High Strain-Rate, Small Strain Response of a NiTi Shape-Memory Alloy

The compressive response of a NiTi shape-memory alloy is investigated at various strain rates using UCSD’s modified 12-in. Hopkinson pressure bar and a conventional Instron machine. To obtain a constant strain rate during the formation of a stress-induced martensite in a Hopkinson test, a copper tube of suitable dimensions is employed as a pulse shaper, since without a pulse shaper the strain rate of the sample varies significantly as its microstructure changes from austenite to martensite, whereas with proper pulse shaping techniques a nearly constant strain rate can be achieved over a certain deformation range. The NiTi shape-memory alloy shows a superelastic response for small strains at all considered strain rates and at room temperature, 296 K. At this temperature and below a certain strain rate, the stress–strain curves of the NiTi shape-memory alloy display two regimes: an elastic austenite regime and a transition (stress-induced martensite) regime. The transition stress of this material and the work-hardening rate in the stress-induced martensite regime increase with increasing strain rate, the latter reaching a steady state level and then rapidly increasing.