Background/Overview
The approach for the assessment of fatigue damage from cyclic variation of stress, strain and temperature as experienced in the hot section of a gas turbine uses the range of stress or strain and the maximum temperature attained in the cycle against which to predict the damage from fatigue curves. This results in a fatigue life based on an elastic prediction which is then assessed against the engine life requirements. Recent methods have proposed the generic use of analyses with plasticity and creep included to determine the relaxed stress and strain state with a revised fatigue damage assessment approach.

In the case of hot section components, the stress or strain may vary in-phase with the temperature, or there may be a phase difference which at its extreme would be completely out-of-phase with the temperature. This case occurs in a hotspot where the maximum temperature condition generates a significant compressive stress.

In other types of cycle where transient effects occur due to the boundary conditions of the external hot gas temperature and the internal cooling temperature combined with the mechanical loading due to rotational and gas loading forces, the peak stress or strain may develop on the rise or fall to the peak temperature condition and would correspond to a much lower temperature than the peak.

Such components do have the added benefit of thermal barrier coatings, but to date, little understanding is currently available that truly captures how the mechanical behaviour differentiates across the two materials.

Project Aims
The aim of this project is to investigate thermo-mechanical fatigue damage in high temperature coated systems through a thorough review of existing data available on single crystal materials, to develop a TMF lifing model that can represent the true service behaviour observed including the influence of creep and oxidation that occurs at high stresses and temperatures, to validate this model with targeted TMF tests and to investigate the fracture behaviour of the TMF experiments in comparison with predicted stress fields from finite element models.

The work could investigate how damage is accumulated in a range of TMF cycles, predicting the first cycle and stabilised cycle responses under these conditions. The incremental accumulation of damage around the cycle may generate an understanding of the driving behaviours in TMF. In particular, a holistic knowledge of the damage evolution experienced in a counter-clockwise -135 degree cycle is of great interest. Research could also incorporate the use of PD crack monitoring and Digital Image Correlation (DIC) for advanced characterisation of the TMF behaviour.

The materials of choice would be the nickel based single crystal superalloys CMSX-3 and CMSX-4 with Pt and PtAl coating systems.

TMF test data from Rolls-Royce would be referred to in addition to that available from open literature.

Outline Plan
1) Familiarisation with previous work, literature, and fatigue data on TMF in single crystals, with particular emphasis on coated materials.
2) Develop TMF testing capability and propose validation tests.
3) Perform validation tests including fractographic studies.
4) Demonstrate application to an engine component.