Time Dependency- Creep and Creep Recovery
The stress and strain rate dependent behavior of a material
may be only part of the picture. In many cases time
dependency has to be considered also. Materials are also
time dependent. Hookean and Newtonian materials respond
immediately upon an input stress or strain rate. When a stress
is imposed on a so-called viscoelastic” material, it does not
immediately respond with constant flow, even though the
stress may be sufficiently above the critical stress or yield point.
Upon removal of the stress, these types of materials recover
to their original state, but slowly, and usually incompletely. This
behavior is referred to as creep. Creep studies can also be
used to determine the yield stress of materials (see Figure 5).
A series of creep and recovery (application of a constant
stress followed by a period of zero stress) can be performed
in incrementally higher and higher stress levels.