Density is one of the
most fundamental physical properties of
any material. It is defined as the ratio
of an objects mass to its volume. Because
most designs are limited by either size
and or weight density is an important
consideration in many calculations.
Density is a function
of the mass of the atoms making up the
materials and the distance between them.
Massive, closely packed atoms characterize
high density materials such as Tungsten
or Neptunium. In contrast light, relatively
distant atoms compose low density materials
such as Beryllium or Aluminum. Density
on a macroscopic level is also a function
of the microscopic structure of a material.
A relatively dense material may be capable
of forming a cellular structure such as
a foam which can be nearly as strong and
much less dense than the bulk material.
Composites including natural constituents
such as wood and bone, for example, generally
rely on microscopic structure to achieve
densities far lower than common monolithic
Availability and manufacturability
requirements are often unseen limiting
factors in materials selection. The importance
of a material being available is obvious.
Materials which are not available cannot
be used. The importance of processibility
is not always so obvious.
Any other desirable qualities
are useless if a material cannot be processed
into the shape required to perform its
function. Most engineering materials in
use today have well known substitutes
which would perform better and often at
lower cost but processes for forming,
cutting, machining, joining, etc. are
not available or commercially viable.
There is often a period of time after
a new material is introduced during which
its application is severely limited while
processing techniques are developed which
facilitate its use.
A materials cost is also
generally a limiting factor. While cost
is universally recognized and perhaps
the easiest of all properties to understand
there are specific cost considerations
for materials selection. Just as materials
and their processing go hand in hand so
do material costs and processing costs.
Understanding the entire processing sequence
is critical to accurately evaluating the
true cost of a material.
Because the appearance
of many mechanical components seems fairly
trivial it is also easy to overlook its
importance in the marketing and commercial
success of a product.