I started my thesis in 2022 at ONERA Châtillon (DMAS/M3S) on "Propagation of short cracks influenced by microstructure" in Nickel-based superalloy INCO718DA, supervised by Julien Réthoré from GeM Nantes and in collaboration with Safran Aircraft Engines which provides us with the Inconel turbine disk. I graduated from INSA Lyon SGM in 2015 and worked as an R&D engineer for several years in the polymer industry in France and abroad before deciding to change my career path and focus on metals subjects.
This work focuses on damage tolerance study and fatigue life prediction of aeronautical parts. The goals are to understand fatigue short crack growth initiated from surface defect and to analyze the microstructure effects on propagation path and velocity in INCO718DA specimens.
The defect size chosen is the smallest experimentally feasible while still bigger than natural microstructural ones (50µm). Three grain sizes – ranging from conventional material size (8µm) to millimetric size – are set up in order to better understand grain boundary impact on crack propagation and on dislocation mechanics.
Modelling of the fatigue crack growth mechanism is investigated within a representative polycrystalline aggregate as simulations of the entire specimen microstructure cannot be computationally realistic (we use a generic multi-model approach recently developed at Onera). Implementation of the digital chain has already been completed for oblate spherical defect.
We will present its application to digital reproductions of experimental tests with crack propagation, still restricted today to simplified materials (heterogeneous elastic behavior).
In parallel, a theoretical work is carried out to understand interactions between crack and grain boundary with an ad-hoc representation of discrete dislocation at the cracktip. Based on the linear elasticity theory of dislocations in bi-materials coupled with the linear elastic fracture mechanics, this approach makes it possible to understand complex interactions, in a heterogeneous anisotropic microstructure, in the vicinity of the cracktip, such as the possible blocking of dislocation stacks by a grain boundary. It’s employed to characterize microstructural barriers role to the plastic flow and crack propagation.
80 - Numerical modeling of high-frequency fatigue testing for short crack propagation from tailored defect