ANALYTICAL METHODS FOR TEXTILE COMPOSITES
independent directions.
1
Thus a 2D composite has distinct layers, which may be separated
without breaking fibers. Of course, yarns in textile composites that are defined to be 2D
may still follow path segments with components in the through-thickness direction, as in a
laminated plain weave; but surfaces may be defined through which no tows pass and which
separate the composite into layers. This cannot be done for a 3D composite.
2.1.1.2 Quasi-Laminar and Nonlaminar Textiles
In modeling their macroscopic properties, all 2D and many 3D textile composites
can be considered to function as laminates, with relatively minor allowance for their textile
nature, even though the routes to their manufacture are very different from conventional
tape lay-up. Most textile composites designed for skin or sheet applications fall into this
category. When high in-plane stiffness and strength are demanded, the majority of fibers
must lie in-plane; relatively few can be dedicated to through-thickness reinforcement
without unacceptable loss of in-plane properties. And indeed for most sheet applications,
damage tolerance and delamination resistance require modest volume fractions of through-
thickness fibers (Sect. 4; [2.1,2.2]). Textile composites that behave in most ways like
laminates will be called "quasi-laminar."
In structures where substantial triaxial stresses exist, the optimal reinforcement will
no longer be a laminate with moderate through-thickness reinforcement. Instead, fibers will
be arranged by some textile process with roughly equal load bearing capacity along all three
axes of a Cartesian system. Such textile composites will be called "nonlaminar."
Nonlaminar textiles are often manufactured to respond to complex part geometry
and triaxial loads, for example the union of a skin and stiffening element or a short beam
with approximately equiaxed cross-section. But even a curved plate designed as a laminate
with through thickness reinforcement must be considered nonlaminar if its curvature is
sufficiently high. As the curvature increases, greater through-thickness tension is generated
by applied bending moments, and so much through-thickness reinforcement is needed to
suppress delamination that the part loses its laminar character (Sect. 4, [2.3]).
Figure 2-2 shows how the main classes of textiles may be categorized as quasi-
laminar or nonlaminar. Occasionally, experience with damage modes obliges categorization
1
Direction vectors are linearly independent if none of them can be expressed as a simple combination of the
others. Thus three in-plane directions cannot be linearly independent.
