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Research Papers

Validating Fatigue Safety Factor Calculation Methods for Cardiovascular Stents

[+] Author and Article Information
Ramesh Marrey

Cordis Corporation, a Cardinal Health company,
1820 McCarthy Boulevard,
Milpitas, CA 95035
e-mail: ramesh.marrey@cardinalhealth.com

Brian Baillargeon, Nuno Rebelo

Dassault Systemes,
Santa Clara, CA 95054

Maureen L. Dreher, Jason D. Weaver, 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, MD 20993

Xiao-Yan Gong

Medical Implant Mechanics,
Aliso Viejo, CA 92656

1Corresponding author.

Manuscript received April 27, 2017; final manuscript received January 22, 2018; published online March 16, 2018. Assoc. Editor: Jeffrey Ruberti.This work is in part a work of the U.S. Government. ASME disclaims all interest in the U.S. Government's contributions.

J Biomech Eng 140(6), 061001 (Mar 16, 2018) (9 pages) Paper No: BIO-17-1179; doi: 10.1115/1.4039173 History: Received April 27, 2017; Revised January 22, 2018

Evaluating risk of fatigue fractures in cardiovascular implants via nonclinical 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 (FSFs). While many methods exist to calculate FSFs, none have been validated against experimental data. The current study presents three methods for calculating FSFs and compares them to experimental fracture outcomes under axial fatigue loading, using cobalt-chromium test specimens designed to represent cardiovascular stents. FSFs 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 107 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.

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Figures

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Fig. 1

Simple example of rotating stresses during loading

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Fig. 2

Schematic of stent test specimen (top) and diagram illustrating the varying connector widths along the stent length (bottom)

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Fig. 3

Material true stress versus true plastic strain response

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Fig. 4

Finite element model boundary conditions

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Fig. 5

Comparison of the experimental and simulation load–deflection behavior due to monotonic axial loading

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Fig. 6

Depiction of the method to calculate gauge length (distance between arrows) for the test specimen

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Fig. 7

Experimental fatigue data showing fractures and survival to 107 cycles

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Fig. 8

Typical fractured test specimen with fractures near the apices of the thinnest connector CONN-4

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Fig. 9

Goodman scatter-plots for axial deformation magnitudes of 1.30% (a), 1.05% (b), zoomed in 1.05% (c), and 0.70% (d). For plots (a), (b), and (d), it should be noted that the scatter plot for the tensor method is not visible due to similar results with the modified tensor method.

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Fig. 10

Surface stresses at the point with lowest FSF (based on both tensor and modified tensor methods) at 1.05% deformation

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Fig. 11

Surface stresses for cyclic torsion (±10 degrees) superimposed on a 1.05% tensile pre-load

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