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R O Ritchie - One of the best experts on this subject based on the ideXlab platform.
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in vitro fatigue crack growth and fracture toughness behavior of thin walled superelastic nitinol tube for endovascular stents a basis for defining the effect of crack like defects
Biomaterials, 2007Co-Authors: Scott W Robertson, R O RitchieAbstract:Abstract Endovascular stents made of the superelastic nickel-titanium alloy Nitinol are subjected in service to tens of millions of loading cycles and even “single-event” overloads, both of which can potentially result in fracture and/or complete failure of the device. A fracture-mechanics-based methodology can provide a means to quantify relevant material parameters critical to the design against such failures. However, there is a dearth of relevant experimental data in the literature on such fracture-mechanics-based approaches to fatigue in Nitinol; furthermore, that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, e.g., bulk Nitinol bar and strip. Consequently, the current work is focused on characterizing in vitro both subcritical and critical crack growth (fatigue–crack growth and R-curve fracture toughness) behavior in thin-walled (∼400 μm thick) Nitinol tubing similar to that used for medical device manufacture (following shape-setting procedures to flatten the material), with a resultant austenite Finish Temperature of Af∼25–30 °C, identical to self-expanding Nitinol stents. Fatigue–crack growth behavior, measured in Hanks’ Balanced Saline Solution over a wide spectrum of growth rates (down to 10−10 m/cycle) and at a range of positive load ratios ( R = 0.1 – 0.7 ), revealed significantly higher fatigue thresholds than had been previously reported for bulk Nitinol material. In addition, we examine the critical effect of test frequency, as most fatigue experiments on Nitinol have been performed at 30 Hz or above, despite the fact that this is far in excess of the frequency of physiological loading. Finally, the fracture toughness properties are characterized in thin-section Nitinol and show marked crack-resistance (R-curve) behavior with a dependence on crack-growth angle (with respect to the tube drawing axis); additionally, measured toughnesses are found to be lower than has been previously reported for bulk Nitinol.
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in vitro fatigue crack growth and fracture toughness behavior of thin walled superelastic nitinol tube for endovascular stents a basis for defining the effect of crack like defects
Biomaterials, 2007Co-Authors: Scott W Robertson, R O RitchieAbstract:Endovascular stents made of the superelastic nickel-titanium alloy Nitinol are subjected in service to tens of millions of loading cycles and even "single-event" overloads, both of which can potentially result in fracture and/or complete failure of the device. A fracture-mechanics-based methodology can provide a means to quantify relevant material parameters critical to the design against such failures. However, there is a dearth of relevant experimental data in the literature on such fracture-mechanics-based approaches to fatigue in Nitinol; furthermore, that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, e.g., bulk Nitinol bar and strip. Consequently, the current work is focused on characterizing in vitro both subcritical and critical crack growth (fatigue-crack growth and R-curve fracture toughness) behavior in thin-walled ( approximately 400microm thick) Nitinol tubing similar to that used for medical device manufacture (following shape-setting procedures to flatten the material), with a resultant austenite Finish Temperature of A(f) approximately 25-30 degrees C, identical to self-expanding Nitinol stents. Fatigue-crack growth behavior, measured in Hanks' Balanced Saline Solution over a wide spectrum of growth rates (down to 10(-10)m/cycle) and at a range of positive load ratios (R=0.1-0.7), revealed significantly higher fatigue thresholds than had been previously reported for bulk Nitinol material. In addition, we examine the critical effect of test frequency, as most fatigue experiments on Nitinol have been performed at 30Hz or above, despite the fact that this is far in excess of the frequency of physiological loading. Finally, the fracture toughness properties are characterized in thin-section Nitinol and show marked crack-resistance (R-curve) behavior with a dependence on crack-growth angle (with respect to the tube drawing axis); additionally, measured toughnesses are found to be lower than has been previously reported for bulk Nitinol.
G Eggeler - One of the best experts on this subject based on the ideXlab platform.
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effect of low Temperature precipitation on the transformation characteristics of ni rich niti shape memory alloys during thermal cycling
Intermetallics, 2010Co-Authors: Martin F X Wagner, H Gugel, J Frenzel, Ch Somsen, G EggelerAbstract:Abstract Thermal cycling of NiTi shape memory alloys is associated with functional fatigue: the characteristic phase transformation Temperatures decrease with increasing number of cycles, and the transformation behavior changes from a single- to a two-stage martensitic transformation involving the intermediate R-phase. These effects are usually attributed to a gradual increase of dislocation density associated with micro-plasticity during repeated cycling through the transformation range. Here, these changes are shown to increase at a higher maximum Temperature (in the fully austenitic state) during differential scanning calorimetric cycling of a Ni-rich alloy. Additional thermal cycling experiments without repeated phase transformations, and post-mortem microstructural observations by transmission electron microscopy, demonstrate that a relevant portion of functional fatigue is due to the formation of nano-scale Ni-rich precipitates of type Ni 4 Ti 3 even at Temperatures relatively close to the austenite Finish Temperature. These results show that both dislocation generation during the diffusion-less phase transformation, and diffusion-controlled nucleation and growth of Ni 4 Ti 3 precipitates, can interact and contribute to the evolution of functional properties during thermal cycling of Ni-rich NiTi.
Scott W Robertson - One of the best experts on this subject based on the ideXlab platform.
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in vitro fatigue crack growth and fracture toughness behavior of thin walled superelastic nitinol tube for endovascular stents a basis for defining the effect of crack like defects
Biomaterials, 2007Co-Authors: Scott W Robertson, R O RitchieAbstract:Abstract Endovascular stents made of the superelastic nickel-titanium alloy Nitinol are subjected in service to tens of millions of loading cycles and even “single-event” overloads, both of which can potentially result in fracture and/or complete failure of the device. A fracture-mechanics-based methodology can provide a means to quantify relevant material parameters critical to the design against such failures. However, there is a dearth of relevant experimental data in the literature on such fracture-mechanics-based approaches to fatigue in Nitinol; furthermore, that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, e.g., bulk Nitinol bar and strip. Consequently, the current work is focused on characterizing in vitro both subcritical and critical crack growth (fatigue–crack growth and R-curve fracture toughness) behavior in thin-walled (∼400 μm thick) Nitinol tubing similar to that used for medical device manufacture (following shape-setting procedures to flatten the material), with a resultant austenite Finish Temperature of Af∼25–30 °C, identical to self-expanding Nitinol stents. Fatigue–crack growth behavior, measured in Hanks’ Balanced Saline Solution over a wide spectrum of growth rates (down to 10−10 m/cycle) and at a range of positive load ratios ( R = 0.1 – 0.7 ), revealed significantly higher fatigue thresholds than had been previously reported for bulk Nitinol material. In addition, we examine the critical effect of test frequency, as most fatigue experiments on Nitinol have been performed at 30 Hz or above, despite the fact that this is far in excess of the frequency of physiological loading. Finally, the fracture toughness properties are characterized in thin-section Nitinol and show marked crack-resistance (R-curve) behavior with a dependence on crack-growth angle (with respect to the tube drawing axis); additionally, measured toughnesses are found to be lower than has been previously reported for bulk Nitinol.
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in vitro fatigue crack growth and fracture toughness behavior of thin walled superelastic nitinol tube for endovascular stents a basis for defining the effect of crack like defects
Biomaterials, 2007Co-Authors: Scott W Robertson, R O RitchieAbstract:Endovascular stents made of the superelastic nickel-titanium alloy Nitinol are subjected in service to tens of millions of loading cycles and even "single-event" overloads, both of which can potentially result in fracture and/or complete failure of the device. A fracture-mechanics-based methodology can provide a means to quantify relevant material parameters critical to the design against such failures. However, there is a dearth of relevant experimental data in the literature on such fracture-mechanics-based approaches to fatigue in Nitinol; furthermore, that which does exist invariably pertains to product forms that are not appropriate for stent manufacture, e.g., bulk Nitinol bar and strip. Consequently, the current work is focused on characterizing in vitro both subcritical and critical crack growth (fatigue-crack growth and R-curve fracture toughness) behavior in thin-walled ( approximately 400microm thick) Nitinol tubing similar to that used for medical device manufacture (following shape-setting procedures to flatten the material), with a resultant austenite Finish Temperature of A(f) approximately 25-30 degrees C, identical to self-expanding Nitinol stents. Fatigue-crack growth behavior, measured in Hanks' Balanced Saline Solution over a wide spectrum of growth rates (down to 10(-10)m/cycle) and at a range of positive load ratios (R=0.1-0.7), revealed significantly higher fatigue thresholds than had been previously reported for bulk Nitinol material. In addition, we examine the critical effect of test frequency, as most fatigue experiments on Nitinol have been performed at 30Hz or above, despite the fact that this is far in excess of the frequency of physiological loading. Finally, the fracture toughness properties are characterized in thin-section Nitinol and show marked crack-resistance (R-curve) behavior with a dependence on crack-growth angle (with respect to the tube drawing axis); additionally, measured toughnesses are found to be lower than has been previously reported for bulk Nitinol.
Martin F X Wagner - One of the best experts on this subject based on the ideXlab platform.
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effect of low Temperature precipitation on the transformation characteristics of ni rich niti shape memory alloys during thermal cycling
Intermetallics, 2010Co-Authors: Martin F X Wagner, H Gugel, J Frenzel, Ch Somsen, G EggelerAbstract:Abstract Thermal cycling of NiTi shape memory alloys is associated with functional fatigue: the characteristic phase transformation Temperatures decrease with increasing number of cycles, and the transformation behavior changes from a single- to a two-stage martensitic transformation involving the intermediate R-phase. These effects are usually attributed to a gradual increase of dislocation density associated with micro-plasticity during repeated cycling through the transformation range. Here, these changes are shown to increase at a higher maximum Temperature (in the fully austenitic state) during differential scanning calorimetric cycling of a Ni-rich alloy. Additional thermal cycling experiments without repeated phase transformations, and post-mortem microstructural observations by transmission electron microscopy, demonstrate that a relevant portion of functional fatigue is due to the formation of nano-scale Ni-rich precipitates of type Ni 4 Ti 3 even at Temperatures relatively close to the austenite Finish Temperature. These results show that both dislocation generation during the diffusion-less phase transformation, and diffusion-controlled nucleation and growth of Ni 4 Ti 3 precipitates, can interact and contribute to the evolution of functional properties during thermal cycling of Ni-rich NiTi.
U Ramamurty - One of the best experts on this subject based on the ideXlab platform.
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combining indentation and diffusion couple techniques for combinatorial discovery of high Temperature shape memory alloys
Acta Materialia, 2013Co-Authors: V V Shastry, Aloke Paul, V D Divya, M A Azeem, David Dye, U RamamurtyAbstract:Abstract We demonstrate the possibility of accelerated identification of potential compositions for high-Temperature shape memory alloys (SMAs) through a combinatorial material synthesis and analysis approach, wherein we employ the combination of diffusion couple and indentation techniques. The former was utilized to generate smooth and compositionally graded inter-diffusion zones (IDZs) in the Ni–Ti–Pd ternary alloy system of varying IDZ thickness, depending on the annealing time at high Temperature. The IDZs thus produced were then impressed with an indenter with a spherical tip so as to inscribe a predetermined indentation strain. Subsequent annealing of the indented samples at various elevated Temperatures, T a , ranging between 150 and 550 °C allows for partial to full relaxation of the strain imposed due to the shape memory effect. If T a is above the austenite Finish Temperature, A f , the relaxation will be complete. By measuring the depth recovery, which serves as a proxy for the shape recovery characteristic of the SMA, a three-dimensional map in the recovery–Temperature–composition space is constructed. A comparison of the published A f data for different compositions with the T a data shows good agreement when the depth recovery is between 70% and 80%, indicating that the methodology proposed in this paper can be utilized for the identification of promising compositions. Advantages and further possibilities of this methodology are discussed.