A new experimental approach for modelling the constitutive behaviour of sheet metals at elevated temperature through interrupted bulge tests

Modelling the deformation behaviour of materials plays a fundamental role in the process design of formed components. In this work, an original methodology based on interrupted hot bulge tests has been proposed for evaluating the effective stress and strain values in a wide range of strain rates. The strain rate value was instantaneously calculated in each test by a new approach based on continuous acquisition of the dome height. The authors conducted bulge tests on the AZ31B magnesium alloy at elevated temperature (450°C) and interrupted the tests at different levels of the strain at the dome apex. The corresponding dome height at which the test had to be stopped was calculated by a predictive model. The strain rate and the stress values evaluated through the analysis of the samples from interrupted bulge tests were correlated using two different constitutive models. The constitutive models calibrated using the proposed approach were finally implemented in the numerical simulations of the bulge tests in order to compare the results with the experimental data. Both the constitutive models revealed to be accurate, showing a good agreement between numerical and experimental dome height versus time curves, especially when using the phenomenological constitutive model, which allowed to keep the discrepancy below 12% in a very large pressure range. Thus, also the effectiveness of the proposed methodology was demonstrated.