Independent Tuning Stiffness and Toughness of Polymer Metal Composites: Modeling, Validation, and Design

Titanium (Ti) alloys are one of the most used metals for biomedical applications, specifically in making implants. The stiffness of the dense Ti is 80-110 GPa, while the stiffness of the compact bone is 12-20 GPa. This high difference between the stiffness of the Ti alloys and compact bone results in stress shielding of the bone and stress concentration at the implant, both of which are undesirable and could result in implant failure. An alternative method to reduce the stiffness of a dense implant and avoid the stress shielding is adding porosity to the structure. This however results in considerable reduction in the toughness of the structure, which is also undesirable for the long-term success of implants. Also, implants such as knee and spine should have high fracture toughness, which is not achievable with porous structures.;In this work, we study a new method for independently tuning the stiffness and toughness of the material by adding various polymers to the additively manufactured Ti structures with engineered porosity. Porous Ti samples with different levels of porosity are fabricated using selective laser melting. Various types of thermoplastic polymers including High Density Polyethylene (HDPE), Polyethylene Terephthalate (PET), and Nylon (MXD6) are used to fill the pores to make the titanium-polymer composite parts. Compression simulations and tests are performed on both porous and composite specimens to compare the mechanical behavior of these structures. A set of finite element simulations is conducted on different structures, and the results are verified with experiments. Simulation results and experimental findings indicate that filling porous Ti with thermoplastic polymers leads to an increase in the toughness of the structure. The percentage increase of the toughness depends on several parameters such as the geometry of the porosity, the percentage of the porosity, and the type of the polymer. Also, a design algorithm is developed based on the simulation and experimental results. This design algorithm receives the desired level of mechanical properties such as desired stiffness and toughness. The algorithm then produces the desired percentage and morphology of porosity. It also recommends the type of the metal and polymer that should be used to create the composite with the desired mechanical properties. Our results pave the way for designing polymer-composite structures with independently tuneable stiffness and toughness for a broad range of applications.