Constant True Stress
Is the applied stress a nominal or a true value. It certainly should be a true value, as this is implicit in the M-N law. However, it is common during testing to fix the load (often in the form of a dead weight), rather than the true stress. Also, most uniaxial creep tests tend to be carried out in tension. Neglecting any inhomogeneity that might arise, such as a necking effect - which is not common during creep testing - the true stress will rise as straining occurs and the cross-sectional area reduces. For all values of n, this will cause the strain rate to rise, whereas it would otherwise be falling and approaching a constant value.