Numerical simulation of high-temperature necking evolution considering strain-rate dependence and strain softening

Necking evolution is a process of non-uniform deformation with varying strain rates, and accurate predictions of this process, taking into account strain rate dependence and strain softening, remain a challenge. In this work, based on the tensile tests with different strain rates at 700°C for Ti6Al4V metal sheets, the effects of constitutive models, element size and time increments are investigated using the finite element method on the simulation of necking evolution. Firstly, physically based and phenomenological visco-plastic models are calibrated and employed to simulate the tension process. It’s found that the necking evolution simulated by the phenomenological model is significantly retarded compared to the physically based model. This delay effect is due to the lack of strain-rate history dependence in phenomenological models, which predict higher stress than physically based models under the condition of increasing strain rates. Furthermore, different element sizes and time increments are used in simulations, and the corresponding results indicate that necking evolution is very sensitive to element size and time increment. Necking develops slowly with coarse elements and large time increments. A small time increment can capture accurate strain-rate gradients in simulations of necking evolution. In summary, considering strain rate dependence, using physically based models with small element sizes and time increments is recommended for accurate simulation of necking evolution or non-uniform deformation.