*3.2.10. Three-dimensional knitted spacer or sandwiched structure*

The 3D knitted spacer fabric consists of two separate fabric layers (top and bottom surfaces) that are connected by intermediary yarns or knitted layers [79]. The top and bottom fabrics can be weft or warp knitted fabrics with or without inlays. Three-dimensional spacer fabrics are renowned for their excellent resilience and air permeability properties. Figure 21 shows schematic and real views of various 3D knitted sandwich fabrics.

## *3.2.11. Three-dimensional nonwoven fabric*

Poisson's ratios throughout its structure [77]. Another multiaxial 3D braided structure has ±bias yarns placed in-plane, and warp (axial), radial (z-yarns), and ±braider yarns placed outof-plane [78]. The braider yarns are intertwined with the axial yarns whereas ±bias yarns are oriented at the surface of the structure and locked by the radial yarns to the other yarn sets. Figure 19 shows the multiaxial cylindrical and conical para-aramid 3D braided structures. The properties of the multiaxial 3D braided structure in the transverse direction can be enhanced

**Figure 19.** (a) The unit cell of multiaxis 3D braided preform [77]; multiaxis 3D braided para-aramid preforms (b) cylin‐

Wunner [32] developed a multiaxis warp knit machine for Liba GmbH. The machine uses a total of four yarn sets such as ±bias, warp and filling. These yarn sets are placed as separate layers and these layers are locked by stitching yarn by using tricot pattern, as

drical Kevlar® preform and (c) conic Kevlar® preform [78].

*3.2.9. Three-dimensional knitted fabric*

**Figure 20.** Multiaxis warp knit structure [32].

shown in Figure 20.

98 Non-woven Fabrics

and the non-uniformity in the directional Poisson's ratios can be decreased [78].

Multiple layers of 2D nonwoven webs are stacked and stitched together in thickness direction to obtain 3D nonwoven fabric. Stitching yarn provides through-the-thickness reinforcement in an effort to impart out-of-plane structural integrity and reduce delamination failures. Olry developed a method called "Noveltex" for 3D nonwoven preform fabrication [80]. This method uses needle punching as a means of fiber entanglement. A 3D nonwoven preform was developed using hydroentanglement method to create through-thickness fiber insertion. Biaxially reinforced nonwoven fabric is another type of 3D nonwoven preforms that is manufactured by employing warp knitting technology. The preform consists of warp and weft yarns along with a fiber web. Warp and weft yarns can be thought of as inlays such that they are laid in fabric structure as separate layers without any interlacements. Warp yarns, weft yarns and fiber web are all connected by stitching yarns to form an integrated structure as shown in Figure 22 [81]. Geogrid structures can be considered as a special type of nonwoven fabric. They can be classified based on their shape such as uniaxial, biaxial and triaxial geogrid structures used in wall, slope and road applications; and manufacturing methods such as punched and drawn geogrids, coated yarn geogrid and laser welded geogrids. The basic functions of geogrid structures are to interlock the aggregates, to redistribute the load over wider area to reduce the vertical stress, and to provide lateral restraint, improved bearing capacity, and tension membrane effect [82].

**Figure 22.** Three-dimensional nonwoven fabric; (a) schematic view of flat 3D nonwoven preform (left) and 3D PANbased graphite felt composite (right); (b) schematic view of circular 3D nonwoven preform (left) and 3D PAN-based graphite felt composite (right); (c) top and side views of 3D biaxially reinforced nonwoven preform [80, 81, 83].
