A numerical investigation of the nucleation and the propagation of NiTi martensitic transformation front under impact loading

A numerical investigation of NiTi SMA alloy under impact loading is presented in this paper. It is aimed at better understanding the observed behavior of NiTi SMA specimen loaded at impact velocities (up to 50m/s). A phenomenological thermodynamic model with an explicit integration frame is chosen and implemented as a user's law in Abaqus explicit commercial code. The parameters of the model are determined from the usual thermal-mechanical testing data without any tuning. The comparison between numerical and experimental results shows a satisfactory agreement, which proves the pertinence of the proposed numerical model. Using this numerical model, the reason of the experimentally observed inconstant strain level ahead of the phase transformation front is found. Instead of a material feature of NiTi SMA under impact loading, such a phenomenon is only due to the loading condition. Virtual tests at very high impact velocities are performed. Steep impulses without a rising time is chosen in order to have only one transformation front propagating inside the specimen, knowing that it is not possible to obtain such a simple condition in a real tensile test. A steady transformation front is observed and there is an asymptotical speed limit of this propagating front when the impact velocity increases. This speed limit is higher than the shear elastic wave speed but lower than the longitudinal elastic wave speed.