Page 26 - Mechanics Analysis Composite Materials

Chapter 1. Introduction

I I I I
0 100 200 300 400 5 IO
Temperature,
Fig. I .8. Temperature degradation of fiber strength normalized by the strength at 20°C.

because viscosity of molten quartz is too high to make thin fibers directly. However,
more complicated process results in fibers with higher thermal resistance than glass
fibers.
The same process that is used for glass fibers can be employed to manufacture
mineral fibers, e.g., basalt fibers made of molten basalt rocks. Having relatively
low strength and high density (see Table 1.1) basalt fibers are not used for high-
performance, e.g. aerospace structures, but are promising reinforcing elements for
pre-stressed reinforced concrete structures in civil engineering.
Development of carbon (or graphite) fibers was a natural step aiming at a rise of
fiber’s stiffness the proper level of which was not exhibited by glass fibers. Modern
high-modulus carbon fibers demonstrate modulus that is by the factor of about four
higher than the modulus of steel, while the fiber density is by the same factor lower.
Though first carbon fibers had lower strength than glass fibers, modern high-
strength fibers demonstrate tensile strength that is 40% higher than the strength of
the best glass fibers, while the density of carbon fibers is 30% less.
Carbon fibers are made by pyrolysis of organic fibers depending on which there
exist two main types of carbon fibers - PAN-based and pitch-based fibers. For
PAN-based fibers the process consists of three stages - stabilization, carbonization,
and graphitization. In the first step (stabilization) a system of polyacrylonitrile
(PAN) filamentsis stretched and heated up to about 400°C in the oxidation furnace,
while in the subsequent step (carbonization under 900°C in an inert gas media) most
elements of the filaments other than carbon are removed or converted into carbon.
During the successive heat treatment at temperature reaching 280OOC (graphitiza-
tion) crystallinecarbon structure oriented along the fibers length is formed resulting
in PAN-based carbon fibers. The same process is used for rayon organic filaments
(instead of PAN), but results in carbon fibers with lower characteristics because
rayon contains less carbon than PAN. For pitch-based carbon fibers, initial organic