Section 3. Structural Materials.
Structural materials are the ones we use in the construction of buildings and
machines where we rely on their response to applied forces.
In buildings and machines,
we rely on the ability of the material to resist deformation under loads; in springs we
utilize the elastic deformation of the materials; we utilize the ability of metals to undergo
extensive plastic deformation in order to give them shape, a method that is not possible
with ceramics and polymers.
In what follows, we will first examine the strength of
metals and their elastic and plastic deformation under stress.
In a following chapter, we
will examine the resistance of metals and ceramics to fracture, fatigue and creep.
mechanical properties of polymers depend very much on their molecular structure; they
are best discussed in conjunction with the properties and processing of these materials.
The Strength of Metals
After having studied this chapter you will be able to:
Distinguish between forces applied to a solid and the resulting stresses.
tensile, compressive and shear stresses.
Draw a stress-strain curve and identify elastic and plastic deformation, yield
stress, ultimate tensile stress, ductility, work-hardening.
Describe the effect of residual stresses on the strength of structural materials.
Perform a hardness test; describe the different hardness tests and their
Explain why the defects make a material strong.
Name and explain the main means of strengthening of a metal: strain hardening
(i.e. work hardening), solution strengthening, precipitation-strengthening, fine
Describe annealing, how it changes mechanical properties and why.
Let us take a wire and bend it around a mandrel into a helical spring. We have
of the material in order to give it a permanent shape.
We can now hang different weights on the ends of the spring and obtain a reversible,
that is proportional to the weight we applied.
If the weight is too
large, the spring elongates permanently, by plastic deformation. We can also use the wire
to support a weight.
If the latter is too heavy, the wire will not hold its shape, it will
yield, become longer, and finally break.
We can perform these experiments with
different metals, a hard or a soft steel wire or a copper wire, and we will observe that the
strength of the wires or possible amount of plastic deformation is different for each metal.
We know from everyday experience that we can not bend a glass or a porcelain plate:
when the stress is too large, it breaks before any deformation.
We also know that we can
bend a piece of polymer but it does not keep its new shape as a metal does.