Date of Award


Document Type

Masters Thesis

Degree Name


Organizational Unit

Daniel Felix Ritchie School of Engineering and Computer Science

First Advisor

Peter Laz, Ph.D.

Second Advisor

Paul Rullkoetter

Third Advisor

Ramakrishna Thurimella


Cyclic hardening, Fatigue, Spinal implant, Titanium


The following study investigates the fatigue behavior of a spinal implant developed by Synthes, Inc. to provide stability and off load pressure from the discs in the lumbar region. The installation process for the StenoFix design utilizes plastic deformations to customize the device to each individual patient, but also has potential to degrade fatigue behavior. Physical testing of two titanium alloys, Ti-6Al-7Nb and Ti-15Mo, was conducted on hydraulic test frames and compared to computational simulations which were carried out for the following scenarios: cyclic excitation following plastic deformation; variation of material property definition; variation of loading location; and three modifications to initial geometry. From the physical testing it was found that Ti-6Al-7Nb had superior fatigue performance when compared to Ti-15Mo, and both materials showed characteristics of cyclic hardening. Computational results display improved predicted fatigue performance when the implant inner and outer wing surface was modified so the derivative of curvature was continuous. The Morrow Strain-Life model was used to predict design life using parameters found in literature and determined by the simulations. The model shows promising results and suggests longer design lives would occur by altering the direction of plastic deformation during installation.

Publication Statement

Copyright is held by the author. User is responsible for all copyright compliance.

Rights Holder

Timothy Helton


Received from ProQuest

File Format




File Size

143 p.


Mechanical engineering