08-08-2012, 11:31 AM
On thermo-mechanical fatigue in single crystal Ni-base superalloys
On thermo-mechanical fatigue in single crystal.pdf (Size: 347.36 KB / Downloads: 41)
Abstract
Thermo-mechanical fatigue (TMF) is a critical damage process in gas turbine jet engines. Reliable life prediction methodologies
are required both for design and life management. Current life estimation approaches are computationally burdensome and/or
semi-empirical, significantly limiting their application. Complexity comes about from the multiplicity of damage processes
which occur during the simultaneously changing temperatures and loads, characteristic of TMF. Furthermore, these processes
interact in ways that are not observed for isothermal LCF. These interactions usually accelerate damage processes and result in
significantly reduced TMF life, when compared to other fatigue scenarios. The results of a multiphased approach to life
prediction will be presented. In phase I the Neu-Sehitoglu (N-S) cumulative damage model was used to: a) provide initial life
predictions and b) identify processes and interactions which most significantly control the life under TMF loading. The N-S
model is based on a linear damage summation rule which both explicitly and implicitly includes damage interactions. In phase II
a sensitivity analysis incorporating statistical concepts was performed on the N-S model parameters. Specifically a nonlinear
optimization was performed to determine optimal parameter values in order to maximize agreement with experimental results
(well within 2X). In phase III, informed by phases I and II, a physics-based fatigue/oxidation (also recognizing creep effects)
model was developed which correctly predicts effects of frequency, hold-time, temperature, and strain range and oxidation
kinetics.
Introduction
Superalloys are a class of metals that exhibit superior high temperature properties such as very high strength at
temperatures up to 85% of their melting points [1] and were developed for use in aero and power generation
turbines. Of the different forms in which superalloys are manufactured, single crystal superalloys (SXSA) are
favored for use as blade materials in the hot section of turbines because of their fortuitous primary crystal orientation
and resulting radial modulus, as well as their superior creep properties resulting from the elimination of grain
Neu-Sehitoglu parameter sensitivity analysis
The primary results from phase I indicate that an environmental-fatigue term dominates life prediction in the N-S
model as applied to OP TMF. Additionally, results indicate that it may not be necessary to calibrate all of the model
parameters for each particular material of interest. Phase II of this work explore these hypotheses.
In order to determine the sensitivity of the Neu-Sehitoglu life estimation results to the various input parameters, a
study was performed in which each parameter was varied by a specified amount while holding all other inputs at
nominal values. Each input was varied by a minimum of +/- 20% in 0.5% increments. The input parameters which
were studied as well as their nominal values are shown in Table 2. Though the sensitivities of all parameters utilized
in the environmental-fatigue module were studied, four parameters had the highest impact on life estimation: they
are the activation energies (Qox and Q