Page 285 - Engineered Interfaces in Fiber Reinforced Composites

266 Engineered interfaces in fiber reinforced composites

Table 6.2
Maximum allowable interface strength for interface delamination"

Composite system (fiber/ a Required interface Calculated transverse
matrix or coating) strength, a; (MPa) strength, 0; (MPa)b

P-55/AI 0.001 475 197
P-lOO/AI 0.153 726 300
P-55jSiC (HBE) -0.605 209 86
P-55jSiC (LBE) 0.574 836 350
P-IOOjSiC (HBE) -0.499 330 I36
P-lOO/SiC (LBE) 0.668 946 390
A1203/SiC (LBE) 0.904 788 326
Nicalon SiC/Al 0.468 1148 470
A1203/TiC -0.085 508 210

"After Gupta et al. (1993).
bBased on transverse stress concentration factor of 2.40.
HBE: high ion beam energy; LEB: low ion beam energy.

from Fig. 6.21 for the corresponding values of CL and B. Table 6.2 presents such
predictions for various combinations of fiber-ceramic matrix (or coating) systems. A
practical implication of Fig. 6.21 is that the level of interface bond strength required
to satisfy the longitudinal splitting can be enhanced by choosing appropriate
combinations of fiber and matrix (or coating) materials, and thereby allowing the
composite to sustain a higher external stress without causing catastrophic failure.

6.4.2.3. Length of longitudinal splitting
In the study of the effect of plasticity and crack blunting on longitudinal and
transverse stress distributions in orthotropic composites materials, Tirosh (1 973)
analyzed the longitudinal splitting problem for uniaxially oriented, continuous fiber
composites with a transverse single edge notch (SEN). For large scale plasticity
where the length of splitting, L,, is comparable to the characteristic dimension of the
specimen which is loaded in axial tension, the J-integral is given by

(6.34)

where zy is the shear yield stress of the fiber-matrix interface, and GLT is the in-plane
shear modulus of the composite. The split length, L,, is obtained by equating the J-
integral to the solution for the crack extension force derived earlier by Sih and
Liebowitz (1968). It is seen that the J-integral in Eq. (6.34) is analogous to the
interface toughness given by Eq. (6.1) or Eq. (6.2) which is obtained from LEFM.
The J-integral can be related to the uniform normal stress, on, acting on the notch
surface. Therefore, the splitting length, L,, is

(6.35)