Page 96 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

Figure 2.52. Illustration of various types of loads (stresses), which result in material strain. Shown are
(a) tensile stress, (b) compressive stress, (c) shear stress, and (d) tortional stress. Reproduced with
permission from Callister, W. D. Materials Science and Engineering: An Introduction, 7th ed., Wiley:
New York, 2007. Copyright 2007 John Wiley & Sons, Inc.

For large stresses, a material will deform plastically, involving the permanent

displacement of atoms. The onset of plastic deformation is referred to as the yield
point (or yield strength) of the material. For most metals, there is a gradual transition
from elastic to plastic deformation; however, some steels exhibit very sharp transi-
tions. After the yield point is reached, plastic deformation continues until the
material reaches its fracture point. Accordingly, the tensile strength represents the
maximum strain in the stress vs. strain curve (Figure 2.54); this property with respect
to its weight is referred to as the specific strength:
tensile strength
ð29Þ specific strength =
specific gravity