Pressure dependence and general states of stress

We have seen that the effect of pressure on the shear yield stress of a polymer can be represented very well by Equation (11.9), x = Xq + ap, which suggests that the Eyring equation may be modified very simply [31] to include the effect of the hydrostatic component of stress p, giving 

Equation (11.11) may be expressed conveniently in terms of the octahedral yield stress 

For a constant strain rate test we therefore have 

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