NUI Galway – UL Alliance First Annual ENGINEERING AND - ARAN ...

Microstructural Modelling of Fatigue Crack Nucleation in Stents
Sweeney, C. 1 , McHugh, P.E. 1 , Leen, S.B. 1
1 Department of Mechanical and Biomedical Engineering & NCBES, NUI Galway
email: (c.sweeney4@nuigalway.ie)
Abstract
A methodology for modelling realistic microstructures
of metallic materials is presented. The approach is
applied to cyclic loading of stainless steel stents with a
view to capturing the role of grain size distribution in
fatigue crack nucleation.
1. Introduction
The use of balloon expandable stents as part of the
angioplasty procedure has revolutionised the treatment
of heart disease. Although stent design is a relatively
mature topic, numerous instances of stent fatigue
fracture have been reported, e.g. [1]. This is particularly
the case recently with the advent of drug eluting stents
where tissue in-growth over stent struts is impeded,
allowing deformed geometries and fractures to be more
clearly revealed than before. Hence, there is a clear
need to gain a deeper understanding of stent fatigue
behaviour and to develop fatigue failure prediction
methods to aid stent design refinements. Typical stent
strut dimensions are comparable with microstructural
geometry, such as grain size [2-3], as shown in Fig. 1.
Crystal plasticity (CP) modelling is therefore necessary
to capture the inhomogeneity effects introduced by
microstructure on deformation and hence fatigue
behaviour of stents. Such a model will allow prediction
and identification of microstructural phenomena during
fatigue, including grain size effects [4] and crack
nucleation mechanisms [5]. Voronoi tessellation is the
first key step in this process, to model random
microstructure of polycrystals [6].
Initial work focuses on a well-known stent material:
316 L stainless steel. A finite element micromechanical
model of a representative volume element of
polycrystalline 316 L was developed and used to
simulate uniaxial cyclic loading for both continuum
plasticity and CP formulations.
2. Materials and Methods
A Voronoi tessellation and Delaunay triangulation
methodology was implemented into a Python script to
generate a microstructural model representing
inhomogeneous polycrystalline geometry. Tessellations
with grain size distribution statistically close to that of a
real microstructure were accepted (example shown in
Fig. 1). An Abaqus user-material subroutine [2],
incorporating an isotropic CP formulation, was
employed. The stabilised cyclic response of the
micromechanical model for strain-controlled uniaxial
cyclic loading was then compared to the experimental
macroscopic response.
50
75 µm
100 µm
Figure 1. Microscopy image of 316L stent strut
[7] (foreground) and simulated microstructure.
3. Results
Good correlation was achieved between grain size
distributions of the real and generated microstructures.
Microscopic stress concentrations were predicted
throughout the micromechanical model. CP material
model constants were identified which agreed with the
macroscopic cyclic stress range.
4. Discussion
A microstructural modelling methodology has been
developed for realistic grain size distributions, capable
of capturing inhomogeneity effects. The CP material
model was calibrated to match macroscopic stress
ranges for different applied strain ranges. Differences in
hysteresis loop shapes, however, suggest that the next
step involve the incorporation of a non-linear kinematic
hardening formulation, e.g. [8], in the CP model. Future
developments also include the introduction of a fatigue
indicator parameter, such as accumulated plastic slip
[5], to predict crack initiation during high-cycle fatigue.
5. Acknowledgements
Research funded by an IRCSET scholarship under
the EMBARK initiative.
6. References
[1] Shaikh, F. et al., 2008. Catheter. .Cardiovasc. Interv.,
71(5), 614-618.
[2] You, X. et al., 2006. Acta. Mater., 54(18), 4825-4840.
[3] Marrey, R.V. et al., 2006. Biomaterials, 27(9), 1988-2000.
[4] Zhang, M., Neu, R. & McDowell, D., 2009. Int. J.
Fatigue, 31(8-9), 1397-1406.
[5] Manonukul, A. & Dunne, F.P.E., 2004. Proc. R. Soc.
London Ser. A, 460(2047), 1881 -1903.
[6] Kovac, M. & Cizelj, L., 2005. Nucl. Eng. Design, 235(17-
19), 1939-1950.
[7] Savage, P. et al., 2004. Ann. Biomed. Eng., 32(2), 202-
211.
[8] Barbe, F. et al., 2001. Int. J. Plasticity, 17(4), 513-536.