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G Eggeler - One of the best experts on this subject based on the ideXlab platform.
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functional and Structural Fatigue of titanium tantalum high temperature shape memory alloys ht smas
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2015Co-Authors: Thomas Niendorf, G Eggeler, P Kroos, E Batyrsina, Alexander Paulsen, Yahya Motemani, Alfred Ludwig, Pio John S Buenconsejo, Jan Frenzel, H J MaierAbstract:Abstract Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel–titanium (Ni–Ti) can only be employed at temperatures up to about 100 °C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni–Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium–tantalum (Ti–Ta) system has been developed to overcome these issues. Binary Ti–Ta provides relatively high M S temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti–Ta–Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and Structural Fatigue of this alloy. Functional Fatigue is dominated by ω-phase evolution, while Structural Fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for Fatigue life extension proposed very recently for binary Ti–Ta, is demonstrated to be also applicable for the ternary Ti–Ta–Al.
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impurity levels and Fatigue lives of pseudoelastic niti shape memory alloys
Acta Materialia, 2013Co-Authors: Mustafa Rahim, Jan Frenzel, M Frotscher, J Pfetzingmicklich, R Steegmuller, M Wohlschlogel, H Mughrabi, G EggelerAbstract:Abstract In the present work we show how different oxygen (O) and carbon (C) levels affect Fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation Fatigue (up to cycle numbers >10 8 ) and scanning electron microscopy (for investigating microStructural details of crack initiation and growth) to study Fatigue behavior. High cycle Fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle Fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher Fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where Fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of Structural Fatigue of pseudoelastic NiTi shape memory alloys.
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design and fabrication of a bending rotation Fatigue test rig for in situ electrochemical analysis during Fatigue testing of niti shape memory alloy wires
Review of Scientific Instruments, 2013Co-Authors: Lakshman Neelakantan, M Frotscher, Jenni Kristin Zglinski, G EggelerAbstract:The current investigation proposes a novel method for simultaneous assessment of the electrochemical and Structural Fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation Fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of Fatigue on corrosion and vice versa. This can be achieved by performing ex situ and∕or in situ measurements. The versatility of the combined electrochemical∕mechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9% NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-Fatigue on the localized corrosion resistance, or the influence of hydrogen on Fatigue life. Ex situ experiments showed that a pre-Fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the Fatigue failure.
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in situ scanning electron microscopic study of Structural Fatigue of struts the characteristic elementary building units of medical stents
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2008Co-Authors: M Frotscher, K Neuking, R Bockmann, K D Wolff, G EggelerAbstract:Abstract The present study addresses the cyclic mechanical behaviour of strut assemblies, the elementary building units of medical stents. Strut assemblies were subjected to approximately 2500 cycles at room temperature in air at a frequency of 5 Hz. They were then integrated into an in situ test rig which allowed to perform pull–pull cycling in a scanning electron microscope. The stress–strain response associated with cyclic loading was monitored. And local failure events, which do not necessarily lead to a total loss of strut assembly integrity, were recorded. Early cracks always form at locations where the presence of notches results in high local stresses. These early failures can result in a redistribution of loads in the entire strut assembly. Miniature tensile testing is a useful method to show that pre-Fatigue exposure weakens the mechanical resistance of a strut assembly. In most cases, crack nucleation seems to control the Fatigue life of individual strut elements and strut assemblies in the austenitic state. It was often observed that micro-crack nucleation is directly followed by fast crack propagation and rupture. During tensile testing, surface flaws were identified as locations where cracks initiate. But we also found micro-cracks around surface inclusions which did not propagate during in situ pull–pull cycling or tensile testing.
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Structural and functional Fatigue of niti shape memory alloys
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004Co-Authors: G Eggeler, Erhard Hornbogen, A Yawny, A Heckmann, Martin F X WagnerAbstract:Abstract Cyclic loading is one of the generic characteristic features of many of the present and potential future applications of NiTi shape memory alloys, no matter whether they exploit mechanical (pseudo-elasticity) or thermal shape memory (one and two way effect). Cyclic loading may well be associated with Structural and functional Fatigue, which both limit the service life of shape memory components. By “Structural Fatigue” we mean the microStructural damage that accumulates during cyclic loading and eventually leads to Fatigue failure. There is a need to understand how microstructures can be optimized to provide good Fatigue resistance. The term “functional Fatigue” indicates that shape memory effects like the working displacement in a one way effect (1WE) actuator or the dissipated energy in a loading–unloading cycle of a pseudo-elastic (PE) damping application decrease with increasing cycle numbers. This is also due to a gradual change in microstructure. In both cases it is important to know how Fatigue cycling affects shape memory properties. The present paper considers Structural and functional Fatigue of NiTi shape memory alloys. It discusses four cases of Fatigue in NiTi shape memory alloys: (1) The evolution of the stress–strain hysteresis in low cycle pull–pull Fatigue of pseudo-elastic NiTi wires. (2) Bending–rotation Fatigue rupture of pseudo-elastic NiTi wires. (3) Strain localization during the stress induced formation of martensite. (4) Generic features of functional Fatigue in NiTi shape memory actuator springs. The paper shows that Fatigue of shape memory alloys is a fascinating research field and highlights the need for further work in this area.
Martin F X Wagner - One of the best experts on this subject based on the ideXlab platform.
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Structural and functional Fatigue of niti shape memory alloys
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004Co-Authors: G Eggeler, Erhard Hornbogen, A Yawny, A Heckmann, Martin F X WagnerAbstract:Abstract Cyclic loading is one of the generic characteristic features of many of the present and potential future applications of NiTi shape memory alloys, no matter whether they exploit mechanical (pseudo-elasticity) or thermal shape memory (one and two way effect). Cyclic loading may well be associated with Structural and functional Fatigue, which both limit the service life of shape memory components. By “Structural Fatigue” we mean the microStructural damage that accumulates during cyclic loading and eventually leads to Fatigue failure. There is a need to understand how microstructures can be optimized to provide good Fatigue resistance. The term “functional Fatigue” indicates that shape memory effects like the working displacement in a one way effect (1WE) actuator or the dissipated energy in a loading–unloading cycle of a pseudo-elastic (PE) damping application decrease with increasing cycle numbers. This is also due to a gradual change in microstructure. In both cases it is important to know how Fatigue cycling affects shape memory properties. The present paper considers Structural and functional Fatigue of NiTi shape memory alloys. It discusses four cases of Fatigue in NiTi shape memory alloys: (1) The evolution of the stress–strain hysteresis in low cycle pull–pull Fatigue of pseudo-elastic NiTi wires. (2) Bending–rotation Fatigue rupture of pseudo-elastic NiTi wires. (3) Strain localization during the stress induced formation of martensite. (4) Generic features of functional Fatigue in NiTi shape memory actuator springs. The paper shows that Fatigue of shape memory alloys is a fascinating research field and highlights the need for further work in this area.
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Structural Fatigue of pseudoelastic niti shape memory wires
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004Co-Authors: Martin F X Wagner, Takahiro Sawaguchi, Gregor Kaustrater, D Hoffken, G EggelerAbstract:In this study, we consider the influence of wire diameter and rotational speed on Fatigue rupture behaviour in bending rotation Fatigue experiments. We show that the dependence of Fatigue life on wire diameter and rotational speed is no longer observed when the bending rotation Fatigue experiments are conducted in a silicon oil bath at constant temperature, or at low rotational speeds in air. Moreover, we qualitatively discuss the stress distribution in the wire during Fatigue testing as a starting point for various mechanical investigations, such as the calculation of the dissipated energy in the wire during bending rotation experiments or the bending moment acting during bending rotation.
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crack initiation and propagation in 50 9 at pct ni ti pseudoelastic shape memory wires in bending rotation Fatigue
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2003Co-Authors: Takahiro Sawaguchi, Martin F X Wagner, A Yawny, Gregor Kaustrater, G EggelerAbstract: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.
Di Song - One of the best experts on this subject based on the ideXlab platform.
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review on Structural Fatigue of niti shape memory alloys pure mechanical and thermo mechanical ones
Theoretical and Applied Mechanics Letters, 2015Co-Authors: Guozheng Kang, Di SongAbstract:Abstract Structural Fatigue of NiTi shape memory alloys is a key issue that should be solved in order to promote their engineering applications and utilize their unique shape memory effect and super-elasticity more sufficiently. In this paper, the latest progresses made in experimental and theoretical analyses for the Structural Fatigue features of NiTi shape memory alloys are reviewed. First, macroscopic experimental observations to the pure mechanical and thermo-mechanical Fatigue features of the alloys are summarized; then the state-of-arts in the mechanism analysis of Fatigue rupture are addressed; further, advances in the construction of Fatigue failure models are provided; finally, summary and future topics are outlined.
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whole life transformation ratchetting and Fatigue of super elastic niti alloy under uniaxial stress controlled cyclic loading
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2012Co-Authors: Guozheng Kang, Chao Yu, Di SongAbstract:Abstract The whole-life transformation ratchetting and Fatigue failure (including functional Fatigue and Structural Fatigue) of super-elastic NiTi shape memory alloy were observed by uniaxial stress-controlled cyclic loading tests and at room temperature. The effects of peak stress, mean stress and stress amplitude were discussed, and then the interaction of transformation ratchetting and Fatigue was then investigated. It is concluded that the whole-life transformation ratchetting of super-elastic NiTi shape memory alloy depends greatly on the stress levels and loading modes, and the dissipated energy in each cycle progressively decreases with the increasing number of cycles, which represents a functional degradation. The results also show that the Structural Fatigue life of the NiTi alloy presented under the stress-controlled cyclic loading is dependent on the peak stress, mean stress and stress amplitude, and the transformation ratchetting shortens the Fatigue life apparently. The Fatigue life decreases with the increasing dissipated energy, and a balance between dissipated energy and Structural Fatigue life should be reasonably considered in the design of damping devices of the NiTi alloy.
H J Maier - One of the best experts on this subject based on the ideXlab platform.
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functional and Structural Fatigue of titanium tantalum high temperature shape memory alloys ht smas
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2015Co-Authors: Thomas Niendorf, G Eggeler, P Kroos, E Batyrsina, Alexander Paulsen, Yahya Motemani, Alfred Ludwig, Pio John S Buenconsejo, Jan Frenzel, H J MaierAbstract:Abstract Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel–titanium (Ni–Ti) can only be employed at temperatures up to about 100 °C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni–Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium–tantalum (Ti–Ta) system has been developed to overcome these issues. Binary Ti–Ta provides relatively high M S temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti–Ta–Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and Structural Fatigue of this alloy. Functional Fatigue is dominated by ω-phase evolution, while Structural Fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for Fatigue life extension proposed very recently for binary Ti–Ta, is demonstrated to be also applicable for the ternary Ti–Ta–Al.
Jan Frenzel - One of the best experts on this subject based on the ideXlab platform.
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functional and Structural Fatigue of titanium tantalum high temperature shape memory alloys ht smas
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2015Co-Authors: Thomas Niendorf, G Eggeler, P Kroos, E Batyrsina, Alexander Paulsen, Yahya Motemani, Alfred Ludwig, Pio John S Buenconsejo, Jan Frenzel, H J MaierAbstract:Abstract Due to their high work output and good mechanical properties, actuators made from shape memory alloys (SMAs) are used in numerous applications. Unfortunately, SMAs such as nickel–titanium (Ni–Ti) can only be employed at temperatures up to about 100 °C. Lately, high-temperature shape memory alloys (HT SMAs) have been introduced to overcome this limitation. Ternary systems based on Ni–Ti have been intensively characterized and alloys are available that can operate at elevated temperatures. However, these alloys either contain substantial amounts of expensive noble elements like platinum and palladium, or the materials are brittle. The titanium–tantalum (Ti–Ta) system has been developed to overcome these issues. Binary Ti–Ta provides relatively high M S temperature combined with excellent workability, but it suffers from fast cyclic degradation. By alloying with third elements this drawback can be overcome: The ternary Ti–Ta–Al alloy shows overall promising properties as will be shown in the present work. In-situ thermo-mechanical cycling experiments were conducted and allowed for evaluation of the factors affecting the functional and Structural Fatigue of this alloy. Functional Fatigue is dominated by ω-phase evolution, while Structural Fatigue is triggered by an interplay of ω-phase induced embrittlement and deformation constraints imposed by unsuitable texture. In addition, a concept for Fatigue life extension proposed very recently for binary Ti–Ta, is demonstrated to be also applicable for the ternary Ti–Ta–Al.
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impurity levels and Fatigue lives of pseudoelastic niti shape memory alloys
Acta Materialia, 2013Co-Authors: Mustafa Rahim, Jan Frenzel, M Frotscher, J Pfetzingmicklich, R Steegmuller, M Wohlschlogel, H Mughrabi, G EggelerAbstract:Abstract In the present work we show how different oxygen (O) and carbon (C) levels affect Fatigue lives of pseudoelastic NiTi shape memory alloys. We compare three alloys, one with an ultrahigh purity and two which contain the maximum accepted levels of C and O. We use bending rotation Fatigue (up to cycle numbers >10 8 ) and scanning electron microscopy (for investigating microStructural details of crack initiation and growth) to study Fatigue behavior. High cycle Fatigue (HCF) life is governed by the number of cycles required for crack initiation. In the low cycle Fatigue (LCF) regime, the high-purity alloy outperforms the materials with higher number densities of carbides and oxides. In the HCF regime, on the other hand, the high-purity and C-containing alloys show higher Fatigue lives than the alloy with oxide particles. There is high experimental scatter in the HCF regime where Fatigue cracks preferentially nucleate at particle/void assemblies (PVAs) which form during processing. Cyclic crack growth follows the Paris law and does not depend on impurity levels. The results presented in the present work contribute to a better understanding of Structural Fatigue of pseudoelastic NiTi shape memory alloys.
