7.
Creep of Kevlar 29, Kevlar 49 and PRD 49-III fibres was investigated. The fibres exhibited transient
creep and the strain-time relationship was represented by a logarithmic time law. The
creep strain recovered with time when the load was removed. Upon reloading to the same
creep stress the strain-time relationship was again logarithmic but the
creep rate was reduced. Modulus measurements were made during the
creep test and these showed that the modulus increased with time. This result indicated a crystallite rotation mechanism which could account for the experimentally observed
creep strain. Creep in PRD 49-III fibres exhibited a small temperature dependence over the temperature range 20°C to 150°C. The apparent
creep activation energy was consistent with the range of values reported for hydrogen bonding. This suggests one possible
creep mechanism in which the combined action of stress and thermal activation causes rearrangement of intercrystalline bonds in the crystallite boundaries resulting in boundary
creep. Boundary
creep allows crystallite rotation which produces the macroscopic
creep strain. Boundary
creep is discussed in terms of the fibre morphology and a model of delayed elasticity.
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