Page 211 - Engineered Interfaces in Fiber Reinforced Composites

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-1.5%

-2.0%
-2.5%

-3.5%

-4.5%

-6.0%

Fig. 5.16. Photoelastic patterns of carbon fibers in an epoxy matrix (a) without and (b) with surface
treatment, at varying applied strains. After Drzal et 211. (1983a). Reproduced by permission of Gordon
and Breach Science Publishers.

This is attributed to the load increase required to cause the interface failure in
transverse tension due to the Poisson effect under compression. In contrast, the
tensile and compressive moduli are relatively insensitive to changes in the interface
bond strength (Fig. 5.17(b)). However, for very high bond strength, the compressive
modulus shows a marginal increase, due presumably to the presence of a brittle
interphase surrounding the carbon fibers. There is little difference, within experi-
mental scatter, between the flexural strength and bending moduli of the composites
with low and intermediate interface bonding. But for the composites with the
strongest interface bond strength, there is a significant increase in the flexural
strength due to a change in the failure mode. That is, from an interface-initiated
mode to a predominantly tensile (or compressive) mode.
The average values of the off-axis properties for three different composites are
summarized in Table 5.7. It is noted that all strength values measured are sensitive
to the level of interface bonding, while the modulus values are relatively insensitive
to the interface adhesion. In particular, the transverse tensile and flexural strengths
are a good indication of the interfacial adhesion. In case of ILSS, the results
obtained from the Iosipescu shear test method show the least scatter amongst the
three types of mechanical tests carried out. SEM study shows that the failure in both
Iosipescu and short beam shear test specimens are matrix-dominated, while the * 45
tension specimen is relatively insensitive to the variation of failure modes. Although
the transverse tensile and transverse flexural modulus values are similar in terms of
absolute magnitude and insensitivity to the interface adhesion level, there are
significant differences between the transverse tensile and transverse flexural
strengths. The transverse flexural strength is more sensitive to interface bonding,
and is much higher than the transverse tensile strength. This observation may be
explained in terms of the non-uniform stress distribution across the thickness of
the specimen in three-point bending. In contrast, in transverse tension, the whole