Creep of polymers

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

The models described so far provide a qualitative illustration of the viscoelastic behaviour of polymers. In that respect the Maxwell element is the most suited to represent fluid polymers the permanent flow predominates on the longer term, while the short-term response is elastic. The Kelvin-Voigt element, with an added spring and, if necessary, a dashpot, is better suited to describe the nature of a solid polymer. With later analysis of the creep of polymers, we shall, therefore, meet the Kelvin-Voigt model again in more detailed descriptions of the fluid state the Maxwell model is being used. 

Various models have been proposed to account for the non-linear creep of polymers, such as the Eyring model this model explains some aspects, but fails with other ones, and will not be discussed here. 

Source Reprinted from Yu. Potapov, O. Figovsky, Yu. Borisov, S. Pinaev, and D. Beilin, Creep of Polymer Concrete at Compressive Loading, J. Scientific Israel Technological Advantages 5, nos. 1-2 (2003) 1-10. With permission. 

Ohama, Y., and Hashimoto, H., Drying Shrinkage and Compressive Creep of Polymer-Modified Concrete (in Japanese), pp. 308-311, Semento-Gijutsu-Nempo 32 (Dec. 1978)... 

L. C. Brinson and T. S. Gates, Ejfects of Physical Aging on Long-Term Creep of Polymers and Polymer Matrix Composites, taternational Journal of Solids and Structures, 32,6-7, pp. 827-846 (1995). 

Bae Beake, B. Modelling indentation creep of polymers a phenomenological approach. J. Phys. D 39 (2006) 4478 85. 

The creep of polymer fibers is composed of the primary creep, which is recoverable with time, and of the non-recoverable or secondary creep [186-188]. The secondary creep is almost negligible if a fiber has been mechanically pre-conditioned, i.e. when it has first been stretched to a strain larger than the strain that will be ultimately reached in the subsequent creep experiment. Thus the results of creep measurements can only be interpreted as true viscoelastic measurements when performed on mechanically pre-conditioned fibers. 

Extrapolation of creep test data to characterise the long term performance of the composite is best achieved by fitting a mathematical equation to the available short term test data. The creep of polymers can be modelled by a power law function identified by the linear nature of the data when plotted on a logarithmic axes. 

Creep describes time-dependent permanent deformation of materials resulting from constant structural stress. The creep of polymers can be divided into two main stages primary creep and steady-state creep. The creep strain rate decreases with time in the primary creep and is constant in the steady-state creep. Strain recovery occurs with the removal of external load after a creep time. Therefore, the total strain (e) consists of three separate parts el, e2, and e3. The el and e2 are the immediate elastic deformation and delayed elastic deformation, respectively. The e3 is the Newtonian flow. It was found that the el and e2 decreased with increasing clay contenf indicating lower creep recovery with the addition of C20A. The creep compHance J, the ratio of strain and applied load, can be expressed as... 

Maksimov RD, Sokolov EA, Mochalov VP (1975) Effect of temperatiffe and humidity on the creep of polymer materials. Uniaxial elongation under variable temperatur e - humidity conditions. Mechanics of Composite Materials 11 (6) 834-839 Maksimov RD, Mochalov VP, Sokolov EA (1976a) Influence of temperature and humidity on the creep of polymer materials. 3. Shear, and Shear and Tensile Strain Acting Together. Mechanics of Composite Materials 12(4) 562-567... 

As discussed in Section 11.3.1, Eyring and collaborators had already considered the application of activated rate theory to the creep of polymers. For polymethyl methacrylate, Sherby and Dorn [68] showed that the creep rate could be fitted to an equation of the form... 

Rostoker W, Galante JO. Indentation creep of polymers for human joint applications. J Biomed Mater Res 1979 September 13(5) 825-8. 

There appears to be no information on the uniaxial creep of polymers used as structural adhesives such as is available referring to the creep of adhesive joints in lap-shear or torsion. The latter is reserved for Chapter 7 where the few data that are available are given. An apparent exception is the careful study of a nylon-epoxy adhesive (FM 1000) by Shen and Rutherford (1972). These authors achieved something approaching uniaxial stress using a cylindrical butt joint in direct tension. However, what they called creep was simply the delayed elastic response. They likened the adhesive to a metal in its behaviour and used the classical but inappropriate concept of separating the creep behaviour into three stages as was done many years ago for metals. 

The creep of polymer fibers is composed of the primary creep, which is recoverable with time, and of the non-recoverable or secondary... 

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