Manufacture And Mechanical Properties Of The New Biaxial Spacer Weft Knitted Reinforced Composites

In this paper, a new biaxial spacer weft knitted structure was firstly knitted from an innovatory flat bed knitting machine, and then the manufacture and mechanical properties of its reinforced composites were investigated. This new structure fabric consists of two biaxial structures separately formed on the front and back needle beds and connected by a set of yarns fed from the vertical direction. High strength and modules yarns can be inserted in warp and weft direction and used as connecting yarns or knitting yarns. The thickness of the structure can easily be varied through adjusting the distance between the front and back needle beds. This new structure combines advantages of both spacer structure and bi-axial reinforced structure. It not only demonstrates good formability, high energy absorption, but also overcomes the shortcoming of traditional spacer structures, such as small thickness. As the reinforcing yarns are inserted in warp and weft direction, the mechanical properties of the structure are improved. It's an excellent composite reinforcement.Due to the structural feature of the biaxial spacer fabric and its potential application in the composite field, the fabrics were firstly knitted with glass fiber yarns and basalt fiber yarns, and the knitting processes and characteristics of these two different kinds of high modulus yarns were analyzed. The tensile properties of the fabrics were also tested and analyzed. The results showed that the utilization of the inserted yarns in the structure mainly depended on their straight state. After the fabrics had been successfully knitted with glass fiber yarns and basalt fiber yarns, their reinforced composites were manufactured through vacuum assisted resin transfer molding (VARTM) and light resin transfer molding (LRTM or RTM-Light) as well as hand laid-up. As LRTM was firstly mentioned and practically used in molding this kind of new structure reinforced composites, a special mould was manufactured.For the composites manufactured from two kinds of yarns and by three molding technologies, the structures produced by VARTM and LRTM were in the solid form while the structures produced by hand laid-up were not solid and in the sandwich form. The resin volume fractions in the LRTM composites were highest and those in the laid-up sandwich structure composites were lowest. For the composites fabricated with the same process, but with two kinds of yarns, the fiber volume fraction of the basalt fiber reinforced composite was a little lower than that of the glass fiber reinforced composite.After the composites had been manufactured, the tensile, flexural, compressive and flatwise compression properties of the composites were tested and analyzed. It has been shown that the tensile and flexural properties of the composites primarily depend on the properties of the inserted biaxial yarns, and that the contribution of knitted yarns is very limited. The compressive properties are not only in connection with the compressive and sheared damages of fiber assemblies, but also with the interfacial cohesive property between fiber assemblies and resin. The different yarn densities in weft and warp directions result in different properties in the two directions and this difference is much larger in tensile and flexural tests than that in compressive test. It has also been shown that the tensile, flexural and compressive properties and damage modes of glass fiber reinforced composite and basalt fiber reinforced composites are quite similar. The flatwise compression strength and modulus are respectively thrice as much as those of foam sandwich constructions. The excellent capability against compress has been achieved.The research work presented in this paper has provided a series of reference data and a solid theoretical basis for the manufacture and practical utilization of this new structure fabric and its reinforced composite in the future.