PSI - Issue 15

Sharath Chavalla et al. / Procedia Structural Integrity 15 (2019) 8–15 Author name / Structural Integrity Procedia 00 (2019) 000–000

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3. Isogeometric modelling 3.1. Stent geometry

In order to perform simulation of the stent structure, a simple closed-cell, self-expandable stent is constructed in Rhinoceros 5. The stent dimensions and design are as shown in the Fig. 1. The stent design is a collection of number of patches. A 2D CAD geometry is generated which is in the form of an unrolled stent geometry. This model is then wrapped on a cylindrical surface to generate the 3D stent model. In the next step, the NURBS data of each patch is exported in the form of text files using the GeoPde plugin Falco et al. (2011) of Rhinoceros. Using in-house matlab code, all the patch data of the stent is used to prepare an input file which is suitable to perform isogeometeric analysis in FEAP solver.

Fig. 1. Generation of 3D NURBS model of the stent with the details of links 1, 2 and 3 including the 2D planar CAD geometry

The complete stent model is a collection of 300 trivariate nurb patches. The order of each patch is maintained to be cubic along the surface direction and quadratic through the stent thickness. As shown in Fig. 1, the stent design consists of 3 links out of which links 1 and 2 are used as vertical and horizontal connectors which assemble along with link 3 to complete the 2D CAD geometry. 3.2. Material model of NiTi To mimic the mechanical behavior of NiTi alloy, the phenomenological model for finite deformations proposed by Christ and Reese (2008) is adopted. The details of the material model are provided in this section. The elastic deformation F e is defined by the multiplicative split.

1 :    F FF F = F F e t e t

(5)

The phase transformation in the polycrystalline NiTi alloy does not steadily proceed due to the presence of internal dislocations. This phenomenon is expressed by the elastic part F e t of the transformation deformation gradient F t .

1 :    F F F F = F F e d e d t t t t t t

(6)

T C = F F is the right Cauchy-Green tensor.

( - ) / 2 E = C 1 , where

The Green-Lagrange strain tensor is defined as The transitional strain tensor is defined by the relation