Due to the interlacement of tows, the architecture of braided composites is complex, especially for the nonorthogonal braid in which the tow has various cross sections along the towpath. The microscopic mechanical analysis is highly sensitive to the geometric architecture, which causes great difficulties for the research on material behavior of nonorthogonal braid. In this article, a finite element model has been proposed to investigate the effective elastic properties and stress distribution of 2D biaxial nonorthogonally braided composites. This research was conducted for two kinds of 2D biaxial braid, 1 × 1 and 2 × 2, using a parallelogram repeated unit cell that is suitable for the description of stress distribution under different load conditions. The differences between 1 × 1 and 2 × 2 in effective elastic properties and stress distribution were discussed. The effect of braid angle on the mechanical properties was also studied. The result reveals that 2 × 2 has greater in-plane Young’s modulus than 1 × 1 with the same tow, fiber volume ratio, and braid angle, although the situation of the out-plane Young’s modulus is on the contrary.

Based on the continuum damage mechanics (CDM) and the cohesive zone model (CZM), a numerical analysis method for the evaluation of damage in composite laminates under low-velocity impact is proposed. The intraply damage including matrix crack and fiber fracture is represented by the CDM which takes into account the progressive failure behavior in the ply, using the damage variable to describe the intraply damage state. The delamination is characterized by a special contact law including the CZM which takes into account the normal crack and the tangential slip. The effect of the interlaminar toughness on the impact damage is investigated, which is as yet seldom discussed in detail. The results reveal that as the interlaminar fracture toughness enhances, the delamination area and the dissipated energy caused by delamination decrease. The contribution of normal crack and tangential slip to delamination is evaluated numerically, and the later one is the dominant delamination type during the impact process. Meanwhile, the numerical prediction has a good agreement with the experimental results. The study is helpful for the optimal design and application of composite laminates, especially for the design of interlaminar toughness according to certain requirements.

To investigate the growth dynamics of the single void during Czochralski silicon growth as well as capture the basic features of the diffusion-controlled dynamic mechanisms, a phase field method has been developed. The free energy of the system involving the chemical free energy and the gradient energy is presented. Numerical tests were performed to examine the capability of this model, and the results show that: the void grows due to the absorption of vacancies in the matrix, which essentially reduces the free energy of the system; with the growth of the void, there forms vacancy concentration gradient towards the void in the matrix; the increase of initial vacancy concentration contributes to a larger void size and growth rate.

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The hierarchical and heterogeneous structure characteristics of composite laminates give rise to the difficulty in the study of the composite laminates damage under low-velocity impact. A numerical method for the evaluation of the impact damage was proposed on the basis of the continuum damage mechanics (CDM) and the cohesive zone model (CZM). The method can bring stable simulations and can effectively establish the relation between the mesoscale structure and the macroscopic response under impact. The evolution of the impact damage and the effect of single ply orientation on the impact damage resistance were investigated. It is found that the tangential delamination is the dominant form of the interface damage and more orientation of fiber can make the impact resistance improved. The prediction has a good agreement with the experimental results. The dynamic analysis is helpful for a thorough understanding of the evolution of low-velocity impact damages in composite laminates.