Page 34 - Using ANSYS for Finite Element Analysis Dynamic, Probabilistic, Design and Heat Transfer Analysis

composite materials • 21

Fiber

Tensile stress FRP composite

Resin

Strain
Figure 2.1. Illustrating the combined effect on
modulus of the addition of fibers to a resin matrix.

The ratio of the fiber to resin derives largely from the manufacturing
process used to combine resin with fiber. However, it is also influenced by
the type of resin system used, and the form in which the fibers are incor-
porated. In general, since the mechanical properties of fibers are much
higher than those of resins, the higher the fiber volume fraction the higher
will be the mechanical properties of the resultant composite. In practice
there are limits to this, since the fibers need to be fully coated in resin to
be effective, and there will be an optimum packing of the generally cir-
cular cross-section fibers. In addition, the manufacturing process used to
combine fiber with resin leads to varying amounts of imperfections and
air inclusions. Typically, with a common hand lay-up process as widely
used in the boat-building industry, a limit for FVF is approximately 30–40
percent. With the higher quality, more sophisticated and precise processes
used in the aerospace industry, FVFs approaching 70 percent can be suc-
cessfully obtained.
The geometry of the fibers in a composite is also important since fibers
have their highest mechanical properties along their lengths, rather than
across their widths. This leads to the highly anisotropic properties of com-
posites, where, unlike metals, the mechanical properties of the compos-
ite are likely to be very different when tested in different directions. This
means that it is very important when considering the use of composites to
understand at the design stage, both the magnitude and the direction of the
applied loads. When correctly accounted for, these anisotropic properties
can be very advantageous since it is only necessary to put material where
loads will be applied, and thus redundant material is avoided.
It is also important to note that with metals the material supplier
largely determines the properties of the materials, and the person who