Page 125 - Engineered Interfaces in Fiber Reinforced Composites

IO8 Engineered interfaces in fiber reinforced composites
11
P = 0.5
-
0.8
Y
=.e .
0.6
5
P
u 0.4 -
2
0
P 0.2 .
E
n-
v
0 0.1 0.2 0.3 0.4 0.5 0.6
Applied stress fJa (GPa)

Fig. 4.8. Variations of debond length, e, as a function of applied stress, ua, for different coefficients of
friction, p, for a XAIOO carbon fiber-epoxy matrix composite. After Zhou et al. (1995a, b).

r 1
(4.48)

for the fully bonded interface, and

(4.49)

for the partially debonded interface. The mean fiber fragment lengths, (2L), are
compared between the theoretical predictions and experimental results for the
carbon fiber-epoxy matrix composites with two different levels of fiber surface
treatments as shown in Fig. 4.9. It is noted that when the applied stress, oa, is greater
than a critical value that corresponds to the initial debonding, the mean fiber
fragment length (2L) consists of two components: namely the bond length and the
debond length. As aa is increased, the debond length gradually increases towards an
asymptotic value, whereas the bond length drops dramatically to a plateau value
within a narrow range of applied stress. Therefore, the contribution of the debond
length to the mean fiber fragment length becomes increasingly more important with
increasing a,.
For both composites good agreement is obtained between theory and experiment
over the whole range of ca. Two major differences can be identified between the
composites of two different fiber surface treatments: shorter mean fiber fragment
length and shorter debond length at a given aa for the XA 100 fibers (Fig. 4.9(b))
than for the XA 1 fibers (Fig. 4.9(a)). This implies that a higher level of surface