A comparative study on the Θ projection and the Ω method

Nowadays, most of plant owners have sought for a deterministic technique to assess the remnant life of their old components. In the case of high temperature equipment serviced in the creep regime, the life assessment is more difficult than that for other damage like corrosion since the accumulated damage is not necessarily detectable. In order to predict the materials' response at high temperatures, a constitutive equation precisely describing their creep behavior is required. In the current work, the Theta projection and the Omega method have been examined using creep data for 1.25Cr-0.5Mo and 2.25Cr-1 Mo steel. Two different tertiary creep behaviors were observed in experiments, namely, proportional rise in strain rate with strain assumed in the former methodology and exponential rise in strain rate with strain assumed in the latter. The Theta projection has unexceptionally described the creep behavior well when the strain was lower than 10%, whereas the tertiary creep behavior observed in not a few experimental cases, in which the proportionality of strain rate-strain correlation is retained in almost the whole tertiary regime, cannot be explained by the Omega method. The apparent exponential rise in the tertiary creep rate found in the final stage of creep should be attributable to the experiment using a constant load technique, in which cross-sectional stress is increasing with strain. The creep behavior of actual components, which are exposed to the condition under the constant stress rather than constant load, would be more precisely described by the Theta projection.