Page 33 - Using ANSYS for Finite Element Analysis Dynamic, Probabilistic, Design and Heat Transfer Analysis

20 • using ansys for finite eLement anaLysis
• Ceramic matrix composites (CMCs)—Used in very high tempera-
ture environments, these materials use a ceramic as the matrix and
reinforce it with short fibers, or whiskers such as those made from
silicon carbide and boron nitride.

2.1.1 PoLyMeR MATRix CoMPoSiTeS

Resin systems such as epoxies and polyesters have limited use for the
manufacture of structures on their own, since their mechanical properties
are not very high when compared to, for example, most metals. How-
ever, they have desirable properties, most notably their ability to be easily
formed into complex shapes. Materials such as glass, aramid, and boron
have extremely high tensile and compressive strength but in “solid form”
these properties are not readily apparent. This is due to the fact that when
stressed, random surface flaws will cause each material to crack and fail
well below its theoretical “breaking point.” To overcome this problem,
the material is produced in fiber form, so that, although the same num-
ber of random flaws will occur, they will be restricted to a small number
of fibers with the remainder exhibiting the material’s theoretical strength.
Therefore a bundle of fibers will reflect more accurately the optimum per-
formance of the material. However, fibers alone can only exhibit tensile
properties along the fiber’s length, in the same way as fibers in a rope.
It is when the resin systems are combined with reinforcing fibers such
as glass, carbon, and aramid that exceptional properties can be obtained.
The resin matrix spreads the load applied to the composite between each
of the individual fibers and also protects the fibers from damage caused by
abrasion and impact. High strengths and stiffnesses, ease of molding com-
plex shapes, high environmental resistance all coupled with low densities,
make the resultant composite superior to metals for many applications.
Since PMCs combine a resin system and reinforcing fibers, the properties
of the resulting composite material will combine something of the proper-
ties of the resin on its own with that of the fibers on their own, as surmised
in Figure 2.1.
Overall, the properties of the composite are determined by:

• The properties of the fiber.
• The properties of the resin.
• The ratio of fiber to resin in the composite (fiber volume fraction
(FVF)).
• The geometry and orientation of the fibers in the composite.