Creep recovery

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

When a fiber is stressed, the instantaneous elongation that occurs is defined as instantaneous elastic deformation. The subsequent delayed additional elongation that occurs with increasing time is creep deformation. Upon stress removal, the instantaneous recovery that occurs is called instantaneous elastic recovery and is approximately equal to the instantaneous elastic deformation. If the subsequent creep recovery is 100%, ie, equal to the creep deformation, the specimen exhibits primary creep only and is thus completely elastic. In such a case, the specimen has probably not been extended beyond its yield point. If after loading and load removal, the specimen fails to recover to its original length, the portion of creep deformation that is recoverable is still called primary creep the portion that is nonrecoverable is called secondary creep. This nonrecoverable elongation is typically called permanent set. 

A different design approach is used in this case. Instead of assuming an apparent modulus of elasticity using a constant creep situation covering the life of the chair, it is better to determine the actual creep deflection over a typical stress cycle, the creep recovery over a non-use cycle, and so on until the creep is determined after a series of what might be considered typical hard usage cycles for the chair. The accumulated creep after a period of two weeks can be assumed to represent the base line for an apparent modulus of elasticity to determine the design life of the chair. 

The instant the load is removed there is a reduction in the elongation of the model equal to aJEx. The equation for subsequent creep recovery is... 

Figure 4 Creep and creep recovery of a four-clement model.

For tensile creep, TJ would be the tensile viscosity. When the viscosity is high (e.g., when working at relatively low temperatures or with very high-molecular-weight polymers) it can be difficult to determine tl-x accurately, so creep recovery measurements are made. Here the load is released after a given creep time and the strain is followed as the specimen shrinks back toward its new equilibrium dimensions. 

Assuming thai the Boltzmann superposition principle holds and that all of the creep is recoverable, what would the creep recovery curve be for I he polymer in Problem 1 if the load were removed after lO.(KM) min ... 

As shown by Fig. 3.11 for an applied force, the creep strain is increasing at a decreasing rate with time because the elongation of the spring is approaching the force produced by the stress. The shape of the curve up to the maximum strain is due to the interaction of the viscosity and modulus. When the stress is removed at the maximum strain, the strain decreases exponentially until at an infinite time it will again be zero. The second half of this process is often modeled as creep recovery in extruded or injection-molded parts after they cool. The creep recovery usually results in undesirable dimensional changes observed in the cooled solid with time. 

Creep recovery response is due to freezing in local deformation of the polymer molecules when the polymer cools rapidly. The polymer molecules are frozen into shapes that distort their Gaussian spherical equilibrium shape. If the polymer is heated or allowed to relax over a very long time there will be dimensional changes as the polymer molecules assume their thermodynamic equilibrium states (Gaussian spherical equilibrium shape). 

The mechanical properties at low strain rates, dynamic mechanical properties, creep-recovery behaviour, thermal expansion and thermal conductivity of foams manufactured from blends of LDPE with an EVA and with an isoprene-styrene block copolymer were studied as a function of the LDPE content in the blends. The experimental results demonstrated important aspects related to the modification of the foam properties by blending. 16 refs. 

CRASH RESISTANCE, 38 297 CRASH SIMULATOR, 297 412 CRASHPAD, 412 CREEP, 33 82 89 90 154 238 246 254 266 303 383 394 CREEP RECOVERY, 178 CREEP RESISTANCE, 184 CRITICAL SHEAR RATE, 319 333... 

Fig. 1.49 The creep and creep recovery of concrete containing a hydroxycarboxylic acid water-reducing agent under saturated conditions (Neville).

In creep and creep recovery experiments, the stress is imposed and the deformation is observed. Equation (3.8) can be inverted to describe those cases ... 

This discrepancy is rather disturbing because the creep recovery experiments were obviously designed and tested with considerable care (186). Furthermore, as pointed out earlier, creep recovery is the most direct method for measuring J°. Most of the likely errors (such as inertial effects in the instrument or nonattainment of steady state) would tend to give values which are too small rather than too large. Similar but smaller effects have been observed by other methods in solutions of polyisoprene (167) and poly (a-methyl styrene) (187). In the latter... 

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 (

Osaki,K., Einaga,Y., Kurata,M., Tamura,M. Creep behavior of polymer solutions. I. A new kind of apparatus for creep and creep recovery. Macromolecules 4, 82-87 (1971). 

In Chap. 13 the creep recovery of a Burgers element was discussed and from Fig. 13.18 it becomes clear that the recoverable shear creep strain is in the present terms equal to... 

Fig. 5.1 Idealized representation of the transient change in fiber and matrix stress that occurs during the isothermal tensile creep and creep recovery of a fiber-reinforced ceramic (the loading and unloading transients have been exaggerated for clarity). It is assumed that the fibers have a much higher creep resistance than the matrix. The matrix stress reaches a maximum at the end of the initial loading transient. After full application of the creep load, the matrix stress relaxes and the fiber stress increases. Upon specimen unloading, elastic contraction of the composite occurs, followed by a time-dependent decrease in fiber stress and increase in matrix stress. Overall, creep tends to increase the difference in stress between constituents and recovery tends to minimize the difference in stress. After Wu and Holmes.15...

Figure 2 Creep-recovery tests of chemically treated woods. U, untreated wood Fs, vapor phase formalization F, liquid phase formalization A, acetylation PO, etherification with propylene oxide MG, treatment with maleic acid and glycerol PFl, impregnation with low molecular weight phenol-formaldehyde resin PEG-ICP, impregnation with polyethylene glycol (PEG-IOOO) WPC, formation of a wood- polymer composite (PMMA) WIC, formation of a wood-inorganic material composite.

Researchers have examined the creep and creep recovery of textile fibers extensively (13-21). For example, Hunt and Darlington (16, 17) studied the effects of temperature, humidity, and previous thermal history on the creep properties of Nylon 6,6. They were able to explain the shift in creep curves with changes in temperature and humidity. Lead-erman (19) studied the time dependence of creep at different temperatures and humidities. Shifts in creep curves due to changes in temperature and humidity were explained with simple equations and convenient shift factors. Morton and Hearle (21) also examined the dependence of fiber creep on temperature and humidity. Meredith (20) studied many mechanical properties, including creep of several generic fiber types. Phenomenological theory of linear viscoelasticity of semicrystalline polymers has been tested with creep measurements performed on textile fibers (18). From these works one can readily appreciate that creep behavior is affected by many factors on both practical and theoretical levels. 

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