Properties of Metals
Stress is the internal resistance, or counterfource, of a material to the distorting effects of an
external force or load. These counterforces tend to return the atoms to their normal positions.
The total resistance developed is equal to the external load. This resistance is known as stress.
Although it is impossible to measure the intensity of this stress, the external load and the area
to which it is applied can be measured. Stress (s) can be equated to the load per unit area or
the force (F) applied per cross-sectional area (A) perpendicular to the force as shown in
s = stress (psi or lbs of force per in.2)
F = applied force (lbs of force per in.2)
A = cross-sectional area (in.2)
Stresses occur in any material that is subject to a load or any applied force. There are many
types of stresses, but they can all be generally classified in one of six categories: residual
stresses, structural stresses, pressure stresses, flow stresses, thermal stresses, and fatigue
Residual stresses are due to the manufacturing processes that leave stresses in a
material. Welding leaves residual stresses in the metals welded. Stresses associated
with welding are further discussed later in this module.
Structural stresses are stresses produced in structural members because of the weights
they support. The weights provide the loadings. These stresses are found in building
foundations and frameworks, as well as in machinery parts.