Time-dependence includes those effects associated with transient
*flow* conditions as well as those effects associated with irreversible changes that result from
*shear* history.

**creep ** The response of a material to the
instantaneous application of a constant
*stress*.

**creep function** In an applied
*stress* test, where an instantaneous and constant stress is applied to a material while the
*shear rate* (or
*shear strain*) is measured over time, the shear rate (or strain)-time function is termed the *creep* function. The function *J*(*t*)
= (*t*) /
is referred to as the creep
*compliance*.

**Deborah number**, *De*** **The ratio of a characteristic
*relaxation time* of a material to the duration of the observation. In
*equilibrium flow*, the effective duration of the experiment is
infinity, and *De* = 0. In oscillatory shear, it is the product of the frequency and the relaxation time of the fluid. In converging flows, the Deborah number is proportional to the
*Weissenberg number*.

**flow hysteresis** A condition resulting from differences in the rate of energy dissipation due to
*shear* history. In a typical rheometric test,
*shear stress* or
*shear rate* is ramped at a fixed speed up to a maximum value, then ramped back down at the same speed to the beginning. In hysteresis, one
*flow curve* lies above the other, forming a continuous loop whose internal area depends on the shear and thermal history of the material, and on how rapidly the stress or shear rate was ramped. If the down-curve lies below the up-curve, then it is referred to as a
*thixotropic* loop, whereas if the down-curve lies above the up-curve, then it is called a
*negative thixotropic* loop.

**negative thixotropy [anti-thixotropy]** A reversible time-dependent increase in viscosity at a particular shear rate. Shearing causes a gradual growth in
*structure* over time.

**relaxation time**,
A time characterizing the response of a
*viscoelastic* material to the instantaneous application of a constant *strain*.

**retardation time**,
A time characterizing the response of a
*viscoelastic* material to the instantaneous application of a
constant
*stress*.

**rheomalaxis** An irreversible decrease of
viscosity during shearing. Attributed to permanent changes in the material
*structure*.

**rheopexy** An effect by which a material recovers some of its pre-sheared viscosity at a faster rate when it is gently sheared compared to when it is allowed to stand. Not to be confused with
*negative thixotropy*.

**stress growth **When an instantaneous and constant *strain *(or
*shear rate*) is applied to a material while
*stress* is measured over time, an increasing stress vs. time or
*modulus* vs. time function is termed stress growth.

**stress relaxation **When an instantaneous
and constant *strain *(or
*shear rate*) is applied to a material while
*stress* is measured over time, a decreasing stress vs. time or
*modulus* vs. time function is termed stress relaxation.

**thixotropy** A reversible time-dependent
decrease in viscosity at a particular
*shear rate*. Shearing causes a gradual breakdown in
*structure* over time.