FEM analysis on balloon expandable stent considering viscoplasticity

Finite element method (FEM) in analysis of stenting process capable to reveal many information, which cannot be accessed in experiment. To obtain qualified result, the procedure should be verified and the analyses results should be validated. In the case of stent simulation, the verification of FEM procedure is more important than the validation of FEM results due to the difficulties in validating FEM results using experimental method. In this study, ANSYS Student R18.0 is used to compare two inelastic behavior analyses in studying the effect of dislocation creep, i.e. rate-independent and rate-dependent plasticity during the same time history. Rate-independent plasticity uses only multilinear behavior (von Mises potential) as isotropic hardening plasticity while rate-dependent plasticity uses creep (Bailey-Norton law) combined with multilinear von Mises potential. Cyclical symmetry of stent and balloon geometry is generated to save computational time. A Palmaz and a Sinusoidal stent type is chosen for the stent models because these stents represent rigid and flexible structure of a slotted-tube stent. To learn the influence of the time hardening formulation, three different periods of creeping time with the equal pressure rate and three different periods of expanding time with the equal creeping time are set in the loading. The study presented that the creep behavior tends to affect the nominal radii of flexible stent (Sinusoidal stent), but no influence to the stent with rigid structural geometry (Palmaz stent). Besides, valuable finding is identified as various expanding periods applied in the stent deployment.