Titanium 64 cold dwell fatigue loading - effect of stress concentrations and fatigue crack growth rates

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The phenomenon of cold dwell fatigue can occur under cyclic loading where the maximum tensile load is maintained for a given duration (e.g. 2 minutes). The phenomenon is called "cold" dwell fatigue as it is prevalent at room temperature and remains pronounced at 120C but fades out above 200C. It has been discovered that this loading regime can lead to a large knock-down in fatigue life in certain titanium alloys, such as Ti64, when there is strong crystallographic texture heterogeneity in the material. Cold dwell fatigue was found to be the cause of several relatively recent failures of aero engines, which experienced Ti-64 front fan-hub ruptures. It is therefore a high priority for aircraft manufactures to better understand the risks this phenomenon might pose to other airframe components, which can have different microstructures and loading regimes. In particular, there is an urgent need to understand if dwell fatigue can lead to a fatigue life debit around fasteners which create a local stress concentration but a more complex local stress field, and during growth of cracks. This is because airframes are designed for 'damage tolerance', where a pre-existing crack is assumed to exist and life is calculated on the basis of the crack growth rate. Both of these topics have not been previously sufficiently addressed in the context of a dwell debit. The aim of this project will therefore be to use advanced materials testing and characterization techniques and modelling to understand the sensitivity of Ti64 materials with heterogeneous textures to damage accumulation around stress concentrations and during crack propagation under dwell fatigue loading conditions. This will involve running high fidelity in-situ cyclic loading tests, while employing a digital image correlation technique to measure the plasticity that occurs locally within the microstructure, using model materials that have been sourced with different types of texture heterogeneity. The results of such experiments will be compared to crystal plasticity simulations, using modelling tools being developed in the LightForm research group (https://lightform.org.uk).