Properties of Metals
Any component, no matter how simple or complex, has to transmit or sustain a
mechanical load of some sort. The load may be one of the following types: a
load that is applied steadily ("dead" load); a load that fluctuates, with slow or fast
changes in magnitude ("live" load); a load that is applied suddenly (shock load);
or a load due to impact in some form. Stress is a form of load that may be
applied to a component. Personnel need to be aware how stress may be applied
and how it effects the component.
DEFINE the following terms:
DISTINGUISH between the following types of stresses by the
direction in which stress is applied.
When a metal is subjected to a load (force), it is distorted or deformed, no matter how strong
the metal or light the load. If the load is small, the distortion will probably disappear when the
load is removed. The intensity, or degree, of distortion is known as strain. If the distortion
disappears and the metal returns to its original dimensions upon removal of the load, the strain
is called elastic strain. If the distortion disappears and the metal remains distorted, the strain
type is called plastic strain. Strain will be discussed in more detail in the next chapter.
When a load is applied to metal, the atomic structure itself is strained, being compressed,
warped or extended in the process. The atoms comprising a metal are arranged in a certain
geometric pattern, specific for that particular metal or alloy, and are maintained in that pattern
by interatomic forces. When so arranged, the atoms are in their state of minimum energy and
tend to remain in that arrangement. Work must be done on the metal (that is, energy must be
added) to distort the atomic pattern. (Work is equal to force times the distance the force