Influence of stored elastic strain energy on fatigue behaviour of NiTi shape memory alloy thermal actuator wire

Nitinol (NiTi), a shape memory alloy (SMA) of nickel and titanium, exhibits two unique properties: the shape memory effect and superelasticity. It is a material of choice for applications demanding extraordinary flexibility and motion. It is subjected to greater fatigue strains compared to ordinary metals. The structural and functional fatigue properties are important for assessing the fatigue life and reliability of the superelastic NiTi. The advances made in the experimental analysis to improve the structural and functional fatigue resistance of superelastic NiTi are reviewed in this paper. Various aspects of fatigue behaviour of NiTi in biomedical and cooling applications, along with fatigue failure mechanism, are elaborated under structural fatigue. Importance of functional fatigue and its connect with structural fatigue performance of NiTi is discussed citing recent research literature. Furthermore, the effect of processing parameters involved in additive manufacturing on the fatigue performance of NiTi is also discussed.

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Effect of Hydrostatic Pressure on Thermodynamic Properties of NiTi Shape Memory Alloy

Archives of Metallurgy and Materials ◽

10.1515/amm-2017-0119 ◽

2017 ◽

Vol 62 (2) ◽

pp. 799-806 ◽

Cited By ~ 1

Author(s):

C. Tatar ◽

Z. Yildirim ◽

O. Kaygili

Keyword(s):

Hydrostatic Pressure ◽

Shape Memory Alloy ◽

Physical Properties ◽

Shape Memory ◽

Applied Pressure ◽

Elastic Strain Energy ◽

Niti Shape Memory Alloy ◽

Sem Images ◽

Transformation Temperatures ◽

Effect Of Hydrostatic Pressure

AbstractThe effect of hydrostatic pressure and different heating/cooling rates on physical properties and microstructure of NiTi shape memory alloy has been investigated. The transformation temperatures and physical properties of the alloy have changed with applied pressure. It has been clearly seen from Differential Scanning Calorimetry (DSC) that with the increase of applied pressure, whileAsandAf, andMftransformation temperatures decrease,Msvalue increase. Moreover, based on the increase of the pressure amount applied on the sample, there was an average increase of 48% for Gibbs free energy and 18% for elastic strain energy. Entropy of the alloys decreases depending on the increase in the amount of applied pressure for all heating rates. Depending on the amount of applied pressure on the sample, an interior strain of 0.177% at most was observed. With the increase of applied pressure on the sample, it was determined that activation energy increased. Additionally, the Scanning Electron Microscopy (SEM) images of the samples show that the grain sizes of the unpressured sample and the samples on which pressure is applied are between 40 and 120 μm, which was determined by Image Analysis Method.

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Strain energy-and stress-based approaches revisited in notch fatigue of ductile steels

MATEC Web of Conferences ◽

10.1051/matecconf/201816514009 ◽

2018 ◽

Vol 165 ◽

pp. 14009 ◽

Cited By ~ 1

Author(s):

Bruno Atzori ◽

Mauro Ricotta ◽

Giovanni Meneghetti

Keyword(s):

Energy Density ◽

Plastic Strain ◽

Strain Energy Density ◽

Elastic Strain ◽

Strain Energy ◽

Elastic Strain Energy ◽

Fatigue Behaviour ◽

Strength Reduction Factor ◽

Plastic Strain Energy ◽

Elastic Strain Energy Density

The constant amplitude, zero-mean stress, axial-fatigue behaviour of plain and bluntly notched AISI 304 L stainless steel specimens is investigated in terms of strain energy density. Concerning plain material, it was found that at the fatigue knee the plastic strain energy density is 1.49 times higher than the elastic strain energy density. In the authors’ opinion, the presence of plasticity at the fatigue knee is responsible for the unsuitableness of classical stress - based approaches to synthesise the fatigue behaviour of this material. On the contrary, the elastic-plastic strain energy density was found an efficient parameter to rationalise in a single scatter band fatigue data of plain and bluntly notched specimens. Based on this result, the classic stress-and the point stress-based approaches were revisited taking into account the presence of plasticity at the fatigue knee, by introducing an equivalent fully elastic material having a linear elastic strain energy density at the fatigue knee equal to that of the actual material. Accordingly, a coefficient of plasticity Kp was successfully introduced to modify the classical definition of fatigue strength reduction factor, Kf.

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Effects of Pre-Strain and Nb Content on Transformation Temperatures in Ti-Ni-Nb Shape Memory Alloy

Materials Science Forum ◽

10.4028/www.scientific.net/msf.475-479.1949 ◽

2005 ◽

Vol 475-479 ◽

pp. 1949-1952 ◽

Cited By ~ 1

Author(s):

Toshio Sakuma ◽

Yuji Mihara ◽

Yasuo Ochi ◽

M. Ozawa ◽

K. Okita ◽

...

Keyword(s):

Shape Memory Alloy ◽

Shape Memory ◽

Strain Energy ◽

Volume Fraction ◽

Elastic Strain Energy ◽

Temperature Hysteresis ◽

Transformation Temperatures ◽

Nb Content ◽

Transformation Hysteresis ◽

Nb Addition

Ti-Ni-Nb shape memory alloy has a wide transformation hysteresis, and has been used as coupling devices and so on. However systematic researches of the influence of the amount of Nb addition on transformation temperatures are few. The purpose of this work is to clarify the influences of pre-strain and Nb content on transformation temperatures in Ti-Ni-Nb shape memory alloys. The specimens were Ti-Ni-Nb alloys (Ni/Ti=1.0, Nb=0-15at%), annealed at 1223 K for 0.3 ks. The variation of transformation temperatures with pre-strain and Nb content were investigated experimentally. The variation of As, Ms and transformation temperature hysteresis with pre-strain and Nb content will be discussed in relation to the elastic strain energy and the volume fraction of slip-deformed martensite.

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