Properties of Metals
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. Cold and hot working are discussed in the next chapter.
The ultimate tensile strength (UTS) is the maximum resistance to fracture. It is equivalent to
the maximum load that can be carried by one square inch of cross-sectional area when the load
is applied as simple tension. It is expressed in pounds per square inch.
area of original cross section
If the complete engineering stress-strain curve is available, as shown in Figure 3, the ultimate
tensile strength appears as the stress coordinate value of the highest point on the curve.
Materials that elongate greatly before breaking undergo such a large reduction of cross-sectional
area that the material will carry less load in the final stages of the test (this was noted in
Figure 3 and Figure 4 by the decrease in stress just prior to rupture). A marked decrease in
cross-section is called "necking." Ultimate tensile strength is often shortened to "tensile
strength" or even to "the ultimate." "Ultimate strength" is sometimes used but can be
misleading and, therefore, is not used in some disciplines.
A number of terms have been defined for the purpose of identifying the stress at which plastic
deformation begins. The value most commonly used for this purpose is the yield strength. The
yield strength is defined as the stress at which a predetermined amount of permanent deformation
occurs. The graphical portion of the early stages of a tension test is used to evaluate yield
strength. To find yield strength, the predetermined amount of permanent strain is set along the
strain axis of the graph, to the right of the origin (zero). It is indicated in Figure 5 as Point (D).