On the effect of geometrical imperfections and defects on the fatigue strength of cellular lattice structures additively manufactured via Selective Laser Melting

Porous structures have great potential in the biomedical field because, compared to traditional fully dense implants, prostheses with a porous structure show reduced stress shielding and improved osseo-integration. Selective Laser Melting (SLM) made possible to obtain metallic cellular materials with highly complex structures characterized by a wide range of cell morphologies that allow to finely tune the mechanical properties of the implant. Nevertheless, there are still several issues to address: among others, detrimental residual stresses and the discrepancy between the as-designed and the manufactured geometry. Micro X-ray computed tomography (mu CT) combined with the Finite Elements (FE) method permits to carry out in-depth investigations on the effect of the number and severity of defects on the mechanical properties. In the current study, the results of fatigue and quasi-static tests were compared with FE calculations based on the as-designed geometry and on the as-built geometry reconstructed from mu CT scans. Both the elastic modulus and the fatigue resistance resulted strongly correlated with the number and severity of defects. Moreover, predictions of the mechanical properties based only on the as-designed geometry were shown not to be accurate. The importance of considering the limitations in accuracy of the manufacturing technique when designing load bearing lattice structures was highlighted.