacturing

Abstract Soft bioinspired fiber networks offer great potential in biomedical engineering and material design due to their adjustable mechanical behaviors. However, existing strategies to integrate modeling and manufacturing of bioinspired networks do not consider the intrinsic microstructural disorder of biopolymer networks, which limits the ability to tune their mechanical properties. To fill in this gap, we developed a method to generate computer models of aperiodic fiber networks mimicking type I collagen ready to be submitted for additive manufacturing. The models of fiber networks were created in a scripting language wherein key geometric features like connectivity, fiber length, and fiber cross section could be easily tuned to achieve desired mechanical behavior, namely, pretension-induced shear stiffening. The stiffening was first predicted using finite element software, and then a representative network was fabricated using a commercial 3D printer based on digital light processing technology using a soft resin. The stiffening response of the fabricated network was verified experimentally on a novel test device capable of testing the shear stiffness of the specimen under varying levels of uniaxial pretension. The resulting data demonstrated clear pretension-induced stiffening in shear in the fabricated network, with uniaxial pretension of 40% resulting in a factor of 2.65 increase in the small strain shear stiffness. The strategy described in this article addresses current challenges in modeling bioinspired fiber networks and can be readily integrated with advances in fabrication technology to fabricate materials truly replicating the mechanical response of biopolymer networks.

Shear response behavior of SSE/Kevlar composite fabric before and after shear interlocking effect

Shear stiffening gel (SSG) material was obtained by high-temperature polymerization with boric acid and hydroxyl silicone oil, shear stiffening elastomer (SSE) material was enhanced by vulcanization, and the rheological properties were analyzed. SSG and SSE/Kevlar composite fabric was obtained by “dip–dry” and “hot-pressing” process, and a picture frame test was performed on neat fabric and composite fabrics under three loading rates. The in-plane shear deformation behaviors of the fabrics were characterized by the DIC marker method, and the shear deformation mechanism of braided cells was analyzed theoretically. The results show that the SSG material has superior shear stiffening characteristics, whereas the SSE material has a much higher initial degree of crosslinking and does not have cold-flow characteristics. The fabrics experience typical shear interlocking behavior in shear deformation, and different shear deformation stiffness and energy absorption capacity before and after the shear interlocking was observed, which is caused by the different resistance mechanisms of braided cell to external shear force. The strain rate phase-change characteristics of SSG and SSE materials make the composite fabric have higher shear deformation stiffness and energy absorption at high speed, and SSE material significantly reduces the contact force between adjacent yarns after shear interlocking.

Liquid metal based triboelectric nanogenerator with excellent electrothermal and safeguarding performance towards intelligent plaster

Polymer-based self-powered wearable sensing devices are easily destroyed under harsh mechanical shocks which largely restricts the development of smart electronic device. Herein, a versatile liquid metal based triboelectric nanogenerator (LM-TENG) device was developed by integrating the shear stiffening elastomer (SSE) with liquid metal (LM) and electrothermal aluminum foil. Based on the electrostatic induction effect, LM-TENG showed the good capacity of harvesting energy. The output voltage and power of the LM-TENG reached to 22.29 V and 55.16 µW with 9 MΩ load resistance at the force and frequency of 40 N and 15 Hz. Besides, LM-TENG demonstrated excellent electrothermal performance by efficiently reaching 69.02 ℃ with the applied voltage of 1.5 V. Additionally, LM-TENG displayed safeguarding performance under quasi-static and dynamic damage by effectively decreasing the impact force from 2.86 to 0.72 kN, endowing the device with excellent anti-impact from sharp objects penetration loading. Moreover, the utility of LM-TENG as intelligent medical plaster with joule-heating, controllable stiffness, self-powering sensing and anti-impact performance was further studied, providing a new strategy for designing multifunctional sensor, and paving the way for intelligent wearable electronics, personal healthcare and safeguards in the future.

Bio‐Inspired Semi‐Active Safeguarding Design with Enhanced Impact Resistance via Shape Memory Effect

AbstractInspired by creatures reducing impact damage, a novel semi‐active protection strategy to enhance impact resistance through structural adjustment is proposed. The heat‐activated shape memory composites (PCLE) consisting of polycaprolactone (PCL) and shear stiffening elastomer are first prepared. The PCLE with 50 wt.% PCL (PCLE‐50%) presents tensile strength of 0.46 MPa, fracture strain of 148%, high shape fixity and recovery rates (97.9% and 91.1%), and stable shape‐memory repeatability. Furthermore, various PCLE‐50%‐based structures are fabricated, which show reinforced anti‐impact performance after shape‐shifting. Loading from 300 mm, the maximum drop hammer impact force of no protection, PCLE‐50%, and deformed assembled single‐branch structure are 3.98 kN, 1.75 kN, and 0.93 kN, respectively. Additionally, assembled complex structures also exhibit exceptional energy absorption properties. Ultimately, intelligent clothing is successfully manufactured. Besides good anti‐impact effect, the clothing shows excellent thermal insulation capabilities whose surface temperature is 31.7 °C after heating for 600 s at 42 °C. This study provides an innovative means to develop deformable, wearable, and intelligent protective devices.

Low-Velocity Impact Response of Sandwich Composite Panels with Shear Stiffening Gel Filled Honeycomb Cores

Low-Velocity Impact Response of Sandwich Composite Panels with Shear Stiffening Gel Filled Honeycomb Cores

Lightweight shear stiffening gel with enhanced mechanical and thermal-insulating performance for advanced fire suit

A lightweight and thermal-insulating shear stiffening gel (LT-SSG) with anti-impact was developed via introducing hollow glass microspheres BR20 into shear stiffening gel (SSG). It was lightweight enough to be placed on a leaf. LT-SSG showed the ability to resist impact. Finally, a multifunctional Kevlar safeguarding suit (MKSS) with flame retardant property was fabricated. It exhibited good adiabatic effect which could isolate heat at high temperatures and prevent heat loss at low temperatures. • LT-SSG with lightweight, heat-insulation and anti-impact properties was prepared. • LT-SSG showed enhanced protective effect due to the existence of BR20 particles. • A multifunctional Kevlar safeguarding suit (MKSS) was assembled. • MKSS was capable to resist fire combustion and displayed good adiabatic effect. • MKSS effectively prevented the heat loss from humans in cold environment. Herein, a lightweight and thermal-insulating shear stiffening gel (LT-SSG) with anti-impact was prepared by introducing insulating glass microspheres BR20 into SSG. The density of LT-SSG was 48.97% lower than that of SSG. Meantime, LT-SSG showed excellent energy absorption effect. The remained impact force of LT-SSG was only 0.23 kN compared with 0.41 kN of SSG when impacted from 20 cm. Moreover, LT-SSG with good thermal insulation performance could effectively isolate surrounding temperature to 42.6 ℃ at 80 ℃ and block external cold temperature to protect the heat loss from human body. While SSG as well as the breakage of BR20 particles absorbed and dissipated much impact energy, proving its excellent protective properties. Finally, a multifunctional Kevlar safeguarding suit (MKSS) was developed. It showed the ability to resist low-speed drop hammer and high-speed ballistic impact. To this end, the reported LT-SSG showed promising application in fire suit and body armors.

Advanced functional safeguarding composites with enhanced anti-impact and excellent thermal properties

Personal safety protection has played an important role in daily life. Developing advanced functional safeguarding composites with enhanced anti-impact and excellent thermal properties will be a significant development for body armor. Herein, Kevlar fiber reinforced polymers (KFRP) were fabricated by introducing short Kevlar fibers (KFs) into a shear stiffening elastomer (SSE). The storage modulus of KFRP with 15 wt% KFs (KFRP-15%) increased from 222.8 kPa to 830.8 kPa when the shear frequency varied from 0.1 Hz to 100 Hz. KFRP-15% achieved a higher tensile strength (2.65 MPa) and fracture toughness (11.95 kJ/m<sup>2</sup>) than SSE in the vertical type, showing superior tear resistance. Additionally, KFRP-15% exhibited promising anti-impact properties, which could dissipate the drop hammer impact force from 1.74 kN to 0.56 kN and remained intact after 10 consecutive impacts. Moreover, KFRP-15% also presented excellent stab-resistant performance. In addition, KFRP-15% also showed improved heat transfer properties, flame retardancy, and smoke suppression capabilities. Finally, functional bracers based on KFRP-15% for protection, thermal-dissipation, and flame-retardant were successfully prepared.

Design and analysis of passive variable stiffness device based on shear stiffening gel

Shear thickening materials are utilized as the core of variable stiffness devices in various engineering applications in recent years. However, due to the used materials being liquid, most of these devices are suffered from the problem of liquid leakage and coagulation. To address this issue, this paper proposes a design method of a passive variable stiffness device based on shear stiffening gel (SSG), which integrates the core SSG and spring into a compact structure to obtain well stability. The used SSG can affect the force transfer within the device, thereby influencing the system’s stiffness dynamically over the external impact load. Due to the core SSG being good shear stiffening properties, the device is highly sensitive to frequency and shows variable stiffness characteristics. When the frequency of external load increased from 0.1 Hz to 5 Hz, the equivalent stiffness of the device could be increased by 105.74%. An equivalent nonlinear model is used to represent the overall stiffness of the device as a function of the frequency and displacement amplitude of the load. The output of the equivalent nonlinear model agrees well with the experimental data. The cross-validation also proves the accuracy of the model fitting, which provides a quantitative description of the dynamic performance of the variable stiffness device. By combining different types of SSGs and springs with different stiffness, the design principle of the variable stiffness device proposed in this paper can be utilized in various impact-loading applications, such as designing the exoskeletal load carriage supporting mechanism and vehicle suspension systems. The work of this paper has guiding significance for the design of adaptive variable stiffness devices.

Nacre-Mimetic Hierarchical Architecture in Polyborosiloxane Composites for Synergistically Enhanced Impact Resistance and Ultra-Efficient Electromagnetic Interference Shielding.

Pervasive mechanical impact is growing requirement for advanced high-performance protective materials, while the electromagnetic interference (EMI) confers severe risk to human health and equipment operation. Bioinspired structural composites achieving outstanding safeguards against a single threat have been developed, whereas the synergistic implementation of impact/EMI coupling protection remains a challenge. This work proposes the concept of nacre-mimetic hierarchical composite duplicating the "brick-and-mortar" arrangement, which consists of freeze-drying constructed chitosan/MXene lamellar architecture skeleton embedded in a shear stiffening polyborosiloxane matrix. The resulting composite effectively attenuates the impact force of 85.9%-92.8% with extraordinary energy dissipation capacity, in the coordinative manner of strain-rate enhancement, structural densification, lamella dislocation and crack propagation. Attributed to the alternate laminated structure promoting the reflection loss of electromagnetic waves, it demonstrates an ultraefficient EMI shielding effectiveness of 47.2-71.8 dB within extremely low MXene loadings of 1.1-1.3 wt %. Furthermore, it serves favorably in impact monitoring and wireless alarm systems and accomplishes performance optimization through the combination of multiple biomimetic strategies. In conclusion, this function-integrated structural composite is shown to be a competitive candidate for sophisticated environments by resisting impact damage and EMI hazards.

Joint inversion of the unified pore geometry of tight sandstones based on elastic and electrical properties

The prediction of the pore geometrical properties is important in the exploration and development of tight-sandstone hydrocarbon reservoirs. To investigate this topic, we have measured the porosity, permeability, P- and S-wave velocities, electrical conductivity, and axial and radial strains as a function of differential (confining minus pore) pressure of tight-sandstone samples, collected from the Zhongjiang gas field of Sichuan, in West China. The results show that the closure of cracks with pressure highly affects these properties. Then, we propose a multiphase reformulated differential effective-medium (R-DEM) model that employs the unified pore geometry (the same pores or cracks with different aspect ratios and volume fractions) for both elastic and electrical modeling. The model gives the pressure-dependence of the P- and S-wave velocities and electrical conductivity, and the experimental porosity and static moduli are used as constraints to estimate the pore geometry. The model describes the elastic properties of sandstones saturated with nitrogen gas, and the electrical conductivity when the pore fluid is brine. The prediction of the wet-rock S-wave velocities is less accurate, due to the presence of shear stiffening and weakening effects. Furthermore, we compare the results with those of the joint elastic-electrical inversion by using the dynamic instead of the static stiffness modulus. The results show that the latter provides a better agreement between theory and experiment. Subsequently, we show that the pore geometry estimated from the elastic or the electrical measurements separately (unjoint inversion) present discrepancies, indicating that a joint inversion is required. The published experimental data are also used to illustrate the model, and the results are satisfactory.

A self-powered three-dimensional integrated e-skin for multiple stimuli recognition

Real-time detection and differentiation of diverse external stimuli remains a huge challenge and largely restricts the development of electronic skins (e-skin). Hence, a versatile e-skin was developed by integrating a thermoelectric graphene/polydimethylsiloxane (PDMS) sponge with piezoelectric array, to minimize the coupling effect between thermal and mechanical excitations and distinguish different mechanical stimuli. Owing to the designed three-dimensional structure, the e-skin was capable of differentiating in-plane and out-of-plane force based on the corresponding induced strains. Besides, graphene/PDMS sponge was utilized as thermal tactile elements with a moderate temperature sensitivity of 0.122 mV/K. A wireless temperature sensing system was proposed for transmitting signals and “hot”/ “cold” tips to terminal devices. Importantly, the e-skin demonstrated excellent sensing performance on recognizing shear directions by 0°-180°. Therefore, a robotic gripper equipped the e-skin with detection of the “holding” and “slipping” motion exhibited promising prospects in artificial tactile feedback system with self-adjusting grip force. Moreover, the self-healing nature originated from shear stiffening elastomer matrix enabled that individual e-skin could be reassembled into an arrayed e-skin with the spatial tactile ability. This work provided a new strategy for designing functional e-skins, and paved the way for intelligent robotic technologies, including adaptive grasping, biomimetic robots, and human-machine interactions.

Impact energy absorption composites with shear stiffening gel-filled negative poisson's ratio skeleton by Kirigami method

A new type of Negative Poisson's ratio skeleton composites (NPRSC) filled with shear stiffening gels (SSG) is presented in this work. The skeleton is manufactured by Kirigami method for a simplest folding process. Combined with the shear stiffening property of SSG and auxetic effect of Negative Poisson's ratio (NPR) skeleton, the flexible composites show excellent impact energy absorption performance. Static compression experiment displays that compared with Positive Poisson's ratio (PPR) skeleton, the NPR structure composed of re-entrant elements is more flexible with a lower initial compression modulus for its special curvature effect. Dynamic impact experiments and numerical simulation reveals that the auxetic effect of NPR skeleton further facilitates the dynamic shear stiffening property of SSG and helps to dissipate the stress transversely. Specifically, the NPR skeleton contributes to a lower peak load, higher energy absorption ratio (EAR) and specific energy absorption (SEA) cooperated with SSG. The NPRSC exhibits improved energy absorption with the increase of impact velocity due to the strain rate sensitivity of SSG. This method provides an innovative idea for the structural design of shear stiffening materials and may be widely applied in the field of intelligent protection.

Flame-retardant triboelectric generator with stable thermal-mechanical-electrical coupling performance for fire Bluetooth alarm system

A novel triboelectric nanogenerators (TENG) device with energy-harvesting, safeguarding properties was developed based on the anti-impact and flame retardant elastomers (AFEs). The AFE composite was fabricated via the introduction of urea and carbon nanotubes (CNTs) into shear stiffening elastomer (SSE). The limiting oxygen index (LOI) value of 6AFE-25 (6AFE contained 25% urea) was 30.5% and could reach V-0 rating. The peak heat release rate of 6AFE-25 achieved 224.79 kW/m2, which was decreased by 29.10% compared with neat SSE. More importantly, the optimized TENG with thickness of 4 mm exhibited a maximum power of 57.25 μW at voltage of 22.70 V which enabled to act as a wearable power source to actuate electronics. Besides, the TENG enabled to dissipate impact force from 5199 to 386 N, exhibiting a remarkable safeguarding performance. Owing to good self-healing property, the thermal-electrical-mechanical coupling performance of the reported TENG device kept stable even after cutting damage and 20 s-fire attack. Finally, a self-powered Bluetooth fire alarm system was developed which could be used in fire rescue.

Intelligent safeguarding Leather with excellent energy absorption via the toughness-flexibility coupling designation

High-performance intelligent body armors with both soft wearability and good energy absorption are increasingly required in modern social life. Leather, a breathable, pliable, and tough natural material has extensive applications in the anti-impact field. In this study, a multifunctional Leather/CNT-SSG/PU sandwich structure with high energy absorption and excellent sensing monitoring properties is designed by integrating tough leather and flexible conductive shear stiffening gel (CNT-SSG). The maximum force dissipation capacity of the Leather/CNT-SSG/PU reaches 84 % in the drop hammer test and the limit penetration velocity up to 142.4 m/s in the ballistic test. Moreover, the Leather/CNT-SSG/PU exhibits excellent stress wave dissipation properties due to the hierarchical structure. By introducing the interdigital electrode array on the non-woven fabric layer, a bulletproof vest (Leather/CNT-SSG/NWF) with intelligent anti-impact performance and in situ monitoring behavior is successfully achieved. This toughness and flexibility coupling designation provides a novel method for next-generation soft body armor.

Suppression of the sutural interface on vibration behaviors of sandwich beam with shear stiffening gel

Sutural composites have been widely adopted as structural components in various engineering applications. In this work, a series of sutural sandwich beam were prepared by integrating the magneto-rheological shear stiffening gel (SSG) and 3D-printed face sheet material. The structural stiffness of beam samples with different sutural interface was tested. It could be found that the sutural interface geometry showed significant influence on the structural stiffness. Due to the specific viscoelastic property of SSG material, the damping ratio of the beam could be improved with the sutural interface. Meanwhile, benefitted from the rate-dependent stiffening and magneto-rheological properties of the SSG core, the sandwich beams exhibited passive and active vibrations suppression capacities. A theory model about the transmission of the beam with SSG core under external vibration excitation was carried out to explain the experimental results. This work proved that the sutural interface could be adopted as a promising strategy to improve the structural stiffness and vibration control capability of the sandwich beam, which was every important in the application of many industry fields.

Material characterization and simulation for soft gels subjected to impulsive loading

For impact and blast experiments of traumatic brain injury (TBI), soft gel materials are used as surrogates to imitate the mechanical responses of brain tissue. To properly model a viscoelastic gel brain in a surrogate head using a finite element (FE) model, material parameters such as the shear moduli and relaxation time at high strain rates are required. However, such information is scarce in the literature and its applicability for a range of dynamic conditions is unclear. We used an integrated experiment and simulation approach to efficiently determine mechanical properties of soft gels at finite strains, as well as over a wide range of strain rates. A novel impact experiment using a gel block was developed to capture the high strain rate behavior by maximizing the inherent shear wave motion at different impact conditions. A corresponding computational model was used to simulate the gel dynamics of the impact. Parametric simulations utilizing optimization and correlation analyses were used to calibrate multiple material parameters in the nonlinear viscoelastic model to the experimental data. The optimal parameters for gels, including Sylgards 184, 3–6636, and 527, were found. We ascertained the initial shear stiffening effect in gels at high strain rate loadings experimentally and incorporated this effect in the simulation. We have verified the integrated approach by comparing the material properties of the gels with analytical results based on shear wave propagation. This study provides a new approach to calibrate the material behavior of soft gels under high strain rate loading conditions.

A lightweight aramid-based structural composite with ultralow thermal conductivity and high-impact force dissipation

<h2>Summary</h2> Material designs for safety protection are increasingly important due to ubiquitous impact damage and thermal hazard. Recent biomimetic architectures achieve extraordinary safeguards but exhibit a single-defense function, which remains a formidable challenge in mechanical-thermal-coupled protection. Herein, a lightweight composite (AFSG) with a shear stiffening gel (SSG)-filled aramid nanofiber (ANF) aerogel structure is developed through infiltration and lamination. Benefiting from the voids-SSG coexistence construction achieved by retaining massive microvoids in densified aerogel networks, AFSG exhibits ultralow thermal conductivity (0.09 W m⁻¹ k⁻¹) and high-impact force dissipation. Specifically, microvoids inside AFSG greatly block heat transfer and achieve a wide insulation temperature (–120 to 300°C). SSG and the laminated-layout structure effectively attenuate 65%–79% of impact force through structure hardening, interlayer sliding, and intralayer cracking. Such a versatile, lightweight structural composite is expected to defend against simultaneous mechanical impact and heat damage as an ideal candidate for next-generation protective materials applied in transportation, military, and aerospace fields.

A novel anti-impact and flame retardant gel towards human protection and high-temperature alarm

A novel anti-impact and flame retardant gel (AFG) was developed by combining ammonium polyphosphate (APP) with shear stiffening gel (SSG). AFG-55% (containing 55% of APP) exhibited excellent thermal property which the peak heat release rate of AFG-55% decreased by 57.48% compared with neat SSG. Its UL-94 vertical burning test reached V-0 rating. The flame-retardant mechanism was attributed to the ammonia and water molecules production during APP degradation which diluted the combustible gases and formed a char layer to block the flame. Besides, AFG effectively dissipated impact force from 768 to 204 N, showing excellent safeguarding properties for protecting human beings from harsh impact. Finally, a functional conductive AFG-55%-based fire warning sensor (FWS) with rapid alarm time of 5 s was developed. FWS stably alarmed the fire even after impact and cutting. Thus, this work proposed a novel AFG-55% polymer with enhanced thermal and safeguarding properties which showed promising application in flame retardance, fire suit and safeguards.

Self-healing and printable elastomer with excellent shear stiffening and magnetorheological properties

This work reports a novel kind of hybrid magnetorheological elastomer (HMRE) that possesses multi-functions including excellent shear stiffening and magnetorheological (MR) effects, as well as self-healing and printable abilities. The HMRE materials are prepared by embedding carbonyl iron particles (CIPs) into a blend of silicon rubber (SR) and low cross-linking gel-like polyurethane (PU). The relative shear stiffening effect of HMRE-3:1 (the mass ratio of PU and SR is 3:1, and 50 wt% CIPs) is 6095% as the shear frequency increases from 0.1 Hz to 100 Hz, which is about 150 times that of SR based magnetorheological elastomer (MRE). With the magnetic flux density from 0 to 1T, MR effect of isotropic HMRE-3:1 is as high as 266%, which is 3.5 times that of SR based MRE. Notably, HMRE obtains extraordinary mechanical and electrical healing capabilities. HMRE with a destructive cut-through injury can sustain an extensibility of 425% (the initial extensibility ≈ 650%) after healing. Meanwhile, possible mechanisms are proposed to explain the shear stiffening properties, MR effect, and self-healing performance of HMRE. Moreover, the extruded modeling 3D printing of flexible HMRE actuators can be achieved due to the plasticine property of pre-polymerized HMRE, which bestows HMRE with magnetic particle distribution programmability and shape designability. This work may provide a new way for the next-generation multifunctional materials with exceptional shear stiffening behavior, magnetically mechanical properties, self-healabilities, and printable performance.

Study on the shear thickening mechanism of multifunctional shear thickening gel and its energy dissipation under impact load

A multifunctional shear thickening gel (MSTG) with both shear thickening performance and magnetorheological effect was prepared by dispersing the carbonyl iron powder into shear stiffening polymer matrix. Upon varying the mass ratio of pyroboric acid to polydimethylsiloxane from 0 to 0.6, the relative shear thickening effect of MSTG first increases and then decreases, demonstrating an excellent shear thickening property between the ratio within 0.2–0.4. A molecular chain model of the cross-linked bond was proposed to illustrate the shear thickening mechanism, and verified by the impact test of MSTG with different proportions. It is believed that more cross-linked bonds were formed at a higher external impact velocity due to the stimuli-response performance of MSTG. • Variation rules of ST effect and chain model of MSTG were put forward. • The chain model was verified by the impact test of MSTG. • An optimization method was proposed to obtain higher shear thickening effect. • Energy dissipation values of MSTG at distinct impact velocities were simulated.