ANALYTICAL METHODS FOR TEXTILE COMPOSITES
if the buckling deflection has a wavelength that is long compared to the spacing of the
through-thickness yarns (Fig. 5-10a). If the through-thickness reinforcement is stiff
enough to limit buckling to intervals between through-thickness tows (Fig. 5-10b),
buckling will occur primarily via axial shear.
This leads to an interesting conjecture about what really matters in modeling the
ultimate strength, including compression strength, of a 3D textile composite when the 3D
reinforcement is sufficiently stiff and closely spaced to inhibit long wavelength lateral
deflections, including buckling (Fig. 5-10b). In this case, the composite can be regarded
as an assembly of tows whose collective strength comes from their internal stiffness.
Only damage that occurs inside tows is significant to the composite's integrity. The most
important interior damage is shear softening, kinking, and tow rupture. Damage between
tows, i.e. matrix cracking, has minimal effect on composite strength, because the
interaction of tows does not depend strongly on their connection via the resin. If the resin
is cracked, friction will still lock tows together as a macroscopic structure, with very little
relative movement of tows permitted by the 3D nature of the tow arrangement. This is
very different to tape laminates or 2D textiles, where interply delamination can
completely detach plies and lead to catastrophic failure at low loads.
5.1.9 Geometrical Irregularity
Geometrical irregularity is found in all textile composites. Common types were
listed in Section 2.3.3. They include inconsistency in tow spacing, tow waviness, and tow
pinching.
Inconsistent tow spacing causes variance in fiber volume content. If fiber content
falls, stiffness falls proportionately. In most models, the effects of variable tow separation
is simply calculated by adjusting input parameters that describe the textile geometry.
Tow waviness and pinching cannot usually be dealt with in this way. Most codes
are based on the assumption that the textile architecture is ideal, as laid out in the design
specifications of the textile manufacturer. Fortunately, the effect of waviness and
pinching on macroscopic elastic constants is almost entirely restricted to reducing the
effective axial stiffness of tows. A simple and reasonably accurate estimate of this
knockdown was presented in Section 3.3.
However, geometrical irregularity has one other very important implication for
modeling stress distributions in the elastic regime. Because irregularity is by nature
