*5.2.6. Three-dimensional fully braided fabric structure*

Geometric relations on the representative 3D fully braided and 3D axial braided structures were investigated in terms of unit cell angle, unit cell yarn length and unit cell yarn path. The results show that the unit cell from 4-step method is influenced by braid patterns for both preform types. Fully interconnected unit cell structures are obtained when patterns on odd numbered rows were used. Patterns on even numbered rows, on the other hand, mostly led to layer-to-layer connection on the unit cell edge forming an empty pocket between each braided layer. The unit cell structure has a fine intertwine in the 1×1 pattern, whereas it has a coarse intertwine for other braid patterns. When the influence of number of layers is consid‐ ered, it was found that, for all braid patterns, the unit cell thickness increases when the number of layers is increased in 3D braided and 3D axial braided structures. Furthermore, for the same number of layers, the unit cell thickness in the 1×1 pattern is less when compared to other patterns. This showed that all braid patterns except 1×1 resulted in a coarse form of unit cell structure [70].

Byun and Chou [141] examined the process-microstructure relationships of 2-step and 4-step braided composites by geometrical modeling of unit cells. The effect of process parameters such as braid pattern and take-up rate on the microstructural properties like braid yarn angle and fiber volume fraction was investigated. They also studied the fabric jamming phenomen‐ on. Three-dimensional braided composites were characterized by using the fabric geometry model (FGM). This model uses the processing parameters as well as the properties of the fibers and the matrix. It basically relies on two parameters such as the fabric geometry and the fiber volume fraction. Fabric geometry is a function of the take-up rate, whereas row and column motions determine the yarn displacement values, which are expressed as number of yarns. The yarn orientation in a 3D preform is dependent upon fabric shape and construction as well as the dimensions of the braiding loom [142].
