ANALYTICAL METHODS FOR TEXTILE COMPOSITES
Figure 4-3. Micrograph of a typical kink band in an aligned tow in an AS4/1895 3D
interlock weave (from [4.1]).
Delamination has been the most serious weakness of conventional tape laminates,
especially around edges or in association with impact damage. When a stress singularity at
an edge or an impact event causes limited delamination, failure can ensue via Euler buckling
of the delaminated plies under in-plane compression; or by delamination crack growth
under shear induced by bending. In the absence of through-thickness reinforcement, the
critical load for buckling and delamination crack growth under in-plane compression falls in
inverse proportion to the size of the delamination to arbitrarily small values; as does the
critical load for delamination crack growth in bending. In either case, resistance to
delamination crack growth depends entirely on the fracture resistance of the matrix. Well
established, low-cost polymer matrices are generally not very tough. Even toughened
polymers and thermoplastics, which are usually relatively expensive, provide only modest
increases in toughness.
In quasi-laminar textile composites such as stitched laminates or 3D interlock
weaves, delamination resistance is greatly enhanced by through-thickness reinforcement.
The through-thickness reinforcement does not entirely eliminate delamination, especially
during impact, but by suppressing buckling under subsequent compressive loads, it
eliminates the driving force for delamination crack growth under in-plane compression; and
