The micro-geometry of three-dimensional (3D) braided tubular preform is more complex than that of rectangular one, especially for the interfaces between yarns. The yarn interfaces are difficult to analyze theoretically, and if two sides of the interfaces do not fit perfectly in the model, the final result will contain overlapped yarns, which is bad for finite element calculation. This paper focuses on the microstructure of 3D four-directional braided tubular composites and presents a modelling approach based on Free Form Deformation (FFD) theory. First, the planar and spatial yarn paths are analyzed and three geometrical mapping equations from rectangular unit-cell to tubular sub unit-cell are derived. The modelling approach was then developed to establish the sub unit-cell model. Finally, the established models are compared with micro computed tomography (l-CT) scans of a dry specimen of the circular braided preform. The model not only solves the yarn-overlapping problem, but also accurately describes the key characteristics of the preform.

三维（3D）编织管状预成型件的微观几何形状比矩形形状的更为复杂，特别是对于纱线之间的界面而言。纱线界面很难从理论上进行分析，如果界面的两侧无法完美地拟合到模型中，则最终结果将包含重叠的纱线，这对有限元计算不利。本文着重研究3D四向编织管状复合材料的微观结构，并提出一种基于自由变形（FFD）理论的建模方法。首先，分析了平面和空间纱线的路径，并推导了从矩形单元到管状子单元的三个几何映射方程。然后，开发了建模方法以建立子单元模型。最后，将建立的模型与圆形编织预成型件干样品的微计算机断层扫描（1-CT）扫描进行比较。该模型不仅解决了纱线重叠的问题，而且准确地描述了预成型件的关键特性。