Compression properties of unidirectional over-braided multilayer composites

Abstract

A unidirectional (UD) braided structure was developed to improve the in-plane mechanical performance of the two-dimensional (2D) braided composites. The basic compression properties of UD braided composites need to be studied to broaden their application. In UD over-braided multilayer composites (UDMLCs), the braid angle gradually increases from the first layer to the last one. This work found that a slight range variation (2°–5°) in the mean braid angle (calculated by averaging the braid angles of all layers) of UDMLCs had a significant effect on the compression modulus and little effect on the compression strength. The reinforcement (braided) yarn off-axis orientation introduced shear under compression load. The gradual increase of the braid angle from the first to the last layer created a load gradient in the thickness direction. These helped to reveal the formation mechanism of the kink band which was the key reason leading to catastrophic compression failure. Compared with 2D over-braided multilayer composites (2DMLCs), the compression modulus and strength of UDMLCs increased by 9.08% and 15.84%, respectively. However, the interweaving of reinforcement yarns and the nesting between layers in 2DMLCs made their integrity better than that of UDMLCs. Meanwhile, the interwoven structure of 2DMLCs also made their structural stability better than that of UDMLCs.