DIC observation of cyclic localized deformation of NiTi wire and localization of fatigue damage

                                 Localized deformation and fatigue degradation

Figure 1: Martensite band front propagating during tensile cycling of NiTi wire  - DIC observation of axial strain field in 8 consecutive partial superelastic cycles.

Compared to the case of common structural materials, the cyclically deformed superelastic NiTi deforms recoverably up to several percent strain which makes the fatigue of NiTi unique. While the axial stress-strain response of the material evolves with number of cycles in parts of the wire where the martensite band front moves (Fig. 2) , it stays the unchanged elsewhere. 

Figure 2: Strain profiles over the martensite band front. During the 10 partial superelastic cycles, nonrecovered strain accummulates in the unloaded state only in parts of the wire, where the martensite band front moves (Unloaded). The moving martensite band front  gradually spreads with increasing number of cycles (Mid-strain), but at the full strain, the strain profile over the martensite band front stays unchanged (Full-strain).

An essential consequence is that upon continuing tensile cycling under same conditions, the martensite band front can not escape from where it has been moving so far, since the stress needed to propagate it through the polycrystal microstructure elsewhere is higher. The cyclic localized deformation thus leads to localized evolution of  microstructure and accummulation of surface damage only in parts of the wire where the martensite band front moves.

In addition to the excessive straining of the wire surface there is local concentration of axial stress on the wire surface when the martensite band front passes by, as clearly evidenced by the 3D-XRD experiment and rationalized by FE simulation.

The accummulation of surface damage in parts of the wire where martensite band fronts move upon cycling thus accelerates the fatigue degradation of the wire. This is the case in tensile tests at any strain (stress) amplitude sufficient to keep the martensite band front in the wire.  Superelastic NiTi wires cycled in tension always fail in places where martensite band fronts move, frequently via crack nucleation at inclusions.

Take away

Localization of deformation in Martensite Band Fronts brings about localization of accumulation of surface damage and consequently fatigue degradation upon cyclic tensile loading.