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FATIGUE OF NITINOL: EXPERIMENTAL AND COMPUTATIONAL
ANALYSIS ON THE EFFECT OF STENT CRIMPING
Abstract
One material that has found particular favour within
the biomedical industry is the near equi-atomic NiTi
alloy, Nitinol. This can be directly attributed to its
characteristic shape memory and superelastic
behaviour. Nitinol self-expanding endovascular stents
are effective in the treatment of peripheral artery
disease, including the superficial femoral, carotid, and
renal arteries. However, fracture rates of 65.4% in
stents used in the superior femoral artery have been
reported [1]. Such failures have been attributed to
cumulative fatigue damage. Accurate characterisation
of the fatigue behaviour of such stents is therefore
essential for their prolonged safe use in human arteries.
1. Introduction
During manufacture, stents are crimped to fit within
a catheter to allow in vivo deployment. This process
exerts a significant crimping strain on the stent
geometry. Consequently, the focus of this study is to
investigate the effect of crimping strain on the fatigue
life of Nitinol stents under strain control conditions.
2. Materials and Methods
Nitinol 'v-strut' stent-like specimens were supplied by
Veryan Medical. Excess material at both ends, along
with support struts, were included in the design to
provide precise alignment, structural stability and secure
gripping during testing. To accurately characterise the
material properties, uniaxial tensile tests were
performed on the support strut. The EnduraTEC
ELF/3200 was employed for this study. The
crimping/deployment process is equivalent to the
loading/unloading path of the super elastic material, see
Figure 1. Therefore, to simulate the crimping process, a
load-unload procedure was performed on the ‘v-strut’
component. The investigation was carried out at 37 o C to
represent in vivo conditions using an environmental
chamber with air-heating fan.
Displacement-controlled fatigue testing was also
performed on the 'v-strut' at 37 o C. Fatigue data was
collected for strains amplitudes from 0.2 to 0.8% for
crimp strains up to 14%. In previous testing of this
nature, stent components are simply subjected to a
single crimp cycle to simulate the deployment process.
In this study, however, the effect of multiple crimping
cycles was also investigated. All fatigue tests were
conducted with 1.5% mean strain and at a frequency of
50 Hz until failure or run-out at 10 7 cycles.
Weafer, F. 1 , Bruzzi, M. 1
1 Mechanical & Biomedical Engineering, NUI, Galway
f.weafer1@nuigalway.ie
70
Figure 1 Typical superelastic stress–strain curve
transposed onto a hoop force–diameter diagram [3]
3. Discussion
The effect of cyclic loading on the fatigue behaviour
of the NiTi stent-like components was successfully
investigated. Fatigue data is presented on a constant life
diagram to demonstrate the effect of strain amplitude on
the fatigue life of the single cell of the stent geometry;
the 'v-strut'. In addition, the influence of the crimping
procedure was identified and explored. The Finite
Element Analysis software ABAQUS/Standard ver6.10,
in combination with the user-defined material
subroutine UMAT, will be used to calculate the stress
and strain fields in the specimens. The FEA models will
be validated against the load-deflection curves from the
load-unload procedure specimens, taking into account
inaccuracies due to geometry factors as well as the nonlinear
behaviour of Nitinol.
Ultimately, it is hoped these models will be capable
of predicting the constitutive fatigue behaviour of
Nitinol and will ultimately aim to contribute to the
developments in design rules for optimising the fatigue
performance of Nitinol medical devices.
4. References
[1] D.E. Allie et al., “Nitinol Stent Fractures in the SFA”,
Endovascular Today Jul/Aug:1-8, 2004
[2] K.N. Melton et al., “Fatigue of NiTi thermoelastic
martensites”, Acta Metallurgica 27:137-144 1979
[3] T.W. Duerig, “The use of superelasticity in modern
medicine”, MRS Bulletin 27:101–104, 2002
Acknowledgements: This study was funded by Science
Foundation Ireland Research Frontiers Programme