On Modeling Assumptions in Finite Element Analysis of Stents

Abstract

Finite Element Analysis (FEA) of Nitinol medical devices has become prevalent in the industry. The analysis methods have evolved in time with the knowledge about the material, the manufacturing processes, the testing or in vivo loading conditions, and the FEA technologies and computing power themselves. As a result, some common practices have developed. This paper presents a study in which some commonly made assumptions in FEA of Nitinol devices were challenged and their effect was ascertained. The base model pertains to the simulation of the fabrication of a diamond shape stent specimen, followed by cyclic loading. This specimen is being used by a consortium of several stent manufacturers dedicated to the development of fatigue laws suitable for life prediction of Nitinol devices. The FEA models represent the geometry of the specimens built, for which geometrical tolerances were measured. These models use converged meshes, and all simulations were run in the FEA code Abaqus making use of its Nitinol material models. Uniaxial material properties were measured in dogbone specimens subjected to the same fabrication process as the diamond specimens. By convention, the study looked at computed geometry versus measured geometry and at the maximum principal strain amplitudes during cyclic loading. The first aspect studied was the effect of simulating a single expansion to the final diameter compared to a sequence of three partial expansions each followed by shape setting. The second aspect was to ascertain whether it was feasible to conduct the full analysis with a model based on the electropolished dimensions or should an electropolish layer be removed only at the end of fabrication, similar to the manufacturing process. Finally, the effect of dimensional tolerances was studied. For this particular geometry and loading, modeling of a single expansion made no discernable difference. The fabrication tolerances were so tight that the effect on the computed fatigue drivers was also very small. The timing of the removal of the electropolished layer showed an effect on the results. This may have been so, because the specimen studied is not completely periodic in the circumferential direction.

Numerical Investigation on the Effect of Inclusions on the Local Fatigue Strain in Superelastic NiTi Alloy

“…Along with the geometric constraints of the inclusion orientation and the drawing direction, further aspects were suspected to affect the local fatigue strains. Several fatigue strain analyses based on FEM have shown that cyclic loading in combination with increasing complexity of the design structures leads to complex local fatigue strain configurations in highly strained regions (see [27]). These complex fatigue strain configurations result in a broad variation of principal strain directions with respect to the drawing direction.…”

“…Strong variance in shape and size for the inclusions (appear in white) is apparent Fig. 2 Exemplary fatigue load case (cyclic flat compression) and the corresponding strain amplitude distribution with principal direction mean strain was defined in correspondence to commonly observed fatigue loads for medical devices subjected to low cycle and high cycle fatigue [27,28]. However, in terms of numerical stability, the applied range for global fatigue strains was capped and thus represents a restricted scope of typical fatigue loads.…”

Computational Methods in Cardiovascular Mechanics

“…In 2010, Auricchio et al [Auricchio et al, 2010] reviewed the properties of the SMA three-dimensional model described in [Auricchio, Petrini, 2004], calibrating the model parameters with respect to experimental data, and showing its application to the simulation of pseudo-elastic Nitinol stent deployment in a simplified atherosclerotic artery model. In 2011, Rebelo and colleagues [Rebelo et al, 2011] presented a study in which some commonly made assumptions in FEA of Nitinol devices were verified. In 2012, Garcia and colleagues [Garcia et al, 2012] using FEA performed a parametric analysis of a commercial stent model to estimate the influence of geometrical variables on the stent radial expansion force.…”

Fatigue-Life Computational Analysis for the Self-Expanding Endovascular Nitinol Stents

“…To shorten the stent development cycle and reduce the associated cost, computer engineering methods and tools are being increasingly utilized. There are several examples in the open literature demonstrating the benefits offered by the computer engineering methods and tools within the vascular stent (Ref [27][28][29][30].…”

“…Through proper selection of the testsample geometry, experimental cyclic-loading procedure, and data reduction (and in combination with the nonlinear finite element analysis), the appropriate strain amplitude vs. fatigue life and constant life strain amplitude vs. mean strain relations for Nitinol are established (Ref [30][31][32]. These plots establish (a) a continuous decrease (at a progressively lower rate) of the maximum principal-strain amplitude with an increase in the fatigue cycle number; and (b) a weak (and potentially negative) dependence of the high-cycle fatigue portion of the strainamplitude vs. fatigue cycle number curve on the mean-strain magnitude (which is contrary to observations made in the majority of engineering materials) (Ref 32).…”