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Title: Estimation of fatigue life under multiaxial loading : modelling and experimentation
Author: Sahadi Cavalheiro, Joao Vitor
ISNI:       0000 0004 9355 4116
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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This doctorate research project investigates the multiaxial deformation and fatigue behaviour of two Nickel-based superalloys used in the discs of aero-engines, named Waspaloy and RR1000, under proportional and non-proportional loading conditions. For that a systematic analysis has been performed to identify and quantify the biaxiality of loading effects on fatigue life, the effects of non-proportional multiaxial loading – including additional cyclic hardening and cross-hardening – on crack initiation and fatigue life, and lastly the role of material micro-structure and crystal orientation on fatigue crack nucleation. An extensive experimental programme with uniaxial and biaxial fatigue tests with Waspaloy and non-proportional tension-torsion tests with RR1000 was carried out. It was experimentally observed that the variation of biaxiality ratio, for the same maximum principal stress, was found to cause a significant change in fatigue life and failure location for Waspaloy. Furthermore, results show that RR1000 presents no load path dependency for stress levels near the yield strength of the material. On the other hand, fatigue lives and cyclic hardening are highly dependent on the degree of non-proportionality. Lives for non-proportional tests were around three times shorter than fatigue lives for proportional tests at comparable stress levels. The experimental programme was used for calibrating and benchmarking a wide range of stress-, strain- and energy-based multiaxial fatigue damage parameters. Most of them could provide a reasonable collapse of both Waspaloy and RR1000 test data. Particularly good fatigue life predictions were obtained with stress-based criteria, especially with Walker stress and Matake’s critical plane approach. Lastly, a GND-coupled crystal plasticity framework and stored energy criterion was implemented for predicting fatigue lives of RR1000. It was concluded that Wan et al.’s stored energy criterion, which combines stress, plastic slip and GNDs density represents an excellent tool for accurately predicting fatigue lives under complex multiaxial loading conditions.
Supervisor: Hills, David ; Nowell, David Sponsor: Brazilian National Council for Scientific and Technological Development (CNPq)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Fatigue of Materials ; Solid Mechanics