PhD opportunities

Fatigue crack growth in generalized scale yielding for thermo-mechanical loading

Thesis proposal

Area of expertiseMécanique
Doctoral SchoolISMME - Ingénierie des Systèmes, Matériaux, Mécanique, Énergétique
SupervisorMAUREL Vincent
Research unitCentre des Matériaux
KeywordsFatigue crack growth, superalloys, Thermal barrier coatings, full field measurements, thermo-mechanical fatigue, Finite Element Analysis
The combustion chambers of aeronautical turbomachines, located between the high-pressure compressor and the high-pressure turbine, are bearing high thermomechanical loading. The high operating temperatures can cause damage and lead to the appearance of cracks. Competitiveness, a major challenge for aeroengine manufacturers, depends in particular on satisfying airlines in terms of reliability and cost of ownership. The service life claimed by engine manufacturers is therefore a crucial factor in customer choice, in addition to performance in terms of specific fuel consumption, efficiency and noise. Increasing the availability of equipment in service and better forecasting of maintenance intervals is a major focus of work.

Previous work carried out by the Materials Center and Safran Aircraft Engines has focused on predicting the behavior and service life of combustion chambers. The methodology for this type of mechanical calculation was validated using a highly instrumented technological test and its digital twin by finite element analysis.

The continuation of the studies relates to the prediction of the propagation of a possible crack having started in order to assess its evolution in exploitation and thus to be able to rule on the maintenance in navigability of these chambers, in particular in the presence of thick thermal barriers.

The thermomechanical fatigue loads observed on the targeted parts are complex, being, on the one hand, made up of a succession of cycles of variable amplitude and, on the other hand, presenting strong stress gradients. High loading levels can also generate general scale yielding condition on the component. This complexity of the loading and the structure raises the question of the modelling of the propagation mechanisms and their experimental validation, in particular on a technological specimen.

The challenge is to develop both experimental and numerical techniques to reproduce the propagation observed on structure. The sensitivity to loading parameters (gradients, temperature) is a key point of the study.

To achieve this, tests of increasing complexity will allow the simulation of spatial gradients and temporal evolutions consistent with those known on the part. The measurement of temperature and displacement fields will allow to refine the knowledge of these loads. A systematic analysis by electron microscopy will allow to specify the mechanisms of evolution of the cracks under these conditions, as well as the role of the thermal barrier coatings in the propagation of these cracks.

The modeling of the experimental conditions will allow to establish and validate associated crack propagation models. The different tests will be simulated using the finite element method and the proposed generalized plasticity propagation criteria. In particular, non-local approaches to evaluate crack growth will be evaluated. The potential of phase field methods will be studied in order to assess the crack path for complex loadings.

The work of the thesis is planned as follows:
The first part consists in experimental data analyses on specimens and combustion chambers in academic and industrial environments: mapping of cracks and their evolution, crack propagation models as well as their implementation in component FEA.
The second phase is to propose, design and carry out characterization and technological tests representative of the types of cracks and cyclic thermomechanical stresses observed on chambers coated with air plasma deposited thermal barriers (APS). The so-called technological tests are of an intermediate complexity between those carried out on simple specimens where the loading is well controlled but which do not represent the complexity of the structure and a motor test presenting uncertainties which can be important on the thermomechanical loading.
The third step will be to propose evolutions of the modelling methodologies in crack propagation based on the different test results: measured thermal and mechanical fields as well as the characteristics of the crack advance (speed, direction...).
Finally, an implementation in the industrial environment is planned to validate the developed models and to compare them with the pre-existing models on cases of increasing complexity up to the calculation of the combustion chamber.
ProfileEngineer and / or Master of Science - Good level of general and scientific culture. Good level of knowledge of French (B2 level in french is required) and English. (B2 level in english is required) Good analytical, synthesis, innovation and communication skills. Qualities of adaptability and creativity. Teaching skills. Motivation for research activity. Coherent professional project.

Prerequisite (specific skills for this thesis):

Skills in metallergy and/or finite element analysis are needed for the project. Courses will be mandatory if one of the topic is not adressed by the applicant.

Applicants should supply the following :
• a detailed resume
• a copy of the identity card or passport
• a covering letter explaining the applicant's motivation for the position
• detailed exam results
• two references : the name and contact details of at least two people who could be contacted
• to provide an appreciation of the candidate
• Your notes of M1, M2
• level of English equivalent TOEIC
to be sent to
FundingConvention CIFRE