An Improved MWK Structure for Composite Reinforcement

Abstract

An improved MWK structure with a double loop pillar stitch is presented in this work. Its structural features, knitting process, and mechanical properties are analyzed and compared with a common tricot MWK structure. The results show that the improved MWK structure with the double loop pillar stitch has better mechanical properties than the common MWK structure with the tricot stitch.One current trend in textile structural composites is to extend their applications from secondary nonload-bearing structures to primary load-bearing structures. This requires a significant improvement in their damage tolerance and reliability. For thin to medium thickness structures, an attractive family of textile preforms, multiaxial warp knitted (MWK) structures, has the potential to meet the demand for structural composites. All layers of the insertion yarns in an MWK structure are placed in a perfect order and reveal the uniformity of the noncrimped parallel yarns [2]. While the insertion yarns play a principal role in plane reinforcement, the stitch yarns provide through-thickness reinforcement, thus significantly increasing the damage tolerance, structural integrity, and out-of-plane strength of the reinforced structure. MWK structures can be produced in a single knitting process. In addition to good ease of handling and production economics, MWK structures also conform to complex shapes, are flexible in the principal yarn directions, and have improved through-thickness strength. The mechanical properties of MWK structural composites, especially out-of-plane strength and impact behavior, could be superior to conventional woven laminated composites due to elimination of fiber crimp in the insertion yarns and the presence of the through-thickness reinforcing stitch loops [1][2][3][4][5].As illustrated in Figure 1, the commonly used basic stitches for MWK structures are the chain stitch and tricot stitch [2]. In such structures, the underlaps of the basic loops form only diagonal lines or vertical lines on the fabric plane. Consequently, the binding effects are not ideal. For better binding effects, we develop an improved MWK structure with a double loop pillar stitch in this work, and we present its structural features, knitting process, and mechanical properties and compare it with common tricot MWK structure as well.

Knitted fabric composites

“…Theoretically, as many layers as preferred can be used but typical commercially available machines only allow four layers (Du and Ko, 1996). The purpose of the knit loops is to hold the layers of unidirectional yarns together, but it has also been proven to be the key to increasing the damage tolerance of the material (Zhou et al, 2005).…”

“…An improved MWK structure for composite reinforcement, developed by replacing the tricot stitch with a 'double loop pillar stitch', has been carried out by Zhou et al (2005) The modification has yielded improvements of up to 7% in breaking strength (Zhou et al, 2005). The tribology or wear resistance of MWK has also been investigated and has been shown to perform better than biaxial nonwoven structures (Mathew et al, 2007).…”

Tensile characteristics of flexible PU-coated multi-axial warp-knitted fabrics

“…For the multi-axial warp-knitted fabrics (MWKF), the following papers are found to be useful to discuss the fabric properties, such as the comparison analysis about the mechanical properties with different polymer-coated membranes or different inserted yarns [11][12][13] and also the manufacture and structure investigations [14,15]. Zhou et al [16] showed that the MWK structure with double loop pillar stitch was better than the common tricot stitch. Gao et al [17,18] studied the properties and meso-scale mechanism of MWK from different architectures, and found that the tear properties were dependent on the structures strongly.…”

An investigation into factors affecting mechanical properties of multi-axial nonwoven structure

“…This unique property of knitted fabrics limits their use as composite reinforcing elements. However, if yarns with high modulus are used in construction of knitted fabrics, then high quality composite materials can be made using these knitted fabrics [4,5]. In this respect, multi-axial warp knitted fabrics due to their inherent mechanical properties are preferred by composite designers.…”