Creep Eyring equation

In both polymers, creep of compression-molded specimens is caused mainly by crazing, with shear processes accounting for less than 20% of the total time-dependent deformation. Crazing is associated with an increasing creep rate and a substantial drop in modulus. The effects of stress upon creep rates are described by the Eyring equation, which also offers an explanation for the effects of rubber content upon creep kinetics. Hot-drawing reduces creep rates parallel to the draw direction and increases the relative importance of shear mechanisms. 

Equation 30 shows that the yield stress is both rate and temperature dependent, hence it captures some important features of yield in polymers. For example, Figure 10 (44) shows a plot of ax/T (or aJT in the notation of Reference (44)) as a function of log strain rate and, as predicted by equation 30, a linear relationship is seen at each temperature. It is worth noting that the Eyring equation (typically in the form of two activated processes acting in parallel) has been successfiilly applied not only to the yield behavior of polsrmers but also to the creep rupture behavior of isotropic and oriented polymers (45- 8). 

Two principal approaches have been used to model the yield behaviour of polymers. The first approach addresses the temperature and strain-rate dependence of the yield stress in terms of the Eyring equation for thermally activated processes [39]. This approach has been applied to many amorphous and crystalline polymers (see Section 12.5.1) and links have been established with molecular relaxation processes determined by dynamic mechanical and dielectric measurements and with non-linear viscoelastic behaviour determined by creep and stress relaxation. The Eyring approach assumes that the yield process is velocity controlled, i.e. the yield process relates to existing thermally activated processes that are accelerated by the application of the yield stress to the point where the rate of plastic deformation reaches the applied macroscopic strain rate. This approach has... 

As shown in Sect. 2, the fracture envelope of polymer fibres can be explained not only by assuming a critical shear stress as a failure criterion, but also by a critical shear strain. In this section, a simple model for the creep failure is presented that is based on the logarithmic creep curve and on a critical shear strain as the failure criterion. In order to investigate the temperature dependence of the strength, a kinetic model for the formation and rupture of secondary bonds during the extension of the fibre is proposed. This so-called Eyring reduced time (ERT) model yields a relationship between the strength and the load rate as well as an improved lifetime equation. 

The Eyring reduced time model provides the framework for the derivation of the creep equation of polymer fibres [10]. The creep shear strain of a domain... 

These equations predict a continuously diminishing rate of creep. Many empirical and semi-empirical models of creep-strain have been made and are described by Ward [24], One of these has been used successfully to describe the later stages of creep in polymers such as oriented polyethylene. The Arrhenius equation was modified by Eyring to apply to the rate of creep (deJdt) in the following way ... 

Creep and yielding are stress- and temperature-activated processes, which in many mateiMs, including pdymers, follow the Eyring rate equation ... 

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... 

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