DIC observation of localized deformation in tensioned NiTi ribbon

Martensite Band Front in NiTi ribbon

Figure: Martensite band front observed during tensile cycling of superelastic NiTi ribbon takes a form of sharp nearly planar macroscopic interface propagating at constant stress. The video shows strain change across the moving interface evaluated experimentally by the Digital Image Correlation /DIC/ method (video shows a looped single cycle DIC experiment).

Stress induced martensitic transformation occurs within ~0.15 mm thin layer and causes strain change 6% in the material.

The macroscopic interface is inclined ~ 55° to the load axis.

The reason for the 55°  inclination is that the macroscopic strain compatibility between deformed and undeformed regions of the ribbon can be fulfilled with least additional elastic deformation at such inclination. In other words the material behind the propagating 55° inclined planar interface can elongate up to 6% with minimum constraint exerted from the elastically deformed austenite across the interface.

Take away
Martensite Band Fronts propagating in tensioned superelastic NiTi ribbon are  planar, sharp and about 55 ° inclined to the load axis.