FAILURE MECHANISMS
around it [4.33]. The notch sensitivity of the composite depends mainly on the strength of
the tows and the work of fracture [4.34]
W
f
= ∫ p du . (4.6)
Very few observations of the mechanisms in cohesive zones have been made in
notched specimens. However, detailed observations during uniaxial tension tests of 3D
interlock weaves [4.22], which were summarized in Sect. 4.3.3, are probably an excellent
guide to what to expect in a notched specimen. The nature of the observed phenomena
themselves also suggests that they are likely to be found in nearly all textile composites
containing significant volume fractions of aligned tows.
Figure 4-10 shows a schematic of a cohesive zone as inferred from the observations
for 3D interlock weaves. At the furthest distances from the notch, the earliest damage in the
band consists of matrix cracking, for example transverse cracks between the orthogonally
disposed tows in an interlock weave. Because this cracking occurs at relatively low loads
and strains, it makes a relatively small contribution to W
f
and therefore to notch effects.
Nearer the notch, the plastic straightening and rupture of aligned tows occurs. As far as
these effects are confined to a band associated with the notch, they may be subsumed in the
relation p(u). (If damage is spread more or less uniformly over the whole composite, this is
large scale yielding and the cohesive zone is no longer an appropriate depiction of events.)
Tow rupture will define the maximum traction, p
max
, supported by the cohesive zone.
Following tow rupture, cohesive tractions will continue to be supported as the broken tows
are pulled out of the composite across the eventual fracture plane. When the tows are fully
pulled out, a traction free crack exists, which occurs first at the notch root.
Tow pullout lengths can be exceptionally large in textile composites. Part of the
reason is the large tow diameters preferred in textiles to minimize manufacturing costs.
When a single tow breaks, debonding from the surrounding composite occurs over a slip
length, l
s
, which scales as the ratio of the tow’s area and circumference, i.e. as its diameter
[4.1]. Pullout lengths also depend on the spatial distribution of flaws in the aligned tows.
since tow strength is probably impaired by tow crimp and pinching, there is scope for
designing a favourable distribution of flaws into the composite by controlling the extent to
which aligned tows are impinged upon by other tows.
After tow failure in 3D interlock weaves, there is an interval where pullout of the
broken tow ends is opposed by unusually strong friction, an effect dubbed lockup (Section
4.3.3; [4.22,4.35]). This peculiarity of textile composites is intimately linked to the
