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research-article

Validating Fatigue Safety Factor Calculation Methods for Cardiovascular Stents

[+] Author and Article Information
Ramesh Marrey

Cordis Corporation, a Cardinal Health company, 1820 McCarthy Blvd., Milpitas, CA 95035 USA
ramesh.marrey@cardinalhealth.com

Brian Baillargeon

Dassault Systemes Simulia Corporation, West Center of Excellence, Santa Clara, CA 95054 USA
brian.baillargeon@3ds.com

Maureen Dreher

U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, Maryland 20993 USA
Maureen.Dreher@fda.hhs.gov

Jason D Weaver

U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, Maryland 20993 USA
jason.weaver@fda.hhs.gov

Srinidhi Nagaraja

U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, Maryland 20993 USA
Srinidhi.Nagaraja@fda.hhs.gov

Nuno Rebelo

Dassault Systemes Simulia Corporation, West Center of Excellence, Santa Clara, CA 95054 USA
nuno.rebelo@3ds.com

Xiao-Yan Gong

Medical Implant Mechanics, Aliso Viejo, CA 92656 USA
xgong@medicalimplantmech.com

1Corresponding author.

ASME doi:10.1115/1.4039173 History: Received April 27, 2017; Revised January 22, 2018

Abstract

Evaluating risk of fatigue fractures in cardiovascular implants via non-clinical testing is essential to provide an indication of their durability. This is generally accomplished by experimental accelerated durability testing and often complemented with computational simulations to calculate "fatigue safety factors". While many methods exist to calculate fatigue safety factors, none have been validated against experimental data. The current study presents three methods for calculating fatigue safety factors and compares them to experimental fracture outcomes under axial fatigue loading, using cobalt-chromium test specimens designed to represent cardiovascular stents. Fatigue safety factors were generated by calculating mean and alternating stresses using a simple Scalar Method, a Tensor Method which determines principal values based on averages and differences of the stress tensors, and a Modified Tensor Method which accounts for stress rotations. The results indicate that the Tensor Method and the Modified Tensor Method consistently predicted fracture or survival (to 10 million cycles) for specimens subjected to experimental axial fatigue. In contrast, for one axial deformation condition, the Scalar Method incorrectly predicted survival even though fractures were observed in experiments. These results demonstrate limitations of the Scalar Method and potential inaccuracies. A separate computational analysis of torsional fatigue was also completed to illustrate differences between the Tensor Method and the Modified Tensor Method. Because of its ability to account for changes in principal directions across the fatigue cycle, the Modified Tensor Method offers a general computational method that can be applied for improved predictions for fatigue safety regardless of loading conditions.

Section 4: U.S. Gov Employees + Reg Authors
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