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research-article

Development of a Flow Evolution Network Model for the Stress-Strain Behavior of Poly(L-lactide)

[+] Author and Article Information
Maureen L. Dreher

US Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics 10903 New Hampshire Ave, Silver Spring MD 20993
maureen.dreher@fda.hhs.gov

Srinidhi Nagaraja

US Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics 10903 New Hampshire Ave, Silver Spring MD 20993
srinidhi.nagaraja@fda.hhs.gov

Jorgen Bergstrom

Veryst Engineering 47A Kearney Rd, Needham Heights, MA 02494
jbergstrom@veryst.com

Danika Hayman

Veryst Engineering 47A Kearney Rd, Needham Heights, MA 02494
dhayman@veryst.com

1Corresponding author.

ASME doi:10.1115/1.4037071 History: Received July 25, 2016; Revised June 02, 2017

Abstract

Computational modeling is critical to medical device development and has grown in its utility for predicting device performance. Additionally, there is an increasing trend to use absorbable polymers for the manufacturing of medical devices. However, computational modeling of absorbable devices is hampered by a lack of appropriate constitutive models that capture their viscoelasticity and post-yield behavior. The objective of this study was to develop a constitutive model that incorporated viscoplasticity for a common medical absorbable polymer. Microtensile bars of poly(L-lactide), i.e., PLLA, were studied experimentally to evaluate their monotonic, cyclic, unloading, and relaxation behavior as well as rate dependencies under physiological conditions. The data were then fit to a viscoplastic Flow Evolution Network (FEN) constitutive model. PLLA exhibited rate dependent stress-strain behavior with significant post-yield softening and stress relaxation. The FEN model was able to capture these relevant mechanical behaviors well with high accuracy. In addition, the suitability of the FEN model for predicting the stress-strain behavior of PLLA medical devices was investigated using finite element simulations of non-standard geometries. The non-standard geometries chosen were representative of generic PLLA cardiovascular stent sub-units. These finite element simulations demonstrated that modeling PLLA using the FEN constitutive relationship accurately reproduced the specimen’s force-displacement curve and therefore, is a suitable relationship to use when simulating stress distribution in PLLA medical devices. This study demonstrates the utility of an advanced constitutive model that incorporates viscoplasticity for simulating PLLA mechanical behavior.

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