Page 212 - Materials Chemistry, Second Edition

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3.2. Metallic Structures and Properties

Figure 3.27. Comparative stress vs. strain curves for pure iron, steel, and hardened steel. Reproduced
with permission from Practical Metallurgy and Materials of Industry, Neely, J. E.; Bertone, T. J., 5th ed.,
Prentice-Hall: New Jersey, 2000.

The typical method used to assess the strength of a metal is a tensile test, where
the metal is clamped to upper and lower jaws, and pulled until it fractures.
Figure 3.27 illustrates typical stress vs. strain curves for iron, steel, and hardened
steel. The elastic limit (EL) is the greatest stress that a material may withstand and
still revert back to its original shape when the stress is removed. In general, the
closer a material is to its elastic limit, the longer it will take for the material to
subsequently return to its original size/shape. For a metal within its EL the crystal
lattice will be elastically lengthened, while becoming thinner at right angles to the
applied stress. The ratio of lateral change to the change in length is referred to as
Poisson’s ratio.
When the elastic limit of a metal has been exceeded, it will undergo plastic flow
beginning at the yield point. It is this property of metals that is exploited for cold and
hot working into desired shapes. When a metal is deformed permanently from the
tension force, it exhibits a property known as ductility. By comparison, the term
malleability refers to the permanent deformation of a metal under a compression
force (e.g., hammering, cold-rolling, etc.). Although most ductile metals are also
malleable, the reverse is not always true. For example, lead is extremely malleable