ANALYTICAL METHODS FOR TEXTILE COMPOSITES
unidirectional tapes reduced according to the volume fractions of stuffers or fillers, it is
characteristically found that tow strengths are knocked down by about 30% [3.11-3.13].
3.5 Out-of-Plane Strength; Delamination and Impact Resistance
The great weakness of tape laminates is their vulnerability to delamination.
Delamination failure can be caused in undamaged laminates by excessive through-thickness
loads, which arise whenever the laminate is attached to some other structural member, such
as a stiffener, near holes or edges, and under in-plane loads if the laminate forms a curved
part. Once a laminate is damaged by impact and contains even a limited delamination crack,
the critical through-thickness stress for failure drops dramatically, since the composite
relies on the matrix toughness alone to resist delamination crack growth. A delamination
crack of length, a, grows essentially as a Griffith crack, with the critical through-thickness
stress falling as a
-1/2
to arbitrarily small values. Under in-plane loads, a partially
delaminated tape laminate will still fail all too readily. It has poor resistance to buckling of
the plies in delaminated areas, which leads to unstable delamination crack growth and
failure. The critical buckling stress again falls as a
-1/2
.
Through-thickness reinforcement in a quasi-laminar textile composite changes the
picture entirely. Even if delamination flaws exist in advance, through-thickness
reinforcement will arrest their growth as long as the stress remains above some critical
value,
σ
1
, which is a material constant for the composite and independent of the
delamination crack length, a. By proper design, delamination crack growth driven by
through-thickness loads in curved parts and buckling of delaminated layers under in-plane
compression can thus be eliminated. Some bounding formulae for the minimum required
volume fractions of through-thickness reinforcement are provided in Sect. 4. The required
volume fractions turn out to be at most a few percent for most applications [3.13-3.15].
3.6 Work of Fracture and Notch Sensitivity
Many textile composites, especially those with 3D reinforcement but also some with
2D reinforcement, have very high values of work of fracture and are exceptionally notch
insensitive. Work of fracture values exceeding 1 MJ/m
2
have been reported for 3D
interlock weaves [3.16], which are an order of magnitude higher than values for tape
laminates or high toughness alloys. High work of fracture generally translates into notch
