*3.1.7. Biaxial knitted fabric*

types are weft knitting and warp knitting. In weft knitting, a continuous yarn forms one horizontal row of loops called a "course" connecting it to the previously formed courses in the process (Figure 4). The vertical columns of loops are called "wale." In warp knitting, yarn loops are connected vertically to form the fabric structure. Knitted fabrics are characterized by their 'wale density' and 'course density.' The wale density is defined as the number of wales per unit length in the course direction. The course density is defined as the number of courses per unit length in the wale direction. Stitch density is the product of course density and wale

**Figure 4.** (a) Two-dimensional weft knitted fabric (b) warp knitted fabric, and (c) spiral knitted fabric [36].

The special looped structure of knitted fabrics results in large gaps in the fabric structure. This reduces the overall fiber volume fraction of the composite leading to low mechanical proper‐ ties. Furthermore, the fabric is loosely formed unlike a woven fabric, which leads to high elongation and low stiffness. These problems have led to structural modifications of knitted fabrics by using inlay yarns either in fabric length or width direction to increase the mechanical properties of the resulting composites. Figure 5 presents the schematic views of these modifi‐ cations. The inlay yarns are trapped inside the knitted loops during the fabric formation. It was shown that the tensile strength of uniaxial knitted fabric composites can be improved

**Figure 5.** (a) Two-dimensional warp in-laid weft knitted fabric (b) 2D weft in-laid weft knitted fabric (c) 2D warp inlaid warp knitted fabric (d) 2D weft in-laid warp knitted fabric, and (e) 2D weft in-laid spiral knitted fabric [49].

density [36, 48].

90 Non-woven Fabrics

*3.1.6. Uniaxial knitted fabric*

significantly in the inlaid directions [49].

Biaxial knitted structures were developed by the insertion of warp (0°), weft (90°) or diagonal (±45°) yarns to the weft or warp knitted fabrics, as shown in Figure 6. The in-laid yarns improve the directional mechanical properties of the resulting composites.

**Figure 6.** (a) Two-dimensional weft in-laid 0° /90° knitted fabric and schematic view (b) warp in-laid 0° /90° knitted fab‐ ric, and (c) warp in-laid ±45° knitted fabric [50-52].

#### *3.1.8. Nonwoven fabric*

Nonwoven fabric is a web structure made up of short fibers that are held together by various techniques. These techniques include needling, knitting, stitching, thermal bonding, chemical bonding, and electrospinning. Needling is a method where vertically positioned barbed needles or water jets strike into the fiber web so as to entangle the fibers and create a mechanical locking between them. Knitting aims to entrap the fibers and fix them in position with the aid of knitting loops. In stitching technique, the fiber web is stitched in through-the-thickness direction. Thermal bonding is generally applied to thermoplastic fibers and powders. Fiber web is subjected to heat treatment which softens and unifies the neighboring fiber surfaces. This process is followed by cooling that solidifies the fibers and gives the web its final form. In the chemical process, polymer dispersions are used as binders to consolidate the nonwoven fabric. In electrospinning method, polymer solution is drawn under high electric energy field by using needles. Various fibers can be used to make nonwoven nano fibers such as polyur‐ ethane, polyvinyl alcohol and carbon. The nonwoven produced from these fibers can provide interesting physical and electrical properties with their high surface area. Nanofibers with diameters in the range of 40-2000 nm (0.04-2 µm) can be made. Fiber diameters can be varied and controlled [53-55]. Figure 7 shows the schematic and real views of 2D nonwoven fabrics manufactured by various methods [56, 57].

**Figure 7.** Schematic view of 2D nonwoven fabric by (a) mechanical needling (b) hydroentanglement (c) schematic view of stitched nonwoven structure (d) knitting loop surface, and (e) knitting loop reverse surface [58].
