Page 27 - Mechanics Analysis Composite Materials

12 Mechanics and anaIysis of composite materials
filaments are made in approximately the same manner as for glass fibers from
molten petroleum or coal pitch and pass through carbonization and graphitization
processes. Because pyrolysis is accompanied with a loss of material, carbon fibers
have a porous structure and their specific gravity (about 1.8) is less than that of
graphite (2.26). The properties of carbon fibers are affected with the crystallite size,
crystalline orientation, porosity and purity of carbon structure.
Typical stress-strain diagrams for high-modulus (HM) and high-strength (HS)
carbon fibers are plotted in Fig. 1.7. As components of advanced composites for
engineering applications, carbon fibers are characterized with very high modulus
and strength, high chemical and biological resistance, electricconductivity and very
low Coefficient of thermal expansion. Strength of carbon fibers practically does not
decrease under temperature elevated up to 1500°C (in the inert media preventing
oxidation of fibers).
The exceptional strength of 7.06 GPa is reached in Toray T-1000 carbon fibers,
while the highest modulus of 850 GPa is obtained in Carbonic HM-85 fibers.
Carbon fibers are anisotropic, very brittle, and sensitive to damage. They do not
absorb water and change their dimensions in humid environments.
There exist more than 50 types of carbon fibers with a broad spectrum of
strength, stiffness and cost, and the process of fiber advancement is not over - one
may expect fibers with strength up to 10 GPa and modulus up to 1000 GPa within a
few years.
Organic fibers commonly encountered in textile applications can be employed as
reinforcing elements of advanced composites. Naturally, only high performance
fibers, i.e. fibers possessing high stiffness and strength, can be used for this purpose.
The most widely used organic fibers that satisfy these requirements are known as
aramid (aromatic polyamide) fibers. They are extruded from a liquid crystalline
solution of the corresponding polymer in sulfuric acid with subsequent washing in a
cold water bath and stretching under heating. Properties of typical aramid fibers
are listed in Table 1.1, and the corresponding stress-strain diagram is presented in
Fig. I .7. As components of advanced composites for engineering applications,
aramid fibers are characterized with low density providing high specific strength
and stiffness, low thermal conductivity resulting in high heat insulation, and
negative thermal expansion coefficient allowing us to construct hybrid composite
elements that do not change their dimensions under heating. Consisting actually of
a system of very thin filaments (fibrils), aramid fibers have very high resistance
to damage. Their high strength in longitudinal direction is accompanied with
relatively low strength under tension in transverse direction. Aramid fibers are
characterized with pronounced temperature (see Fig. 1.8) and time dependence for
stiffness and strength. Unlike inorganic fibers discussed above, they absorb water
resulting in moisture content up to 7% and degradation of material properties by
15-20%.
The list of organic fibers was supplemented recently with extended chain
polyethylene fibers demonstrating outstanding low density (less than that of water)
in conjunctions with relatively high stiffness and strength (see Table 1.1 and
Fig. 1.7). Polyethylene fibers are extruded from the corresponding polymer melt in