Cold Creep of Titanium: Analysis of stress relaxation using synchrotron diffraction and crystal plasticity simulations

There is a long standing technological problem in which a stress dwell during
cyclic loading at room temperature in Ti causes a significant fatigue life
reduction. It is thought that localised time dependent plasticity in soft
grains oriented for easy plastic slip leads to load shedding and an increase in
stress within a neighbouring hard grain poorly oriented for easy slip.
Quantifying this time dependent plasticity process is key to successfully
predicting the complex cold dwell fatigue problem. This work uses a novel
approach of in situ synchrotron X-ray diffraction during stress relaxation
tests, to quantify the time dependent plasticity. Measured lattice strains from
multiple lattice families (21 diffraction rings) were compared with simulated
lattice strains from crystal plasticity finite element (CPFE) simulations. The
prism slip parameters were found to show stronger strain rate sensitivity
compared to basal slip, and this has a significant effect on stress
redistribution to hard grain orientations during cold creep.