Page 315 - Engineered Interfaces in Fiber Reinforced Composites

296 Engineered interfaces in $fiber reinforced composites

distinct from those of the bulk fiber and matrix materials. In a broad sense, the
interphase can also include interlayers of various nature and thickness that are
formed between the fiber and matrix as a result of the application of coating
materials on the fiber before being incorporated into the matrix. Apart from the
polymeric coatings that are applied to improve the fracture toughness of brittle
polymer matrix composites as discussed in the foregoing section, coatings of
different materials are also used extensively in MMCs and CMCs for various other
purposes. In particular, compatibility of the coating material with the composite
constituents during the manufacturing processes and in service conditions is the
most important for MMCs and CMCs. The coating should also prevent deterio-
ration of fiber strength and stiffness and enhance the fiber-matrix wettability and
adhesion. In this section, a review is given of theoretical advances on the roles of the
interphase/interlayers and the effects of various parameters on the mechanical
performance of fiber composites containing such an interphase/interlayer.
Previous studies of the interphaselinterlayer have mainly focused on the
coefficient of thermal expansion (CTE) and residual thermal stresses. The impor-
tance of residual thermal stresses cannot be overemphasized in composites
technology because the combination of dissimilar materials in a composite creates
inevitably an interphase across which residual stresses are generated during
fabrication and in service due to the difference in thermo-mechanical characteristics.
The importance of an interlayer is clearly realized through its effects in altering the
residual stress fields within the composite constituents.
7.3.1. Theoretical studies of interphase

Many publications have appeared in the literature, which analyze the effects of
interphase/interlayers on stress distribution, in particular those arising from
differential shrinkage between fiber and matrix. Also specifically studied are the
overall thermo-mechanical properties of the composites, including Young’s mod-
ulus, CTE and strength under various loading conditions. The idea behind these
interphase/interlayer models is ultimately to provide practical guidance for
controlling the local failure mode, and thus for the optimum design of the
interphase/interlayer. Jayaraman et al. (1993) and Jayaraman and Reifsnider (1993)
have recently given a comprehensive review on theoretical analyses of composites
containing an interphaselinterlayer.
The thermo-mechanical properties of the interlayer can be assumed to be either
uniform or non-uniform. The properties of the non-uniform interphase can vary
continuously or in a step-wise manner across the thickness between the bulk fiber
and the matrix material. For varying interphase/interlayer properties, several
different models have been proposed. The longitudinal shear modulus of the
interphase was expressed by an exponential law (Van Fo Fy, 1967) based on a
hexagonal fiber arrangement. The representative longitudinal modulus of the
interphase was also proposed following the relationship involving heat capacity
jump and volume fraction of the fiber in a calorimetric analysis for unidirectional
glass reinforced epoxy matrix composites (Theocaris, 1984). Reciprocal and cubic