Читать книгу The Rheology Handbook - Thomas Mezger - Страница 237

1 Immediate step in stress from τ = 0 to τ0, then keeping constantly τ0 = const; this stress interval lasts from the time point t0 to t2 (τ0 = const, stress phase)

2 Immediate step in stress from τ0 back to τ = 0, then remaining constantly at τ = 0; this interval at rest lasts from t2 to t4 (stress removal phase, or rest phase)

Both steps should be performed as fast as possible. This requires the use of a highly dynamic rheometer drive.

Figure 6.1: Preset of two intervals when performing creep tests:

1) stress phase: After an immediate step onto a constant stress value follows the creep phase

2) rest phase: After the immediate removal step to zero-stress follows the creep

recovery phase

Figure 6.2: Creep and creep recovery curve, showing also the final values of the re-formation γe and the permanently remaining deformation γv

1 For polymers (uncrosslinked and unfilled, solutions and melts):γmax ≤ 50 %, however, sometimes even up to γmax = 100 %

2 For most dispersions (i. e. emulsions, suspensions, foams), crosslinked polymers (such as elastomers, rubbers, thermosets), and gels:γmax ≤ 1 %, however, sometimes counts γmax < 0.1 % only

When applying higher deformation values, there is the risk of exceeding the limiting value of the linear viscoelastic (LVE) range. In this case, the basic laws of rheology are no longer valid, i. e. viscosity law (according to Newton) and elasticity law (acc. to Hooke) and also the relations of Maxwell, Kelvin/Voigt and Burgers. More information about the LVE-range and further γmax values, see Chapter 8.3.3.1: oscillatory tests, amplitude sweeps.