Page 15 - 3D Fibre Reinforced Polymer Composites

4 30 Fibre Reinforced Polymer Composites

1400 r - In-Plane Property

1200 L---l Through-Thickness Property
m
a
z 1000

620 MPa

+
Carbo n/Epoxy E-glass/Epoxy Kevlar/Epoxy
(c>
Figure 1.2 (continued) Comparison of in-plane and through-thickness mechanical
properties of some engineering composites.

A further problem with 2D laminates is their poor impact damage resistance and low
post-impact mechanical properties. Laminates are prone to delamination damage when
impacted by low speed projectiles because of the poor through-thickness strength. This
is a major concern with composite aircraft structures where tools dropped during
maintenance, bird strikes, hail impacts and stone impacts can cause damage. Similarly,
the composite hulls to yachts, boats and ships can be damaged by impact with debris
floating in the water or striking a wharf while moored in heavy seas. This damage can
be difficult to detect, particularly when below the waterline, and can affect water-
tightness and structural integrity of the hull. Impact damage can seriously degrade the
in-plane mechanical properties under tension, compression, bending and fatigue loads.
For example, the effect of impact loading on the tension and compression strengths of
an aerospace grade carbodepoxy laminate is shown in Figure 1.3. The strength drops
rapidly with increasing impact energy, and even a lightweight impact of several joules
can cause a large loss in strength. The low post-impact mechanical properties of 2D
laminates is a major disadvantage, particularly when used in thin load-bearing structures
such as aircraft fuselage and wing panels where the mechanical properties can be
severely degraded by a small amount of damage. To combat the problem of
delamination damage, composite parts are often over-designed with extra thickness.
However, this increases the cost, weight and volume of the composite and in some cases
may provide only moderate improvements to impact damage resistance.
Various materials have been developed to improve the delamination resistance and
post-impact mechanical properties of 2D laminates. The main impact toughening
methods are chemical and rubber toughening of resins, chemical and plasma treatment
of fibres, and interleaving using tough thermoplastic film. These methods are effective
in improving damage resistance against low energy impacts, although each has a
number of drawbacks that has limited their use in large composite structures. The