ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

In this study, a biodegradable material model of magnesium alloy was proposed and the
corrosion simulation was applied to two FEA models of MAS designs for comparison
of the corrosion resistance. Preliminary corrosion experiments were followed to verify
the corrosion simulation of the two FEA models. The aim of this study is to verify the
FEA model can simulate the corrosion process of MAS and prove the optimized design
can improve the corrosion resistance of MAS.
3. MATERIALS AND METHODS
3.1. Corrosion simulation of two stent models
A MAS model (Model A) from an optimized design [4] and another model (Model B)
from a patented design [5] are shown in Fig. 1. Only one ring of both designs was
chosen for the 3D model to save simulation time.
Fig. 1. Two MAS models and detailed dimensions
The material property was from a commercial magnesium alloy AZ31 with the
chemical composition of Al 0.03, Zn 0.01, Mn 0.002 and Mg balance (mass percentage).
The AZ31 has a modulus of 43.5 GPa, Poisson’s ratio of 0.35 and density of 1.77
g/mm 3 . The yield stress is 175 MPa and the tensile strain limit is 0.17. A biodegradable
material model was proposed based on the continuum damage approach to scale linearly
with the scalar damage parameter (D) for both the elastic and plastic properties of the
undamaged material [6]. Two corrosion mechanisms are considered in a cooperating
way. The former is the uniform corrosion damage DU, which has phenomenological
correspondence to the micro galvanic mechanism experimentally observed in Mg alloys
that results in corrosive attack uniformly distributed on the surface exposed to the
aggressive environment [7]. The latter is the stress corrosion damage DSC, describing
the damage related to stress corrosion process, namely the localization of the corrosion
attack in the areas of the material where the maximum principal stress is more
concentrated and the corrosive phenomenon evolves mediated by the stress field [8].
The corrosion damage variable D responsible for the global degradation is considered as
a linear superposition of the two mechanisms under the isotropic damage assumption :
D D D
(1)
U
SC
When D equals 0, it means the element of the FEA stent model is intact; while when D
equals 0.9, it means that the element is completely damaged and will be deleted in the
model during simulation. A detailed description of the damage model adopted in this