The fracture mechanics of bone

The objectives of this study are to apply the discipline of fracture mechanics to hone, and develop a test method to measure a relevant fracture property. Some of the more conventional mechanical properties are first examined. It is demonstrated that hone is heterogeneous, anisotropic and anelastic. It is also relatively brittle when compared with other common load hearing materials. These properties are not suitable for the use of linear elastic fracture mechanics, and it is argued that a more general approach is adopted, centred around the Griffith energy balance. The theory of quasistatic crack growth is based on this principle, and is described. The concept of crack stability is introduced. The theory and various practical criteria are then used to design a test rig and specimen for measuring the work of fracture (R). Both rig and specimen are thoroughly tested for accuracy and consistency. Tests are carried out on specimens of Bovine femoral cortex, and the resulting values of work of fracture correlated with ash weight and ultimate tensile stress. It is shown that, for the first 10 mm of cracking, R increases rapidly with crack extension. There is also a significant correlation between R and crack velocity. On several specimens, cracking became unstable. This was characterised by a very dramatic increase in crack velocity, and a change in the appearance of the fracture surfaces. A second series of tests are described in which attention is focussed on the first 10 mm of cracking over a range of different cracklengths. From the resulting data, it is suggested that fracture may he divided into 4 regions. A theoretical model, based on the effects of tied crack faces, is developed and shown to fit the experimental results. In the penultimate chapter, the dependance of R on crack velocity is examined, and compared with the behaviour of other materials. The strain rate dependancy of Young's modulus and the ultimate tensile stress is related to the work of fracture and crack velocity. It is shown that, to an approximation, the analysis fits the experimental results. The concept of adhesive and cohesive fracture is introduced. It is suggested that the change from adhesive to cohesive fracture, which will occur at a critical crack speed, is also synonymous with the transition from stable to unstable fracture. Finally, some developments of the test method and other suitable topics for future research are suggested.