Viscoelasticity - Orthopaedic Biomaterials in Research and Practice - page 97

PROBLEM 4.13
Considering the viscoelastic material in Figure 4.1, testing the material
under a higher strain rate will generally change the appearance of the
material behavior according to the following (true or false):
1. Decreasing ductility
2. Decreasing stiffness
3. Increasing amount of recoverable strain
4. Decreasing the elongation to failure
ANSWER:
T, F, T, T. Increasing the strain rate will give the material a response
more closely aligned with the curves on the left of Figure 4.1. Because
testing at a faster rate will not allow as much time for molecular rear-
rangement, plastic flow behavior will not develop as sufficiently. As a
result, the material response is characterized as having a stiff elastic
curve, with more recoverable (elastic) strain, less ductility, and less strain
to failure.
PROBLEM 4.14
Viscoelastic properties can be affected by the following (true or
false):
1. Temperature
2. Moisture content
3. Load history
4. Atomic bonding type (e.g., ionic vs. covalent)
ANSWER:
T, T, T, T. Because viscoelasticity is associated with larger-scale molecu-
lar rearrangement, temperature and moisture content will directly affect
this flow potential of the material. Similarly, the atomic bond type will
directly affect the ability of a material to rearrange over time. We also
know that viscoelastic materials can display significant hysteresis, impli-
cating load history as having a direct effect on current creep and relax-
ation behavior.
PROBLEM 4.15
Polymers typically have a critical property termed the glass-transition
temperature. At temperatures below the T g , the polymer behavior is char-
acterized as glassy, while above T g the material behavior is described as
more rubbery. High-density polyethylene (HDPE) has a T g of −110°C
and polyetheretherketone (PEEK) has a T g of 145°C. Which would we
expect to have a greater potential for creep when implanted in the body?