Recovery from creep

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. 

Creep measurements on single PpPTA filaments have been performed by Walton and Majumdar. They observed creep strains amounting to less than 20% of the initial elastic strain after several years under stress. The graphs of the creep plotted against the logarithm of time were not linear but showed an increasing slope. Ericksen observed logarithmic creep curves and, in addition, found that the strain after recovery from creep also... 

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. 

Figure 9.11. Recovery from tensile creep of an acetal copolymer at 20°C and 65% relative humidity. (From TCI Technical Service Note G117, reproduced by permission of ICI Plastics Division )...

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. 

Fig. 5.14. Reduced compliance vs molecular weight for undiluted polystyrenes of narrow molecular weight distributions. Symbols are O from creep recovery (163), Cr from G (w) (192), O- from flow birefringence (180), (X from (189), 9 from G (a>) (M>105 only) (124), jO extrapolated from steady state creep (191), -O from stress relaxation (165), and...

Fig. 5.15. Reduced compliance vs the cMw product for solutions and undiluted samples of narrow distribution polystyrenes. Symbols are O from flow birefringence (179), O from several methods (185 O from Nt (178), CX from G (co) and iVt (177), Q from G (oS) (175, 176), JD from Nl (184), from creep recovery (163), 4 from G (

A variety of rheological tests can be used to evaluate the nature and properties of different network structures in foods. The strength of bonds in a fat crystal network can be evaluated by stress relaxation and by the decrease in elastic recovery in creep tests as a function of loading time (deMan et al. 1985). Van Kleef et al. (1978) have reported on the determination of the number of crosslinks in a protein gel from its mechanical and swelling properties. Oakenfull (1984) used shear modulus measurements to estimate the size and thermodynamic stability of junction zones in noncovalently cross-linked gels. 

The loss of modulus caused by crazing becomes less pronounced as the draw ratio is increased, especially in tests carried out at lower stress levels. This observation supports earlier conclusions drawn from creep studies on other rubber-toughened plastics (6) if the specimen can reach a strain of 5% largely or entirely by shear mechanisms, the loss of modulus resulting from the creep and recovery program is quite small if, on the other hand, crazing is the dominant mechanism, the loss in modulus is large. 

Recovery from fermentation broth and separation of carboxylic acids, amino acids were tested by many authors using layered BLM, rotating, creeping, spiral-type FLM, HELM, HLM, and MHS-PV techniques of the OHLM processes. The research works for the last 15 years in this field classified according to the OHLM techniques with types of membrane walls and carriers used are provided in Table 13.7. 

This means that for the boards at 14-in. span and 24-in. span, the residual creep (after 24-h load) was about the same, namely 75-76% from under 2 X design load and 69-71% from under 2.5 X design load (82% recovery from the instant deflection for the both boards if we consider only the second creep). 

Fractional recovered strain vs. reduced time (= recovery time/creep time) for acetal copolymer at 20 °C and 65% humidity. The data from different creep times and stresses do not superimpose (from Thermoplastics and Mechanical Engineering Design, ICI Plastics Division, booklet GI 17). 

Figure 3.17 Strain versus time representation of the stress-controlled programming performed at 263 kPa followed by free recovery. The creep effect in the foam during step 1 is shown in the subplot. Source [41] Reproduced with permission from Elsevier...

The mechanical properties of textile fibers, yarns, and fabrics may be more fully determined by subjecting the substrate to small forces in directions other than along the fiber axis. Tear, bending, and shear strengths, as well as recovery from bending and abrasion resistance, etc., also influence the wear properties of textiles. Finally, time-dependent extension and recovery, termed creep, or creep recovery, respectively, is of importance in determining the performance of fibers in industrial applications. A discussion on the measurement of these parameters, however, is beyond the scope of this article. 

PI Enthalpy recovery and creep evolution (down-jumps) Relaxation times from fit of data are longer for enthalpy than for creep. 65... 

The shelf is assumed to be loaded and unloaded several times during the design life of 5 years. When the load is removed, the link element recovers partially toward its original length. From our analysis, using the principle of superposition, the recovery would be the elastic strain and a portion of the viscoelastic strain, depending on the length of time allowed for recovery. The creep portion of the strain is permanent. 

Recovery curves from creep under constant stress are also illustrated in Figure 4.2, which indicates that at any selected time the extent of recovery (and so the unrecovered strain) is directly proportional to the stress that had been applied formerly. The relation between recovery and creep will be derived in Section 4.2.1. 

Figure 4.26. A schematic representation of creep and recovery [From Lakes (1999) see http //silver.neep.wise.edu/ lakes/].

Fig. 2.25. The equilibrium compliance and high-frequency limiting compliance JoQ plotted as functions ofT — T. Jq values for various molecular weights (580 (V), 3500 (o), and 10200 (A)) are from cyclic shear data of Gray et al. [202]. /e for molecular weight 3400 (-h) is from creep-recovery data of Plazek and O Rourke [90]. Curves for the M = 3500 and 10200 samples are calculated from the equation given in table 5 of [90]. The dashed curve is an extrapolation outside the range of measurement. The dotted lines are the values predicted by the Rouse theory, 0.4M/(p 7). Values of J o for all polymers were obtained from measurements at temperatures from Tg to Tg -h 20 K and extrapolated to higher temperatures.

The general characteristics of creep compliance to steady state and recovery from the steady state are sketched in Fig. 3.6. 

Two quantities that play important roles in flow behavior of polymeric liquids are the steady-state viscosity at zero shear rate, rto, and the steady-state recoverable shear compliance, J°. Both are obtained quite directly from creep results, from (To and the shear rate fss in the steady-state region of the creep phase and J° from the total recoil strain (yr) in the recovery phase ... 

The steady-flow viscosity qo and the steady-state compliance can easily be determined from creep data in the region of linear viscoelastic behavior as shown-in Fig. 1-12, from equation 40 of Chapter 1, provided steady-state flow has been attained. However, it is easy to be misled into believing prematurely that the linear portion of the creep curve has been reached in general, it cannot be expected to become linear until the flow term t/vo is at least as large as the intercept / . It is always desirable to perform the recovery experiment shown in Fig. 1-12 to conflrm the calculation. 

Leaderman [37] carried the type of analysis used by Smith one stage further in analysing the creep and recovery of a sample of plasticised polyvinyl chloride (PVC). The apparently remarkable result was obtained here that the initial rate of recovery from a given load was larger than the initial creep under that load (see also Section 11.4). The situation is illustrated in Figure 11.4. 

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... 

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