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Research Papers

Constitutive Behavior of Ocular Tissues Over a Range of Strain Rates

[+] Author and Article Information
Wonsuk Kim

 Department of Mechanical Engineering, University of Michigan-Dearborn, Dearborn, MI 48128wskim@umich.edu

Alan Argento1

 Department of Mechanical Engineering, University of Michigan-Dearborn, Dearborn, MI 48128aargento@umich.edu

Frank W. Rozsa

 Department of Ophthalmology and Visual Sciences, Kellogg Eye Center, University of Michigan, Ann Arbor, MI 48105rozsa@umich.edu

Kaitlyn Mallett

 Department of Mechanical Engineering, University of Michigan-Dearborn, Dearborn, MI 48128kmallett@umd.umich.edu

1

Corresponding author.

J Biomech Eng 134(6), 061002 (Jun 08, 2012) (8 pages) doi:10.1115/1.4006847 History: Received September 20, 2011; Revised April 28, 2012; Posted May 18, 2012; Published June 08, 2012; Online June 08, 2012

The constitutive behavior of bovine scleral and corneal tissues is measured in tension and compression, at quasi-static and moderate strain rates. Experiments are conducted at strain rates up to about 50 strain per second by a pneumatic testing system developed to overcome noise and measurement difficulties associated with the time dependent test of low impedance materials. Results for the tissues at room and the natural bovine body temperatures are similar and indicate that ocular tissue exhibits nonlinear stiffening for increasing strain rates, a phenomena termed rate hardening. For example, at a tensile strain rate of 29/s, corneal tissue is found to develop 10 times the stress that it does quasi-statically at the same strain. Thus, conventional constitutive models will grossly underpredict stresses occurring in the corneo-scleral shell due to moderate dynamic events. This has implication to the accuracy of ocular injury models, the study of the stress field in the corneo-scleral shell for glaucoma research and tonometry measurements. The measured data at various strain rates is represented using the general framework of a constitutive model that has been used to represent biological tissue mechanical data. Here it is extended to represent the measured data of the ocular tissues over the range of tested strain rates. Its form allows for straightforward incorporation in various numerical codes. The experimental and analytical methods developed here are felt to be applicable to the test of human ocular tissue.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 2

Test system. (a) Overall system. (b) Fixture base and slide device in tensile arrangement. (c) Compressive arrangement. (Specimens shown are generic.)

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Figure 4

Measured tensile stress-strain curves for bovine cornea at 4 mm/min actuator speed when tests were performed at body temperature (solid line) and room temperature (dashed line)

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Figure 5

Measured mean tensile stress-strain curves for bovine cornea at 0.0065, 0.16, and 29/s along with a comparative result from [6]

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Figure 6

Linear curve fitting of the experimental data of bovine cornea at 0.0065/s

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Figure 7

Variation of constitutive constants as functions of strain rate

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Figure 3

Typical tensile stress-strain curves of bovine cornea obtained using the linear strain (solid line) and nonlinear strain (dashed line)

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Figure 1

Specimen extraction locations. (a) Cornea: Schematic of location and orientation of tissue samples from bovine cornea. One dog-bone sample is centered longitudinally while the other is slightly off-center. Circular samples (8 mm) were positioned as shown. Outer ring indicates rim of sclera tissue outside cornea. (b) Sclera: Schematic of location and orientation of tissue samples taken from bovine sclera. Dorsal view after removal of cornea. Dog bones from the two uppermost sections are oriented along the longitudinal axis (back to front) in the intact globe. Dog bones from the lower flap represent circumferential sections. The position of the optic nerve is represented by the central shaded circle.

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Figure 8

Bovine cornea in tension: (a) Piecewise linear fit of the model constants and (b) constitutive model curves and experimental data showing strain rate dependence

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Figure 9

Bovine sclera in tension: (a) Piecewise linear fit of the model constants and (b) constitutive model curves and experimental data showing strain rate dependence

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Figure 10

Bovine cornea in compression: (a) Piecewise linear fit of the model constants and (b) constitutive model curves and experimental data showing strain rate dependence

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Figure 11

Bovine sclera in compression: (a) Piecewise linear fit of the model constants and (b) constitutive model curves and experimental data showing strain rate dependence

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