Page 18 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 1

INTRODUCTION

Fiber composite technology is based on taking advantage of the high strength and
high stiffness of fibers, which are combined with matrix materials of similar/
dissimilar natures in various ways, creating inevitable interfaces. In fiber composites,
both the fiber and the matrix retain their original physical and chemical identities,
yet together they produce a combination of mechanical properties that cannot be
achieved with either of the constituents acting alone, due to the presence of an
interface between these two constituents. The growing number of uses for fiber
reinforced composites in many engineering applications has made the issue of
interfuce (or more properly termed, interphase (Drzal et al., 1983)) a major focus of
interest in the design and manufacture of composite components.
A classic definition of the interjiuce in fiber composites is a surface formed by a
common boundary of reinforcing fiber and matrix that is in contact with and
maintains the bond in between for the transfer of loads. It has physical and
mechanical properties that are unique from those of the fiber or the matrix. In
contrast, the interphase is the geometrical surface of the classic fiber-matrix contact
as well as the region of finite volume extending therefrom, wherein the chemical,
physical and mechanical properties vary either continuously or in a stepwise manner
between those of the bulk fiber and matrix material. In other words, the interphase
exists from some point in the fiber through the actual interface into the matrix,
embracing all the volume altered during the consolidation or fabrication process
from the original fiber and matrix materials. Therefore, the earlier definition of
Metcalfe (1974) for interface can be used for interphase as well: “An interface is the
region of significantly changed chemical composition that constitutes the bond
between the matrix and reinforcement”. Fig. 1.1 schematically illustrates the
concept of the interphase according to Drzal et al. (1983). Also shown in Fig. 1.1 are
the various processing conditions that are imposed on the interphase to allow
chemical reactions to take place and volumetric changes and residual stresses to be
generated. It is the latter definition of interface that is in general use in this book.
However, for analytical purposes in micromechanics the interface is still conve-
niently considered to be infinitely thin and the properties of the mating fiber and
matrix are isotropic and homogeneous.

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