Crack initiation and propagation in 50.9 at. pct Ni-Ti pseudoelastic shape-memory wires in bending-rotation fatigue

Sawaguchi, T.A.; Kausträter, G.; Yawny, A.; Wagner, M.; Eggeler, G.

doi:10.1007/s11661-003-0186-x

Crack initiation and propagation in 50.9 at. pct Ni-Ti pseudoelastic shape-memory wires in bending-rotation fatigue (bibtex)

by T.A. Sawaguchi, G. Kausträter, A. Yawny, M. Wagner, G. Eggeler

Abstract:

The structural fatigue of pseudoelastic Ni-Ti wires (50.9 at. pct Ni) was investigated using bending-rotation fatigue (BRF) tests, where a bent and otherwise unconstrained wire was forced to rotate at different rotational speeds. The number of cycles to failure (Nf ) was measured for different bending radii and wire thicknesses (1.0, 1.2, and 1.4 mm). The wires consisted of an alloy with a 50-nm grain size, no precipitates, and some TiC inclusions. In BRF tests, the surface of the wire is subjected to tension-compression cycles, and fatigue lives can be related to the maximum tension and compression strain amplitudes (ɛ_a) in the wire surface. The resulting ɛ_a-N_f curves can be subdivided into three regimes. At ɛ_a > 1 pct rupture occurs early (low N_f) and the fatigue-rupture characteristics were strongly dependent on ɛ_a and the rotational speed (regime 1). For 0.75 pct < ɛ_a < 1 pct, fatigue lives strongly increase and are characterized by a significant statistical scatter (regime 2). For ɛ_a < 0.75 pct, no fatigue rupture occurs up to cycle numbers of 10⁶ (regime 3). Using scanning electron microscopy (SEM), it was shown that surface cracks formed in regions with local stress raisers (such as inclusions and/or scratches). The growth of surface cracks during fatigue loading produced striations on the rupture surface; during final rupture, ductile voids form. The microstructural details of fatigue-damage accumulation during BRF testing are described and discussed.

Reference:

Sawaguchi, T.A., Kausträter, G., Yawny, A., Wagner, M., Eggeler, G.: Crack initiation and propagation in 50.9 at. pct Ni-Ti pseudoelastic shape-memory wires in bending-rotation fatigue, Metallurgical and Materials Transactions A 34, 2847-2860, 2003.

Bibtex Entry:

@Article{Sawaguchi2003,
  Title                    = {{Crack initiation and propagation in 50.9 at. pct {Ni-Ti} pseudoelastic shape-memory wires in bending-rotation fatigue}},
  Author                   = {Sawaguchi, T.A. and Kaustr\"{a}ter, G. and Yawny, A. and Wagner, M. and Eggeler, G.},
  Journal                  = {Metallurgical and Materials Transactions A},
  Year                     = {2003},
  Number                   = {12},
  Pages                    = {2847--2860},
  Volume                   = {34},

  Abstract                 = {The structural fatigue of pseudoelastic Ni-Ti wires (50.9 at. pct Ni) was investigated using bending-rotation fatigue (BRF) tests, where a bent and otherwise unconstrained wire was forced to rotate at different rotational speeds. The number of cycles to failure (Nf ) was measured for different bending radii and wire thicknesses (1.0, 1.2, and 1.4 mm). The wires consisted of an alloy with a 50-nm grain size, no precipitates, and some TiC inclusions. In BRF tests, the surface of the wire is subjected to tension-compression cycles, and fatigue lives can be related to the maximum tension and compression strain amplitudes (ɛ\textsubscript{a}) in the wire surface. The resulting ɛ\textsubscript{a}-N\textsubscript{f} curves can be subdivided into three regimes. At ɛ\textsubscript{a} > 1 pct rupture occurs early (low N\textsubscript{f}) and the fatigue-rupture characteristics were strongly dependent on ɛ\textsubscript{a} and the rotational speed (regime 1). For 0.75 pct < ɛ\textsubscript{a} < 1 pct, fatigue lives strongly increase and are characterized by a significant statistical scatter (regime 2). For ɛ\textsubscript{a} < 0.75 pct, no fatigue rupture occurs up to cycle numbers of 10\textsuperscript{6} (regime 3). Using scanning electron microscopy (SEM), it was shown that surface cracks formed in regions with local stress raisers (such as inclusions and/or scratches). The growth of surface cracks during fatigue loading produced striations on the rupture surface; during final rupture, ductile voids form. The microstructural details of fatigue-damage accumulation during BRF testing are described and discussed.},
  Doi                      = {10.1007/s11661-003-0186-x},
  ISBN                     = {1073-5623},
  ISSN                     = {1073-5623}
}