Page 210 - Engineered Interfaces in Fiber Reinforced Composites

192 Engineered interfaces in fiber reinforced composites

3.0

120
-

101-
7
- 2.0 .E
U 80 >5
m
a L
r Q
t
Q
.E 60 L
In 0
In - 1.0 6
=! 40 0 Y
.$-8- -8-

0 1 2 3
Treatment time in min
Fig. 5.15. Effect of carbon fiber surface treatment level on ILSS (0) and impact energy (0) for a carbon
fiber-epoxy matrix composite. After Goan et al. (1973).

relationship between the interface bond strength and various mechanical properties
of carbon fiber-epoxy matrix composites. A summary is given in the following for
the composite containing AS4 PAN-based carbon fibers (Hercules, Inc.) with three
different surface conditions. These are designated as AU4, AS4 and AS4C, that
stand for ‘as received’ without any surface treatment, ‘surface-treated’ with an
optimal electrochemical oxidation procedure, and ‘surface treated and coated’ with
a 100-200 pm thick layer of epoxy. Fig. 5.16 shows a series of photoelastic stress
patterns with increasing strain obtained in fiber fragmentation tests of AU and AS
fibers embedded in an epoxy matrix. Examination of the different stress patterns has
revealed that the AU fiber interacts with the matrix only through weak frictional
force, whereas a relatively high stress is built up along the whole AS fiber. The
interfacial bond strengths, TI,, calculated based on the critical transfer lengths are
37.2, 68.3 and 81.4 MPa for the composites with AU4, AS4 and AS4C fibers,
respectively.
The longitudinal tensile strength of the composite is found to increase with
interfacial bond shear strength, Fig. 5.17(a), in particular when the failure process is
dominated by the interface. When the interface bond strength is very high, the
failure location changes from the interface to the surrounding matrix and the
composite becomes brittle. With increasing interface bond strength, the compressive
strength is also shown to be enhanced to a greater extent than the tensile strength.