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Fatigue life prediction on Inco 718 part subjected to service induced damage

Periodic Reporting for period 2 - FLECTION (Fatigue life prediction on Inco 718 part subjected to service induced damage)

Reporting period: 2021-04-01 to 2022-09-30

The successful integration of the contra-rotating open rotor (CROR) technology requires an accurate prediction of the fatigue life of IN718 parts. To this end, the life assessment approach for parts with visible or barely visible surface damages generated during service life or inspection activities needs significant improvement.

The typical damage generated can be classified as nicks, dents and scratches (NDS) and they are usually small and difficult to characterise in shape. Moreover, there is a lack of knowledge on how they affect the local residual stresses and, consequently, the fatigue life. Scientific literature on typical materials for aeronautic applications (Ti-based and Ni-based alloys) have proven that the residual stresses around the NDS may have a beneficial effect on fatigue life. Nevertheless, a thorough study that correlates NDS with fatigue life in IN718 in a statistically robust way has never been developed. Consequently, the current design codes neglect the potential benefit of impact residual stresses leading to a conservative approach.

FLECTION seeks to advance the state-of-the-art by developing and validating a combined numerical-experimental approach to assess, with a robust statistical prediction, safe life in IN718 components damaged with NDS. This is expected to contribute to the specific impacts of the Clean Sky Engines AIDP-LPA Platform 1 to reach flight test of the CROR demo-engine and ensuring TRL 6, fulfilling future market needs and enhancing the competitiveness of the European aeronautical industries.
Throughout FLECTION, a range of experimental and modelling activities were undertaken for the damage generation and fatigue performance prediction.

A new experimental set up for the experimental simulation of foreign object damages was developed, which involved the dynamic indentation of specifically designed specimens through a SHPB. The final experimental set up was the result of a development process where challenges such as the occurrence of multiple impacts and the avoidance of excessive specimen distortion were overcome. The characterisation of the damage geometries through 3D SEM scanning confirmed the good quality of the indentations.

An extensive experimental campaign for the characterisation of the purchased IN718 was performed in order to have data for the numerical model calibration and have a baseline against which to compare the test results of the damaged material. This involved tests for the characterisation of the elasto-plastic, fracture and fatigue properties at room temperature and 550°C.

The testing of the damaged material encountered additional challenges, which made the scope deviate from the initial program. The initially planned axial fatigue testing proved to be unviable for the manufactured specimens since crack nucleation was developing at the end of the gauge section rather than from the indentation. A 4-point bending test was then set up which provided useful data on the fatigue life of the damaged material.

All of the experimental data was used to develop, calibrate and validate the numerical models. The developed finite element model for the prediction of the residual stresses due to the dynamic indentation showed a good agreement with the scans of the damages and with the residual stress measurements. However, for the latter, the resolution of the measurement techniques was found to be too low to validate smaller damages.

Finally, the fatigue model was developed for the prediction of the fatigue life taking into account the residual stresses. The model showed good agreement with the deep-blunt fatigue test results in terms of crack front and predicted number of cycles.
FLECTION went beyond the current state-of-the-art through the implementation of an advanced numerical-experimental approach to assess the fatigue life in IN718 components damaged with NDS.

More specifically, the novel experimental setup for the laboratory simulation of foreign object damages, together with the novel numerical model implemented for the prediction of impact residual stresses and fatigue life is expected to contribute to the specific impacts of the Clean Sky Engines AIDP-LPA Platform 1 to reach flight test of the CROR demo-engine and ensuring TRL 6, fulfilling future market needs and enhancing the competitiveness of the European aeronautical industries.

This combined numerical-experimental program of work provided a major step forward from the state-of-the-art, which will enable reducing the number of experimental effort during the design process of newly configure aero-structures.
Impact analysis