**5. Properties of fabrics and composites**

#### **5.1. Two-dimensional fabric**

warp yarns are laid by warp yarn guide track. Two layers of weft yarns are laid over the warp layers by weft yarn carriers. The stitching yarn locks the warp and weft yarn sets using multiple latch needles in which stitched yarns were structured as weft loops. Simple as well as complex sectional knitted preforms were fabricated by the special take-up device. The critical process parameters are warp and weft densities, stitching density, yarn feeding, and fabric take-up

**Figure 41.** (a) Schematic views of the warp knitting action to form the 2D warp knitted fabric structure using the latch

**Figure 42.** (a) Schematic views of 3D weft knitting methods (b) 3D knitting machine; and (c) weft yarn carrier during

Wunner [32] designed a multiaxis warp knit machine for Liba GmbH. The machine is equipped with a pinned conveyor bed, a fiber carrier for each yarn set, a stitching unit, yarn creels and a take-up unit. It employs ±bias, warp and filling (90° yarn) yarn sets together with stitching yarn. Stitching yarn unites all the layers and provides structural integrity (Figure 43). Tricot

ratios [51, 109].

116 Non-woven Fabrics

(b) Actual 2D warp knitting machine [36, 49, 107].

knitting [108, 109].

*4.4.3. Multiaxis 3D knitted fabric*

*4.4.3.1. By warp knitting method*

pattern is generally used for this process.

#### *5.1.1. Woven fabric structure*

The 2D biaxial woven fabric is produced with a simple and highly automated process. It is by far the most economical structure in the composite industry in terms of fabric and composite production costs. The fabric is very stable and easy to handle during processing as well as has good drapability, which facilitates the fabrication of various countered parts. The fabric, however, contains numerous warp/weft interlacement points throughout its structure which impair fiber alignment and thus the load distribution capability of the reinforcing fibers. For this reason, the in-plane properties of 2D woven composite are somewhat lower than those of an equivalent UD composite. However, 2D woven composite still provides acceptably high in-plane properties especially in 0˚ and 90˚ direction due to fiber orientation in these directions. On the other hand, the in-plane properties of 3D woven composites are significantly lower than 2D woven composites for a number of reasons. Firstly, the 3D woven structure has zyarns inserted in through-the-thickness direction in order to improve weak out-of-plane properties of 2D layered woven composites such as delamination resistance and impact strength. However, the incorporation of z-yarns reduces the in-plane directional volume fraction of the composite and leads to lower in-plane properties. One of the problematic issues with the biaxial fabric composites is their low mechanical properties in bias directions such as ±45˚ and ±60˚. Triaxial fabrics bring a solution with their multidirectional fiber architecture. Scardino and Ko [110] reported that triaxial fabric has better properties in the bias directions when compared to biaxial fabric. The study revealed a 4-fold tearing strength and 5-fold abrasion resistance compared with a biaxial fabric with the same setting. Elongation and strength properties were found to be roughly the same. Schwartz [111] compared triaxial fabrics with leno and biaxial fabrics. He defined the triaxial unit cell and proposed the fabric moduli at crimp removal stage. He concluded that it is crucial to strictly define the fabric equivalency before comparing various kinds of fabrics. It was shown that triaxial fabric shows better isotropy compared to leno and plain fabric. This brings a clear advantage since isotropy plays an important role in fabric bursting and tearing strength as well as shearing and bending properties. Skelton [112] proposed a relation between the bending rigidity and the angle of orientation. It was found that the bending behavior of highly isotropic structures like triaxial fabrics is not dependent upon the orientation angle. Triaxial fabric is more stable in comparison with an orthogonal fabric with the same percentage of open area. Triaxial fabric also shows a greater isotropy in flexure and has greater shear resistance when compared to an equivalent orthogonal fabric.
