عنوان مقاله [English]
The present paper represents a new analytical method for calculation of the stiffness of three-dimensional, four-directional braided composites. In most previous work, the analytical model has been largely neglected in favor of the finite element model. Among those who have used the analytical model, the braided preform has been considered to be made of one, or three, types of representative unit cells, while microscopic evidence of the microstructure of pre-forms reveals different configurations of the yarns in the interior, surface and corner regions of a braided pre-form. This paper presents a multi-unit cell model, in which four kinds of unit cell, namely; interior,
interior surface, exterior surface and corner unit, have been introduced as representative cells. Each type of unit cell in a braided composite has different micro-structural parameters and possesses unique mechanical
properties. In this model, each type of unit cell has been considered a uni-directional composite. Next, using rotation matrices, the angle between yarns and longitudinal direction has been considered in general coordinates of the model. Finally, using a volume averaging method, the total stiffness of the braided composite is calculated. The results are compared with those obtained from experimental methods and the effect of braiding angle on the stiffness of braided composites is discussed.