Page 68 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 3. Measurements of interfacelinterlaminar properties 51

P
U
3
m
- IO2
.-
0-
4-
2
-4-
U
W
n
v)
m
- IO’
.- 3
+
.-
L
u
1 IO’ 102 104
E, /Em

Fig. 3.5. Dependence of fiber critical aspect ratio, (2L),/d, on the Young’s modulus ratio of fiber to
matrix material, EfIE,,,. (0) Experimental data from Asloun et al. (1989); (-) Termonia (1993); (---)
Cox (1952).

also complicate the interpretation of test results. For example, extensive splitting of
highly oriented organic fibers, such as Kevlar and PBT (Morgan and Allred, 1993),
into small fibrils on the fiber surface makes the test results doubtful (Kalanta and
Drzal, 1990; Scherf et al., 1992). The fiber straightening pretension applied during
specimen preparation is also found to influence the fragmentation behavior, causing
significant data scatter unless carefully controlled (Ikuta et al., 1991; Scherf and
Wagner, 1992). Another important drawback of this test is that the matrix must
possess sufficient tensile strain and fracture toughness to avoid premature failure of
the specimen, which is induced by fiber breaks, as mentioned earlier. A technique
has been devised to circumvent this problem in that a thick layer of the brittle matrix
material is coated onto the fiber, which is subsequently embedded in a ductile resin
(Favre and Jacques, 1990).

3.2.4. Fiber pull-out test

In the fiber pull-out test, a fiber(s) is partially embedded in a matrix block or thin
disc of various shapes and sizes as shown in Fig 3.6. When the fiber is loaded under
tension while the matrix block is gripped, the external force applied to the fiber is
recorded as a function of time or fiber end displacement during the whole debond
and pull-out process. There are characteristic fiber stresses that can be obtained
from the typical force (or fiber stress). The displacement curve of the fiber pull-out