Liquid nitrogen temperature effects on the mechanical behaviors of three-dimensional seven-directional braided composites

Three-dimensional (3D) braided composites are broadly adopted in aerospace cryogenic fuel tanks. This work designed and fabricated 3D seven-directional braided composites (3D7dBCs) with different braiding angles, and their compression properties at room and liquid nitrogen temperatures (RT and LNT) were explored and discussed, while the mechanical properties were compared with other braided structural composites. Temperatures and braiding angles considerably influenced the properties of 3D braided composites. The multi-directional compression strength of 3D7d21-LNT and 3D7d34-LNT was 392.7 MPa, 426.1 MPa, 270.3 MPa, 210.3 MPa, and 268.8 MPa, 204.1 MPa, which was 18.1% and 18.6% higher than that of 3D7d21-RT and 3D7d34-RT in the out-of-plane direction, 5.1% and 6.5% in the longitudinal direction, 74.2% and 9.7% in the transverse direction, whereas the longitudinal strength decreased as braiding angles increased, and the out-of-plane and transverse compression strength increased. Additionally, shear cracking was the main reason for the failure of 3D7dBCs at RT, while fiber loosening, pulling-out, and interfacial debonding were exhibited at LNT. Compared to 3D five-directional and 3D six-directional braided composites (3D5dBCs and 3D6dBCs), the incorporation of seventh yarns significantly improved the out-of-plane properties of 3D7dBCs.