Low-energy tensile-impact behavior of superelastic NiTi shape memory alloy wires

Thermo-Mechanical Behaviour of NiTi at Impact

“…As the strain rate is raised from the quasi-static limit, the forward transformation stresses increase while the reverse transformation stresses decrease widen then the hysteresis, Fig. 1, as is widely corroborated in the literature (Shaw & Kyriakides, 1995;Vitiello et al, 2005;Schmidt, 2006;Zurbitu et al, 2009a). Above certain strain rate this trend changes, Fig.…”

“…2. Thus, the forward transformation stresses stop increasing and become constant after 10 -1 s -1 , and the reverse transformation stresses change their tendency and increase for strain rates higher than 10 -3 -10 -2 s -1 (Tobushi et al, 1998;Vitiello et al, 2005;Pieczyska et al, 2006a;Schmidt, 2006;Zurbitu et al, 2009a). The combination of these factors narrows the hysteresis.…”

“…At high strain rates, the time for heat exchange between the surroundings and the specimen is reduced so part of the transformation heat is spent in heating up or cooling down the specimen changing the transformation stresses. It is known that this trend continues until the impact range, 10 -2 s -1 (Zurbitu et al, 2009a), but it is unknown the temperature evolution at those high strain rates. At impact, the time for heat exchange is drastically reduced and the deformation process is closer to adiabatic conditions, so it is necessary a deeper understanding of the adiabatic nature of the stress induced transformation on the thermo-mechanical behaviour of NiTi.…”

“…Variation of the transformation stresses with strain rate: (a) forward SIM transformation stresses, and (b) reverse SIM transformation stresses for a load-unload cycle with complete SIM transformation. [After (Zurbitu et al, 2009a)]. …”

“…For instance, these alloys are highly attractive for impact damping, seismic protection or health monitoring for impact damage detection (Dolce & Cardone, 2001;Tsoi et al, 2003;Qiu et al, 2006). Nevertheless, the thermo-mechanical behaviour of these alloys at impact, deformation rate in the order of 1 -10 3 s -1 , is not yet well known and only a few works deal with the dynamics of the superelastic transformation at impact strain rates, from the parent phase B2 to the martensitic phase B19', studying the mechanical behaviour in compression mode (Chen et al, 2001;Xu et al, 2006), in tensile mode (Zurbitu et al, 2009a), or analyzing the dynamics of propagating phase boundaries (Niemczura & Ravi-Chandar, 2006;Zurbitu et al, 2009b). Unfortunately, there is a lack of knowledge on the thermal evolution of NiTi when it is subjected to impact loads, so this chapter will address this issue.…”