DIC observation of localized deformation in tensioned NiTi wire

Martensite Band Front in NiTi wire

Figure: Martensite band front observed during tensile cycling of superelastic NiTi wire takes a form of radially symmetrical taper perpendicular to the load axis propagating  along the wire at constant stress. Relatively wide strain gradient over the taper on the surface was evaluated experimentally by the Digital Image Correlation /DIC/ method (video shows a looped single cycle DIC experiment).

Stress induced martensitic transformation occurs within the taper formed by the propagating martensite band front and causes strain change 6.5 % in the material. The width of the propagating front is roughly equal to the wire diameter.

Take away
Martensite Band Fronts propagating in tensioned superelastic NiTi wire look from outside like a taper associated with a broad strain gradient perpendicular to the load axis.