Martensite band front in tensioned NiTi wire

Martensite Band Front in tensioned NiTi wire

Figure 1: Macroscopic internal stress, strain and phase fraction fields within the Martensite band front in tensioned NiTi wire evaluated by the 3D-XRD experiment and simulated by FE implemented SMA model.

The Martensite Band Front propagating in tensioned NiTi wire extends over thousands of grains. It consists  of a nose cone shaped macroscopic internal interface surrounded by gradients of stress, strain and phase fraction.

The  macroscopic interface in a wire is cone shaped due to the radial symmetry of the wire and the need for the macroscopic compatibility of strain between the elastically strained austenite (axial strain ~1%) and the heavilly deformed martensite band (axial strain ~10%).

The problem is that, compared to the case of the thin NiTi ribbon, the macroscopic strain compatibility at the propagating interface  in the NiTi wire is hard to achieve due to more severe constraints. It appears that  the buried austenite cone interface  is also inclined ~55° to the load axis as in the ribbon but much wider gradients of phase fraction, strain and stress are necessary to maintain the macroscopic strain compatibility across the cone shaped interface in the wire.

The macroscopic internal stress field associated with the martensite band front  in the wire gives rise to the nose cone shape of the macroscopic interface and vice versa - i.e. they are linked so that one cannot exist without the another.

There is very good agreement between the experimentally determined and simulated  macroscopic internal stress field in the wire.

Figure 2: Stress strain curve of NiTi wire prestrained to 4.5% in the 3D-XRD experiment.

The Martensite Band Front propagates along the wire at constant applied stress 420 MPa which is far below the equivalent stresses ~570 MPa determined experimentally at the cone interface (compare the sigma_zz stress component and sigma_eq stress in the figure above.

Take away

Martensite Band Front propagating in tensioned superelastic NiTi wire consists of a nose cone buried macroscopic interface surrounded by internal stress, strain and phase fraction fields observed experimentally and confirmed via finite element simulation using SMA model.

The band propagates at applied tensile stress which is much lower than would correspond to the equivalent stress in grains transforming at the austenite cone due to the internal stress gradient and change of the stress state within the propagating martensite band front.