An experimental and modelling investigation of local deformation during reverse loading of an aerospace nickel alloy

Abstract

Cyclic loading is of great importance in aerospace applications but the microstructural origins of the change in flow stress with load path are poorly understood and difficult to predict when a change is made to the microstructure. Many crystal plasticity models are capable of modelling macroscopic flow behaviour during non-monotonic load paths but it remains unclear whether they capture reversibility at the local microstructural scale, or simply fit the macroscopic hardening response using more material parameters. A methodology is developed here to apply uniaxial load reversals and measure local surface deformation using high resolution digital image correlation (HRDIC) at several points during the load cycle. These experiments reveal the discrete nature of deformation in the form of localised crystallographic slip bands, which are found to not reverse their plastic deformation with macroscopic strain in all cases. Post-mortem analysis in the bulk of samples by electron backscattered diffraction (EBSD) also showed microstructure scale non-reversal, with misorientation formed in forward deformation not being removed by reverse deformation in all grains. This local behaviour is shown to be linked to grain orientation and the formation of slip bands on multiple slip planes. The experimental results are compared statistically, over many hundreds of grains, to crystal plasticity simulations of reverse loading. Simulation results are taken from both finite element method (CPFEM) and fast Fourier transform (CPFFT) model implementations, encompassing the two popular numerical techniques used for full-field crystal plasticity models. The two models predict strain localisations in corresponding locations of the microstructure but the CPFFT model shows larger strains in the localised regions. The models do not predict the local non-reversal behaviour seen in the experimental results and instead show universal reversal of local deformation. Slip bands are not seen in the models and the interaction between slip bands not being present is proposed as the reason for the difference in local reversal behaviour to the experimental results.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Preuss (Supervisor) & Joao Quinta Da Fonseca (Supervisor)