Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 57

atomic origin of mechanical properties

These different types of stress-strain behavior derive from the funda-
mental physics of interaction between the atoms of the solid involved. It
is important to consider these, at least qualitatively, as these interactions
provide some insight into the origin of mechanical behavior not predict-
able from the data presented on stress-strain curves or in their tabular
summaries.
Atomic association within solids (and liquids) at body temperature is
chemical in nature and depends on the fundamental attraction between
electrical charges of opposite sign, expressed in Coulomb's law:

kq q
d

⋅ ⋅ 12

2

F =

where F is the interactive force (attractive when the charges are of oppo-
site sign), q 1 and q 2 are the magnitudes of the charges, d is their physical
separation (1-3 Å in solids), and k is an experimentally derived constant
that adjusts the units appropriately. At the extremely short distances
between atoms in solids and liquids, coulombic forces are very large and
are the origin of the structural cohesion of materials.
Charges are formed on atoms (atoms are converted to ions) by the addi-
tion and subtraction of electrons. The “structure” of electrons around the
nucleus of any atom produces preferred numbers of electrons, described as
constituting a “filled outer electron shell”; thus, ions derived from any atom
have one or more favored net charges.

Interatomic bond
types

Combination of these two effects in solids produces four types of bonds
or interactions between atoms (Figure 3.3):

1. Metallic. Atoms that form metallic solids, such as iron, nickel,
chromium, cobalt, and so on, are able to lose one or more elec-
trons easily. This is seen in their ability to conduct current (in this

e - e -

M + M + M +

e - e - e - e -

C

C

M + M + M + M +

C

C

e - e - e - C

C

C

M + M + M +

e -

C

Metallic Covalent

Na + Cl - Na + Cl -

C

C

H

Cl - Na + Cl - Na +

C

-

C

O H

C

Na + Cl - Na + Cl -

C

Ionic van der Waals

FIGUre 3.3 types of bonds between atoms in solids.