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Braided composites are gaining attention in the most industrial applications. To design rods with optimal tensile properties against combined loads, experimental studies were conducted to investigate the effect of using axial yarn and core in different categories on the tensile properties of braided reinforced composite rods. In this study, six types of braided composite rods with different arrangements of braid components (axial yarn or core type) were produced using glass and polyester fibers with epoxy resin as the matrix. The elastic modulus, ultimate tensile strength, and work of fracture of these rods were tested, and the experimental elastic modulus of samples were compared with previously developed models. Moreover, comparing the stress-strain results of the samples revealed that the produced hybrid samples demonstrate pseudo-ductile tensile behavior, showing varying trends as the type of reinforcement is altered. The results indicate that using a double-layer triaxial braid as reinforcement for the composite can increase the elastic modulus up to 13% compared to the similar single-layer sample. Additionally, employing a double-layer triaxial braid as reinforcement for the composite leads to a greater work of fracture. However, in terms of ultimate tensile strength, biaxial braid reinforcement demonstrates better performance in reinforcing the composite rods.

Advanced composite components are increasingly applied in the aerospace industry, and engineers have high hopes to reduce overall weight and maintain structural stability according to new composite materials. In this paper, a three-dimensional four-directional braided composite material consisting of T300-3K carbon fibers and TDE-85 epoxy resin is presented and its equivalent mechanical property parameters are calculated using the bridging model and volume averaging approach. Our calculations are in good agreement with values in the available literature. We focus on a typical cylindrical shell structure and analyze the effects of external excitation, fiber volume fraction, aerodynamic force, structural damping, and braiding angle on the nonlinear resonant responses of this braided composite cylindrical shell. Additionally, we also study the nonlinear dynamic characteristics of the cylindrical shell considering the variation of the external excitation. Our findings indicate that complex environmental factors and material parameters play a significant role in the nonlinear vibrations of the braided composite cylindrical shell.

Three-dimensional braided composites have become one kind of critical engineering material for applications in extreme environments. The 3D stepwise rotary braiding process is one vital technique for manufacturing preforms with high efficiency and flexibility. However, the fabric topology is decided by the combination of switch rotation directions, which affects the mechanical properties, and the full carrier configuration results in a loose four-directional structure which is supposed to be improved by adding axial yarns. Therefore, experiments are carried out to illustrate the effect of fabric topology and axial yarn condition on the compressive properties of 3D stepwise rotary braided composites. Samples with three types of fabric topologies named Type A, B, and C are prepared under four axial yarn conditions including no axial yarn addition, 12K axial yarn addition, 24K axial yarn addition, and 36K axial yarn addition, which are fabricated with braiding angles of 20°, 30° and 40°. Longitudinal and transverse compression tests are conducted, and the morphology is observed. It shows that the braiding angle has more influence on the longitudinal compressive properties than transverse compressive properties, and the effect of fabric topology and axial yarn condition depends on the braiding angle. The fabric topology affects a lot on the longitudinal compressive properties when the braiding angle is small, resulting in a gap of up to 40%. The longitudinal compressive properties are improved significantly by adding axial yarns especially for the composites with large braiding angles, making the strength more than double. With the increase in axial yarn size, the strength increment gradually decreases while the modulus declines after a certain size for smaller braiding angles.

In this paper, 3D braided composites are considered for high-performance piezoelectric energy harvester designing, and a 3D braided composite piezoelectric energy harvester (3D BCPEH) is proposed. The advantage of 3D BCPEH is that the natural frequency of the device can be adjusted by adjusting the stiffness of the spring, making the natural frequency of the device close to the vibration frequency of the environment, thereby achieving the best harvesting effect of the energy harvester. During the vibration process, spring support can cushion the impact and stress on the piezoelectric energy harvester, thereby protecting the device from damage and helping to improve the stability and performance of the system. Finite element analysis is used to obtain the elastic modulus, shear modulus, and Poisson’s ratio of 3D braided composites with varying braiding angles. Based on Hamilton’s variational principle, the vibration control equation of the spring supported 3D BCPEH is derived. The effects of spring rate, braiding angle, external excitation acceleration, external load resistance, and structure size on the output response of 3D BCPEH are simulated and analyzed. The validity of the proposed 3D BCPEH with spring support is confirmed by the simulation results.

Three-dimensional (3D) braided composites have received much attention in the fields of aerospace, the automotive industry, , due to their unique structure and excellent performance. However, due to the complexity of their preparation process, the generation of defects is almost inevitable. With its wide application, it has become crucial to understand and control its defects. This article reviews the research progress on the defects of braided composites. First, it analyses the connection between the molding process parameters and the formation of void defects. It also introduces some nondestructive testing methods and principles for defects in 3D braided composites. Second, for the yarn deformation defects caused by the extrusion of component materials, the relevant assumptions about cross-sectional deformation and uniform yarn orientation are summarized, and the law of the influence of yarn distortion randomness on the material properties is summarized. For void defects, the differences and connections between the matrix porosity model and the matrix/yarn porosity model are discussed. In addition, the effects of other types of defects on the material are summarized. Finally, based on the summary of existing results, the prospect of future research directions is proposed.

Resin matrix composites (RCs) have better thermal and chemical stability, so they are widely used in engineering fields. In this study, the aging process and mechanism of two different types of resin-based three-dimensional four-way braided composites (H15 and S15) under different hygrothermal aging conditions were studied. The effect of aging behavior on the mechanical properties of RCs was also studied. Three different aging conditions were studied: Case I, 40 °C Soak; Case II, 70 °C Soak; and Case III, 70 °C-85% relative humidity (RH). It was found that the hygroscopic behavior of RCs in the process of moisture-heat aging conforms to Fick’s second law. Higher temperatures and humidity lead to higher water absorption. The equilibrium hygroscopic content of H15 was 1.46% (Case II), and that of S15 was 2.51% (Case II). FT-IR revealed the different hygroscopic mechanisms of H15 and S15 in terms of aging behavior. On the whole, the infiltration behavior of water molecules is mainly exhibited in the process of wet and thermal aging. At the same time, the effect of the aging process on resin matrices was observed using SEM. It was found that the aging process led to the formation of microchannels on the substrate surface of S15, and the formation of these channels was the main reason for the better moisture absorption and lower mechanical strength of S15. At the same time, this study further found that temperature and oxygen content are the core influences on post-aging strength. The LVI experiment also showed that the structural changes and deterioration effects occurring after aging reduced the strength of the studied material.

Three-dimensional braided composites (3D-BCs) have better specific strength and stiffness than two-dimensional planar composites (2D-PCs), so they are widely used in modern industrial fields. In this paper, two kinds of 3D four-directional braided composites (3D4d-BCs) with different braided angles (15°, denoted as H15, and 30°, denoted as H30) were subjected to hydrothermal aging treatments, low-velocity impact (LVI) tests, and compression after impact (CAI) tests under different conditions. This study systematically studied the hygroscopic behavior and the effect of hygrothermal aging on the mechanical properties of 3D4d-BC. The results show that higher temperatures and smaller weaving angles can significantly improve the moisture absorption equilibrium content. When the moisture absorption content is balanced, the energy absorption effect of 3D4d-BC is better, but the integrity and residual compression rate will be reduced. Due to the intervention of oxygen molecules, the interface properties between the matrix and the composite material will be reduced, so the compressive strength will be further reduced. In the LVI test, the peak impact load of H15 is low. In CAI tests, the failure of H15 mainly occurs on the side, and the failure form is buckling failure. The main failure direction of H30 is 45° shear failure.

A three-dimensional helix geometry unit cell is established to simulate the complex spatial configuration of 3D braided composites. Initially, different types of yarn factors, such as yarn path, cross-sectional shape, properties, and braid direction, are explained. Then, the multiphase finite element method is used to develop a new theoretical calculation procedure based on the unit cell for predicting the impacts of environmental temperature on the thermophysical properties of 3D four-direction carbon/epoxy braided composites. The changing rule and distribution characteristics of the thermophysical properties for 3D four-direction carbon/epoxy braided composites are obtained at temperatures ranging from room temperature to 200 °C. The influences of environmental temperature on the coefficients of thermal expansion (CTE) and the coefficients of thermal conduction (CTC) are evaluated, by which some important conclusions are drawn. A comparison is conducted between theoretical and experimental results, revealing that variations in temperature exert a notable influence on the thermophysical characteristics of 3D four-directional carbon/epoxy braided composites. The theoretical calculation procedure is an effective tool for the mechanical property analysis of composite materials with complex geometries.

In this work, a novel optimization method is proposed to pursue high-performance 3D-braided composite structure under thermo-mechanical loads, which optimizes structural topology and braiding parameters concurrently. To realize such design under affordable computational cost, we decompose the optimization method into offline and online stages. In the offline stage, a parameterized geometric model controlled by two braiding parameters, i.e., fiber volume fraction and braiding angle, is established to characterize the microstructure of composite. Subsequently Energy-based Homogenization Method is applied to calculate equivalent composite properties including elastic tensor, thermal conductivity tensor, and Coefficient of Thermal Expansion. A surrogate model based on Radial Basis Network is established to map braiding parameters to equivalent material properties. In the online stage, the surrogate model is integrated into Rational Approximation of Material Properties to build a systematic design scheme for structural topology and braiding parameters. Taking manufacturability into account, the proposed method is combined with stiffener layout design to obtain easy-to-manufacture braided composite structures. Finally, several numerical examples are provided to demonstrate the effectiveness of the proposed optimization method, indicating that braiding parameters have essential impacts on the composite structural design and performance.

The low delamination tendency and high damage tolerance of three-dimensional (3D) braided composites highlight their significant potential in handling defects. To enhance the engineering potential of three-dimensional four-directional (3D4d) braided composites and assess the failure mode of hole defects, this study introduces a series of 3D4d braided composites with prefabricated holes, studying their compressive properties and failure mechanisms through experimental and finite element methods. Digital image correlation (DIC) was used to monitor the compressive strain on the surface of materials. Scanning acoustic microscope (SAM) and scanning electron microscopy (SEM) were used to characterize the longitudinal compression failure mode inside the material. A macroscopic model is established, and the porous materials are predicted by using the general braided composite material prediction theory. While reducing the forecast cost, the error is also controlled within 21%. The analysis of failure mechanisms elucidates the damage extension mode, and the porous damage tolerance ability aligns closely with the bearing mode of braided material structure. Different braiding angles will lead to different bearing modes of materials. Under longitudinal compression, the average strength loss of 15° specimens is 38.21%, and that of 30° specimens is 8.1%. The larger the braided angle, the stronger the porous damage tolerance. Different types of prefabricated holes will also affect their mechanical properties and damage tolerance.