ANALYTICAL METHODS FOR TEXTILE COMPOSITES
4.5 Shear Delamination in Bending
When shear stresses are created by bending near a free edge or notch, delamination
crack growth can ensue under pure Mode II or mixed mode conditions. In a 2D laminate,
the crack growth is catastrophic for common loading configurations, with the critical stress
falling with the length of the initial delamination length.
Through-thickness reinforcement changes the mechanics of Mode II delamination in
a similar way to its effect on Mode I delamination, bridging the crack and imposing
shielding shear tractions on the fracture surfaces. The resulting fracture behaviour has been
well illustrated by models and tests of stitched laminates, especially using the end-notch
flexure test configuration [4.30-4.32]. Provided the stitches remain intact, the critical
applied load,
τ
d
, for delamination crack propagation approaches a steady-state value,
τ
ss
,
which is independent of the delamination crack length for long enough cracks. Like the
critical stress,
σ
1
, for bridged Mode I delaminations (Sect. 4.4),
τ
ss
is a material property
of the composite. Since all delaminations must remain relatively short for smaller values of
the applied shear stress,
τ
ss
can be used as a design limit. However, the stitching must also
be strong enough to remain intact. If it fails, catastrophic delamination crack growth will
resume, but the Mode II delamination toughness will still be enhanced by the work required
to stretch and break the stitches [4.30,4.32].
4.6 Notch Sensitivity
Textile composites are generally remarkably notch insensitive. In both tension and
compression, cohesive or nonlinear process zones form at the notch and redistribute loads
in such a way as to reduce the stress concentration factor and minimize the degradation of
ultimate strength. Stress concentration factors can also be limited by splitting cracks
running from the notch root parallel to the load axis, which isolate the material ahead of the
notch from the notch stress fields. Both of these mechanisms are familiar in tape laminates,
but they can both be much more effective in textile composites.
4.6.1 Cohesive Zones
The effectiveness of a cohesive zone in redistributing loads around a notch is
determined by mechanisms that permit significant local displacements without complete
loss of strength. The cohesive zone is characterized by the relation between the tractions, p,
it supports and the displacement discontinuity, 2u, it introduces into the elastic material
