Finite Element Simulation of the Impact of Inclusion Size and Shape On Mechanical Stability of Nitinol Vascular Implants

Nonmetallic inclusions are known as one of the major fracture initiation factors during structural fatigue of Nitinol vascular implants. Typical nonmetallic inclusions are carbides (TiC) and intermetallic oxides (Ti₄Ni₂O_x). In Nitinol medical devices these randomly distributed inclusions vary in size and shape and have significant influence on fracture.

In this study parameters like inclusion size and shape within the matrix material were analyzed in vicinity of a failure-critical location of a Nitinol generic diamond shape sample.

In the first part of the study diamond shape samples manufactured from medical-device grade Nitinol tubing were analyzed for inclusion size and shape using scanning electron microscopy (SEM). The diamond shape samples were subjected to tensile and fatigue testing employing typical in-vivo displacements of a stent. Afterwards, the diamond shape samples were analyzed again by SEM and results were compared to the images of the previous inspection to detect any changes at the inclusion locations. The experimental investigation was the basis of the Finite Element Analysis (FEA).

In the second part of the study a FEA model was set up including and excluding inclusions, and simulated using parameters as determined in the experimental part. To verify the simulation, results were compared to experimental strain measurements using Digital Image Correlation of unpolished, flat diamond shape samples at different displacements. Subsequently, inclusion shape, size and location within the model were systematically varied and resulting strains were evaluated.

By analyzing the strain distributions, the influence of inclusion size and shape on fatigue failure of a generic Nitinol stent is discussed and classifications from higher to lower strain distributions are presented.