Materials such as metals (aluminum, iron, copper, etc.),
ceramics (silicon carbide, porcelain) or polymers (milk jugs
made of polyethylene) are tested by scientists and engineers to
reveal certain mechanical properties, such as the maximum stress
a material can withstand before it fails. The stress at which a
material breaks is a measure of its strength. During use a
material may degrade, which may cause it to fail at much lower
stresses. For example, if a material is loaded over and over
again and then fails it has undergone what is known as fatigue.
Fatigue is a very common mode of failure for materials and has
been studied for centuries. Fatigue occurs every day in objects
that you’re familiar with. For example, airplane wings fatigue
thousands of cycles on every flight and bridges fatigue every
time a car drives over them. However, just because a material is
undergoing fatigue does not mean that it will always break. In
fact, engineers run careful experiments so that they can be sure
that things will not