Investigation on fatigue behaviors of NiTi polycrystalline strips under stress-controlled tension via in-situ macro-band observation

A digital image correlation study of a NiTi alloy subjected to monotonic uniaxial and cyclic loading-unloading in tension

“…The literature on fatigue tests has suggested that during unloading, the formation of stable remnant B19′ variants, along with de-twinning, grain reorientation effects and the retention of lattice defects within B2 and remnant B19′ grains hinder the latter phase′s reverse transformation back to B2 [14,15,[22][23][24][25][26][27][28]. The combined effect of the above mechanisms serves to increase the magnitude of residual strains upon unloading after each cycle along with a progressive degradation in superelasticity.…”

“…Since then, researchers have routinely applied DIC to characterise strain fields during the mechanical testing of thermo-mechanically processed NiTi alloys [3,[9][10][11][12][13][14]. For example, DIC investigations revealed the development of a region of strain concentration upon deviation from linearity towards the end of the elastic region; suggesting the initiation of phase transformation just prior to the onset of the macroscopic stress plateau [1].…”

“…For example, DIC investigations revealed the development of a region of strain concentration upon deviation from linearity towards the end of the elastic region; suggesting the initiation of phase transformation just prior to the onset of the macroscopic stress plateau [1]. Thereafter, further localisation in the strain fields results in Lüders/transformation band(s) that propagate along the gauge length with higher macroscopic tensile strains [1,3,11,12,[14][15][16]. The bands are typically inclined at ~60° with respect to the loading direction [12,[17][18][19][20][21] and divide the gauge length into domains with varying local axial strain extrema comprising fully martensitic and as-yet untransformed B2 regions [11,12,14,15].…”

“…The bands are typically inclined at ~60° with respect to the loading direction [12,[17][18][19][20][21] and divide the gauge length into domains with varying local axial strain extrema comprising fully martensitic and as-yet untransformed B2 regions [11,12,14,15]. The axial strain values within the mobile band fronts do not saturate but record a continuous increase with higher macroscopic tensile strains [1,14].…”

“…Fatigue testing of NiTi alloys typically involves repeated cycles of loading up to the end of the macroscopic stress plateau region and then unloading back to zero load. Such investigations reported increases in the irreversible strain component (through highly accumulated dislocation densities and the formation of stable martensite variants with successive cycles) contributing to an increase in the residual strain upon unloading and a progressive degradation in superelasticity involving the retention of an increasing phase fraction of B19′ [14,15,[22][23][24][25][26][27][28][29].…”