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Strength
is the ability of a
material to resist deformation. The strength of a component
is usually considered
based on the maximum load that can be borne before failure is
apparent. If under simple
tension the permanent deformation (plastic strain) that takes
place in a component before
failure, the load-carrying capacity, at the instant of final
rupture, will probably be less than
the maximum load supported at a lower strain because the load
is being applied over a significantly
smaller cross-sectional area. Under simple compression, the
load at fracture will be
the maximum applicable over a significantly enlarged area
compared with the cross-sectional area
under no load.
This obscurity can be overcome by utilizing a
nominal stress figure for tension and shear. This is
found by dividing the relevant maximum load by the original
area of cross section of the component.
Thus, the strength of a material is the maximum nominal
stress it can sustain. The nominal
stress is referred to in quoting the "strength" of
a material and is always qualified by the
type of stress, such as tensile strength, compressive
strength, or shear strength.
For most structural materials, the difficulty
in finding compressive strength can be overcome by substituting
the tensile strength value for compressive strength. This
substitution is a safe assumption
since the nominal compression strength is always greater than
the nominal tensile strength
because the effective cross section increases in compression
and decreases in tension. When
a force is applied to a metal, layers of atoms within the
crystal structure move in relation to
adjacent layers of atoms. This process is referred to as slip.
Grain boundaries tend to prevent slip.
The smaller the grain size, the larger the grain boundary
area. Decreasing the grain size through
cold or hot working of the metal tends to retard slip and
thus increases the strength of the
metal.
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