The Hybrid Effects
One phenomenon associated with the interaction
in a blended structure that greatly complicates
analysis is the so-called “hybrid effect,” which is
defined in Marom et al.48 as the deviation of behavior
of a hybrid structure from the Rule of
Mixtures. A positive hybrid effect means that the
property is above the prediction given by the Rule
of Mixtures, whereas a negative hybrid effect
means the property is below the prediction. There
is a second definition for the hybrid effect as the
difference between the performance of a fiber in a
hybrid structure and in a nonhybrid structure.
Again the hybrid effect can be positive
or negative depending on whether the property in
the hybrid system is greater or smaller than that
in the nonhybrid system.
In previous articles by Pan and Postle,three physical mechanisms have been identified
and demonstrated to be responsible for causing
the hybrid effects in a hybrid structure. The first
(Mechanism I) is the protection or enhancement
afforded by the system to the fibers through the
so-called fragmentation process during structure
extension. This enhancement effect is largely dependent
on the along-fiber property variation
(variation along a fiber length) and on the fiber–
fiber gripping. Because of this enhancement, the
(in situ) behavior of the fibers in a structure will
be different from the original (ex situ) fiber behavior
determined before the fibers are embedded
into the system. The second mechanism (Mechanism
II) is related to the between-fiber property
variation, i.e., variation of breaking properties
between fibers of the same type. Because of this
variation, fibers of the same type in the structure
will break gradually according to the statistical
distribution of their breaking strain, eventually
reducing the values of strength and breaking
elongation of the structure because of the fiber–
fiber interaction of the same type. Because the
along-fiber property variation and the betweenfiber
property variation exist in both hybrid and
nonhybrid systems, Mechanisms I and II are effective
in a yarn of single-fiber type as well.
The third (Mechanism III) is the cross coupling
effects between the different fiber types in thestructure; this mechanism was found52 in general
to enhance the fiber in situ properties of the
lower-breaking-strain fiber type but depresses
those of the higher-breaking-strain fiber type,
leading to a positive hybrid effect associated with
the first peak and a negative hybrid effect associated
with the second peak on the stress–strain
curve of a blended structure, when compared with
nonblended structures.