A homogenization-based five-step multi-scale finite element (FsMsFE) simulation framework is developed to describe the time-temperature-dependent viscoelastic behavior of 3D braided four-directional composites. The current analysis was performed via three-scale finite element models, the fiber/matrix (microscopic) representative unit cell (RUC) model, the yarn/matrix (mesoscopic) representative unit cell model, and the macroscopic solid model with homogeneous property. Coupling the time-temperature equivalence principle, multi-phase finite element approach, Laplace transformation and Prony series fitting technology, the character of the stress relaxation behaviors at three scales subject to variation in temperature is investigated, and the equivalent time-dependent thermal expansion coefficients (TTEC), the equivalent time-dependent thermal relaxation modulus (TTRM) under micro-scale and meso-scale were predicted. Furthermore, the impacts of temperature, structural parameters and relaxation time on the time-dependent thermo-viscoelastic properties of 3D braided four-directional composites were studied.

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.

The effect of carbon fibers (Cf) and Ni nanoparticles addition on the melting point, microstructure, shear strength, indentation hardness and indentation creep of SnBi₅₈/Cu solder joints were explored. Composite solder with various Cf percentages (0, 0.02, 0.04 and 0.06 wt.%) and 0.1 wt.% Ni nanoparticles were prepared by solder paste stirring method. The results indicated that the adulteration of Cf and Ni nanoparticles refines the microstructure and improves the mechanical properties of Sn58Bi (SnBi₅₈)/Cu solder joints. However, the added materials have a relatively small impact on the melting point of the composite solder. Solder joints containing 0.04wt.% Cf and 0.1 wt.% Ni nanoparticles showed the best performance in all samples. Compared with SnBi₅₈/Cu solder joints, the shear strength and indentation hardness of SnBi₅₈-0.1Ni-0.04Cf/Cu solder joints was increased by 28.35% and 16.84%, respectively. Furthermore, the indentation creep decreased by 19.61%. When the addition of 0.06 wt.% Cf, there were more voids and Cf accumulation in SnBi₅₈-0.1Ni-0.06Cf/Cu solder joints, which resulting in the shear strength and indentation hardness of SnBi₅₈-0.1Ni-0.06Cf/Cu solder joints were reduced.

Sn58Bi solder is considered a promising lead-free solder that meets the performance requirements, with the advantages of good wettability and low cost. However, the low melting point characteristic of Sn58Bi poses a serious threat to the high-temperature reliability of electronic products. In this study, Sn58Bi solder alloy based on nickel (Ni) functionalization was successfully synthesized, and the effect of a small amount of Ni on creep properties and hardness of Cu/Sn58Bi/Cu micro solder joints at different temperatures (25 °C, 50 °C, 75 °C, 100 °C) was investigated using a nanoindentation method. The results indicate that the nanoindentation depth of micro solder joints exhibits a non-monotonic trend with increasing Ni content at different temperatures, and the slope of the indentation stage curve decreases at 100 °C, showing that the micro solder joints undergo high levels of softening. According to the observation of indentation morphology, Ni doping can reduce the indentation area and accumulation around the indentation, especially at 75 °C and 100 °C. In addition, due to the severe creep phenomenon at 100 °C, the indentation hardness rapidly decreases. The indentation hardness values of micro solder joints of Cu/Sn58Bi/Cu, Cu/Sn58Bi-0.1Ni/Cu, and Cu/Sn58Bi-0.2Ni/Cu at 100 °C are 14.67 ± 2.00 MPa, 21.05 ± 2.00 MPa, and 20.13 ± 2.10 MPa, respectively. Nevertheless, under the same temperature test conditions, the addition of Ni elements can improve the high-temperature creep resistance and hardness of Cu/Sn58Bi/Cu micro solder joints.

Three-dimensional (3D) braided composites have received much attention in the fields of aerospace, the automotive industry, etc. , 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.