The four-year research project BE 96-3911 set out to provide an improved basis for accounting for the anisotropic mechanical behaviour of single crystal super-alloys under conditions relevant to high temperature service. Two micro-mechanical models were used for predicting deformation and failure of single crystal (SX) gas turbine blading. The models were tuned on the basis of uniaxial creep and fatigue tests. Later they were validated on the basis of discriminatory mechanical testing, such as multiaxial creep and thermal fatigue tests. New results were obtained in a number of research areas. The project output includes:
- The establishment of a database of creep and low cycle fatigue (LCF) tests for carefully characterised crystal orientations.
- Metallographic identification and characterisation of deformation and damage mechanisms using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in combination with image analysis.
- Micromechanical modelling to establish a physically based life prediction methodology, which addresses damage accumulation and failure.
- Multiaxial and thermal fatigue benchmark testing of single crystal specimens, including specimen surface imaging.
- Validation and refinement of deformation and damage models with the previous results.
- Assessment of the validated model with appropriate material databases using the results of multiaxial creep and thermal fatigue tests as benchmarks.
Further progress in the field:
The results of the present project have increased the basic understanding of deformation and damage processes in SX super alloys under complex loading conditions. The final industrial assessment of the two modelling approaches developed, highlights the complexity of high temperature anisotropic materials behaviour.
Accordingly, certain limitations have been identified, which can be partly resolved through improved fitting of the models to experimental data or can be the focus for further detailed research. Trial finite element (FE) computations with a candidate blade, using robust numerical implementations of the models developed here, have however demonstrated the feasibility of carrying out inelastic FE analyses of actual blade components, although high computational effort is still a practical restriction. Therefore, although industry cannot immediately introduce the models developed in the blade design process, significant steps forward have been made and the present project will remain a milestone for further advancements in the field.
