A theoretical model for impact protection of flexible polymer material

The relationship between the protective performance of flexible polymer material and material parameters(elasticmodulus,viscosity coefficient)is explored,an impact collision motion equation between two bodies is establishedfrom the viscoelastic material constitutive,and the relationship between the kinematic response and the materialparameters is obtained.Based on the Kelvin constitutive model,a theoretical model for impact between the pro-tective body and the protected body is established,then the dynamic response is obtained.The feasibility of themodel was verified by drop hammer experiment,and the material parameters(elastic modulus,viscosity coeffi-cient)were obtained by formula.The model is discretized and the relationship between local impact response andmaterial parameters is analyzed.The discussion results on the relationship between the impact response and theprotective material performance indicate that adjusting the elastic modulus,viscosity coefficient,and thicknessof the protective material can effectively improve protective effect.

Abrasion test of flexible protective materials on hydraulic structures

In this study, several kinds of flexible protective materials sprayed with polyurea elastomers （hereinafter referred to as polyurea elastomer protective material） were adopted to meet the abrasion resistance requirement of hydraulic structures, and their abrasion resistances against the water flow with suspended load or bed load were studied systematically through tests. Natural basalt stones were adopted as the abrasive for simulation of the abrasion effect of the water flow with bed load, and test results indicate that the basalt stone is suitable for use in the abrasion resistance test of the flexible protective material. The wear process of the polyurea elastomer protective material is stable, and the wear loss is linear with the time of abrasion. If the wear thickness is regarded as the abrasion resistance evaluation factor, the abrasion resistance of the 351 pure polyurea is about twice those of pure polyurea with a high level of hardness and aliphatic polyurea, and over five times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with suspended load. It is also about 50 times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with bed load. Overall, the abrasion resistance of pure polyurea presented a decreasing trend with increasing hardness. Pure polyurea with a Shore hardness of D30 has the best abrasion resistance, which is 60 to 70 times that of high-performance abrasion-resistant concrete under the abrasion of the water flow with bed load, and has been recommended, among the five kinds of pure polyurea materials with different hardness, in anti-abrasion protection of hydraulic structures.

Recent Advances in Design Strategies and Multifunctionality of Flexible Electromagnetic Interference Shielding Materials

With rapid development of 5G communication technologies,electromagnetic interference(EMI)shielding for electronic devices has become an urgent demand in recent years,where the development of corresponding EMI shielding materials against detrimental electromagnetic radiation plays an essential role.Meanwhile,the EMI shielding materials with high flexibility and functional integrity are highly demanded for emerging shielding applications.Hitherto,a variety of flexible EMI shielding materials with lightweight and multifunctionalities have been developed.In this review,we not only introduce the recent development of flexible EMI shielding materials,but also elaborate the EMI shielding mechanisms and the index for"green EMI shielding"performance.In addition,the construction strategies for sophisticated multifunctionalities of flexible shielding materials are summarized.Finally,we propose several possible research directions for flexible EMI shielding materials in near future,which could be inspirational to the fast-growing next-generation flexible electronic devices with reliable and multipurpose protections as offered by EMI shielding materials.

Research progress in lead-less or lead-free three-dimensional perovskite absorber materials for solar cells

The trend toward lead-free or lead-less perovskite solar cells(PSCs) has attracted increasing attention over the past few years because the toxicity of lead(Pb) is one of the substantial restrictions for large-scale applications. Researchers have investigated the viability of substituting Pb with other elements(group 14 elements, group 2 elements, transition-metal elements, and group 13 and 15 elements) in the three-dimensional(3 D) perovskites by theoretical calculations and experimental explorations. In this paper, recent research progress in Pb-less and Pb-free PSCs on the perovskite compositions, deposition methods, and device structures are summarized and the main problems that hinder the enhancement of device efficiency and stability are discussed in detail. To date, the fully Sn-based PSCs have shown a power conversion efficiency(PCE) of 8.12% and poor device stability. However, lead-less PSCs have shown higher PCE and a better stability. In addition, the introduction of double-perovskite materials also draws researchers' attention. We believe that the engineering of elemental composition, perovskite deposition methods, and interfacial modification are critical for the future development of Pb-less and Pb-free PSCs.

Flexible SERS substrates for hazardous materials detection: recent advances

This article reviews the most recent advances in the development of flexible substrates used as surface-enhanced Ra-man scattering(SERS)platforms for detecting several hazardous materials(e.g.,explosives,pesticides,drugs,and dyes).Different flexible platforms such as papers/filter papers,fabrics,polymer nanofibers,and cellulose fibers have been investigated over the last few years and their SERS efficacies have been evaluated.We start with an introduction of the importance of hazardous materials trace detection followed by a summary of different SERS methodologies with par-ticular attention on flexible substrates and their advantages over the nanostructures and nanoparticle-based solid/hybrid substrates.The potential of flexible SERS substrates,in conjunction with a simple portable Raman spectrometer,is the power to enable practical/on-field/point of interest applications primarily because of their low-cost and easy sampling.

Recent progress in 2D materials for flexible supercapacitors

High performance supercapacitors coupled with mechanical flexibility are needed to drive flexible and wearable electronics that have anesthetic appeal and multi-functionality. Two dimensional（2D） materials have attracted attention owing to their unique physicochemical and electrochemical properties, in addition to their ability to form hetero-structures with other nanomaterials further improving mechanical and electrochemical properties. After a brief introduction of supercapacitors and 2D materials, recent progress on flexible supercapacitors using 2D materials is reviewed. Here we provide insights into the structure–property relationships of flexible electrodes, in particular free-standing films. We also present our perspectives on the development of flexible supercapacitors.

Flexible electronics and optoelectronics of 2D van der Waals materials

Flexible electronics and optoelectronics exhibit inevitable trends in next-generation intelligent industries,including healthcare and wellness,electronic skins,the automotive industry,and foldable or rollable displays.Traditional bulk-material-based flexible devices considerably rely on lattice-matched crystal structures and are usually plagued by unavoidable chemical disorders at the interface.Two-dimensional van der Waals materials(2D VdWMs)have exceptional multifunctional properties,including large specific area,dangling-bond-free interface,plane-to-plane van der Waals interactions,and excellent mechanical,electrical,and optical properties.Thus,2D VdWMs have considerable application potential in functional intelligent flexible devices.To utilize the unique properties of 2D VdWMs and their van der Waals heterostructures,new designs and configurations of electronics and optoelectronics have emerged.However,these new designs and configurations do not consider lattice mismatch and process incompatibility issues.In this review,we summarized the recently reported 2D VdWM-based flexible electronic and optoelectronic devices with various functions thoroughly.Moreover,we identified the challenges and opportunities for further applications of 2D VdWM-based flexible electronics and optoelectronics.

Nature-inspired materials and designs for flexible lithium-ion batteries

Flexible lithium-ion batteries(FLBs)are of critical importance to the seamless power supply of flexible and wearable electronic devices.However,the simultaneous acquirements of mechanical deformability and high energy density remain a major challenge for FLBs.Through billions of years of evolutions,many plants and animals have developed unique compositional and structural characteristics,which enable them to have both high mechanical deformability and robustness to cope with the complex and stressful environment.Inspired by nature,many new materials and designs emerge recently to achieve mechanically flexible and high storage capacity of lithiumion batteries at the same time.Here,we summarize these novel FLBs inspired by natural and biological materials and designs.We first give a brief introduction to the fundamentals and challenges of FLBs.Then,we highlight the latest achievements based on nature inspiration,including fiber-shaped FLBs,origami and kirigami-derived FLBs,and the nature-inspired structural designs in FLBs.Finally,we discuss the current status,remaining challenges,and future opportunities for the development of FLBs.This concise yet focused review highlights current inspirations in FLBs and wishes to broaden our view of FLB materials and designs,which can be directly“borrowed”from nature.

Recent advances in flexible alkaline zinc-based batteries:Materials,structures,and perspectives

The development of wearable electronic systems has generated increasing demand for flexible power sources.Alkaline zinc(Zn)-based batteries,as one of the most mature energy storage technologies,have been considered as a promising power source owing to their exceptional safety,low costs,and outstanding electrochemical performance.However,the conventional alkaline Zn-based battery systems face many challenges associated with electrodes and electrolytes,causing low capacity,poor cycle life,and inferior mechanical performance.Recent advances in materials and structure design have enabled the revisitation of the alkaline Zn-based battery technology for applications in flexible electronics.Herein,we summarize the up-to-date works in flexible alkaline Zn-based batteries and analyze the strategies employed to improve battery performance.Firstly,we introduce the three most reported cathode materials(including Ag-based,Ni-based,and Co-based materials)for flexible alkaline Zn-based batteries.Then,challenges and modifications in battery anodes are investigated.Thirdly,the recently advanced gel electrolytes are introduced from their properties,functions as well as advanced fabrications.Finally,recent works and the advantages of sandwich-type,fiber-type and thin film-type flexible batteries are summarized and compared.This review provides insights and guidance for the design of high-performance flexible Zn-based batteries for next-generation electronics.

Gel-Based Triboelectric Nanogenerators for Flexible Sensing:Principles,Properties,and Applications

The rapid development of the Internet of Things and artificial intelligence technologies has increased the need for wearable,portable,and self-powered flexible sensing devices.Triboelectric nanogenerators(TENGs)based on gel materials(with excellent conductivity,mechanical tunability,environmental adaptability,and biocompatibility)are considered an advanced approach for developing a new generation of flexible sensors.This review comprehensively summarizes the recent advances in gel-based TENGs for flexible sensors,covering their principles,properties,and applications.Based on the development requirements for flexible sensors,the working mechanism of gel-based TENGs and the characteristic advantages of gels are introduced.Design strategies for the performance optimization of hydrogel-,organogel-,and aerogel-based TENGs are systematically summarized.In addition,the applications of gel-based TENGs in human motion sensing,tactile sensing,health monitoring,environmental monitoring,human-machine interaction,and other related fields are summarized.Finally,the challenges of gel-based TENGs for flexible sensing are discussed,and feasible strategies are proposed to guide future research.

Wearable flexible zinc-ion batteries based on electrospinning technology

Flexible wearable batteries are widely used in smartwatches, foldable phones, and fitness trackers due to their thinness and small size. Zinc-based batteries have the advantages of low cost, high safety, and ecofriendliness, which are considered to be the best alternative to flexible lithium-ion batteries(LIBs).Therefore, wearable flexible zinc-ion batteries(FZIBs) have attracted considerable interest as a promising energy storage device. Electrospun nanofibers(ESNFs) have great potential for application in wearable FZIBs due to their low density, high porosity, large specific surface area, and flexibility. Moreover, electrospinning technology can achieve the versatility of nanofibers through structural design and incorporation of other multifunctional materials. This paper reviews a wide range of applications of electrospinning in FZIBs, mainly in terms of cathode, anode, separator, polymer electrolyte, and all-inone flexible batteries. Firstly, the electrospinning device, principles, and influencing parameters are briefly described, showing its positive impact on FZIBs. Subsequently, the energy storage principles and electrode configurations of FZIBs are described, and some of the common problems of the batteries are illustrated, including zinc anode dendrite growth, corrosion, cathode structure collapse, and poor electrical conductivity. This is followed by a comprehensive overview of research progress on the individual components of FZIBs(cathode, anode, separator, and polymer electrolyte) from the perspective of electrostatically spun fiber materials and an in-depth study of all-in-one flexible batteries. Finally, the challenges and future development of FZIBs are individually concluded and look forward. We hope that this work will provide new ideas and avenues for the development of advanced energy technologies and smart wearable systems.

Finite Element Simulation of Flexible Roll Forming with Supplemented Material Data and the Experimental Verification

Flexible roll forming is a promising manufacturing method for the production of variable cross section products. Considering the large plastic strain in this forming process which is much larger than that of uniform deformation phase of uniaxial tensile test, the widely adopted method of simulating the forming processes with non-supplemented material data from uniaxial tensile test will certainly lead to large error. To reduce this error, the material data is supplemented based on three constitutive models. Then a finite element model of a six passes flexible roll forming process is established based on the supplemented material data and the original material data from the uniaxial tensile test. The flexible roll forming experiment of a B pillar reinforcing plate is carried out to verify the proposed method. Final cross section shapes of the experimental and the simulated results are compared. It is shown that the simulation calculated with supplemented material data based on Swift model agrees well with the experimental results, while the simulation based on original material data could not predict the actual deformation accurately. The results indicate that this material supplement method is reliable and indispensible, and the simulation model can well reflect the real metal forming process. Detailed analysis of the distribution and history of plastic strain at different positions are performed. A new material data supplement method is proposed to tackle the problem which is ignored in other roll forming simulations, and thus the forming process simulation accuracy can be greatly improved.

Smart Material Based Complex Mode Active Control of Flexible Manipulators with Kinematic Redundancy

An active control methodology is presented for suppressing the vibratoryresponse of flexible redundant manipulators with bonded piezoceramic actuators and strain gagesensors. Firstly, the dynamic equation of the manipulator is decoupled by means of the complex modetheory and the state-space expression of the controlled system is developed. Secondly, a continuouslinear quadratic regulator (LQR) state feedback controller is designed based on the minimumprinciple. Thirdly, a full-order Luenberger state observer featuring an assigned degree of stabilityis determined via the duality between control and estimation. Finally, a numerical simulation iscarried out on a planar 3R flexible redundant manipulator. The simulation results reveal that thedynamic performance of the system is improved rapidly and significantly.

FEM simulation of flexible roll forming based on different material models

Flexible roll forming is a new roll forming process that produces parts with variable cross sections. This forming process is proposed to meet the demand of weight reduction of automobile industry. In order to study the mechanisms and material flow rules in this new forming process,the finite element mothod( FEM) model of a nine-step flexible roll forming of an ultra-high-strength steel bumper is established based on deep understanding and reasonable simplification of the process.Given that the material model is an important factor that influences the simulation accuracy,three material models which consist of different yield criteria and hardening models are adopted in the FEM models. Sheet thickness and springback amount calculated with three material models are studied comparatively. According to sheet thickness reduction and springback amounts,it is found that the MKi( Mises yield criterion and kinematic hardening law) model's result is larger than MI( Mises yield criterion and isotropic hardening law) model and HI( Hill's yield criterion and isotropic hardening law) model. Therefore,it is concluded that material models do have influences on the flexible roll forming simulation and need to be determined carefully.

The Algorithm of Intelligent Compensation in Flexible Material Processing

Flexible material is easy to be distorted in high speed sewing process,so it must be tracked in the process. In this paper,The mathematics model is established with computer visual measure and digital image process for distorted flexible material. By the means of optimizing initial iterative value,the distorted variable of two correlated subareas evaluated. The method based on improved steepest descent algorithm and simulation annealing algorithm is proposed. It has been proven that the evaluating the algorithm’s error and convergent speed by the method of numerical emulate is the effective approach.

Preparation and characterization of iron-ore-imbedded silicone rubber materials for radiation protection

Iron-ore-imbedded silicone rubber materials were produced for radiation shielding. Samples were tested against a Co-60 gamma source, which is widely used in nuclear technology and medicine. Decreasing the particle size of iron ore resulted in better gamma radiation protection owing to more homogenous distribution. In addition, the materials had flexible properties up to the addition of 60 wt% iron ore content. Further, 0.5 mm Pb E gamma protection was provided by using 2.06-mm-thick SDT-60 as the Co-60 source. Iron ore–silicone rubber composites are candidate materials for lead-free flexible radiation protection systems owing to their relatively inexpensive and easy production.

Flexible MXene films for batteries and beyond

MXenes add dozens of metallic conductors to the family of two-dimensional(2D)materials.A top-down synthesis approach removing A-layer atoms(e.g.,Al,Si,and Ga)in MAX phases to produce 2D flakes attaches various surface terminations to MXenes.With these terminations,MXenes show tunable properties,promising a range of applications from energy storage devices to electronics,including sensors,transistors,and antennas.MXenes are also excellent building blocks to create flexible films used for flexible and wearable devices.This article summarizes the synthesis of MXene flakes and highlights aspects that need attention for flexible devices.Rather than listing the development of energy storage devices in detail,we focus on the main challenges of and solutions for constructing high-performance devices.Moreover,we show the applications of MXene films in electronics to call on designs to construct a complete system based on MXene with good flexibility,which consists of a power source,sensors,transistors,and wireless communications.

New shakedown criterion and permanent deformation properties of unbound granular materials

Unbound granular material specifications for road pavements in Australia are primarily based on physical material specification rather than mechanical characterisation. This simplified approach does not reflect the actual material performance under repeated dynamic traffic loads. There is a little information available on the influence of the local crushed rock properties and compacted layer properties on permanent deformation (PD). This study aims to characterise the local unbound granular materials in Victoria according to their PD behaviour under repeated loads and to develop a suitable shakedown criterion that could describe the PD of the tested materials to simplify the flexible pavement design. Repeated-load triaxial tests were conducted over several samples with a range of moisture contents, gradations, densities, and stress conditions. The laboratory test results showed that PD behaviour was influenced by several factors. In addition, the tested subbase-specified unbound granular materials reflect high PD resistance that is almost equivalent to basequality unbound granular materials. This may indicate that current requirements for the subbase-quality unbound granular materials are over-prescribe. Moreover, as the existing shakedown criterion was not applicable for the multi-stage repeated-load triaxial test and the local tested materials, a new shakedown criterion and new boundaries are proposed based on the PD behaviour. In the proposed criterion, the shakedown ranges are identified based on the curve angle of the PD vs. logarithm of the number of loading cycles, and this new criterion was validated using several materials from existing literature. The local tested base and subbase materials can be assigned as Range A when PD\1%, Range B when 1%\PD\3%, and Range C when PD[3%. The proposed criterion could provide a useful and quick approach to assess the PD of the unbound granular materials with both single and multistages of stresses.

Mechano-electrochemical perspectives on flexible lithium-ion batteries

With the advent of flexible/wearable electronic devices,flexible lithium-ion batteries(LIBs)have attracted significant attention as optimal power source candidates.Flexible LIBs with good flexibility,mechanical stability,and high energy density are still an enormous challenge.In recent years,many complex and diverse design methods for flexible LIBs have been reported.The design and evaluation of ideal flexible LIBs must take into consideration both mechanical and electrochemical factors.In this review,the recent progress and challenges of flexible LIBs are reviewed from a mechano-electrochemical perspective.The recent progress in flexible LIB design is addressed concerning flexible material and configuration design.The mechanical and electrochemical evaluations of flexible LIBs are also summarized.Furthermore,mechano-electrochemical perspectives for the future direction of flexible LIBs are also discussed.Finally,the relationship between mechanical loading and the electrode process is analyzed from a mechano-electrochemical perspective.The evaluation of flexible LIBs should be based on mechano-electrochemical processes.Reviews and perspectives are of great significance to the design and practicality of flexible LIBs,which is contributed to bridging the gap between laboratory exploration and practical applications.

Effect of vertical stress rest period on deformation behaviour of unbound granular materials:Experimental and numerical investigations

Repeated load triaxial test is used to assess the deformation behaviour of unbound granular materials(UGMs) in flexible road pavements. Repeated load pulse characteristics(i.e. shape, loading period and rest period) are the stress configurations used in the experimental set-up to simulate the passing axle loads. Some researchers and standard testing protocols suggest a rest period of varying durations after a loading phase. A thorough review of existing literature and practises has revealed that there is no agreement about the effect of the rest period of vertical stress pulse on the deformation behaviour of the UGMs. Therefore,the main objective of this study is to investigate the effect of repeated stress rest period on the deformation behaviour of UGMs experimentally. Experiments are conducted, both with and without rest period, using basalt and granite crushed rocks from Victoria, Australia. Furthermore, in order to gain insight into the effect of the rest period, finite element modelling is also developed. Both the experimental and modelling results show that the rest period has a noticeable effect on both resilient and permanent deformation behaviours of UGMs. It is, therefore, recommended to take extra precautions while adopting a particular standard testing protocol and to supplement the results by additional tests with different loading configurations.