An Examination of Primary Creep in Gas Turbine Applications

Abstract Primary creep is possibly the most neglected phase of creep in the structural analysis of gas turbine components. Significant testing and literature have focused on the characteristics of minimum creep rate and stress rupture, but primary creep is largely ignored or simply described as instantaneous strain. At higher stress levels, non-trivial primary creep strains and plastic strains may begin accumulating even as load is applied. Neglecting primary creep may be acceptable for materials with minimal primary. For longer life gas turbine components like rotors, that never approach rupture, primary creep may be the main factor to redistribute stress, and the resultant mean stress relaxation may drastically affect fatigue life. For alloys such as single-crystal nickel and directionally solidified gamma prime strengthened super alloys, the magnitude of primary creep strain may be nontrivial. This paper explores the influence of temperature and stress on primary creep. The similarity between the hardening mechanism of primary creep and plasticity are explored. As stress increases and the temperature drops, it becomes difficult to draw a dividing line between plasticity and primary creep. It is suggested that primary creep and plasticity not be separated. An empirical formula describing the primary creep with plasticity in finite element analysis is proposed. Progressive and stepped creep tests were performed on several rotor alloys to demonstrate the increase of primary strain with stress. These tests demonstrate a potential to characterize creep behavior with a reduced number of tests.