ANALYTICAL METHODS FOR TEXTILE COMPOSITES
thread. The glass or polyester threads, which contribute around 2% of the total weight,
serve mainly to hold the primary yarns together during subsequent handling. The fibers in
the primary yarns remain close to unidirectional. Multiple layers of uniweave are often
stitched together to form a sheet of any desired thickness.
Figure 2-5. Four-step braiding process.
2.1.2.5 Warp Knits
The multi-axial warp knit process ties yarns of primary fibers together in layers
with 0°, ±45°, and 90° orientations. The knitting is done with fine polyester threads, which
amount to a small percentage of the total weight. During knitting, the polyester threads are
passed around the primary yarns and one another in interpenetrating loops (Fig. 2-6). The
mechanical properties of the stack of layers can be controlled by selecting the yarn weight
in each of the four orientations. The knitted stacks form building blocks which can be
laminated to form the thickness desired for some structure. The knitted stacks can also be
stitched together in a secondary operation. The major advantage of the knitting process is
its low cost compared to conventional tape lay-up.
Stacks most commonly contain four oriented layers. However, a symmetric, seven
layer stack may be preferred to control twist and bending in the final laminate.
