The complexity of the fiber-matrix interphase in a composite is largely due to the
myriad of variables (material, processing, and design) that affect its formation. The interphase,
thus formed, has to be characterized at several levels (micro-structural, chemical, and
mechanical) in order for one to fully understand the nature of the bond between the fiber and
matrix and in order to perform a stress analysis of the fiber-interphase-matrix assemblage.
A thorough thermo-mechanical characterization of the interphase is difficult, at
present, due to the necessity of studying the interphase in situ, its small dimension (usually
on the order of a micrometer), and its general complexity. However, a cursory glance at the
literature shows that great progress has been made in all of the three levels of characterization
mentioned above for various composite systems. Several recent attempts have focused
on the physical characterization (evaluation of volume fraction, thickness, Young's modulus,
shear modulus and coefficient of thermal expansion) of the interphase.
Models of physical properties (thickness, Young's modulus, Poisson's ratio and
coefficient of thermal expansion) of the interphase have been considered by several researchers
in an effort to study the influence of the interphase on overall composite properties
and behavior. Hypotheses on interphase formation and properties have been proposed and
tested by some researchers. Both experimental characterization as well as modeling studies
are necessary to achieve a more profound understanding of the interphase and its behavior.
The interphase, in a composite, is usually modeled as a homogeneous region,
despite the fact that it may have spatial property variations.
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