Polymer fibre creep rupture

An understanding of the mechanism of creep failure of polymer fibres is required for the prediction of lifetimes in technical applications. Coleman has formulated a model yielding a relationship similar to Eq. 104. It is based on the theory of absolute reaction rates as developed by Eyring, which has been applied to a rupture process of intermolecular bonds [54]. Zhurkov has formulated a different version of this theory, which is based on chain fracture [55]. In the preceding sections it has been shown that chain fracture is an unlikely cause for breakage of polymer fibres. 

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 time-dependent behaviour is different in the two types of highly oriented, chain-extended polymer fibres. Table 1 gives the results of studies in FIBRE TETHERS 2000 (1995), which were made because creep rupture is a concern in deep-water mooring of oil-rigs. The low-load creep in aramid fibres is due to a straightening of the initial. structure. It reduces in rate, even on a logarithmic scale, with time and is not a source of creep rupture. In Vectran, the creep is less and is absent after 10 days under load. 

Thermosetting polymers are viscoelastic at ambient temperature and unreinforced are susceptible to creep. Fibre reinforcements are generally elastic and creep resistant but some types may show the phenomena of stress rupture if exposed to high stresses in adverse environments. Composites reinforced with unidirectional stiff fibres and stressed in the fibre direction can be highly creep resistant as the fibre attracts a high proportion of the load in line with its stiffness relative to the polymer matrix. The behaviour of composites under static loading is a function of many material variables polymer type... 

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