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284 CHAPTER ELEVEN
COST
Most of the cost of materials will be related to the machining costs. Materials, except
for very hard metals and special composites, do not cost all that much.
AVAILABILITY

Metals, wood, and plastics are relatively easy to procure. Composite materials are not
that hard to find either. Most of these materials can be purchased in preformed shapes
like pipes, sheets, spheres, and so on. Consider starting the design using preformed
parts; they can offer great strength and accuracy.

STRENGTH

Materials have several different characteristics that quantify their strength under vari-
ous kinds of loads.

Tensile

This is a rough measure of how strong the material is when stretched (like a string).
Glass and composite materials excel at this.

Compression
This is a measure of how well the material can hold up weight resting on it (like a post).
Metals excel at this.

Flexing

This is a measure of how the material deforms with sideways pressure. In some designs,
the material must not bend at all. In other designs, the material must bend (like a tree
withstands a strong wind). As such, failure can come by bending too much, by break-
ing, and by failing to return to the original shape. All the materials mentioned can be
used (by proper design) to suit the requirements of bending or nonbending applications.

Shock

This is a measure of the material’s ability to survive a sudden shock. Shocks present a
sudden increase in pressure on a material and radiate shock waves in ways that slow,
steady pressures do not.