Abstracts Category : Other

Independent tuning stiffness and toughness of polymer metalcomposites: modeling, validation, and design

by Esfahani Sajedeh Nasr

Titanium (Ti) alloys are one of the most used metalsfor biomedical applications, specifically in making implants. Thestiffness of the dense Ti is 80-110 GPa, while the stiffness of thecompact bone is 12-20 GPa. This high difference between thestiffness of the Ti alloys and compact bone results in stressshielding of the bone and stress concentration at the implant, bothof which are undesirable and could result in implant failure. Analternative method to reduce the stiffness of a dense implant andavoid the stress shielding is adding porosity to the structure.This however results in considerable reduction in the toughness ofthe structure, which is also undesirable for the long-term successof implants. Also, implants such as knee and spine should have highfracture toughness, which is not achievable with porous structures.In this work, we study a new method for independently tuning thestiffness and toughness of the material by adding various polymersto the additively manufactured Ti structures with engineeredporosity. Porous Ti samples with different levels of porosity arefabricated using selective laser melting. Various types ofthermoplastic polymers including High Density Polyethylene (HDPE),Polyethylene Terephthalate (PET), and Nylon (MXD6) are used to fillthe pores to make the titanium-polymer composite parts. Compressionsimulations and tests are performed on both porous and compositespecimens to compare the mechanical behavior of these structures. Aset of finite element simulations is conducted on differentstructures, and the results are verified with experiments.Simulation results and experimental findings indicate that fillingporous Ti with thermoplastic polymers leads to an increase in thetoughness of the structure. The percentage increase of thetoughness depends on several parameters such as the geometry of theporosity, the percentage of the porosity, and the type of thepolymer. Also, a design algorithm is developed based on thesimulation and experimental results. This design algorithm receivesthe desired level of mechanical properties such as desiredstiffness and toughness. The algorithm then produces the desiredpercentage and morphology of porosity. It also recommends the typeof the metal and polymer that should be used to create thecomposite with the desired mechanical properties. Our results pavethe way for designing polymer-composite structures withindependently tuneable stiffness and toughness for a broad range ofapplications.Advisors/Committee Members: Elahinia, Mohammad (Advisor).