Mechanical properties - Orthopaedic Biomaterials in Research and Practice - page 61

acetylene, may be used to further strengthen the material through
incorporation during cross-linking.
Particulate reinforcement. Weak materials, such as polymers, may
be strengthened by adding small quantities of very strong particles
or fibers. In materials in which the limitation on strength is easy
crack propagation, such materials produce increases in strength
since cracks must either break through them or “detour” around
them to propagate. This effect can be produced in some metal
alloys by careful heat treatment producing precipitation of very
strong materials, usually metallic carbides, or by the introduction
of oxygen to form internal dispersions of metallic oxides. (These
latter processes may degrade corrosion resistance; see Chapter 12.)

Other mechanisms of deformation

In addition to elastic (recoverable) and plastic (nonrecoverable) deforma-
tion, there are three other principal modes of materials deformation that
must be taken into consideration in selection of materials. None of these
types of behavior are predictable from standard stress-strain curves but
must be characterized by other experimental means.

Creep Creep is simply plastic deformation at stresses below the yield point.

The phenomenon is usually observed as a time-dependent increase in
strain when a material is maintained at a constant stress below the yield
stress. Most materials exhibit creep, but it is most noticeable in metals at
elevated temperatures and in polymers and tissues at body temperatures.
It may or may not be recoverable, depending on the nature of the internal
deformation process.
There are three distinct stages of the creep process (Figure 3.4),
which are often referred to as types of creep:

Primary (type 1) creep. Occurs at an initial high rate, which rapidly
decreases.

σ 0

3

ε

2

ε ´´
ε ´
ε 0

1

1 min 1 h 1 day 1 month 1 year 10 year

Time (log scale)

FIGUre 3.4 Creep curves.