Publication Date

12-2016

Date of Final Oral Examination (Defense)

1-15-2016

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Trevor Lujan, Ph.D.

Advisor

Darryl Butt, Ph.D.

Advisor

Tyler Brown, Ph.D.

Abstract

Meniscal injuries due to tissue tearing are prevalent in the U.S. yet the failure behavior of the meniscus is poorly understood. Clinical studies indicate that fatigue failure causes many of these tears. The highly circumferentially aligned fibers result in transversely isotropic material properties. Tears preferentially align bi-directionally to the fiber orientation. The aim of this study is to present the bi-directional fatigue life behavior of meniscal fibrocartilage. A novel fatigue life approach was developed to achieve this aim. Forty-eight bovine specimens were subjected to cyclic sinusoidal tension-tension stress at 2 Hz until rupture. Normalized peak tensile stresses were determined at prescribed stress levels (SL) from 60-90% of ultimate tensile stress. A novel method was developed to estimate the ultimate tensile stress (UTS) per specimen from tangent modulus of an 8% precondition strain wave. UTS standard errors were reduced from 6.96/1.09 (LG/TR) to 1.96/0.48 (LG/TR) based on a correlation from a set of 64 UTS tests. In total 32 fatigue specimens were tested. The Weibull distribution was used to determine 50% mean lifetimes for each SL. S-N plots followed a linear-log form SL = -5.9 N_0 + 108 (LG) and SL = -5.2 N_0 + 112 (TR) for mean cycles to failure (N0) with an average fit of R2= 0.800.18. Fatigue life results yielded a four-fold increase of mean cycles to failure between LG and TR orientations, but were not able to be shown statistically significant (p = 0.12). The slope of the damage rate decreased by 11% from LG to TR orientations, supporting the hypothesis that bi-directional material properties affect fatigue life behavior of the meniscus.

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