ADVANCES IN COMPOSITE MATERIALS - ECODESIGN AND ANALYSIS

Classification of textile reinforced composites

Two main criteria can be used to characterise the textile reinforcements: the material structure/geometry and the technological process (Hu, 2008).

Fukuta et al. (1984) gives a classification of the textile reinforcement based on the significant dimensions of the textile material and its specific geometry. Fukuta considers not only the 3 dimensions, but also the preset fibres directions used in the material structure.

According to Scardino (1989), the textile reinforcements can be divided into 4 groups, depending on their architecture: discrete, continuous, with plane geometry and with spatial geometry, as illustrated in Table 1.

When considering the technological process, all textile processes can be used to produce reinforcement for composite materials, but the specifics of each type of process and the resultant material geometry lead to differences in possibilities and behaviour. The main processes employed in the production of textile reinforcements are: weaving, braiding, knitting and non-woven. Also there are other processes, such as filament winding and poltrusion, which process filaments. Most used reinforcements are woven fabrics (2D and 3D) and nonwovens (fibre mats), but the knitted fabrics, especially warp knitted structures, present a good development potential.

The selection of a certain process is based on the architectural possibilities, the material characteristics and behaviour (dimensional stability, mechanic strength, drape and formability, etc) and its suitability with regard to the composite processing and its application.

Potential of knitted fabrics for composite reinforcement

The main advantages of knitted fabrics for composite reinforcement are:

• the possibility of producing knitted fabrics with 3D complex shapes

• improvement of fabric handling and matrix injection during composite processing

• acceptable processability of high performance fibres (glass, aramid, PES HT or HM)

• rapid manufacturing of knitted fabrics for reinforcements

• controlled anisotropy (yarn in-laid under preferential angles).

When considered in reference to other types of textile materials, knitted fabrics are not as well developed, mainly due to their lower mechanical properties (Leong et al., 2000). According to Verpoest et al. (1997), knitted fabrics present lower in-plane strength and stiffness in comparison to materials such as woven, braiding, non-crimps. Another problem limiting the use of knitted fabrics for composite reinforcement is the low value for volume fraction, due to the specific geometry of knitted stitches, characterised by areas without yarns.

The reduced mechanical behaviour is determined by the specific bending of fibres in the knitted stitches. Mechanical properties are controlled through fabric structure, structural parameters, yarn characteristics and process parameters. Structure is an effective way of improving properties by the use of float stitches and in-laid straight yarns placed under certain angles. Stitch density also affects the tensile behaviour and fabric stiffness,

Yarns are also important, their properties being transferred to the fabric level. The specifics of the knitting process make bending strength and rigidity the most important characteristics. This situation is essential, considering that high performance fibres are rigid and therefore must be processed carefully. Apart from carbon fibres, all other high performance yarns can be bent around a needle hook and transformed into stitches. The problems related to their processing are the fibre destruction and the modifications brought by the strains during knitting that lead to reduced mechanical characteristics of the fabric. The use of in-laid straight yarns eliminates the problem of fibre damage and also increases the volume fraction.

The multiaxial warp knitted fabrics, presented in part 3 of this chapter, are the most used for the production of composites. They have a laminar structure, with layers of yarns under preset angles, according to the application. The layers are connected and the risk of delamination is reduced.

Another development direction is the production of preforms with complex shapes for advanced composite materials. This is an interesting development direction, considering the complexity of the fabric architecture that can be achieved through knitting. The literature presents a significant amount of references concerning the development, characterisation and mechanical behaviour of these 3D fabrics.