The IDERPLANE H2020 research project has recently being funded by the European Community – Call CleanSky2 - and is led by the Department of Mechanical Engineering of Politecnico di Milano with the scientific direction of prof. Stefano Beretta. Topic Manager is GE Avio, and the consortium is composed by the University of Brescia, the “Institut national des sciences appliqués” INSA-Lyon, the Free University of Bozen-Bolzano and the team of Argo Srl.
The high power transmissions that have to be designed for modern highly efficient turbofans need the extensive application of epicyclical gears with planet gear containing an integrated bearing. These components are subjected to severe rolling contact fatigue (RCF) conditions as many others (e.g. wheels/rails of high speed trains): propagation of micro-cracks starting from the bearing race surface and leading to spalling is a typical damage mode of these components. The main objective of the project is to provide innovative, effective and validated criteria for the design and assessment of more reliable planet bearings for aerospace application. Compared other applications, there is no surface wear to remove the surface damage. In detail, in some specific cases that have led to catastrophic failures, planet gears are affected by cracks starting from the spalls that can bifurcate into the body of the gear wheel, leading to the complete failure of the component. Based on experimental observations, the analyses have shown interactions between spalling and sub-surface cracks driven by shear stresses: in-core cracks are the result of a competition between the two phenomena.
The main idea behind IDERPLANE is to analyze the problem not in terms of the usual stress-based design of gears, but rather on damage tolerance concepts. This kind of analysis is meant at understanding/measuring the risk of a catastrophic failure in the case of development of subsurface propagation of cracks driven by shear stresses. This could be seen an established route, but unfortunately RCF is a grey area where there are no data available for such an analysis (that should be based on reliable crack growth curves for different driving mechanisms), because it is very difficult to make cracks propagate under shear as it happens in RCF (and as it was shown in the known failures of planet gear containing an integrated bearing).
This is the strength and unicity of the IDERPLANE consortium, where the partners have special technology and abilities:
- to obtain crack growth curves under RCF conditions on specimens (PoliMI);
- to reproduce the subsurface crack propagation in bi-disks experiments (UniBS);
- to follow and track the development of sub surface cracks under complex load conditions (INSA), to analyse by FE the real service conditions (UniBZ);
- to validate the defect tolerance and the design criteria by means of full scale tests (PoliMI).
The design approach proposed is based on a preliminary damage tolerance analysis, aimed at identifying the maximum size of the allowable defects, followed by the subsequent crack growth investigation. An effective prediction of the crack growth path, aimed at the maximization of the reliability, favoured by paths which produce spalling instead of in core crack propagation, can be achieved only if several influence parameters are considered. In particular, the properties of the base material, the geometry of the component, the heat treatment process, the profile of the residual stresses and the hardness profile, with its case-core transition, are taken into account.
Validation. The validation of the proposed design criteria will be performed by means of full scale test articles subjected to load conditions representative of the real application together with the presence of artificial spalls. New monitoring approaches based on acoustic emission will be also implemented.
Timing. The project started in November 2018 and will end in November 2021.
