cr] is smaller than the actual plastic strain at failure, since in reality the strain rate gradually increases during tertiary creep.
Unfortunately, although creep tests are easy to perform, it is rather difficult to estimate in advance how long the one should test to reach the secondary or tertiary creep regime.
The evolution of the strain rate with strain clearly shows the three distinctive regions: primary creep, secondary creep, and tertiary creep up to failure, see Fig.
You should be looking for the phenomena that cause the tertiary creep
acceleration and fracture.
As is generally the case for creep behavior, the creep curve can be divided into three main regions; the primary creep region where the strain rate decreases with the number of load cycles applied; the secondary creep region where the strain rate is almost constant, otherwise known as the steady state strain rate; and the tertiary creep
region where the strain rate increases rapidly up to failure.
Last, the tertiary creep
occurs when a dramatic increase in creep rate is followed by an eventual rupture.
Life prediction Is usually based on the secondary creep path until the onset of tertiary creep.
Since MOPE is an isotropic volumetrically stable material for which the secondary creep path is known to be established within days , semi-logarithmic techniques can be used  to extrapolate the above behavior prior to the onset of tertiary creep.
No tertiary creep
was observed, even for the highest stress level of 25.