Page 208 - Materials Science and Engineering An Introduction

180 • Chapter 6 / Mechanical Properties of Metals

Plastic Deformation

For most metallic materials, elastic deformation persists only to strains of about 0.005.
As the material is deformed beyond this point, the stress is no longer proportional to
strain (Hooke’s law, Equation 6.5, ceases to be valid), and permanent, nonrecoverable,
plastic deformation or plastic deformation occurs. Figure 6.10a plots schematically the tensile stress–strain
behavior into the plastic region for a typical metal. The transition from elastic to plastic
is a gradual one for most metals; some curvature results at the onset of plastic deforma-
tion, which increases more rapidly with rising stress.
From an atomic perspective, plastic deformation corresponds to the breaking
of bonds with original atom neighbors and then the re-forming of bonds with new
neighbors as large numbers of atoms or molecules move relative to one another; upon
removal of the stress, they do not return to their original positions. The mechanism of
this deformation is different for crystalline and amorphous materials. For crystalline
solids, deformation is accomplished by means of a process called slip, which involves
the motion of dislocations as discussed in Section 7.2. Plastic deformation in noncrystal-
line solids (as well as liquids) occurs by a viscous flow mechanism, which is outlined in
Section 12.10.

6.6 TENSILE PROPERTIES
Yielding and Yield Strength
Most structures are designed to ensure that only elastic deformation will result when
a stress is applied. A structure or component that has plastically deformed—or expe-
: VMSE rienced a permanent change in shape—may not be capable of functioning as intended.
Metal Alloys It is therefore desirable to know the stress level at which plastic deformation begins,
yielding or where the phenomenon of yielding occurs. For metals that experience this gradual

Figure 6.10 (a) Typical stress– Elastic
strain behavior for a metal showing +
elastic and plastic deformations, the Elastic Plastic
proportional limit P, and the yield
strength s y , as determined using the Upper yield
0.002 strain offset method. (b) point
Representative stress–strain behavior
found for some steels demonstrating
the yield point phenomenon.
Stress P Stress Lower yield
point

Strain Strain
0.002
(a) (b)