Elementary Deformation and Damage Mechanisms During Fatigue of Pseudoelastic NiTi Microstents

Abstract

The blood circulatory system in the human body is associated with a pulsating blood flow which results in cyclic widening of the vessels in the rhythm of the heart beat. For a medical stent, which is supposed to stabilize vessels, this results in cyclic mechanical loading, which can lead to fatigue phenomena, such as fracture of strut elements of the stent. It is important to understand the elementary deformation and damage processes which govern the fatigue of microstents. [1] We have previously shown that fatigue life is affected by surface-related defects such as intermetallic surface particles, laser burrs, and electropolishing artifacts. [2] The purpose of the present work is to document that mechanical cycling of microstents also affects the microstructure. The mechanical experiments performed in the present study are not designed to simulate in vivo conditions of microstents. Instead, the purpose of this study is to contribute to a better understanding of the role of microstructure during mechanical cycling of microstents. Materials and Methods Microstents and Phase Transition TemperaturesPseudoelastic NiTi microstents with a length of 20 mm and an expanded outer diameter of 3 mm were obtained from Abbott Vascular Instruments Deutschland GmbH (Rangendingen, Germany). The stents had a Ni content of 50.8 at.%. Stent struts, the elementary building units of the stents, have rectangular cross-sections of 90 Â 112 mm 2 . The phase transformation behavior of the as-received microstent can be characterized by differential scanning calorimetry (DSC) using a DSC 2920 from TA Instruments. DSC experiments were performed according to ASTM-2004-03 in the temperature range of AE 150 8C at a cooling/heating rate of 10 K Á min À1 , with 3 min hold times at the highest and lowest temperatures. [3] In the as-received condition, the stents showed a two-step martensitic transformation upon cooling from B2 to R phase and from R phase to B19 0 . Upon heating, a two-step reverse transformation back to austenite is observed, as shown in Figure 1. An A f temperature of 21 8C suggests that the material is fully austenitic at room temperature. After 30 million fatigue cycles, the phase transformation temperatures increased slightly and the peaks are broader and less pronounced. Similar observations were made by Grossmann et al. [4] with spring actuators, where it was found that dislocations accumulate in the microstructure duringIn the present study, we investigate the fatigue behavior of Nickel Titanium (NiTi) microstents at 22 8C (room temperature) and 37 8C up to 30 Â 10 6 load cycles. We briefly describe our test procedure, which applies displacement-controlled pull-pull fatigue cycling between displacements corresponding to apparent strains of 5 and 7.5%. The response of the microstents to mechanical loading indicates cyclic softening during 30 Â 10 4 cycles. Subsequently, the maximum load remains constant throughout the remainder of the test. We use transmission electron microscopy (TEM) to clarify the microstructural reasons...

Fatigue of Nitinol: The state-of-the-art and ongoing challenges

“…Second, the stress-strain response of the material stabilizes after several cycles of loading, i.e. ~100-150 cycles (Eggeler et al, 2004;Frotscher et al, 2011;Gall et al, 1998;Maletta et al, 2014;Mao et al, 2006;Miyazaki et al, 1986b;Moumni et al, 2005;Nemat-Nasser and Guo, 2006;Sehitoglu et al, 2001). Comparable changes for thermally martensitic Nitinol (Kang et al, 2012;Melton and Mercier, 1979a) and mixedmode Nitinol (Nayan et al, 2008) were also reported in the literature.…”

“…They also indicated that the fatigue life of strut elements in medical stents was controlled by the crack nucleation stage and there was no distinct crack growth stage observed on the fracture surface. In subsequent research, Frotscher et al (Frotscher et al, 2011) observed an increase in dislocation density in some grains with an increase in the number of cycles. This localized dislocation density may form stressinduced martensite in some areas within the material.…”

Modeling of torsion fatigue in shape memory alloys

“…Eggeler et al (2004) first defined functional fatigue as degradation of the SMAs' functional properties such as the dissipated energy or maximum recoverable strain during cyclic loads. Many efforts have been put into investigating the functional behavior and cyclic degradation of superelastic Nitinol (Eggeler et al, 2004;Frotscher et al, 2011;Gall et al, 1998;Maletta et al, 2014;Mao et al, 2006;Miyazaki et al, 1986a;Moumni et al, 2005;Nemat-Nasser and Guo, 2006;Sehitoglu et al, 2001), martensitic Nitinol (Kang et al, 2012;Melton and Mercier, 1979a), or a mixture of two phases (Nayan et al, 2008) under uniaxial loading. These studies have also been extended not only to torsional loading (Jensen, 2005;Wang et al, 2010) but also to multiaxial loading (Song et al, 2014;Wang et al, 2012).…”

Analysis of the compression process and dimensional optimization of self‐expanding stent of a nickel titanium alloy

“…Peltona et al have performed fatigue experiments on diamond shaped specimens and speculated that pulsatile fatigue is not the primary cause of fractures in the superficial femoral artery [2]. M. Frotscher et al have carried out a series of experiments on NiTi struts to study the fatigue properties of NiTi stents and found that the fatigue fracture occurred at the fillet areas [3][4][5]. However, during the compression process, the stent can undergo great radial deformation.…”