Polymers - Orthopaedic Biomaterials in Research and Practice - page 144

in describing these properties, probably because the general inferiority
of mechanical behavior (with respect to metals) generally relegates poly-
mers to low-stress, low-strain-rate applications. Thus, it should be under-
stood that, in this discussion, “modulus” means the modulus obtained by
engineering tests at a typical laboratory strain rate, say between 0.1 and
1.0 mm s −1 , at 25°C. Thus, these moduli are typically between the true
relaxed and unrelaxed values.
Figure 6.7 shows the family of stress-strain curves for a typical poly-
mer of low crystallinity. As for other viscoelastic materials, the initial
slope of the stress-strain curve (“modulus”) increases with increasing
strain rate. Note also the effect of temperature.
Small deformations of polymers produce deformation and failure of
weak intermolecular bonds and are frequently recoverable. Larger defor-
mations produce uncoiling of the polymer chains, elongation, and failure
of covalent cross-link and backbone bonds and thus are more likely to be
irreversible. Thus, it is also possible, at any given temperature, to define
an engineering value of “yield” stress. This has the same difficulty as the
previously described “modulus,” since it is both strain rate and tempera-
ture dependent. Most such values are determined under the conditions
previously cited for “modulus.”
The mechanical behavior of polymers has a very strong dependence
on temperature. In Figure 6.8 we can see this, again for a generic poly-
mer with a variety of crystallinities. The middle curve, which is the most
characteristic, is for a material with 50% crystallinity, such as that previ-
ously seen represented by the stress-strain curves in Figure 6.7.
There are four regions in the “modulus” versus temperature curves
shown in Figure 6.8:

Glassy . Below a specific temperature, T G , called the glass transition
temperature (or point ), both crystalline and amorphous polymers
are rigid and brittle, much like metals and ceramics.
Leathery . As the temperature rises above T G , the “modulus” drops
very significantly (note that the vertical scale is a logarithmic one)
and deformations are slowly recoverable.

σ

ε

FIGUre 6.7 typical stress-strain curves for a polymer.