Physical Concepts and Basic Fluid Mechanics
49
B
Fig.
3.1.1
:
Diagram illustrating the characteristics
of
a viscoelastic material such as the
arterial wall: (a) stress relaxation: a step increase in length, L, or diameter results in an
increase in force
or
tension which declines over time
to
a lower level, (b) creep
phenomenon: with a step increment in applied force, the arterial length or diameter
increases, but gradually and (c) hysteresis; with increase in stress, strain increases, but
when the stress is removed, the strain follows a different path, resulting in a “hysteresis
loop”
indicating energy
loss.
If
a
strip
of
artery
is
subjected to a step change in length, it will result
in an initial increase in stress, and then decays to a lower value. This is
known
as
stress-relaxation. There is a finite amount of time the vessel
takes to relax. This is described by a time constant, which differs in
different arteries. When an artery
is
subjected to
a
stepwise change in
stress, its length will gradually increase to a constant value. This is the
so-called, ”creep phenomenon”.
As
with stress relaxation, the increase in
length or diameter, also takes
a
finite amount
of
time and
is
also
