Date of Award

Spring 2-2016

Document Type

Thesis

Degree Name

Master of Science in Biomedical Engineering

Department

Biology

First Advisor

Glen Livesay

Second Advisor

Renee Rogge

Third Advisor

Simon Jones

Abstract

Instantaneous tensile testing and stress-relaxation testing are forms of mechanical testing used to determine the elastic and viscoelastic properties of biological tissue. Equilibrium testing is a form of testing that combines both of these testing approaches at different strain points to determine the elastic properties of a material and also assess their viscoelastic properties in the same test. This testing method is commonly used on highly viscoelastic materials such as cartilage but has never been fully described in dense collagenous materials such as tendon or ligament. This analysis utilizes different strain points selected to capture the classic non-linear behavior of stress-strain curves at lower strain values of dense collagenous materials. During an equilibrium test, the material is loaded instantaneously to the first strain point and held there, such that the material then experiences stress-relaxation. After a pre-determined holding period, the material is then pulled by the same elongation to the second strain point and allowed to experience another stress-relaxation. In this experiment, this was conducted for six cycles for each specimen. Strain points were spaced equally by using the transition point as a basis for analysis to include three points within the toe region, one at the transition point, and the last two in the linear region. During these stress-relaxation tests at different strain points, the material was expected to experience an exponential growth during the elastic pulls and then exponential decay during stress relaxation. It was found that both of these parameters were highly dependent on the strain states, indicating a potential additive model for viscoelasticity as well as casting some doubt on the validity of superposition as an assumption of collagenous materials. It was also found that the equilibrium modulus was different between the toe and the linear regions. Based upon these findings, the modulus determined for rat tail tendon using the equilibrium testing approach did display a similar pattern to moduli determined using the instantaneous tensile method.

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