Page 224 - Mechanics Analysis Composite Materials

Chapter 4. Mechanics of a composite layer 209

Stiffness and strength of fabric composites depend not only on the yarns and
matrix properties, but on material structural parameters, i.e., on fabric count and
weave, as well. The fabric count specifies the number of warp and fill yarns per inch
(25.4 mm), while the weave determines how the warp and the fill yarns are
interlaced. Typical weave patterns are shown in Fig. 4.81 and include plain, twill,
and satin. In the plain weave (see Fig. 4.81a) which is the most common and the
oldest, the warp yarn is repeatedly woven over the fill yarn and under the next fill
yarn. In the twill weave, the warp yarn passes over and under two (as in Fig. 4.8 1b)
or more fill yarns in a regular way. A structure with one warp yarn passing over four
and under one fill yarn is referred to as a five harness satin weave (Fig. 4.81~).
Being formed from one and the same type of yarns plain, twill, and satin weaves
provide approximately the same strength and stiffness of the fabric in the warp and
the fill directions. Typical stress-strain diagrams for a fiberglass fabric composite
of such a type are presented in Fig. 4.82. As can be seen, material demonstrates
relatively low stiffness and strength under tension at the angle of 45" with respect to
the warp or fill directions. To improve these properties, multiaxial woven fabrics,
one of which is shown in Fig. 4.81d, can be used.
Fabric materials whose properties are more close to those of unidirectional
composites are made by weaving a great number of larger yarns in longitudinal
direction and fewer and smaller yarns in the orthogonal direction. Such weave is
called unidirectional. It provides materials with high stiffness and strength in one
direction, which is specific for unidirectional composites and high processability
typical for fabric composites.
Being fabricated as planar structures, fabrics can be shaped on shallow
surfaces using the material high stretching ability under tension at 45" to the yarns'

(c) (4
Fig. 4.81. Plain (a), twill (b), satin (c),and triaxial (d) woven fabrics.