Page 20 - Engineered Interfaces in Fiber Reinforced Composites

Chapter I. Inlroduction 3

composite. There is now a considerable amount of evidential data rcgarding the
influences of interfaces on fracture toughness in both transverse and interlaminar
fractures, and strength and stiffness of fiber composites in various failure modes and
loading configurations (Kim and Mai, 1991; Drzal and Madhukar, 1993). although
the relationship between documented material properties and the actual perfor-
mances of composites is still in question. It follows therefore that a thorough
knowledge of the microstructure-property relationship at the interface region is an
essential key to the successful design and proper use of composite materials.
Further, the interface properties are becoming gradually accepted as design and
process variables to be tailored for particular end applications (Kim and Mai, 1993).
Although there is no simple quantitative relation known for interface optimization
of a given combination of fiber and matrix, various chemical-physical and
thermodynamic-mechanical principles along with previous experience are invalu-
able sources of information to design the interface qualitatively. A number of
potential solutions have been suggested to improve specific properties of the
composites, particularly the interface bond quality for efficient stress transfer and
the fracture resistance/damage tolerance of inherently brittle composites without
sacrificing other important mechanical properties.
This book is concerned mainly with interfaces in advanced composites made from
high performance fibers, such as glass, carbon, aramid and some other organic (e.g.
ultrahigh molecular weight (UHMW) polyethylene) and inorganic (e.g. B/W,
A1203, Sic) fibers and useful matrix materials encompassing polymer, metals/
alloys and ceramics. To control the interface properly and thereby to provide the
composite with improved mechanical performance and structural integrity, it is
essential to understand the mechanisms of adhesion which are specific to each fiber-
matrix system, and the physico-chemical characterization of the interface with
regard to the origin of adhesion. This is the focus of Chapter 2. A number of
theoretical and experimental methods developed to assess the quality of the interface
bond are summarized. Several common experimental techniques that have been
developed to assess the fiber-matrix interface bond quality on a microscopic scale of
the so-called ‘single fiber microcomposite test’, are presented in Chapter 3 along
with the interlaminar/intralaminar strengths and fracture toughness of various
failure modes using composite laminates. Their applicability and limitations are
critically discussed with regard to the loading geometry and interpretation of the test
data based on the underlying mechanics. A proper load transfer across the interface
region is also of particular importance in composites technology. Chapter 4
considers from the load transfer and fracture mechanics angles, extensive and in-
depth theoretical analyses based on a shcar-lag model for the single fiber composite
test with different loading geometry. Of special interest are the stress states in the
composite constituents and debond process along the interface depending on the
nature of the interface bond. This is followed in Chapter 5 by comparisons of the
theories with experimental results of several different composite systems. Particular
emphasis is placed on the various techniques of surface treatments on a range of
technologically important fibers to improve bond strength as well as to enhance
fiber-matrix compatibility and stability during processing or fabrication of the