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

Periodic Reporting for period 1 - FLECTION (Fatigue life prediction on Inco 718 part subjected to service induced damage)

Reporting period: 2019-10-01 to 2021-03-31

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.
In the first period of FLECTION the work performed has been focused on the detailed investigation of the the state-of-the-art of modelling and experimental activities for the generation of NDS and prediction of their residula stresses and conquent fatigue life. Moreover, experimental activities have been performed to set up an advanced experimental procedure for the generation of NDS in a laboratory environment. More specifically:

A detailed review of the state-of-the-art on how to predict with numerical models the residual stresses and fatigue life of materials damaged with NDS has been produced.

A detailed review of the state-of-the-art on how to accurately measure the residual stresses produced by NDS has been produced.

Experiments involving the generation of NDS at different strain rates (impact energies), shapes and sizes have been performed to understand the feasibility of damage fatigue specimens in a high repeatable way.

Design of high quality specimens for material characterisation and to be damaged in a control and repeatable way.

Part of the mechanical characterisation of the base material.

Implementation of numerical models to support the set up of the experimental procedure for the generation of NDS in fatigue specimens.
FLECTION will go beyond the current state-of-the-art through the implementation of an advanced numerical-experimental approach to assess, with a robust statistical prediction, safe life in IN718 components damaged with NDS. In particular, the main foreseen achievements include:

The implementation of a new state-of-the-art methodology to generate artificial NDS in a controlled on fatigue specimens and its characterisation.

A robust, statistically-meaningful set of experimental data on fatigue life of inconel 718 material damaged by NDS with different geometry.

A new advanced numerical model for the prediction of the fatigue life of damaged inconel 718 material affected by NDS and its exploitation to understand the effect of the different NDS geometry features on fatigue life.

These achievements will give the opportunity to develop a European capability, including both advanced experimental and numerical modelling methodologies, for the assessment of safe life in super alloys used in high strain rate and high temperature applications. This will enable Europe to play a forefront role in this area, competing with America-based facilitates and capabilities.
Impact analysis