Seyed Mohammad AHMADI
One of the main functions of bone is to support the human body mechanically. To fulfil its complex role, bone possesses unique mechanical properties: it is stiff enough to resist deformation while being able to absorb energy.
Looking at bone at the macroscopic scale, two distinct types of tissues are detectable, namely trabecular and cortical. Cortical (compact) bone has a high density and low porosity with a great compressive strength while trabecular (cancellous) bone is highly porous and provides internal support of bone. Cortical bone which constitutes up to 80% of bone mass endures the mechanical loads while trabecular bone which has 50-90% of total bone volume, make mechanical loads distribute evenly by acting as energy absorbent. Additively manufactured porous metallic biomaterials are promising candidates for application as bone-mimicking implants since they are capable of delivering both above-mentioned functions.
In this thesis, a comprehensive study has been carried out using analytical methods, numerical methods and experiments to gain a better understanding of these structures. A wide range of porous structures with several topological designs and material types were considered. The quasi-static and fatigue behaviour of those AM porous biomaterials were determined experimentally and were compared with analytical solutions and computational (i.e. finite element modelling) results. The quasi-static mechanical properties and fatigue S-N curves were analysed both in absolute and in normalized terms. In the case of quasi-static mechanical properties, normalization was performed with respect to the mechanical properties of the bulk (i.e. matrix) material from which the porous biomaterials were made while stress levels in the S-N curves were normalized with respect to the yield or plateau stress of the porous biomaterial.
In general terms, both topological design and material type were found to influence the quasi-static mechanical properties and fatigue behaviour of AM porous metallic biomaterials. In the case of quasi-static mechanical properties, the effects of the topological design were dominant but the material type had considerable effects too. The analytical solutions and computational results were generally in agreement with experimental observations particularly for the lower values of the relative density. As for the fatigue behaviour, the absolute S-N curves were found to be highly dependent on the porosity of the biomaterials. Once normalized with respect to the yield or plateau stress, the effects of porosity largely disappeared. The normalized S-N curves were, however, highly dependent on topological design as defined by the type of the repeating unit cell. The material type was also found to play an important role in defining the S-N curves. Indeed, for a group of unit cells considered in this thesis, the material type had a larger effect than the type of the unit cell on the normalized S-N curves of AM porous metallic biomaterials.
Due to the uniqueness of each patient’s bone structure and differences in various types of orthopaedic procedures, there is no specific porous structure or material type with best properties. However, the methods and results presented in this thesis provide some useful information that could be applied when designing a dedicated implant. In particular, they could be used for selecting the right topological design (i.e. the type of repeating unit cell), the appropriate porosity, and strut size, and for deciding about the material type. The ultimate goal often is to mimic the properties of the bone that is being replaced. The results of this thesis show AM porous metallic biomaterials could mimic several aspects of bone tissue properties and are therefore promising candidates.