Page 225 - Mechanics Analysis Composite Materials

210 Mechanics and analysis of composite materials
directions. Much more possibilities for such shaping are provided by implementa-
tion of knitted fabrics whose strain to failure exceeds 100%. Moreover, knitting
allows us to shape the fibrous preform in accordance with the shape of the future
composite part. There exist different knitting patterns, some of which are shown in
Fig. 4.83. Relatively high curvature of the yarns in knitted fabrics and possible fiber
breakage in the process of knitting result in materials whose strength and stiffness
are less than those of woven fabric composites, but whose processability is higher,
and the cost is lower. Typical stress-strain diagrams for composites reinforced by
knitted fabrics are presented in Fig. 4.84.
Material properties close to those of woven composites are provided by
braided structures which, being usually tubular in form are fabricated by mutual
intertwining, or twisting of yarns about each other. Typical braided structures
are shown in Fig. 4.85. Biaxial braided fabrics in Fig. 4.85 can incorporate
longitudinal yarns forming a triaxial braid whose structure is similar to that shown
in Fig. 4.81d. Braided preforms are characterized with very high processability
providing near net-shape manufacturing of tubes, and profiles with various cross-
sectional shapes.
Although microstructural models of the type shown in Fig. 4.80 and leading to
equations similar to Eq. (4.167) have been developed to predict stiffness and even

c, MPa
400

300

200
45"

100

E,%
0 2
0 I 2 3 4 5
Fig. 4.82. Stress-strain curves for fiber glass fabric composite loaded in tension at different angles with
respect to the warp direction.

Fig. 4.83. Typical knitted structures.