Structural Design Of Three-Dimensional Woven Double-Layer Spacer Fabrics And Mechanical Properties Of Composites

Three-dimensional woven spacer composites are newly sandwich structure composites and have excellent properties such as low weight,high strength,thermal insulation,good wave permeability,strong designability and delamination resistance,which have a wide application space in aircrafts,high-speed rails,oil storage tanks,insulated vans,wind blades,radomes and other fields.3D woven double-layer spacer composites designed with reference to the design concept of multilayer sandwich structure have more excellent mechanical properties,and can overcome the shortcomings of easy delaminating of traditional multilayer sandwich structure materials.However,the research and application of 3D woven double-layer spacer composites are limited by the complexity of fabric structure and the secrecy of weaving process.Based on the structural design of 3D woven double-layer spacer fabrics,its weaving process was explored and optimized by using multi-eyelet heddle and multi-rapier weft insertion device.Then,two double-layer spacer composites（3DWSC-6+6,3DWSC-4+8）with the pile height of（6+6）mm and（4+8）mm were successfully prepared by using the hand lay-up molding process,and their compression,bending and low velocity impact properties were systematically studied by comparing with the single-layer spacer composites（3DWSC-12）with the pile height of 12mm.Firstly,the flatwise and edgewise compression properties of double-layer and single-layer spacer composites were tested and analyzed.The results show that the flatwise and edgewise compression properties of double-layer spacer composites are obviously improved due to the addition of the middle panel.Among them,the flatwise compressive strength（11.45 MPa and8.62 MPa）of 3DWSC-6+6 and 3DWSC-4+8 is 57.71%and 18.73%higher than that of 3DWSC-12（7.26 MPa）,respectively.The edgewise compressive strength（16.90 MPa and 15.54 MPa）increased by 54.48%and 42.05%in the warp direction,respectively,and the edgewise compressive strength（19.76 MPa and 19.44 MPa）increased by 50.72%and 48.28%in the weft direction,respectively.In addition,the flatwise compression failure mode of the double-layer spacer composites is the fractures of pile yarns layer by layer,and the edgewise compression failure mode is crack propagation of panel,which show greater energy absorption characteristic.Secondly,the warp and weft bending properties of double-layer and single-layer spacer composites were investigated.The results show that the bending peak load of double-layer spacer composites can be effectively increased when its middle layer panel is close to the upper panel or the lower panel.The warp bending peak loads of 3DWSC-4+8 and 3DWSC-8+4（the upper pile height is 8 mm and the lower pile height is 4 mm）are 998.24 N and 807.81 N,respectively,and the weft bending peak loads are 1227.86 N and 1221.55 N,respectively,which are significantly higher than that of 3DWSC-6+6 and 3DWSC-12.In addition,the weft bending stiffness of double-layer spacer composites is significantly enhanced due to the addition of the middle panel.The weft bending stiffness of 3DWSC-4+8,3DWSC-6+6 and 3DWSC-8+4 is14.17 N·m~2,14.23 N·m~2 and 13.84 N·m~2,respectively,which are all significantly higher than that of 3DWSC-12（9.69 N·m~2）.Besides,double-layer spacer composites can maintain a higher bending load after reaching the bending peak load,and have a higher energy absorption value.Finally,the low velocity impact responses of double-layer and single-layer spacer composites at impact energies of 5 J and 10 J were explored,and then the residual strength of specimens after impact was evaluated.The results show that the maximum contact force between double-layer spacer composites and impactor is significantly higher than that of single-layer spacer composites,and the damping index（the ratio of the absorbed energy to elastic energy）of double-layer spacer composites is significantly lower than that of single-layer spacer composites,which indicate double-layer spacer composites have better impact resistance.In addition,under the impact energy of 5 J,the impact resistance of double-layer spacer composites increases with the decrease of the distance between the middle layer and the upper layer.Among them,the maximum contact force of 3DWSC-4+8 is the largest（1929.33 N）,and the damping index is the smallest（20.50）.Besides,under the impact energy of 10 J,the strength retention rate（>80%）of double-layer spacer composites after impact is higher than that of single-layer spacer composites（<70%）.