عنوان مقاله [English]
As the application of 3-dimensional braided composites is becoming more common, many models have been developed to analyze their mechanical properties. Although much research work has been done, no theoretically perfect method has been successfully applied to the prediction of stiffness and strength parameters for three dimensional braided composites. 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 had been largely neglected in favor of the finite element model. Among those who have used the analytical model, the braided pre-form has been considered as being made of one or three types of representative unit cell, 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 types 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 onsidered as 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 obtained results were compared with experimental results. Finally, after evaluation of analytical methods and their comparison with experimental methods, the relationships between different braid angles and longitudinal and transverse stiffness, longitudinal and transverse shear stiffness and longitudinal and transverse Poisson ratios were predicted. It was shown that they are affected by changes in the braided angle.