A critical plane approach for LCF evaluation of gas turbine disks and rotors

In order to safely increase gas turbine efficiency without issues of early damage and failures, component life evaluation must neither be too conservative nor too optimistic. The method used for designing the parts is supposed to be as accurate as possible, so that the subsequent application of the appropriate statistically defined safety factors will not excessively reduce the component life estimation. In order to achieve such important targets, all phenomena that can be detrimental for fatigue damage, such as multiaxiality and non-proportionality, need to be properly addressed by the Low Cycle Fatigue (LCF) assessment method. In particular, high temperature gas turbine rotating parts are characterized by a superposition of thermal and centrifugal stresses, which act in the same location but along different directions (i.e. multiaxiality) in different moments of the start-up/base-load/shut-down cycle (i.e. non-proportionality). In this framework, critical plane approaches are the most appropriate methods for an accurate and reliable low cycle fatigue life estimation. These methods search the most critical plane where the greatest damage will be accumulated by defining a damage parameter that is calculated for all planes. Among several methods developed in the academic community and considered in this work, the Fatemi-Socie method was identified as the most effective for the studied materials and components. The Fatemi-Socie damage parameter is a combination of in-plane shear strain and normal stress, which accounts for the fact that fatigue damage (of the materials object of the study) is developed along shear strain with an important contribution of tensile normal stress, which is responsible for crack opening and propagation. An extensive testing campaign was performed to identify the most appropriate approach for the studied materials and to accurately fit all the model parameters. The testing campaign therefore included high temperature LCF tests and several types of multiaxial tests (tension-torsion, notched specimens, etc.). Since the search for the critical plane can be demanding from the point of view of computing resources, an in-house software was developed with the scope of reducing the calculation time consistently with the speed required by the industrial design loops. This paper will cover the background and the assumptions behind the development of a complete industrial workflow for the evaluation of LCF life of rotors and disks, prior to its systematic application for a significant number of Ansaldo components.