An experimental investigation of the nucleation and the propagation of NiTi martensitic transformation front under impact loading

The paper presents an experimental investigation of the NiTi superelastic shape memory alloy (SMA) under high-speed impact loading. A Split Hopkinson tensile bar (SHTB) and special short strikers are used to reach impact speeds exceeding 50 m/s. A very high-speed camera allows for the capture of full resolution images (924 × 768 pixels) at a sampling rate of 2 million frames/s. Digital image correlation (DIC) analysis provides good measurements of the strain field as well as the particle velocity field of the whole specimen. Experimental results shows that the increase of the critical transformation stress is much higher at strain rates around 10³/s instead of a moderate increase at lower strain rates under 10²/s. The heterogeneous stress field due to elastic wave propagation at high impact velocity leads to a heterogeneous transformation zone inside the specimen. The loading and the distal ends are the privileged sites of the martensitic transformation nucleation because of stress wave reflections. Transformation fronts will afterwards occur and propagate, driven by the stress increase at the loading end or that due to the loss of particle velocity (kinetic energy) at the distal end respectively. The jump condition across the front is respected. Finally, the cumulated martensitic phase length is traced when multiple fronts occur. It is found that the growing speed of this cumulated martensitic phase length remains proportional to the loading speed and that it can exceed the shear wave speed in the martensitic phase of the material.