Page 174 - Fiber Fracture

FRACTURE OF CARBON FIBERS 159

PAN Fiber
TI
Fe
Mo
AI
Au
cu

As
MP Fiber
Advanced
MP Fiber
100 200 300 400 500 600 700 800 900
Thermal Conductivity, W rn-’K“
Fig. 2. Thermal conductivity of structural materials.

sheets, these are randomly oriented relative to each other, are described as ‘turbostratic’
(turbulent and stratified) and are shown in Fig. 3b. Many physical properties depend
merely on the large aromatic sheets. The aromatic character of the isotropic carbon is
shown in Fig. 3c.

PHYSICAL PROPERTIES

Because of the rich variety of carbon fibers available today, physical properties vary
over a broad domain. Fig. 4 shows a plot of strength versus modulus. ‘General purpose’
fibers made from isotropic pitch have modest levels of strength and modulus. However,
they are the least expensive pitch-based fiber, and are useful in enhancing modulus
or conductivity in many applications. PAN-based fibers are the strongest available;
however, when they are heat treated to increase modulus the strength decreases.
Mesophase pitch fibers may be heat treated to very high modulus values, approaching
the in-plane modulus of graphite at 1 TPa. The Achilles heel of mesophase pitch-
based fibers in composite applications is low compressive strength; this is illustrated in
Fig. 5. Electrical and thermal conductivity are important in many applications, and these
are illustrated in Figs. 6 and 7 respectively. Mesophase pitch fibers have the highest
conductivity and lowest resistivity.
Finally, there is a property of high-performance carbon fibers, both PAN and
mesophase pitch-based, which sets them apart from other materials. They are not
subject to creep or fatigue failure. These are important characteristics for critical
applications. In a comparison of materials for tension members of tension leg platforms
for deep-sea oil production described by Salama (1997), carbon fiber strand survived
2,000,000 stress cycles between 296 and 861 MPa. In comparison, steel pipe stressed
between 21 and 220 MPa failed after 300,000 cycles. Creep studies on PAN and