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Rostock, Germany

Neelakantan L.,Ruhr University Bochum | Neelakantan L.,Indian Institute of Technology Madras | Zglinski J.K.,Ruhr University Bochum | Frotscher M.,Ruhr University Bochum | And 2 more authors.
Review of Scientific Instruments | Year: 2013

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 andor in situ measurements. The versatility of the combined electrochemicalmechanical 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. © 2013 American Institute of Physics.

Young M.L.,Ruhr University Bochum | Young M.L.,University of North Texas | Frotscher M.,Ruhr University Bochum | Frotscher M.,Cortronik GmbH | And 5 more authors.
International Journal of Materials Research | Year: 2012

Depending on the processing method, pseudoelastic NiTi alloys can have small, lenticular Ni4Ti3 precipitates; however, the mechanical properties of these precipitates are not well understood. By performing nanoindentation with a spherical indenter, Ni4Ti 3 precipitates within a pseudoelastic NiTi alloy were examined. Scanning electron microscopy was used to examine the indents after nanoindentation. After unloading, the hardness and remnant depth ratios of the indents in the Ni4Ti3 precipitates, the NiTi matrix, and the "average" NiTi alloy were compared. To decouple the effects of elasticity from those of pseudoelasticity, similar nanoindentation experiments were performed on an NiAl sample and compared with results from the NiTi sample. © 2012 Carl Hanser Verlag GmbH & Co. KG.

Rahim M.,Ruhr University Bochum | Frenzel J.,Ruhr University Bochum | Frotscher M.,Ruhr University Bochum | Frotscher M.,Cortronik GmbH | And 5 more authors.
Acta Materialia | Year: 2013

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 >108) 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. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Quosdorf D.,University of Rostock | Brede M.,University of Rostock | Leder A.,University of Rostock | Lootz D.,Cortronik GmbH | And 2 more authors.
Technisches Messen | Year: 2011

This article de-scribes a Micro-Particle-Image-Velocimetry measuring setup (Micro-PIV), which allows to measure the three-dimensional flow field inside a transparent 1: 1 model of an original co- ronary stent. The treatment of the coronary heart disease is of utmost importance due to it's high prevalence. Often coronary stents are implanted which lead to adverse effects originating from the altered blood flow through the vessel treated. © Oldenbourg Wissenschaftsverlag.

The invention relates to a device for coating a stent, comprising a holder for the stent, a spraying unit comprising a spray mandrel and an air nozzle. The spray mandrel, the air nozzle and the holder are designed and disposed relative to each other such that the spray mandrel projects from one side into the stent during coating and the air nozzle projects into the stent from the opposing side. The invention further provides a method for coating a stent which employs the device according to the invention. Finally, the invention relates to stents that can be obtained according to the method.

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