Figure 1-1.Stress applied to a materiaI.
Very rarely do Steelworkers work with elements
in their pure state. We primarily work with alloys and have
to understand their characteristics. The characteristics
of elements and alloys are explained in terms of
physical, chemical, electrical, and mechanical
properties. Physical properties relate to color, density,
weight, and heat conductivity. Chemical properties
involve the behavior of the metal when placed in
contact with the atmosphere, salt water, or other
substances. Electrical properties encompass the
electrical conductivity, resistance, and magnetic
qualities of the metal. The mechanical properties
relate to load-carrying ability, wear resistance,
hardness, and elasticity.
When selecting stock for a job, your main
concern is the mechanical properties of the metal.
The various properties of metals and alloys were
determined in the laboratories of manufacturers and
by various societies interested in metallurgical
development. Charts presenting the properties of a
particular metal or alloy are available in many
commercially published reference books. The
charts provide information on the melting point,
tensile strength, electrical conductivity, magnetic
properties, and other properties of a particular metal
or alloy. Simple tests can be conducted to determine
some of the properties of a metal; however, we
normally use a metal test only as an aid for
identifying apiece of stock. Some of these methods
of testing are discussed later in this chapter.
Strength, hardness, toughness, elasticity, plasticity,
brittleness, and ductility and malleability are
mechanical properties used as measurements of how
metals behave under a load. These properties are
described in terms of the types of force or stress that
the metal must withstand and how these are resisted.
Common types of stress are compression, tension,
shear, torsion, impact, 1-2 or a combination of these
stresses, such as fatigue. (See fig. 1-1.)
Compression stresses develop within a material
when forces compress or crush the material. A column
that supports an overhead beam is in compression, and
the internal stresses that develop within the column are
Tension (or tensile) stresses develop when a
material is subject to a pulling load; for example, when
using a wire rope to lift a load or when using it as a
guy to anchor an antenna. Tensile strength is defined
as resistance to longitudinal stress or pull and can be
measured in pounds per square inch of cross section.
Shearing stresses occur within a material when
external forces are applied along parallel lines in
opposite directions. Shearing forces can separate
material by sliding part of it in one direction and the
rest in the opposite direction.
Some materials are equally strong in compression,
tension, and shear. However, many materials show
marked differences; for example, cured concrete has a
maximum strength of 2,000 psi in compression, but
only 400 psi in tension. Carbon steel has a maximum
strength of 56,000 psi in tension and compression but
a maximum shear strength of only 42,000 psi;
therefore, when dealing with maximum strength, you
should always state the type of loading.
A material that is stressed repeatedly usually fails
at a point considerably below its maximum strength in
tension, compression, or shear. For example, a thin
steel rod can be broken by hand by bending it back and
forth several times in the same place; however, if the
same force is applied in a steady motion (not bent back
and forth), the rod cannot be broken. The tendency of
a material to fail after repeated bending at the same
point is known as fatigue.