Recovery After Creep

Variation of cell gas pressure and initial yield stress for PP foam of density 20 kg m 2, versus logarithm of the 

Although, in principle it is now possible to study deformation of internal cells in foams using CT microtomography, no such research has been reported. 

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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. 

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. 

Figure 3-38. An example of strain behavior under various intermittent and cyclic loads, a) Recovery after creep b) a strain increment caused by a stress step function c) strain with stress applied 1) continuously and 2) intermittently.

Recovery after creep is a slow process [84,85[. For LDPE, EVA, and PP foams, subjected to creep for 10 s, it appears that 100% recovery from... 

Viscoelasticity can also be determined by a controlled stress rheometer. The shape of a creep curve can show that a fluid is viscoelastic, and the amount of recovery after the stress is removed gives a measure of elasticity. 

Fig. 15. Measured shear strain during creep under a constant shear stress and viscoelastic recovery after cessation of shear for PDMS near the gel point [71] plotted against the time. The solid lines are predicted by the gel equation for finite strain...

Compressive creep experiments were performed on an EVAC foam from a rnnning shoe. The recovery process after creep is discussed. Modelling was performed of gas diffusion perpendicular to the stress axis and along the stress axis. 16 refs. 

Creep, stress relaxation and set are all methods of investigating the result of an applied stress or strain as a function of time. Creep is the measurement of the increase of strain with time under constant force stress relaxation is the measurement of change of stress with time under constant strain and set is the measurement of recovery after the removal of an applied stress or strain. It is important to appreciate that there are two distinct causes for the phenomena of creep, relaxation and set, the first physical and the second chemical. The physical effect is due to rubbers being viscoelastic, as discussed in Chapter 9, and the response to a stress or strain is not instantaneous but develops with time. The chemical effect is due to ageing of the rubber by oxidative chain scission, further crosslinking or other reaction. 

The rubber industry has traditionally paid more attention to measuring the recovery after removal of an applied stress or strain, i.e. set, than to creep or stress relaxation. This is partly because relatively simple apparatus is required and it is a convenient way to get an indication of the state of cure, but also because it appears at first sight that set is the important parameter when judging sealing efficiency. Set correlates with relaxation only generally and it is actually the force exerted by a seal that usually matters, rather than the amount it would recover if released. 

To determine if either of these answers were correct, the properties of a series of polyurethane networks which contained photo labile disulfide bonds were studied (7). The creep curve of the s imple in the initial crosslink density state (comparable to Vq) was determined in the usual way. Then, after complete recovery, another creep experiment was started but, at time t during this run, the sample was subjected to ultraviolet irradiation which photochemlcally broke some of the network chains. 

After the irradiation was extinguished, the creep experiment was continued for the usual period. Finally, after complete recovery, the creep response of the network at the new crosslink density... 

Lack of recovery after compression loading at elevated temperature probably results from a combination of creep and recrystallization of the sample while in the stressed condition. That recrystallization plays a... 

Measurement of set is effectively restricted to rubbers and flexible cellular materials, where it has traditionally been paid rather more attention than stress relaxation and creep tests. Its popularity has a lot to do with the simple apparatus required. If set is measured on plastics it is usually made by following recovery after removal of load in a creep test. 

The recovery from creep, which is the decrease in strain at any given time after completely unloading the test piece. It is expressed as a percentage of the strain just prior to the removal of the load. 

The steady-state recoverable compliance can be measured by fitting Eq. (6.13) to creep data at long times, or more accurately, jf is determined from the recovery after removal of the stress subsequent to attainment of steady-state flow. 

Moreover, a total recovery of an initial creep rate was observed after a reverse jump in temperature. This assumed that the structural state of a specimen remained unchanged during the measuring process. In addition, in the extension experiments a specimen was set into the special clamps providing high friction and preventing the errors from a slip the latter was confirmed by a fuU recovery after unloading. 

When a load is removed from a metal part, the recovery from strain, if it occurs, is instantaneous. With plastics, much of the strain is recovered reasonably quickly but a proportion is recovered only after a period of time. Thus, recovery, like creep, is time-dependent. Recovery data are provided as fractional recovered strain vs. reduced time (Figure 2.8), where ... 

Mechanical Properties. Water-equilibrated fibrin film can be stretched to two to three times its initial length, and, unless held for some time in the stretched state, returns to its original shape when the load is removed. The response to stress is not as rapid as in true rubberlike materials, and the film undergoes slow creep under constant load, as well as slow creep recovery after removal of load. Furthermore, stress-strain curves over cycles of elongation and contraction exhibit marked hysteresis. 

Figure 7.10 Experimental and theoretical prediction for UHMWPE strain under creep and creep-recovery after 1000 s loading conditions for different stress levels.

Elastic creep recovery can be observed after short term loading (10 sec) as illustrated in Figure 7. Since this recovery is primarily induced by the filler, one can observe distinct differences in the properties of the RAC s as the filler content is increased. Addition of 10% powdered rubber to an AC10 produced a mixture with a creep recovery better than that of an AC30 from the same source. The stress relaxation process is rather slow creep recovery after medium term loading (100... 

Figure 7. Creep recovery after 10 sec loading of powdered rubber AC 10-3 mixtures. Reference binder AC30-4.

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