Page 304 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 7. Improvement of transverse fracture toughness with interface control 285

The tensile debonding model associated with the intermittently bonded interface,
schematically shown Fig. 7.1, appears to be rather unrealistic in unidirectional fiber
composites as the stress state near the crack tip should be three-dimensional in
nature (Kim and Mai, 1991a). The model certainly needs further verification as it
requires complicated stress conditions to be satisfied. Nevertheless, there is no doubt
that the longitudinal splitting promoted by the weakened interface increases the
interfaced debonding and subsequent fiber pull-out with large contributions to the
composite fracture toughness. The beneficial effect of the tensile debonding
mechanisms with crack bifurcation may be more clearly realized in the delamination
promoter concept which is discussed in Section 7.4.

7.2.2. Fiber coating for improved energy absorption capability

It has been confirmed in Chapter 6 that for brittle polymer matrix composites,
typically CFWs, a strong interface favors a brittle fracture mode with relatively low
energy absorption, but a weak interface allows high energy absorption through
multiple shear failure (Novak, 1969; Bader et al., 1973). Carbon fibers coated with a
silicone fluid resulted in the fibers being surrounded by an inert film which reduced
the interfacial bond strength with increased toughness (Harris et al., 1971;
Beaumont and Phillips, 1972). The major source of fracture toughness for CFRP
was found to be fiber pull-out following interface debonding (Harris, 1980). It
follows then that a sufficiently high frictional shear stress, zf, is needed while
maintaining the lowest possible shear bond strength, Zb, so that the work required to
pull-out the fibers against friction can be enhanced.
Several different viscous fluids have been investigated as interlayer for several
different combinations of composite constituents. Sung et al. (1977) were the first to
use the concept of strain rate sensitive coatings, e.g. SVF and silicone grease, to
improve the impact toughness of glass fiber polyester matrix composites (GFRPs).
Provided the silicone fluid is Newtonian and the shear stress is uniform, the pull-out
toughness of a composite with short fibers of embedded length, le and pull-out
distance, Epo is given by

where q and t are the viscosity and thickness of the viscous fluid, and vo is the
velocity of fiber pull-out. The fiber pull-out toughness is proportional to the viscous
shear stress acting on the fibers during pull-out at a given strain rate, which could be
maximized by selecting appropriate coatings of high fluid viscosity and small
thickness. Fig. 7.3 shows the inverse relationship between fracture toughness and
coating thickness, with a higher viscosity giving a higher fracture toughness for a
given coating thickness.
Rubbers of various kinds have been among the major coating materials that
received significant interest. The toughness of carbon fiber composites was improved