Page 352 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interlaminar fracture toughness with interface control 333

the crack initiation. Hunston et al. (1987) noted that the composite qc is actually
greater than the resin for values below about 0.5 kJ/m2. Further indication
from Fig. 8.2 is that as the resin increases above this value, the incremental
increase in the composite qc is much smaller than the resin G;E, and there may be
little gain in the composite cfc for increase in the resin qc above 2.0 kJ/m2. The fact
that the composite cfc is higher than the resin G;", with brittle resins suggests the full
development of an intrinsic small plastic deformation zone is possible so that full
transfer of the resin G;", to the composite can be achieved. In addition, the failure
mechanisms, such as interface debonding and fiber bridging, can also contribute
significantly to the total energy dissipation in these composites when the matrix
materials are brittle (Hunston et al., 1987). For tougher matrices with qc greater
than 0.5 kJ/m2, the high resin q is only partly transferred to the composites.
Many investigators have attempted to clarify the relationship between the resin
qc and the composite G;c with varying degree of success. An established
explanation is that for tough resins the poor translation of G;E into Gfc is mainly
due to the suppression of the toughening effect in a thin epoxy film between
reinforcing fibers which act as rigid fillers and constrain plastic deformation and/or
microcracking at the crack tip (Bascom and Cottington, 1976). This has been
confirmed by the strong bond-line thickness dependence of G;E in adhesive joints
(Scott and Phillips, 1975; Kinloch and Shaw, 1981). Recent work using large
deformation finite element analysis carried out by Daghyani et al. (1995a, b, 1996)
on rubber-modified epoxies adhesively bonded between two aluminum or carbon
fiber composite adherends has confirmed that the adherends impart constraints that
have prevented the full toughness of the modified adhesive to be transferred to the
joints. The size (or volume) of the crack tip deformation zone can be treated as the
ability of the resin to suppress the onset of unstable and rapid crack propagation,
which in turn determines the amount of energy dissipated prior to fracture.
Other important parameters for the correlation between and qc include the
ductility or the failure strain, particularly the non-linear strain (Jordan and Bradley,
1988; Jordan et al., 1989) of the matrix resin, the bond strength of the fiber-matrix
interface (Jordan and Bradley, 1987; Bradley 1989a, b), and the fiber V, and their
distributions in the composites (Hunston et al., 1987). A high failure strain promotes
the intrinsic capacity of the resin to permit shear deformation, and is shown to
increase the and qc values almost linearly, the rate of increase being steeper for
G;", than for Gic.
To understand better the relationship between the neat resin fracture toughness
and the composite interlaminar fracture toughness, in-situ observations have been
made specifically on the crack tip damage zone in interlaminar fracture of carbon
fiber composites using scanning electron microscopy (Chakachery and Bradley,
1987; Hibbs et al., 1987). Comparisons between composites containing two different
types of epoxy resins with and without rubber modifications, namely T6T145/
F155NR (q = 167J/m2; qc = 335 J/m2) and T6Tl45/F185 (q = 6400J/m2;
qc = 2000 J/m2), have been carried out and their major difference in fracture
behavior identified in Fig. 8.3. For the unmodified epoxy matrix T6T145/F155NR
system, crack extension occurs by void formation in the matrix and interfacial