A model for uniaxial creep based on internal-stress redistribution

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Abstract
The changes of creep-strain rate associated with primary and tertiary creep are attributed to a process of internal-stress redistribution between the strong and weak portions of material while for each portion the creep rate depends only on its stress. An expression for the primary-creep curve is derived which is shown to be in substantial agreement with the usual empirical (time)1/3 power law. The model describes the changes in creep rate when the stress is constant and implies as a natural extension a method of calculating creep rate when the stress changes because memory effects are attributed solely to changes of internal stress. The calculated rates obtained according to the method show the creep-recovery phenomenon and in this respect are superior to the ‘hardening hypotheses’ which fail to simulate creep recovery but are normally used for stress-redistribution calculations. According to the model the strain during primary creep is dependent on the stress derivative of the secondary creep rate. Analysis of results for the aluminium alloy DTD 5070A has helped confirm the model by showing a correlation between these quantities.

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