Page 173 - Fiber Fracture

158 J.G. Lavin

INTRODUCTION

Carbon fibers come in many different forms. The most important type commercially
is the fiber made from polyacrylonitrile (PAN), which was initially developed for
aircraft applications. It was attractive because of its high strength and modulus, and
because it is not subject to creep or fatigue failure. Composites made from PAN-based
carbon fiber allowed for reduction in aircraft weight, and improvement in range, payload
and performance. The composites were first adopted in military aircraft, but rapidly
spread to commercial aircraft and then to other applications such as sporting goods.
Pitch-based general purpose (isotropic) fibers have been used in Japan for large-volume
reinforcement of cementitious matrices, especially exterior building panels. Pitch-based
high performance (mesophase) fibers were developed for space applications. They are
capable of very high Young’s modulus (up to that of in-plane graphite) and have a
high negative coefficient of thermal expansion along the fiber axis. This makes possible
composites with a zero coefficient of thermal expansion, which is important for space
applications. When a panel is facing the sun, it may reach 200°C, and when it is facing
away from the sun, it may drop to 200°C below zero.
Carbon fibers are either the strongest or stiffest materials available, when corrected
for density, as illustrated in Fig. 1. The mesophase pitch fibers also have high levels of
thermal conductivity, as shown in Fig. 2.
The word ‘graphite’ is much misused in carbon fiber literature. The word refers to a
very specific structure, in which adjacent aromatic sheets overlap with one carbon atom
at the center of each hexagon as shown in Fig. 3a. This structure appears very rarely
in carbon fibers, especially in PAN-based fibers, even though they are conventionally
called graphite fibers. While high-performance fibers are made up of large aromatic

4
HT Carbon
m h
E Spectra63
0
--r
E
93-
m
0-
c3 0
r HM Carbon
52- Kevlam 49

0’ I I I I
100 200 300 400 500
Specific Stiff ness, G Pa/( gm/cm3)
Fig. 1. Specific strength and stiffness of strong fibers.