In the onset of necking

With an increase in strain-rate sensitivity of the flow stress a change in the condition of onset of necking can be expected, since a change in deformation rate results in a change of flow stress without much change in the strain-hardening rate. 

This response is best explored through a different phenomenological statement of the non-linear visco-plastic constitutive relation considered by Hutchinson and Neale (1977) that is given as 

Recognizing that the flow stress is affected both by plastic strain and by its hardening rate, as well as by the strain rate, in a rigid plastic behavior the increase in flow stress is 

In homogeneous, non-localized deformation of a rigid plastic solid as idealized here. 

This gives immediately an expression for the uniform extension strain s at impending localization beyond which straining ceases to be uniform, i.e.. 

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