Multiscale characterization of mechanical and fracture properties of cortical bone specimens

Bone is a hierarchical material with multiple length scales ranging from mineralized collagen fibrils at the nanoscale, single lamella at the sub microscale, lamellar structure at the microscale, osteons at the mesoscale to a whole bone at the macroscale. Several fracture testing methods are available including the three-point bending test, the compact tension test and the double cantilever test. However, these methods are confined to the macroscopic scale. At the microscopic scale, Vickers Fracture Indentation was proposed. Nevertheless, the study observed instances of varied crack lengths making it irreproducible and thus there is a need for further advancement on the testing front at the lower length scales. The research objective is to measure the fracture properties of bone at the level of osteons and at lower length scales using scratch testing. Cortical Bone Specimens obtained from porcine and bovine femurs have been cut, ground, polished and tested using an arduous experimental procedure. The nature of the fracture observed exhibited a strong anisotropy with toughening mechanisms and a competition between plastic flow and brittle fracture. Scanning Electron Microscopy revealed the presence of micro cracking, crack bridging, crack deflection and flaking or chipping along the length of the scratch. The process of applying nonlinear fracture mechanics methodologies such as the J-integral or the energetic size effect law in the analysis used to determine the fracture toughness of bone was meticulous. The results of this investigation showed not only a coupling between elasticity and fracture characteristics but also a mixed mode fracture involved in the determination of the fracture resistance. The investigation employs scratch tests which determine the fracture properties not only at the microscale but also at the lower length scales due to the scalability of the scratch tests. Furthermore, nanoindentation tests were conducted on cortical bone specimens to determine their mechanical properties. Due to the viscoelasticity and viscoplasticity exhibited by cortical bone specimens, creep induced rate effect studies were carried out and their influence on the fracture properties was studied. The presented research paves the way towards a deeper understanding of the fragility of hard biological tissues. The knowledge gained could be applied to inform novel treatment for bone diseases, prevention of bone brittlement and design of advanced structural materials