Page 348 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8

IMPROVEMENT OF INTERLAMINAR FRACTURE
TOUGHNESS WITH INTERFACE CONTROL

8.1. Introduction

The superior specific modulus and specific strength along with other unique
properties as well as manufacturing advantages and design freedom offered by fine
diameter fibers have made polymer matrix composites ideally suited to weight
critical applications. Composites are produced normally in the form of layers or
laminates which are extremely susceptible to crack initiation and growth preferen-
tially along the laminar interfaces in various failure modes (Kim and Mai, 1991). As
reiterated in Section 3.4, delamination is the most prevalent life-limiting crack
growth mode in laminate composites. When subjected to complex three-dimensional
load paths, delamination may cause severe reduction in in-plane modulus and
strength which can possibly lead to catastrophic failure of the whole structure. It
has been shown that delamination may be introduced due to the external loading,
whether in static tension and bending, in cyclic fatigue or by low-velocity and
low-energy impact, during manufacturing and in service. Potential delamination
sites are locations with discontinuities in the load path (see Fig. 3.28). These
discontinuities give rise to interlaminar stresses even under in-plane loading
(Wilkins, 1983).
Much attention has been directed toward fundamental understanding of the root
causes and the corresponding failure mechanisms of delamination which are specific
to different combinations of fiber and matrix materials and interface characteristics.
A number of experimental techniques have also been developed to characterize the
interlaminar fracture toughness of various modes (see Section 3.4). Many techniques
have been devised to combat the problem of delamination. These techniques involve
either material improvement or fiber architecture modification. Material improve-
ment requires the enhancement of fracture toughness of matrix material and fiber-
matrix interface bonding because delamination initiates and propagates preferen-
tially in the matrix material and interface region. The use of tough matrix materials,
typically rubber-toughened epoxies and high performance engineering thermoplas-
tics have been studied extensively. The interleaving technique is also shown to bc
very promising where soft, tough strips of adhesive or composite are interleaved

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