Synchrotron X-Ray diffraction measurements of 3D residual strain fields below surface anomaly in a Nickel-base superalloy
Résumé
Some aircraft components, such as turbine disks, are subjected to very strict certification stages. Other than the fact that the material has a non-linear behaviour (Nickel base superalloy), the complex loading it experiences makes the definition of a fatigue cycle difficult. Moreover, during operation or maintenance those components are exposed to the creation of surface anomalies such as dents or scratches. They create an initial residual strain state that is multiaxial and with steep gradients in the bulk. Preliminary studies showed that induced compressive residual strain field beneath the anomalies are influencing the first stages of crack propagation and thus are responsible for conservative results. An exhaustive measurement of these residual strain fields, including spatial distribution and evolution inside the bulk is then of prime importance along with the development of a new crack growth model accounting for residual stresses. This residual strain field evolves with the thermo-mechanical loading the turbine experiences, so describing its relaxation is also an important aim in this study. In a second step, a model for the flaw introducing will be developed so that it will be possible to calibrate it with the measurements. The goal is to non-destructively capture the residual strain field in terms of multiaxiality and gradients. The X-Ray diffraction method has been chosen to describe it. But in order to work in transmission, it is mandatory to have a high-energy X-Ray beam. At the European Synchrotron Radiation Facility (ESRF), the beamline ID22 offers an energy up to 80keV which is sufficient to go through 1.8mm of our material Inconel 718 DA. V-type dents and scratches of various depth such as 200µm, 150µm and 100µm are studied. In order to describe the relaxation of the residual strain field, the same flaws are submitted to a mechanical loading under temperature. Samples no thicker than 1.8mm are then extracted for characterization at the ESRF. The results show a good description of the gradients and all the components have been captured. A model representing the introducing of a V-type dent is developed and fortified to account for the sample extraction so that it is possible to reach the initial residual stress field. Synchrotron X-Ray diffraction has been used in order to non-destructively capture the 3D residual strain field. These datawill be used to calibrate the model developed for the introducing of the surface flaw and the extraction of the sample. The goal is to have a handy model where every depth can be numerically simulated and output the associated residual strain field to be an input for the new crack growth model.