Page 298 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 7

IMPROVEMENT OF TRANSVERSE FRACTURE
TOUGHNESS WITH INTERFACE CONTROL

7.1. Introduction

In view of the interface-related fracture toughness theories presented in Chapter 6,
it is seen that the transverse fracture toughness of composites containing brittle
fibers and brittle matrix, typically CFRPs and GFRPs, increases with increasing vf
and cr; of the fiber, but decreases with increasing interfacial bond strength, Zb. High
Zb discourages interfacial debonding and subsequent fiber pull-out. Along with the
techniques specifically designed to improve the delamination resistance discussed in
Chapter 8, significant research efforts have been directed towards the development
of techniques to improve the fracture toughness of brittle fiber-brittle matrix
composites in the transverse direction without impairing other important mechan-
ical properties. These techniques can be classified into two major approaches: one
relies on the improvement of the intrinsic properties of the composite constituents,
whether the reinforcement or the matrix phase; and the other depends on the control
of interface and/or interlaminar properties. Examples of the first approach include
the use of fiber hybrids, tough matrices and large diameter or bundle fibers. The
second approach includes fiber coatings with appropriate polymers, delamination
promoters and reduction of shrinkage stresses in the matrix through the modifi-
cation of interface properties. Comprehensive reviews have been given on these
toughening methods by Kim and Mai (1991a, 1993a), and they are briefly described
in the following.
(1) Different types of fibers can be incorporated into a matrix material to produce
a hybrid fiber composite. Typical hybrid fiber composites are made from glass or
aramid fibers that are added to otherwise brittle carbon fiber composites to enhance
the fracture toughness resulting from the toughening mechanisms associated with
the ductile fibers, while maintaining a high strength and high modulus gained from
carbon fibers. The effect of toughening relies heavily on how the hybrid fibers are
mixed and the ply layup is arranged. A review on this topic has been presented by
Hancox (198 1).
(2) Tough matrices, such as thermoplastics and rubber-modified epoxies, are
particularly useful for high fracture toughness and damage tolerance against

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