Creep measurement

3 Creep Measurements Creep measurements are basically very simple and 

Exact relationships between creep rate and diffusion coefficients have been proposed, such as the Bird-Dorn-Mukherjee equation  

As we have seen, if any one of the material functions of linear viscoelasticity, for example G(t), is known over the full range of times, or frequencies, the linear behavior is completely specified. However, it is never possible to obtain such a complete characterization, and it may be necessary to use more than one measurement technique to probe the widest possible range of times or frequencies. 

Since it is the long-time behavior that is closely related to molecular structure, this is the information that is most interesting in the present context. For example, the zero-shear viscosity describes behavior in the limit of zero frequency and is very sensitive to molecular weight. However, for a material whose longest relaxation time is quite large, neither step-strain nor oscillatory shear experiments are useful to probe the behavior at very long times or very low frequencies. The main problem is that the stress is so small that it is not possible to measure it precisely. It is in this region that creep measurements are most useful. This is because it is possible to make a very precise measurement of a displacement, and it is also possible to apply a very small controlled stress. Controlled-torque (controlled-stress) rheometers are available from several manufacturers. 

The creep compliance is only measured directly from f = 0 to fj. However, solving Eq. 4.54 for /(f), we find that we can use the recovery data to extend /(f) up to f = 2 fj as follows  

Now that /(f) has been determined up to f = 2 fj, this information can be used in combination with the next portion of the y(f) curve to determine the compliance at times up to 3 fj. This procedure can be repeated until the terminal zone is reached, i.e., until /(f) becomes linear with time. Finally, the zero-shear viscosity can be calculated as the reciprocal of the slope of this line. 

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Controlled stress viscometers are useful for determining the presence and the value of a yield stress. The stmcture can be estabUshed from creep measurements, and the elasticity from the amount of recovery after creep. The viscosity can be determined at very low shear rates, often ia a Newtonian region. This 2ero-shear viscosity, T q, is related directly to the molecular weight of polymer melts and concentrated polymer solutions. 

The Metravib Micromecanalyser is an inverted torsional pendulum, but unlike the torsional pendulums described eadier, it can be operated as a forced-vibration instmment. It is fully computerized and automatically determines G, and tan 5 as a function of temperature at low frequencies (10 1 Hz). Stress relaxation and creep measurements are also possible. The temperature range is —170 to 400°C. The Micromecanalyser probably has been used more for the characterization of glasses and metals than for polymers, but has proved useful for determining glassy-state relaxations and microstmctures of polymer blends (285) and latex films (286). 

Stress relaxation. In a stress-relaxation test a plastic is deformed by a fixed amount and the stress required to maintain this deformation is measured over a period of time (Fig. 2-33) where (a) recovery after creep, (b) strain increment caused by a stress step function, and (c) strain with stress applied (1) continuously and (2) intermittently. The maximum stress occurs as soon as the deformation takes place and decreases gradually with time from this value. From a practical standpoint, creep measurements are generally considered more important than stress-relaxation tests and are also easier to conduct. 

Creep and stress-relaxation tests measure the dimensional stability of a material, and because the tests can be of long duration, such tests are of great practical importance. Creep measurements, especially, are of interest to engineers in any application where the polymer must sustain loads for long periods. Creep and stress relaxation are also of major importance to anyone interested in the theory of or molecular origins of Viscoelasticity. 

The viscoelastic behavior of concentrated (20% w/w)aqueous polystryene latex dispersions (particle radius 92nm), in the presence of physically adsorbed poly(vinyl alcohol), has been investigated as a function of surface coverage by the polymer using creep measurements. From the creep curves both the instantaneous shear modulus, G0, and residual viscosity, nQ, were calculated. 

In this paper we report some rheological studies of aqueous concentrated polystyrene latex dispersions, in the presence of physically adsorbed poly(vinyl alcohol). This system has been chosen in view of its relevance to many practical systems and since many of the parameters needed for interpretation of the rheological results are available (15-18). The viscoelastic properties of a 20% w/w latex dispersion were investigated as a function of polymer coverage, using creep measurements. 

Rheological Measurements Three types of rheological measurements have been carried out. In the first type, transient (creep) measurements were performed on a 20% w/w dispersion of latex A, as a function of coverage by PVA. These experiments were carried out using a "Deer" rheometer (PDR 81, Integrated Petronic Systems, London) fitted with a stainless steel concentric cylinder. The procedures used have been described in detail before (21,22). 

Creep measurements of PpPTA show that 0.025strength loss is 0.04observed value for PpPTA fibres is about 5%. 

Creep feed grinding wheels, 1 20-21 Creep feeding, for young animals, 10 873 Creep measurements, 21 741-742 Creep rate, 13 472 Creep resistance, 13 473... 

The presumed effect of this pressure difference on the zero-creep measurements was rarely mentioned in the literature, but its existence would not be too difficult to test. If Pc were as important as believed71, then the W0 of a prismatic wire would have been twice that of a cylindrical wire of an identical cross section, and the WQ of a wire with a flat horizontal end would be quite different from that of a wire ending in a hemisphere. Neither of these differences ever was observed. 

Foamed blends of ethylene-styrene interpolymer and LDPE were subjected to a range of mechanical tests, including compressive impact testing, Instron compression and Poisson s ratio measurements, compressive creep measurements and compression set and recovery measurements. The data obtained were compared with those for EVA and the suitability of these foamed blends as replacements for EVA in the manufacture of soccer shin guards and midsoles for sports shoes was evaluated. 20 refs. 

Table 2.6 Data on creep measurements for control and water-reduced superplasticized concrete...

Fig. 35. Apparatus for creep measurements (241). Courtesy of Marcel Dekker, Inc.

The sample shown in Figure 1 contains a molded-in thermocouple. Such samples were used not in creep measurements, but to study the temperature rise during irradiation. 

The temperature rise in %-inch thick samples was also measured and calculated, although no %-inch samples were used for creep measurements. The measured maximum temperature rise was 41.5°C., and the calculated temperature rise was 41.7°C. The bulk of the study reported here is for 0.033-inch thick polystyrene samples, and for the typical conditions of this work the average temperature rise is 63.5% of the maximum temperature rise or 1.8°C. 

Experiment III. The sample was stressed (2300 p.s.i.g.) and allowed to creep for 10 minutes, at which time the beam was turned on for 5.5 minutes. The beam was then turned off while the stress remained on. Deflection measurements continued for 2 minutes after the beam was turned off. This run was a typical creep measurement before, during, and after irradiation (Figure 6, III). 

Creep. Creep measurements were carried out in a Frank apparatus preheated to the desired temperature by an air bath, using tensile microspecimens 0.1 cm thick, obtained by milling, as described by ASTM D-1708. 

Creep measurements involve the application of a constant stress (usually a shearing stress) to the sample and the measurement of the resulting sample deformation as a function of time. Figure 9.6 shows a typical creep and recovery curve. In stress-relaxation measurements, the sample is subjected to an instantaneous predetermined deformation and the decay of the stress within the sample as the structural segments flow into more relaxed positions is measured as a function of time. 

Admixtures Mixture proportions Properties of fresh Creep measurements on 150 > 300 mm... 

Some characteristic values for engineering plastics have been derived from the literature data on creep measurements (see, e.g., Ogorkiewicz (1970)) and are given in Table 13.11. 

Fig. 10. Comparison of calculated [32] and measured shift factor of PVAc. Circles are data from the shear creep measurement [33]...

The influence of addition of sodium bentonite (a commonly used antisettling system) on the rheological behaviour of a pesticide suspension concentrate (250 g dm ) has been investigated. Steady state shear stress-shear rate curves were carried out to obtain the yield value and viscosity as a function of shear rate. The shear modulus was also measured using a pulse shearometer, and the residual viscosity was obtained in afew cases from creep measurements. The rheological parameters Tg (Bingham yield value),... 

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