*5.1.3. Nonwoven fabric structure*

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

Nishimoto et al. [113] investigated 2D circular biaxial braided fabrics. They used a step response model to examine the temporal change in braiding angle under unsteady-state conditions. An examination of the flow pattern during the consolidation revealed that the permeability of the fabric is determined by spaces between the fibers especially in the case of low braid angle. Permeability and porosity may result in a non-uniform flow pattern during liquid molding [114]. The effect of braiding angle on the mechanical properties of 2D biaxial braided composite was analyzed. It was shown that when the braiding angle is increased, the bending modulus and strength decreased [115]. Smallest braiding angle (approximately 15˚) resulted in the highest bending properties [116]. The mechanical properties of 2D biaxial 2×2 pattern braided fabric composites were studied by a 3D finite element micromechanics model and compared with equivalent 2×2 twill fabrics to analyze their fracture modes under various loading requirements [117]. The biaxial compressive strength properties of 2D triaxial braided cylinders were investigated. It was reported that the fiber waviness affects the axial compres‐ sion strength. The composites exhibited considerably higher compression and tension strength in the axial direction when compared to those in the braid direction [118]. Smith and Swanson [119] studied the response of 2D triaxial braided composites under compressive loading. It was shown that the laminated plate theory provided good stiffness predictions for low braid angle, whereas a fiber inclination model yielded close estimations for various braid angles. Tsai et al. [120] investigated the burst strengths of 2D biaxial and triaxial braided cylindrical composites. It was reported that the crack formation in biaxial braided composite starts in tow direction. In a triaxial fabric composite, on the other hand, the cracks first appear in the

Byun [121] developed an analytical approach to predict the geometric characteristics and mechanical properties of 2D triaxial braided textile composites. The model is based on the unit cell geometry of the braided structure. It was reported that the geometrical model can accurately predict some important properties such as the braid angle and fiber volume fraction. An averaging technique based on the engineering constants was used to calculate the stiffness properties of the composites. It was shown that the averaging technique yields more precise results when worked with small braid angles. It was also reported that the model gives more accurate results when the bundle size of the axial yarns is much larger in comparison with that of the braider yarns. Yan and Hoa [122] used an energy approach for predicting the mechanical

orthogonal fabric.

118 Non-woven Fabrics

*5.1.2. Braided fabric structure*

longitudinal direction.

behavior of 2D triaxially braided composites.

Properties of 2D nonwoven fabric structure depend on fiber type and size, packing density (fiber volume fraction), pore size and distribution in the web volume, and fiber orientation in the web [56]. The packing density (α) of a web is defined as the ratio of the volume occupied by the fibers to the whole volume of the web as defined by Eqs. (3) and (4):

$$\alpha = \text{total fiber volume} / \text{total web volume} = \frac{V\_f}{V\_{\text{web}}} = \frac{W\_f \mid \rho\_f}{t \, A} = \frac{\text{Basis weight}}{t \, \rho\_f} \tag{3}$$

where, Vf is the volume of fibers; Vweb is the volume of the web; Wf is the weight of fibers = weight of the web; *ρ*<sup>f</sup> is the fiber or polymer density; t is the thickness of the web and A is the area of the web.

Porosity (*ε*) can be obtained by following relation:

$$
\omega = 1 - \alpha \tag{4}
$$

Nonwovens are composed of short fibers that are entangled or bonded together to form a continuous fabric structure. Therefore the mechanical properties of the fabric and its composite strictly depend upon fiber strength/stiffness and the bonding strength between the fibers. Unlike many other forms of reinforcement, nonwoven fabrics with randomly oriented fibers can be regarded as isotropic structures bearing similar properties in all possible in-plane directions. The main parameters that determine the mechanical properties of a nonwoven composite are fiber modulus/tenacity, packing density/fiber volume fraction, fiber orientation distribution and fiber length distribution [123].

It is demonstrated that various polymer solutions can be used in electrospinning process to make various sectional nanofibers including cylinder, rod, ellipse, flat ribbons and branched fibers. The arrangement of fibers in the nano-web is generally random, with a slight bias to the machine direction due to movement of the collector and the air drag/suction [124-126]. The process parameters in electrospinning are electric voltage, the distance between the spinneret and the collector, the polymer concentration, the diameter of the spinneret, and the web structural design parameters [127]. It is noted that the critical part of the electrospinning is the fluid instability. At high electric fields, the jet becomes unstable and has an appearance of inverted cone, which is a single, rapidly whipping jet [128, 129]. The dominant instability strongly depends on the fluid parameters of the jet, namely, the viscosity, the dielectric constant, the conductivity and the static charge density on the jet [130].

#### *5.1.4. Knitted fabric structure*

The 2D knitted fabric is thicker than an equivalent woven structure due to its special loop elements which are buckled toward an additional (third) fabric dimension. The fabric is highly extensible with a low flexural rigidity [131]. It was reported that the knit preform properties are greatly influenced by the fiber strength and modulus, knitted structure, stitch density, prestretch parameters and incorporation of inlays [36]. The deformation behavior of knitted preforms can be predicted by initial load-elongation properties of knitted fabrics. The knitting process parameters influence the knitted preform during fabrication. Loop formation during the knitting process imposes dramatic bends and twists on fibers that cause fiber/machine element failures when working with high modulus/brittle fibers. It was shown that the knittability of these fibers depends on frictional properties, bending strength, stiffness, and fiber/yarn strength [49]. The knittability of a given yarn can be improved by certain machine parameter adjustments including low tension application during yarn input, fabric take down tension setting, and loop length control which is adjusted by stitch cam settings [109]. Also, the knittability of high performance yarns mainly depends on yarn-to-metal friction charac‐ teristics. Positive yarn feeding control and tension compensator improve the dimensional stability of the knitted preform. It was demonstrated that yarn bending rigidity and inter-yarn coefficient of friction are very important determinants for loop shape while the loop length of high performance yarns, glass yarns in particular, was found to vary with needle diameter, stitching cam setting and machine setting [49, 79, 109].
