Properties of Metals
Assessment of mechanical properties is made by addressing the three basic stress types.
Because tensile and compressive loads produce stresses that act across a plane, in a direction
perpendicular (normal) to the plane, tensile and compressive stresses are called normal stresses.
The shorthand designations are as follows.
For tensile stresses: "+SN" (or "SN") or "s" (sigma)
For compressive stresses: "-SN" or "-s" (minus sigma)
The ability of a material to react to compressive stress or pressure is called compressibility.
For example, metals and liquids are incompressible, but gases and vapors are compressible.
The shear stress is equal to the force divided by the area of the face parallel to the direction
in which the force acts, as shown in Figure 1(c).
Two types of stress can be present simultaneously in one plane, provided that one of the
stresses is shear stress. Under certain conditions, different basic stress type combinations may
be simultaneously present in the material. An example would be a reactor vessel during
operation. The wall has tensile stress at various locations due to the temperature and pressure
of the fluid acting on the wall. Compressive stress is applied from the outside at other
locations on the wall due to outside pressure, temperature, and constriction of the supports
associated with the vessel. In this situation, the tensile and compressive stresses are considered
principal stresses. If present, shear stress will act at a 90 angle to the principal stress.