ANALYTICAL METHODS FOR TEXTILE COMPOSITES
proportion to the maximum misalignment angle of a wavy tow. Since waviness in textiles
is usually higher than in tape laminates, the compressive strength is likely to be lower.
However, it need not be much lower. In 3D architectures that permit nominally straight in-
plane tows (e.g. the interlock weaves of Sect. 2.3.1.6), waviness can be controlled by high
quality processing.
How waviness might lower tensile strength is less clear, but some experiments
suggest that it does. Unfortunately, verified models are unavailable for estimating waviness
effects on tensile strength; and, in most data sets for tensile strength, it is difficult to
identify trends with degree of waviness amidst the noise of the data. Knockdowns due to
fiber damage are much more important in tension than they are in compression (Sect. 4)
and will always give some scatter in strength.
3D textiles are often equal or superior in notched strength to equivalent tape
laminates. The large strain to failure of many 3D textile composites allows the development
of extensive damage process zones next to stress concentrators, which decreases notch
sensitivity. This superiority is not generally shared by 2D textiles, which lack some of the
energy absorbing mechanisms present in 3D textiles (Sect. 4).
2D Weaves
The ultimate tensile strengths of woven graphite/epoxy laminates and equivalent
tape laminates are compared in Figure 3-2 (from [3.4] and [3.5]). The data represent 5-
harness satin weave fabric manufactured with 3k yarns of carbon and consolidated with
two resin systems. The woven laminates had a fiber volume fraction approximately 10%
lower than in the tape laminates and a higher areal weight, and were accordingly 12-17%
thicker. The ratio of the fabric strength to the tape laminate strength appears in parenthesis
above each pair of bars.
The unnotched strength of the textiles (Fig. 3-2(a)) is significantly lower (15-23%)
when the load is aligned with the 0° fiber axis. For a ±45° laminate, the difference
disappears. Tensile axial tow strength, which is fiber dominated, is most affected by
waviness and fiber damage. Shear strength, which is resin dominated, is affected less. The
quasi-isotropic laminates have knockdowns similar to the 0/90° laminates. Notched strength
(Fig. 3-2(b)) shows similar trends, although the knockdowns for the 0/90° laminates are
greater (30-33%).
