This paper describes high temperature cyclic and creep relaxation testing and modelling of a high nickel-chromium material (XN40F) for application to the life prediction of superplastic forming (SPF) tools. An experimental test programme to (i) characterise the high temperature cyclic elastic-plastic-creep behaviour of the material over a range of temperatures between 20°C and 900°C, including cyclic controlled strain-range tests at two different strain-rates and creep relaxation tests, and (ii) identify the constants relevant to thermo-mechanical fatigue (TMF) life prediction for the material, is described. The objective of the material testing is the development of a high temperature material model for cyclic analyses and life prediction of superplastic forming (SPF) dies for SPF of titanium aerospace components. A two-layer visco-plasticity model, available within the commercial FE code ABAQUS, which combines both creep and combined isotropic-kinematic plasticity is chosen to represent the material behaviour. A number of thermo-mechanical life prediction methodologies are investigated, including the Zamrik and Ostergren models. The process of material and failure constant identification is presented and the predicted results are compared with the rate-dependent (isothermal) experimental results. The temperature-dependent material model is furthermore applied to simulative TMF tests, designed to represent the temperature and stress-strain cycling associated with the most damaging phase of the die cycle. The two-layer visco-plasticity model is shown to give good correlation with the test data, thus vindicating future application of the material model in thermo-mechanical analyses of SPF dies. For life prediction, the Zamrik model was consistently conservative although the Ostergren model was generally more accurate. A Coffin-Manson approach, based on maximum temperature, was consistently non-conservative and significantly over-predicted TMF life for a ‘representative; test.
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