Page 302 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 7. Improvement of transverse fracture toughness with interface control 283

5.29) allows non-uniform application of coatings on the fiber with a resulting
pattern of intermittently coated and uncoated regions. This condition provides good
bonding in the low spots, while high points are shielded from the matrix by the
coating, allowing extensive interface crack propagation. Graphite and polytetra-
fluoroethylene (PTFE) coatings on boron fibers and a viscous polyurethane coating
on carbon fibers were shown to be effective in avoiding fiber-initiated catastrophic
failure of the composites.
The intermittent bonding concept was further developed by Marston et al. (1974)
and Atkins (1 975). The argument is that as far as there are enough regions of strong
interface, ensuring that the rule of mixtures (RoM) composite strength is retained,
the rest of the composite could have quite a weak interface which may serve to blunt
the running crack by the tensile debonding mechanism (Cook and Gordon, 1964). It
follows then that if a composite is laid up randomly with respect to weak and strong
interface regions, as schematically shown in Fig. 7.1, both high strength and high
toughness should be simultaneously obtained. A remarkable 400% improvement in
fracture toughness was achieved with a 10% loss of tensile strength in the
unidirectional boron fiber-epoxy resin (BFRP) system with an 80% coating of
polyurethane varnish (PUV) (Atkins, 1974, 1975). In contrast, the improvement
with silicone vacuum ffuid (SVF) coating for the same composite system was only
10-15%, even though the interface shear bond strengths, Zb, of the fibers coated
with the two coating materials were similar. This indicates that similar Tb does not
necessarily mean similar fracture toughness. The explanation lies probably with the
tensile debonding mechanism which might have taken place with the PUV coating

T

- High strength
region

- Low strength
region

1

Fig. 7.1. Fibers containing deliberately produced regions of low interfacial strength are shown in the
staggered manner. Aftcr Atkins (1975).