ANALYTICAL METHODS FOR TEXTILE COMPOSITES
irregularity endemic to textile products. Lockup is the impingement of irregular features on
neighbouring tows as they are dragged by one another during the pullout process. It is
abetted by the through-thickness compression generated by the through-thickness
reinforcement in a 3D composite when it is loaded in in-plane tension. Through-thickness
compression forces sliding tows into close contact. Lockup allows the cohesive traction,
p, to remain near its peak value, p
max
, while displacement discontinuities 2u ~ 1 mm are
developed. The resulting contribution to W
f
is of the order of 1 MJm
-2
in some 3D
interlock weaves. The characteristic length, l
ch
, of the cohesive zone will correspondingly
be ~ 100 mm. Notch sensitivity will be found only for notches exceeding this dimension,
as discussed in Section 3.6. For tape laminates, W
f
~ 100 MJm
-2
and l
ch
~ 3 mm at most.
Figure 4-10. Schematic of a nonlinear damage band growing from a stress
concentrator in a textile composite.
4.6.2 Splitting at a Notch
The mechanics of splitting cracks running parallel to the load axis from notches
have been much studied (e.g. [4.36]). A splitting crack will be energetically favoured when
the toughness, G
s
, of the material through which it must propagate is much lower than the
composite toughness, G
Ic
, for propagation of a mode I crack normal to the applied load. In
a textile composite, G
Ic
is large; e.g., ~ 1 MJm
-2
for 3D angle interlock weaves, as
discussed above. But the arrangement of fibers into discrete tows also lowers G
s
, since it
creates resin rich layers which are easy paths for fracture. The mechanics of splitting cracks
in textile is a subject of current research.
