Page 110 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 4

MICROMECHANICS OF STRESS TRANSFER
ACROSS THE INTERFACE

4.1. Introduction

One of the most important phenomena in fiber composite technology for
applications to load bearing primary structures is the stress transfer between the
fiber and matrix across the interface when the composites are subjected to various
loading conditions (Kim and Mai, 1991a, b, 1993, Kim et al., 1994~ Zhou et al.,
1995b). In the past several decades, a significant effort has been put into
understanding the stress transfer in various forms of microcomposite tests as a
means of evaluating the bond quality at the fiber-matrix interface region. This
endeavor has been prompted by the rapid development of technologically important
fibers and matrix materials and the corresponding new fiber surface treatment
techniques of various natures which must not only be compatible with the composite
fabrication processes, but also function properly in adverse service environments.
From the stress-transfer mechanics viewpoint, theoretical analyses dealing with
the stress state at the interface region are vital to understanding how and to what
extent the interface properties influence the mechanical performance and fracture
behavior of the composites. Since the early pioneering work by Cox (1952) and
Rosen (1964), a number of models have been developed to predict the response of
composite materials in terms of thermo-mechanical properties and microfailure
mechanisms under various loading conditions and different environmental situa-
tions. These range from simplified physical models such as the Kelly-Tyson model
(1965) to numerical solutions of stress and strain fields in the composite constituents
based on rigorous finite element (FE) analyses. The fiber fragmentation test has a
significant analogy with practical composites containing aligned short fibers when
subjected to uniaxial tension along the fiber direction as it exhibits the fundamental
damage modes that are present in the multiple fiber composites in service.
Apart from the elastic stress transfer at the perfectly bonded interface, another
important phenomenon that must be taken into account is the stress transfer by
friction, which is governed by the Coulomb friction law after the interface bond
fails. Furthermore, matrix yielding often takes place at the interface region in
preference to interfacial debonding if the matrix shear yield strength, z, is
significantly smaller than the apparent interface bond strength, Zb. It follows thus

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