哈尔滨工业大学（深圳）学术讲座

演讲人Speaker: Prof. Fionn Dunne

题目Title: Stress redistribution and temperature sensitivity in dwell fatigue of titanium alloy from in-situ characterisation and crystal plasticity modelling

时间Date：2024年3月15日  Time：14:00 − 16:00  地点Venue： T3202

内容摘要Abstract: Cold dwell fatigue in Titanium aero-engine alloys is the degradation and failure process in which microcracks, or facets, nucleate typically within 15° of basal planes of (hard) HCP grains orientated with their c-axes at or about parallel to principal stress direction. A key driving force has been argued, through use of crystal plasticity (CP) modelling methods, to be creep deformation in an adjacent (soft) grain well-orientated for slip leading to stress redistribution onto the hard grain which occurs during cycle hold times (the dwell period), and over progressive cycling. In addition, dwell facet nucleation and growth has been found to be associated with macrozones which are millimeter-sized polycrystal regions with strong texture. So far as we are aware, the key mechanistic argument for soft-grain creep and load shedding onto an adjacent hard grain have not yet been demonstrated or measured in experiment. The work presented in this paper addresses this important absence.

Titanium alloy Ti-6Al-4V samples containing macrozones have been characterized with EBSD and speckled to facilitate DIC displacement measurement to allow in-situ full-field spatial intra-macrozone strain measurement in three-point bend test samples under dwell fatigue loading. A novel (but with simplifications) methodology to extract out spatial elastic strains, and hence stresses with knowledge of anisotropic stiffnesses, at peak load at the beginning and end of the cyclic dwell period is presented and supported by considerations of stress equilibrium checks and CP modelling. Hence, full-field intra-macrozone stresses both during dwell periods and over cycles of fatigue loading have been obtained and are presented.

Both soft-grain creep and redistribution of stress (load shedding) onto hard grains during the load hold time have been observed and quantified over a range of temperatures, and the associated dwell time constant for 66% of redistribution to have occurred has also been quantified, thus addressing the importance of the duration of the cycle dwell period. In addition, the in-situ DIC studies have allowed full-field strain and stress quantification over multiple cycles such that the cyclic change to redistributed stresses from cycle to cycle has also been quantified. The experimental observations reinforce that the hold period in dwell fatigue of Ti alloys does generate creep and load shedding and that cycling progressively drives up the stresses on the hard macrozones thus supporting many of the previous hypotheses from CP modelling. A quantitative assessment of how well CP models reflect experimental measurements of stress redistribution onto hard grains is also discussed. It is also shown that load shedding is temperature dependent, being most damaging at about 120°C, and diminishing away above about 200°C, thereby supporting previous theoretical studies.

个人简介(About the speaker):

Fionn Dunne is Chair in Micromechanics at Imperial College London. His research is in the fundamentals of deformation and fracture particularly relating to hcp polycrystal and Ni alloys and includes experiment, characterisation, computational crystal plasticity and discrete dislocation plasticity. Applications include micro-deformation, fatigue crack nucleation, microstructure-sensitive crack growth, and polycrystal sonics for NDE. He has published over 180 research papers and is co-author of Introduction to Computational Plasticity, OUP, 2005. He is a consultant to Rolls-Royce, a member of their Core Materials Working Group, served as a member of the MOD's Research Programmes Group and was a Royal Society Industry Fellow with Rolls-Royce. He was Royal Academy of Engineering/Rolls-Royce Research Chair 2015-20. He led the EPSRC programme grant Heterogeneous Mechanics in Hexagonal Alloys across Length and Time Scales, and directed the Imperial Rolls-Royce Nuclear University Technology Centre. He is Honorary Professor with the Beijing Institute of Aerospace Materials, and is Emeritus Fellow of Hertford College Oxford. He was elected a Fellow of the Royal Academy of Engineering in 2010, was awarded the IoM3 Harvey Flower Prize (2016), and shared the 2017 Imperial President's Award for Outstanding Research Team.