Journal ArticleDOI
Crack initiation and propagation in 50.9 at. pct Ni-Ti pseudoelastic shape-memory wires in bending-rotation fatigue
Tak Ahiro Sawaguchi,Gregor Kausträter,Alejandro Andres Yawny,Martin F.-X. Wagner,Gunther Eggeler +4 more
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TLDR
The structural fatigue of Ni-Ti wires was investigated using bending-rotation fatigue (BRF) tests, where a bent and otherwise unconstrained wire was forced to rotate at different rotational speeds as discussed by the authors.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 (N
f
) 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.read more
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Journal ArticleDOI
Structural and functional fatigue of NiTi shape memory alloys
TL;DR: In this paper, structural and functional fatigue of NiTi shape memory alloys is investigated and four cases of fatigue are discussed: (1) the evolution of the stress-strain hysteresis in low cycle pull-pull fatigue of pseudo-elastic NiTi wires.
Journal ArticleDOI
High quality vacuum induction melting of small quantities of NiTi shape memory alloys in graphite crucibles
TL;DR: In this article, the authors present experimental results and discuss thermodynamic and kinetic aspects of the reaction of NiTi melts with graphite crucibles; a method is suggested to keep the carbon dissolution into the melt at a minimum.
Journal ArticleDOI
Phase volume fractions and strain measurements in an ultrafine-grained NiTi shape-memory alloy during tensile loading
TL;DR: In this paper, an ultrafine-grained pseudoelastic NiTi shape-memory alloy wire with 50.9% Ni was examined using synchrotron X-ray diffraction during in situ uniaxial tensile loading (up to 1 GPa) and unloading.
Journal ArticleDOI
Fracture mechanics and microstructure in NiTi shape memory alloys
S. Gollerthan,Marcus L. Young,Alberto Baruj,Jan Frenzel,Wolfgang W. Schmahl,Wolfgang W. Schmahl,Gunther Eggeler +6 more
TL;DR: In this article, cracks propagate at similar stress intensities of 30 ± 5 MPa m into martensite and pseudoelastic austenite in both materials, and a miniature CT specimen was developed, which yields reliable critical fracture mechanics parameters.
Journal ArticleDOI
Fatigue of Nitinol: The state-of-the-art and ongoing challenges.
TL;DR: A review of the present state of the art on the fatigue behavior of superelastic Nitinol is presented and several conclusions are made and recommendations for future works are offered.
References
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Journal ArticleDOI
An overview of nitinol medical applications
T M Duerig,A.R. Pelton,D Stockel +2 more
TL;DR: In this article, the authors present 10 less-obvious, but very important, reasons for nitinol's success, both past and future, including the quickly growing and technologically demanding stent applications.
Journal ArticleDOI
Non-medical applications of shape memory alloys
TL;DR: In this article, the authors stress the need for further exploration of the 4P-relation: principles-properties-processing-products as well as in companies as well in universities or other research laboratories, illustrated by describing some actual applications indicating why they are successful, other applications why they failed and still others that can only be realised if some further, probably possible, material improvement can be realised.
Journal ArticleDOI
Ni4Ti3-precipitation during aging of NiTi shape memory alloys and its influence on martensitic phase transformations
TL;DR: In this paper, the microstructure of a Ni-rich NiTi shape memory alloy and its influence on the thermal characteristics of martensitic transformations were studied. But the authors did not consider the effect of stress on the precipitation process of Ni4Ti3-precipitates.
Journal ArticleDOI
Science and Technology of Shape-Memory Alloys: New Developments
TL;DR: The shape-memory effect and superelasticity of shape memory alloys (SMAs) have been studied extensively in the literature as discussed by the authors, where the authors present an overview of recent progress in this field.
Journal ArticleDOI
Fatigue of NITI thermoelastic martensites
K.N. Melton,O. Mercier +1 more
TL;DR: In this paper, the authors studied the mechanical fatigue of NiTi shape memory alloys and found that the fatigue limit is lower than predicted from the phenomenological law relating growth rates to the elastic modulus.