Common metals for stable long-term implants (e.g. stainless steel, Titanium and Titanium alloys) are much stiffer than spongy cancellous and even stiffer than cortical bone. When bone and implant are loaded this stiffness mismatch results in stress shielding and as a consequence, degradation of surrounding bony structure can lead to disassociation of the implant. Due to its lower stiffness and high reversible deformability, which is associated with the superelastic behavior, NiTi is an attractive biomaterial for load bearing implants. However, the stiffness of austenitic Nitinol is closer to that of bone but still too high. Additive manufacturing provides, in addition to the fabrication of patient specific implants, the ability to solve the stiffness mismatch by adding engineered porosity to the implant. This in turn allows for the design of different stiffness profiles in one implant tailored to the physiological load conditions. This work covers a fundamental approach to bring this vision to reality. At first modeling of the mechanical behavior of different scaffold designs are presented as a proof of concept of stiffness tailoring. Based on these results different Nitinol scaffolds can be produced by additive manufacturing.
Multi-axial behavior of shape memory alloy (SMA) bars with circular cross section is studied by considering the effect of temperature gradient in the cross section as a result of latent heat generation and absorption during forward and reverse phase transformations. The local form of energy balance for SMAs by taking into account the heat flux effect is coupled to a closed-form solution of SMA bars subjected to multi-axial loading. Non-Mises definitions are employed for the effective stress and strain to enable the model to capture the coupling between tension and torsion. The resulting coupled thermo-mechanical equations are solved for SMA bars with circular cross sections. A number of experiments were conducted and the results were then successfully compared with the model. It is shown that the isothermal solution is valid only for specific combinations of ambient conditions and loading rates. The present approach is a beneficial platform in modeling and analysis of applications with high loading rates.