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.
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).
Elastic limit is the maximum stress to which a test specimen may be subjected and still
return to its original length upon release of the load. A material is said to be stressed within
the elastic region when the working stress does not exceed the elastic limit, and to be
stressed in the plastic region when the working stress does exceed the elastic limit. The
elastic limit for steel is for all practical purposes the same as its proportional limit.
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