On instabilities and autocatalytic effects associated with the nucleation of martensite bands in polycrystalline SMA flat tensile specimens

On the origin of Lüders-like deformation of NiTi shape memory alloys

“…LDM has been found to occur during tensile deformation via any of the three processes: (1) stress-induced B2-B19 0 transformation; (2) stress-induced R-B19 0 and (3) stress-assisted martensite reorientation in B19 0 phase. Whereas LDM has been observed in a variety of specimen geometries, including thin wires, thick bars, ribbons, plates and ''dog-bone''-shaped specimens, the effect appears to be more pronounced in specimens of large length-to-thickness aspect ratios (Shaw and Kyriakides, 1998;Messner et al, 2000). The stress peak at the onset of LDM is greater in specimens of large aspect ratios (Messner et al, 2000).…”

“…Band fronts are typically inclined at $551 to the load axis in plate-shaped specimens (Shaw and Kyriakides, 1998;Messner et al, 2000), apparently perpendicular to load axis in wires and bars, but likely of conical shapes with the cone tip in the bar center, and spiral shaped in thin-walled tubes (Sun and Li, 2002).…”

“…Strain localization and LDM deformation of NiTi sheets have been modelled by FEM-implemented continuum level models (Messner et al, 2000;Shaw, 2000;Shaw, 2002;Sun and Zhong, 2000;Iadicola and Shaw, 2004). Shaw (2000) at first used a highly unrealistic trilinear s À curve (softening in the second stage) to represent the NiTi constitutive behavior.…”

“…These alloys tend to deform in tension via propagation of macroscopic shear bands, in which the deformation is localized under constant stress level and exhibit a plateau-type s À curves, as documented in numerous research publications (e.g. Brinson et al, 2004;Liu and Xiang, 1998;Tan et al, 2004;Shaw and Kyriakides, 1998;Messner et al, 2000;Sun and Li, 2002;Liu et al, 2000). Since this has not been always recognized, inconsistencies between experimentally observed localized deformation of NiTi in tension and its modelling counterparts assuming homogeneous deformation can be found in the literature.…”

“…The experimentally measured plateau stress and plateau strain thus do not necessarily correspond to the martensitic transformation stress and strain of NiTi as often assumed in modelling. There are continuum level SMA models (Messner et al, 2000;Shaw, 2000;Shaw, 2002;Sun and Zhong, 2000;Iadicola and Shaw, 2004) describing the features of the Lu¨ders-like deformation mode (LDM) of NiTi polycrystals, such as the stress drop at the onset, the band inclination, rate and thermal sensitivities of the band propagation, etc. Most of these models assume strain softening material behavior on the macro-level.…”