Evaluation of the Theta (Θ) Projection Technique for Estimating the Remaining Creep Life of GTD-111DS Turbine Blades

Remaining life estimates for hot section turbine components often involves the use of creep models based on parametric extrapolations of short-term stress rupture tests. For the assessment of components such as gas turbine blades fabricated from nickel based superalloys, strain based approaches may be more useful since life is often evaluated in terms of the time to reach a critical strain limit. In the case of GTD-111DS, General Electric (GE) recommends a retirement limit in the range of 0.5%–1% strain at any point in the airfoil. Tertiary models such as the MPC Omega method, which has gained some popularity in the refining industry, are generally adequate for predicting rupture or retirement times in situations where life is dominated by secondary/tertiary stage creep. However, a constitutive model that describes the full creep curve is needed to adequately evaluate life for highly creep resistant alloys such as GTD-111DS, where a significant portion of service life is consumed with the initial small increments of strain. In this paper, the Theta (Θ) projection technique is used to analyze the creep behavior of GTD-111DS. The statistical variability of extrapolating short term creep tests to actual service conditions using the Θ model is reviewed, and use of the model is discussed in relation to evaluating the life of GTD-111DS turbine blades.