STRENGTH OF MATERIALS

STRENGTH OF MATERIALS 195
STRENGTH OF MATERIALS
Strength of Materials
Strength of materials deals with the relations between the external forces applied to elastic
bodies and the resulting deformations and stresses. In the design of structures and
machines, the application of the principles of strength of materials is necessary if satisfactory
materials are to be utilized and adequate proportions obtained to resist functional
forces.
Forces are produced by the action of gravity, by accelerations and impacts of moving
parts, by gasses and fluids under pressure, by the transmission of mechanical power, etc. In
order to analyze the stresses and deflections of a body, the magnitudes, directions and
points of application of forces acting on the body must be known. Information given in the
Mechanics section provides the basis for evaluating force systems.
The time element in the application of a force on a body is an important consideration.
Thus a force may be static or change so slowly that its maximum value can be treated as if
it were static; it may be suddenly applied, as with an impact; or it may have a repetitive or
cyclic behavior.
The environment in which forces act on a machine or part is also important. Such factors
as high and low temperatures; the presence of corrosive gases, vapors and liquids; radiation,
etc. may have a marked effect on how well parts are able to resist stresses.
Throughout the Strength of Materials section in this Handbook, both English and
metric SI data and formulas are given to cover the requirements of working in either
system of measurement. Formulas and text relating exclusively to SI units are given
in bold-face type.
Mechanical Properties of Materials.—Many mechanical properties of materials are
determined from tests, some of which give relationships between stresses and strains as
shown by the curves in the accompanying figures.
Stress is force per unit area and is usually expressed in pounds per square inch. If the
stress tends to stretch or lengthen the material, it is called tensile stress; if to compress or
shorten the material, a compressive stress; and if to shear the material, a shearing stress.
Tensile and compressive stresses always act at right-angles to (normal to) the area being
considered; shearing stresses are always in the plane of the area (at right-angles to compressive
or tensile stresses).
Fig. 1. Stress-strain curves
In the SI, the unit of stress is the pascal (Pa), the newton per meter squared (N/m2).
The megapascal (newtons per millimeter squared) is often an appropriate sub-multiple
for use in practice.
Unit strain is the amount by which a dimension of a body changes when the body is subjected
to a load, divided by the original value of the dimension. The simpler term strain is
often used instead of unit strain.
Proportional limit is the point on a stress-strain curve at which it begins to deviate from