Bulk creep

Bulk creep and recovery in systems with viscosity dependent upon free volume. Trans. Soc. Rheology 5, 285—296 (1961b). 

As indicated in another section, the response to an isotropic pressure as a step function of time gives the bulk creep compliance B(t). However, the response to a sinusoidal pressure gives the complex bulk compliance ... 

In other words, the lifetime of a part should decrease exponentially with increasing temperature as well as with increasing applied stress both predictions are borne out by experiments. It is worth noting that Eqs. (12.42) to (12.44) are only valid if the rate of damage generation was controlled by the bulk creep response of the material and steady-state conditions are established during the experiment. 

There is little experimental data and only a small amount of theoretical analysis relevant to the behavior of coarse porosity. Two theoretical approaches will be described, one using the bulk creep properties of the fuel and one based on stress-directed diffusion. Also an empirical approach is outlined. The theoretical treatments assume spherical porosity and cannot cope with changes in void shape as deformation proceeds. 

As the bulk creep properties are used and the effect of microstructure is not considered this analysis can be used to define the lower limit of densification rate. 

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