Session: 18-05: Failure Prediction & Life Assessment II

Paper Number: 151365

An Examination of Primary Creep in Gas Turbine Applications 

Primary creep is possibly the least understood and most neglected phase of creep. 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, but for alloys such as single-crystal nickel and directionally solidified gamma prime strengthened super alloys, the magnitude of primary creep strain may be nontrivial. Primary creep may be the main factor to redistribute stresses and the resultant mean stress may drastically affect fatigue life. Gas turbine components that never approach their rupture lifetime will still experience primary creep. This paper explores the influence of temperature and stress on primary creep. The similarity between the hardening mechanism or primary creep and plasticity are explored. An empirical formula describing the progression of creep is proposed for implicit creep simulation in finite element analysis.  It is demonstrated that this formulation can simulate a wide range of primary creep curve shapes, including delayed primary creep.  The accumulation of primary creep strain during loading is calculated and a finite element creep simulation will demonstrate the need for consideration during fitting.  Example material tests for multiple nickel super alloys will be shown, including stepped testing that exhibits additional primary creep with load additions.

Presenting Author: William Day Power Systems Mfg LLC

Presenting Author Biography: 38 Years working in the area of mechanical integrity for turbomachinery. Former chair of the MMM Committee. Chief Engineer at PSM. Currently completing a Phd at UCF.

Authors:

William Day Power Systems Mfg LLC
Firat Irmak Florida Institute of Technology
Ali P. Gordon University of Central Florida