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7. Time-Dependent Effects

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.


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