Fabrication And Mechanical Properties Of The Biaxial Weft Knitted Thermoplastic Composites

In recent years, the proportion of industrial textiles in the whole textile products grows rapidly, especially thermoplastic composites reinforced with high performance fibers have fastly developed. The process of weft-knitted fabrics is easy and it has high productivity. So as a kind of the reinforcements, they can be used with thermoplastic matrix to produce composites which have many advantages, such as better mold ability and impact resistance, lower cost and so on. Because of the excellent tensile properties, good shear resistance and rupture resistance, etc., the weft-knitted fabrics with pre-oriented reinforcement yarns are popular in the area of composites. In order to promote the development of pre-oriented knitted reinforcement structures and their application in composites, investigations related to their fabrication using new techniques and mechanical properties are very significant.This paper mainly investigates the fabrication of biaxial weft knitted thermoplastic composites with GF/PP commingled yarns and their mechanical properties. The work includes knitting of the biaxial weft knitted performs, thermo-pressing forming of the composites, tensile testing and analysis. By experimental work and analysis on the knittability, three different kinds of biaxial weft knitted fabrics were firstly produced on a modified hand flat knitting machine. At the same time, the appropriate knitting parameters and some problems encountered during knitting process were also investigated.Thermoplastic composites were then fabricated by laying-up of single and double layer knitted fabrics through thermo-pressing forming process. During the fabrication, the single layer fabrics were cut in five directions according to the tensile testing requirements; the double layer fabrics were laid-up with two method. Besides, the influencing factors such as duration, temperature, cooling method during the thermo-pressing forming process were also analyzed.The tensile testing and analysis were finally carried out, including tensile strengths, elastic modulus, stress-strain curves and the damage modes. The influences of the different fabrics knitted with various GF/PP commingled yarns and the different laying-up methods were discussed. Based on the experimental results, the one-linear regression was used to establish the relations between the tensile strength, modulus and the GF volume fraction. The results show that the use of the GF/PP commingled yarns as weft and warp inserted yarns favorites the PP resin distribution and wetting as well as interface between the glass fiber and resin. This not only improves the manufacturing quality of the composites, but also enhances their mechanical properties. The tensile properties of the biaxial weft knitted structural composites joined with bolting and riveting were also tested and analyzed. The different damage modes in different tensile directions were compared. It was found that the tensile strength of composites after jointing together with bolting considerably decreased.