ANALYTICAL METHODS FOR TEXTILE COMPOSITES
In triaxial braids loaded along the axial tows, microcracking similar to that depicted
in Fig. 4-1 usually occurs first in the bias rovings, where the dominant stress is shear
[4.2]. At higher strains, plastic straightening of the aligned tows may generate further
measurable nonlinearity prior to peak load. Whether plastic tow straightening is significant
depends on the degree of waviness of the aligned tows, which depends strongly on
architecture and processing.
4.3.2 Stitched, Stitched-Woven, and Stitched-Knitted Laminates
The principal systems of microcracks in stitched prepreg, stitched uniweave, and
stitched-knitted laminates are roughly periodic cracks normal to the applied load in the
transverse plies; and shear cracks in off-axis plies. These crack systems are very similar to
those found in tape laminates, although their shapes and spacing are influenced in a
complicated way by the stitches. Ultimate failure accompanies rupture of the aligned plies,
much as in a tape laminate. The stitching minimizes delamination during large strains, but
this has a minor effect on the ultimate strength or strain to failure. Because the in-plane
fibers lie in approximately flat plies, tow straightening is not evident.
4.3.3 3D Weaves
In 3D interlock weaves, the first cracks observed usually run orthogonal to the
applied load between transverse tows [4.1]. Cracks within the transverse tows are
comparatively rare. Other cracks develop along the trajectories of warp weavers (interlock
tows). At loads typically exceeding half the peak load (~500 MPa in typical graphite/epoxy
composites), significant softening occurs because of plastic tow straightening. Although
the stuffers and fillers (the in-plane load bearing tows) are nominally straight in interlock
weaves, they tend in current materials to be substantially more wavy than the majority of
plies in stitched-woven or stitched-knitted laminates. Plastic tow straightening may
contribute 0.1-0.25% to the strain to peak load [4.22].
The strength limiting process in 3D interlock weaves is the rupture of aligned tows.
Tows fail as discrete entities. When one tow fails, the damage does not generally propagate
into neighboring tows. Rather, tow rupture sites are broadly distributed, leading to tow
pullout over 5-10 mm lengths across the ultimate fracture plane [4.1,4.22].
Following the attainment of peak load, a sharp drop occurs in tensile tests, after
which tow pullout sustains much smaller loads (Fig. 4-7). However, the load drop occurs
at remarkable high strains, typically 2.5-4% for loads aligned with the stuffers in
