Design Of Composite Materials Research Articles

Effect of annealing and carbon nanotube infill on the mechanical and electrical properties of additively manufactured polyether-ether-ketone nanocomposites via fused filament fabrication

The light-weight, high-strength poly-ether-ether-ketone (PEEK) and its composite materials are the ideal candidate for a plethora of engineering applications. However, the inherent high viscosity and melting temperature of PEEK pose great challenges for the extrusion-based 3D printing process, leaving a wide knowledge gap on the properties and functionalities of novel PEEK-based composites. In this work, the PEEK/CNT composite filaments with different CNT content ranging from 1 to 7 wt% are prepared by single-screw extruder and subsequently printed with a custom-made high temperature 3D printer. The effects of CNT content and annealing on the mechanical strength and electrical conductivity of the 3D printed parts are investigated in detail. The tensile test result shows that the presence of CNT significantly enhances the mechanical strength of the composite. With 7 wt% CNT content, the maximum tensile strength and elastic modulus reaches 107.7 MPa and 1908.0 MPa, respectively, which amounts to a rise of 27.4% and 17.0% compared with the neat PEEK sample. The annealing treatment is similarly effective in strengthening the printed composites. The addition of CNT is also found to greatly improve the electrical conductivity to the PEEK, reaching 101 S/m for 7 wt% CNT. With such superior electrical conductivity, the printed PEEK/CNT composite sample exhibits outstanding electromagnetic interference (EMI) shielding performance, reaching 32.4 dB. However, additional annealing treatment decreases the electrical conductivity as wells as EMI shielding performance. Such decrease is mainly attributed to the growth of the crystals during annealing, which expels the CNTs to the grain boundaries and leads to remarkable agglomeration, which disconnects the pervasive conductive networks. The finding in this work can provide insights on the new designs of PEEK composite feedstock materials and cultivate new 3D printing strategies targeted specifically for highly-functional, high-strength novel composites.

In situ study on the hierarchical interfacial “global regulation – equilibrium iteration” bearing-toughening mechanisms in Strombus gigas shell by synchrotron radiation computed tomography technique

Strombus gigas shell is a natural biological composite material with excellent strength and toughness, and its excellent performance is closely related to its cross-laminated multistage structure. Studying the damage evolution process to reveal the strengthening mechanism is very important. Through synchrotron radiation computed tomography experiments, the important damage evolution phenomenon from outside to inside of the whole structure, from single interface to multistage interfaces (i.e. first-order, second-order and third-order interfaces), were observed. The first-order interface plays the role of global regulation of stress distribution and failure mode in the whole structure, and the second-order and third-order interfaces play the role of regulating stress distribution during crack propagation. These interfaces cooperate with each other while playing respective roles, showing the optimization and control mechanism of “iterative to balance”. New comprehensions of multistage interfaces are expected to provide new strategies for the optimization design and manufacturing of artificial composite materials.

Organic Single Crystal/Polymer Hybrid Actuator with Waveguide, Low Temperature, and Humidity Actuation Properties

Organic single crystal/polymer hybrid actuators that can be applied in extreme conditions such as polar regions and space are extremely important and rare. Herein, we report an extremely simple but efficient strategy to fabricate hybrid organic crystalline materials with low-temperature, humidity actuation, and waveguide properties. First, needle-like crystals composed of 10-(trifluoromethyl)anthracene-9-carbonitrile (TFMAC) exhibited excellent elastic properties in the temperature range of 77–423 K. Second, we prepared a crystal/polymer hybrid by coating a layer of polyvinyl alcohol (PVA) polymer on the crystal surface. Fortunately, the flexible hybrids had wet and low-temperature actuation properties. The mechanical claws made of hybrids could transport objects at low temperature, showing excellent actuation behavior. Meanwhile, organic crystals and hybrids exhibited excellent optical waveguide properties, with optical loss coefficients as low as 0.29–0.45 dB mm–1 before and after bending. This article has opened up new ideas for the design and fabrication of temperature-sensitive flexible composite functional waveguide materials.

Periodic rhomboidal cells for symmetry-preserving homogenization and isotropic metamaterials

In the design and analysis of composite materials based on periodic arrangements of sub-units it is of paramount importance to control the emergent material symmetry in relation to the elastic response. The target material symmetry plays also an important role in additive manufacturing. In numerous applications it would be useful to obtain effectively isotropic materials. While these typically emerge from a random microstructure, it is not obvious how to achieve isotropy with a periodic order. We prove that arrangements of inclusions based on a rhomboidal cell that generates the face-centered cubic lattice do in fact preserve any material symmetry of the constituents, so that spherical inclusions of isotropic materials in an isotropic matrix produce effectively isotropic composites.

Enhanced solar-driven hydrogen evolution over ultrathin g-C3N4/ReSe2 heterojunction-like nanosheets with surface selenium vacancies

The design of composite photocatalytic materials is essential for efficient photocatalytic hydrogen production. In the present work, we have successfully designed a simple synthetic process to ultrathin ReSe2 nanosheets using an ultrasound-assisted liquid-phase synthetic route through self-assembling ReSe2 onto g-C3N4 (shortened as g-C3N4/ReSe2 nanosheets) to improve photocatalytic performance. Based on density functional theory (DFT) estimation along with experimental characterization, it is found that the integrated heterojunction-like ultrathin g-C3N4/ReSe2 nanosheets possess more edge active sites, enhanced electron transfer efficiency and conductivities, which significantly accelerates the dissociation and migration of photogenerated electron-hole pairs. Meanwhile, selenium vacancies could be introduced into nanosheets via controlling the synthetic procedure with a reduced Se source that will favor to enhance the photocatalytic efficiency. The ultrathin g-C3N4/ReSe2 heterojunction with Se vacancies performed highly improved photocatalytic hydrogen production. In typical, the H2 production of the g-C3N4/ReSe2 nanosheets under visible light is 1055.50 μmol g−1 h−1, which is 21 times higher than that of bare g-C3N4, indicating that it overcomes the high charge complexation rate of g-C3N4. These results do not only provide insight into the application of two-dimensional ReSe2 nanosheets in photocatalysis, but also open up new avenues for the rational design of two-dimensional hybrid materials in solar energy conversion.

Predictive Modeling and Non-linear Optimization Techniques for Composite Materials Design

With the rise in the use of composite materials for product design, research has been performed in determining the optimal way to produce materials for given desired outputs. As of now response surface methodology and the Taguchi method are the front running methods for optimizing material production methods at the design level. This research investigates why these methods are not a one size fits all solution to optimizing composite materials production for material properties. It proposes utilizing predictive modeling and non-linear optimization techniques from historical manufacturing data of a non-highly controlled manufacturing process. The method is examined with the manufacturing and testing data of a local concrete product manufacturer. The models and optimization methods are validated with residual values to the true data and sensitivity analysis of the problem. The initial testing of the method offers promise to companies who have not found Taguchi or surface response methodology, applicable to their specific business solutions.

Application of Polypyrrole Cellulose Nanocrystalline Composite Conductive Material in Garment Design

The Chinese nation has a long cultural history and has deep attainments in food, clothing, art, and other cultural fields. With the development of science, technology, economy, and culture, new materials continue to appear, providing new ideas for clothing design. Polypyrrole is a common conductive polymer. The pure pyrrole monomer presents a colorless oily liquid at room temperature, slightly soluble in water and nontoxic. Nanocrystals, also called nanoscale crystals, use high-energy polymer spheres to pack calcium, magnesium ions, and bicarbonate in water to produce a water-insoluble crystal structure. Conductive composite materials mainly refer to composite conductive polymer materials, which are composed of polymers and various conductive substances through a certain composite method. This article aims to study the application of polypyrrole cellulose nanocrystalline composite conductive material in clothing design. Starting from the structural characteristics of the polypyrrole cellulose nanocrystalline composite conductive material, this article uses case analysis to study deeply the suitable polypyrrole cellulose nanocrystalline composite conductive material. This article can effectively use the innovative application method of its appearance style, so as to realize its application in clothing design. Starting from the functional properties of the polypyrrole cellulose nanocrystalline composite conductive material, the specific application of the polypyrrole cellulose nanocrystalline composite conductive material in different clothing designs is analyzed. Combining the postmodernist clothing style characteristics, aesthetic habits, and the characteristics of polypyrrole cellulose nanocrystalline composite conductive materials, this paper studies the innovative style design of polypyrrole cellulose nanocrystalline composite conductive materials. The experimental results in this paper show that when the reaction time is 2 min, the reaction rate at this time is zero, indicating that this time is in the initial stage of the reaction. After 4 minutes, as the reaction time increases, the reaction rate shows an increasing trend; when the reaction time is longer than 10 minutes, the reaction rate increases slowly and has a downward trend, which indicates the end of the reaction. The highest average reaction rate is about 7.5 mg/min.

Acoustic Tweezer-Modulated Biomimetic Patterned Particle-Polymer Composite for Water Vapor Harvesting.

With the recent threat of climate change and global warming, ensuring access to safe drinking water is a great challenge in many areas worldwide. Designing functional materials for capturing water from natural resources like fog and mist has become one of the key research areas to maximize the production of clean water. From this aspect, nature is a great source for designing bioinspired functional materials as some of the plant leaves and animal exoskeletons can harness water and then store it to save themselves from arid, xeric conditions. Inspired by the Stenocara beetle, we have designed a composite surface structure with periodic islands made of aluminum microparticles surrounded by poly(dimethylenesiloxane) (PDMS). An acoustic tweezer-based method was used to fabricate the bioinspired composite structures, where surface acoustic waves at specific frequencies and amplitudes are applied to align the microparticles as islands in the polymer matrix. An oxygen plasma etching step was applied to expose the microparticles on the PDMS surface. The average water harvesting efficiencies for structures made with 120 and 80 kHz acoustic frequencies and 1 hour etching time were found to be 9.41 and 8.84 g cm-2 h-1, respectively. The acoustically patterned biomimetic composite surface showed higher water harvesting efficiency compared with completely hydrophobic PDMS and hydrophilic aluminum surfaces, demonstrating the advantages of the bioinspired composite material design and acoustic-assisted manufacturing technique. The biomimetic fog water harvesting material is a promising avenue to fulfill the demand for a cost-effective, sustainable, and energy-efficient solution to safe drinking water.

A mixed formulation for physics-informed neural networks as a potential solver for engineering problems in heterogeneous domains: Comparison with finite element method

Physics informed neural networks (PINNs) are capable of finding the solution for a given boundary value problem. Here, the training of the network is equivalent to the minimization of a loss function that includes the governing (partial differential) equations (PDE) as well as initial and boundary conditions. We employ several ideas from the finite element method (FEM) to enhance the performance of existing PINNs in engineering problems. The main contribution of the current work is to promote using the spatial gradient of the primary variable as an output from separated neural networks. Later on, the strong form (given governing equation) which has a higher order of derivatives is applied to the spatial gradients of the primary variable as the physical constraint. In addition, the so-called energy form of the problem (which can be obtained from the weak form) is applied to the primary variable as an additional constraint for training. The proposed approach only required up to first-order derivatives to construct the physical loss functions. We discuss why this point is beneficial through various comparisons between different models. The mixed formulation-based PINNs and FE methods share some similarities. While the former minimizes the PDE and its energy form at given collocation points utilizing a complex nonlinear interpolation through a neural network, the latter does the same at element nodes with the help of shape functions. We focus on heterogeneous solids and check the performance of the proposed PINN model against the solution from FEM on two prototype problems: elasticity and the Poisson equation (steady-state diffusion problem). It is concluded that by properly designing the network architecture in PINN, the deep learning model has the potential to solve the unknowns in a heterogeneous domain without any available initial data from other sources. Finally, discussions are provided on the combination of PINN and data from physical FE simulations for a fast and accurate design of composite materials in future developments.

Self‐Assembly Construction of Carbon Nanotube Network‐Threaded Tetrathiafulvalene‐Bridging Covalent Organic Framework Composite Anodes for High‐Performance Hybrid Lithium‐Ion Capacitors

Hybrid lithium‐ion capacitors (HLICs), a special class of electrochemical energy storage devices composed of battery‐type anodes and capacitor‐type cathodes, have the potential to bridge the gap between high‐energy‐density batteries and high‐power‐density capacitors. Nevertheless, the key challenge for developing high‐performance HLICs is the imbalances of the electrochemical kinetics and lifespans between the battery‐type anodes and capacitor‐type cathodes. Herein, the self‐assembly preparation of the 3D‐crosslinked carbon nanotube (CNT) network‐threaded tetrathiafulvalene‐bridging covalent organic framework (TTF‐COF) composite via in situ growth of the 2D‐stacked TTF‐COF capping layer alongside the outer walls of 3D‐interlaced CNTs is reported. Originated for the electron‐donating and redox‐switchable TTF units, the TTF‐COF component has abundant active sites and high charge conductivity for reversible Li+ storage. Moreover, due to the 3D‐assembled architecture, the TTF‐COF/CNT composite possesses abundant open nanochannels for ion transfer and 3D‐interconnected conductive CNT network for electron transfer. When used in HLICs, the TTF‐COF/CNT composite anodes exhibit ultrahigh specific capacity (609 mAh g−1 at 100 mA g−1) and outstanding power density (12 000 W kg−1 at 4000 mA g−1). Herein, the design of advanced COF composite materials with high activity, porosity, and conductivity can be a promising route for boosting the overall performances of high‐power‐type energy storage devices.

Chitosan-Gelatin Films: Plasticizers/Nanofillers Affect Chain Interactions and Material Properties in Different Ways.

Biopolymers, which are biodegradable and inherently functional, have high potential for specialized applications (e.g., disposable and transient systems and biomedical treatment). For this, it is important to create composite materials with precisely defined chain interactions and tailored properties. This work shows that for a chitosan–gelatin material, both glycerol and isosorbide are effective plasticizers, but isosorbide could additionally disrupt the polyelectrolyte complexation (PEC) between the two biopolymers, which greatly impacts the glass transition temperature (Tg), mechanical properties, and water absorption. While glycerol-plasticized samples without nanofiller or with graphene oxide (GO) showed minimal water uptake, the addition of isosorbide and/or montmorillonite (MMT) made the materials hydrolytically unstable, likely due to disrupted PEC. However, these samples showed an opposite trend in surface hydrophilicity, which means surface chemistry is controlled differently from chain structure. This work highlights different mechanisms that control the different properties of dual-biopolymer systems and provides an updated definition of biopolymer plasticization, and thus could provide important knowledge for the future design of biopolymer composite materials with tailored surface hydrophilicity, overall hygroscopicity, and mechanical properties that meet specific application needs.

Prediction of Fracture Toughness of Pultruded Composites Based on Supervised Machine Learning.

Prediction of mechanical properties is an essential part of material design. State-of-the-art simulation-based prediction requires data on microstructure and inter-component interactions of material. However, due to high costs and time limitations, such parameters, which are especially required for the simulation of advanced properties, are not always available. This paper proposes a data-driven approach to predicting the labor-consuming fracture toughness based on a series of standard, easy-to-measure mechanical characteristics. Three supervised machine-learning (ML) models (artificial neural networks, a random forest algorithm, and gradient boosting) were designed and tested for the prediction of mechanical properties of pultruded composites. A considerable dataset of mechanical properties was acquired as results of standard tensile, compression, flexure, in-plane shear, and Charpy tests and utilized as the input to predict the fracture toughness. Furthermore, this study investigated the correlations between the obtained mechanical characteristics. Analysis of ML performance showed that fracture toughness had the highest correlations with longitudinal bending and transverse tension and a strong correlation with the longitudinal compression modulus and tensile strength. The gradient boosting decision tree-based algorithms demonstrated the best prediction performance for fracture toughness, with an MSE less than 10% of the average value, providing a prediction within the range of experimental error. The ML algorithms showed potential in terms of determining which macro-level parameters can be used to predict micro-level material characteristics and how. The results provide inspiration for future pultruded composite material design and can enhance the numerical simulations of material.

Influencing Factors on Synthesis and Properties of MXene: A Review

MXene has a unique two-dimensional layered structure, a large specific surface area, and good electrical conductivity, stability, magnetic properties, and mechanical properties. Attributed to its structure and properties, MXene has entered various application fields, such as tumors, antibiotics, batteries, hydrogen storage, sensors, electromagnetic shielding, etc. Studies on MXene are implemented in various fields. The preparation methods, surface functional groups, and the preparation technology of composite materials directly affect the performance of MXene and determine its application fields. As significantly important issues, many influencing factors exist in the preparation process of MXene and the process of composite material design, which directly impact the properties and applications of MXene. This paper investigates and analyzes the factors influencing the preparation, properties, and application of MXene, explores the research progress of MXene, and provides critical insights for synthesizing and modifying effective MXene adsorbents. Moreover, this review discusses the influencing factors on applications, i.e., CO2 reduction, energy storage, heavy metal removal, etc., and compares key factors, including surface modification, pH, reaction time, and adsorption capacity, etc. Furthermore, this review provides an overview on current status and offers recommendations.

Recent Advances in Peptide Engineering of PEG Hydrogels: Strategies, Functional Regulation, and Biomedical Applications

AbstractPolyethylene glycol (PEG) is hydrophilic, biocompatible, and electrically neutral in an aqueous solution due to its unique internal structure. PEG is widely used to prepare hydrogels for applications in drug delivery and tissue engineering. In addition, PEG can be used as a functionalization agent of biomaterials to prolong the presence of drug systems in vivo in biomedical applications. The functionalization of PEG hydrogels using functional peptides gives PEG hydrogels the advantages of better biocompatibility, higher loading efficiency, more adjustable lifetime, and lower cost, which have exhibited promising applications in biomedical fields. In this review, recent advances in the functionalization of PEG hydrogels with peptides for biomedical applications are presented. For this aim, first, the basic properties of PEG hydrogels are described by introducing various preparation methods, and then, the functional regulation of PEG hydrogels is demonstrated with different motif‐designed peptides to achieve specific properties. Finally, biomedical applications of peptide‐engineered PEG hydrogels in controlled drug delivery, tissue engineering, medical diagnostics, bioimaging, and biosensing are described in detail. This work provides a systematic literature review on this promising topic and facilitates readers in the design of composite materials and functional tailoring of PEG hydrogels with biomolecules.

Molecular mechanics-based design of high-modulus epoxy to enhance composite compressive properties

The performance of carbon fiber reinforced composites has been shown to be constrained by the low compression to tensile strength ratio, which blocks its lightweight application. Designing and preparing high-strength, high-modulus resins has become a popular direction in the field of synthetic materials and increasing resin modulus has become one of the key goals for improving composite compression properties. This study improves the interactions between molecular segments and reduces chain motility by introducing polar groups into resin molecules and increasing the reactive functionality. Four typical polar epoxy resins with high purity were synthesized, with a modulus of 6500 MPa and a tensile strength of 86 MPa when cured. Hydrogen bonds are detected and the complex process of group changes as a function of temperature is investigated using in-situ IR. The compression properties of the composites made with the newly synthesized resins were significantly improved, which suggests the significance of high-performance resin design in lightweight carbon fiber composite materials.

Knowledge database creation for design of polymer matrix composite

We present a mechanistic data science (MDS) framework capable of building a composite knowledge database for composite materials design. The MDS framework systematically leverages data science to extract mechanistic knowledge from composite materials system. The composite response database is first generated for three matrix and four fiber combinations using a physics-based mechanistic reduced-order model. Next, the mechanistic features of the composites are identified by mechanistically analyzing the composites stress–strain responses. A relationship between the composite properties and the constituents’ material features are established through a mechanics constrained data science-based learning process after representing materials in latent space, following a dimension reduction technique. We demonstrate the capability of predicting a composite materials system for target properties (material elastic properties, yield strength, resilience, toughness, and density) from the MDS created knowledge database. The MDS model is predictive with reasonable accuracy, and capable of identifying the materials system along with the tuning required to achieve desired composite properties. Development of such MDS framework can be exploited for fast materials system design, creating new opportunity for performance guided materials design.

Insight into the existent state of nitrogen-doped carbon dots in titanate nanotubes and their roles played toward simultaneous removal of coexisted Cu2+ and norfloxacin in water

In this work, nitrogen-doped carbon dots (NCDs) were introduced in different existent sites of titanate nanotubes (TNTs) by a facile synthesis, and their effects on surface potential, photoelectrochemical properties and simultaneous removal of coexisted Cu2+ and norfloxacin (NOR) performance in water were systematically investigated. Constructed NCDs-TNTs composite displayed superior performance towards the adsorption (ion exchange/coordination) of Cu2+ and adsorption-oxidization of NOR over the two individuals, mainly benefiting from the collaboration of NCDs in different existent states. The existence of TiNH chemical linkage was identified between TNTs and NCDs-OT (NCDs on the outer surface of TNTs), which not only modulates the surface potential to favor the external diffusion of Cu2+ or NOR+ from aqueous solution to the negatively charged NCDs-TNTs, but also facilitates the intraparticle transfer of contaminants to the reactive sites. In addition, the up-conversion light property of NCDs-OT and the interstitial NCDs-IT (NCDs on the inner surface of TNTs) doping in TNTs interact together to enable NCDs-IT-TNTs to fully absorb and utilize all visible light. The photoexcited electrons were further trapped by NCDs-OT to promote the photogenerated carrier separation. Adsorbed Cu2+ could also improve the performance of NCDs-TNTs toward NOR oxidization, mainly owing to the self-synchronous doping of adsorbed Cu2+ broadening light absorption area and acting as mediators for delivering electrons. This work provides unique insights into the structural design of composite materials for such combined contamination remediation in water.

A study on in vitro and in vivo bioactivity of silk fibroin / nano-hydroxyapatite / graphene oxide composite scaffolds with directional channels

As an important carrier of cells and growth factors, scaffold materials have attracted attention in bone tissue engineering, but how to construct materials with both component and structural advantages is still a challenge. In this study, silk fibroin (SF) / nano-hydroxyapatite (nHAp) / graphene oxide (GO) composite scaffolds with directional channels were prepared by directional temperature field freeze-drying technique. The characterization, morphology, degradation rate, biocompatibility and bioactivity of directed channel SF/nHAp/GO scaffolds were studied systematically. As a comparison, the series of studies were also conducted in SF scaffolds, SF/nHAp scaffolds and undirected SF/nHAp/GO scaffolds. The characterization results showed that the directed channel SF/nHAp/GO scaffolds had better connectivity, more suitable porosity, degradation rate for osteogenesis and ability to induce bone-like apatite. The cell compatibility results showed that the directed channel SF/nHAp/GO scaffolds were more beneficial to the adhesion and proliferation of bone marrow mesenchymal stem cells (BMSCs). And the expression of osteogenic genes was all up-regulated by RT-PCR. The in vivo experimental results showed that the directed channel SF/nHAp/GO scaffolds showed stronger biological activity and osseointegration ability. The composite scaffolds were non-toxic in vitro and in vivo. Therefore, this material may have great application potential in repairing bone tissue defects. It provides a meaningful reference for the further study of scaffold materials with directional channels such as dental and orthopedical repair materials from the material composition and structure design.

Color carbon fiber and its discoloration response

Carbon fibers have been manufactured industrially for several decades and are widely used in the construction of aircraft, racing cars or sports appliances et al. However, carbon fiber showing no response and adaptation to environmental stimulation has prevented the widespread use of this excellent mechanical reinforcement as functional material. Here, we demonstrate that carbon fibers have not only excellent mechanical properties but also intelligent properties with color responsiveness. We coated graphene oxide (GO) on carbon fiber surface, where the two-dimensional GO orderly stack together and self-assemble as a lamellar stacked structure. Such self-assemble GO film exhibits structural color since the stacked structure is analogous to one-dimensional photonic crystals. Specifically, color carbon fiber with GO one-dimensional photonic crystal coating (colored-CF) is realized for the first time by uniformly surface depositing with vacuum filtration method. We further investigated the colored-CF's intelligent color response and associated coloring mechanism, as well as its microstructure, tensile mechanical properties. The results show that GO coating can repair the surface defects of carbon fiber and improve its specific tensile strength and modulus. Moreover, we found that the colored-CF has interesting characteristics of color response under different solvents, temperature and stress. Our approach gives inspiration for a future design of intelligent responsive carbon fiber composite materials.

Effect of Fiber Geometry on Fracture and Fatigue of Composite Hydrogels

Abstract Hydrogel-based biomedical applications are under rapid development. These applications usually demand hydrogels to have high toughness and high fatigue threshold. Recently, various fiber-reinforced composite hydrogels have been developed to meet this challenge. However, the effect of fiber geometry on the fracture and fatigue of composite hydrogels is still elusive. Here, we use a model composite hydrogel to study the influence of fiber width, fiber spacing, and fiber configuration on these properties. It is found that the toughness of the composite hydrogel does not increase monotonically with the fiber width or fiber spacing, but presents a peak. This is because the variation of fiber width and fiber spacing not only affects the volume of fiber in the fracture process zone but also influences the dissipated elastic energy density in that volume, which is affected by the stress concentration. The peak is a consequence of the trade-off between these two factors. Our study further shows that the shape of the fiber network affects the stress concentration in the fiber dramatically, thereby leading to a huge difference in the toughness and fatigue threshold of the composite hydrogels. This work highlights the importance of fiber size as well as the shape of fiber networks on the mechanical properties of composite hydrogels. It may help the design of tough and fatigue-resistant stretchable composite materials.