Page 349 - Engineered Interfaces in Fiber Reinforced Composites

330 Engineered interfaces in jiber reinforced composites

selectively at delamination prone laminar interfaces, particularly at or near free
edges. Nevertheless, with an ever-increasing need for large allowable design strains
and improved reliability of aerospace structural parts, material improvement alone
has proven to be insufficient. Therefore, modification of fiber architecture using
through-the-thickness reinforcements has been considered as an alternative solution
to the improvement of interlaminar and intralaminar mechanical properties.
In this chapter, the underlying physics and the efficiency of these modifying
techniques are critically examined, which have been developed specifically for
enhanced interlaminar fracture resistance and damage tolerance of fiber composites.
Particular emphasis is placed on the discussion of the advantages achieved and
disadvantages induced by the modifications. The property relationships between
ductility and toughness of the matrix material, fiber-matrix interface bond strength,
composite interlaminar fracture toughness and impact response are specifically
discussed.

8.2. Effects of matrix materials on interlaminar fracture resistance

8.2.1. Introduction

The first generation of resins developed for use in high performance carbon fiber
composites emphasized high modulus and high glass transition temperature, 7''. Due
to the low interlaminar fracture resistance of these resins, in particular under hot
and wet conditions, a second generation of matrix materials has been developed
with special focus on the resistance to interlaminar fracture of composites.
The development of the second generation resins stems from the early work of
McGarry (1969) and Sultan et al. (1971) who found that the fracture toughness of
epoxy resins could be improved by adding certain liquid rubber, particularly
carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer. In addition to
being used as a matrix material for high performance fiber composites, the
toughened epoxies have also been used as structural adhesives, tooling compounds,
moldings, potting and encapsulating materials.
When epoxy resins are suitably modified to impart optimized composition and
microstructure, they possess a balance of desired engineering properties, such as
fracture toughness, tensile and flexural strengths and stiffness. Complicated
mechanical and fracture properties have been observed for toughened epoxy resins,
and significant research efforts have been directed toward disclosing the origins of
toughening in these materials. Indebted to many investigators, including especially
Bascom et al. (1975), Kinloch and Shaw (1981), Yee and Pearson (1986), various
intrinsic (microstructural) and extrinsic (mechanical, thermal and environmental)
factors have been identified, which control the fracture properties, deformation and
failure processes in toughened epoxies. Besides using rubber as a toughening agent
for resins, several inorganic fillers such as alumina, silica, barium titanate, glass
beads and aluminum hydroxide have been employed extensively as reinforcements
for other applications. Many comprehensive reviews on this topic can be found in