Page 356 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interlaminar fracture toughness with interface control 337

Table 8.2
Interlaminar fracture toughness of various composite materials"

Composite system Neat resin Mode 1 Mode I1 GclCl9
(fiberimatrix) GK (kJ/m*) 4 (kJ/m2) (kJ/mZ)

AS4/350 1-6 0.07 1.15 8.0 7.0
AS4/Dow P4 0.08 0.8 5.0 6.3
AS4/Dow P6 (Novolac) 0. I5 1.75 10.9 6.2
AS413502
T6T145/F155NR 0.07 0.57 3 5.3
T6T145/F155 0.167 1.66 5 3.0
T6T145/F185NR 0.73 1.5 2.1 1.8
T300/BP907 0.34 1.05 2.3 2.2
0.325
T6T 145iF I 85
C6000/HX206 6.4 2.2 1.1
C6000/HX2 10 2.2 - -
AS4W6/Lexan 2.8 - -
T6T145IIIX205 8.1 1.7 1.06
0.34 - -
"After Bradley (1 989).
3501-6, 3502, F15SNR. F185NR. unmodified epoxies; F155, F185, rubber-modified epoxies; Lexari,
polycarbonate.

The effectiveness of the modified matrix on interlaminar fracture toughness is
strongly dependent on the fiber-matrix interfacial properties, such as the bond shear
strength Zb, or the interface fracture toughness. It is important to reiterate that the
full utilization of the intrinsic toughness of modified resins require a sufficiently
strong bond at the fiber-matrix interface so that the resin can be strained to failure
before the interface fails (Hibbs et al., 1987; Bradley, 1989a, b). This view is rather
different from the beneficial effect of interfacial debonding in brittle matrix
composites which may promote fiber bridging of fracture surfaces and thus
contribute to the total interlaminar fracture toughness (Hunston et al., 1987). Even
in simple compressive or shear loading conditions, the interfacial properties play a
decisive role in determining the failure mode during fracture of modified matrix
composites (Drzal and Madhukar, 1991), and there is a significant mutual
correlation between the properties of the interface and the matrix material (Drzal,
1990). In this regard, the issue of the interface should always be taken into account
in the study of matrix modifications.
The local fiber volume fraction, vf, plays an important role in determining the
delamination resistance. The fiber vf and the distribution across the composite
thickness determine the effective thickness of the resin rich region along the crack
path, which in turn influences significantly the development of crack tip deformation
zone (Kim et al., 1992). Strain energy density (c&,/E,, a,, being the yield strength
of the matrix material) and residual stress arising from the matrix shrinkage are also