Abstract
Micro-pillar tests on α and α-β colony Ti alloys in combination with crystal plasticity finite element analysis has enabled the extraction of a and b phase slip strength and rate sensitivity properties. Faithfully representative α-β microstructure polycrystal plasticity models have then been established in order to investigate dwell fatigue in isothermal rig test behaviour and anisothermal thermomechanical flight loading conditions. The role of thermal alleviation in diminishing dwell sensitivity has been demonstrated.
Highlights
Cold dwell fatigue was first recognised through an air accident in the 1970s due to the failure of two titanium fan disks in Rolls-Royce RB211 engines [1,2]
A significant lifetime reduction, known as dwell debit, occurs when titanium alloys are subjected to cyclic loading with stress hold at maximum magnitude in each cycle at low temperatures [3,4,5,6]
Facet crack nucleation on basal slip planes of hexagonal closed packed (HCP) crystals is argued to be responsible for the early service failure of near α titanium alloys [7]
Summary
Cold dwell fatigue was first recognised through an air accident in the 1970s due to the failure of two titanium fan disks in Rolls-Royce RB211 engines [1,2]. The modelling work by Zhang et al [20] found that load shedding in α Ti-6Al alloy increases with temperature from 20°C and peaks at about 120°C followed by a diminution to zero at about 220°C This results from the influence of temperature on both the slip system strengths (CRSSs) and strain rate sensitivity. The atomistic simulations carried out by Ready et al (Ready et al, 2017) demonstrated that Mo atoms in titanium alloys are not able to trap the vacancies for long enough at room temperatures in order to give rise to the observed strain rate sensitivity. Orientations facilitating basal or prismatic slip give rise to considerable creep under load with the former showing the higher strain rate sensitivity These rate-sensitive behaviours are argued to be key to understanding the cold dwell fatigue phenomenon together with its thermal alleviation, which are addressed
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