1.1 Fundamental Definitions
Static Stresses
TOTAL STRESS on a section mn through a loaded body is the resultant force S exerted by
one part of the body on the other part in order to maintain in equilibrium the external
loads acting on the part. Thus, in Figs. 1, 2, and 3 the total stress on section mn due to
the external load P is S. The units in which it is expressed are those of load, that is,
pounds, tons, etc.
UNIT STRESS, more commonly called stress , is the total stress per unit of area at section
mn. In general it varies from point to point over the section. Its value at any point of a
section is the total stress on an elementary part of the area, including the point divided
by the elementary total stress on an elementary part of the area, including the point divided
by the elementary area. If in Figs. 1, 2, and 3 the loaded bodies are one unit thick and
four units wide, then when the total stress S is uniformly distributed over the area,
P/A P/4. Unit stresses are expressed in pounds per square inch, tons per square foot,
etc.
TENSILE STRESS OR TENSION is the internal total stress S exerted by the material fibers to
resist the action of an external force P (Fig. 1), tending to separate the material into two
parts along the line mn. For equilibrium conditions to exist, the tensile stress at any cross
section will be equal and opposite in direction to the external force P. If the internal total
stress S is distributed uniformly over the area, the stress can be considered as unit tensile
stress S/A.
COMPRESSIVE STRESS OR COMPRESSION is the internal total stress S exerted by the fibers
to resist the action of an external force P (Fig. 2) tending to decrease the length of the
material. For equilibrium conditions to exist, the compressive stress at any cross section
will be equal and opposite in direction to the external force P. If the internal total stress
S is distributed uniformly over the area, the unit compressive stress S/A.
SHEAR STRESS is the internal total stress S exerted by the material fibers along the plane mn
(Fig. 3) to resist the action of the external forces, tending to slide the adjacent parts in
opposite directions. For equilibrium conditions to exist, the shear stress at any cross section
will be equal and opposite in direction to the external force P. If the internal total stress
S is uniformly distributed over the area, the unit shear stress S/A.

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.

Edited by Myer Kutz, 2006 by John Wiley & Sons, Inc.

Read More......
Edited by Myer Kutz, 2006 by John Wiley & Sons, Inc.