Cooperation of length scales and orientations in the deformation of bovine bone.

Bone has a complex hierarchical structure. Combined wide angle X-ray diffraction and small angle X-ray scattering were used together with in situ tensile testing to investigate the deformation and failure mechanisms of bovine cortical bone at three material levels: (1) the atomic level, corresponding to the mineral crystal phase; (2) the nano level, corresponding to the collagen fibrils; (3) the macroscopic level. It was found that deformation was linear at all three levels up to a strain of 0.2% in the longitudinal tensile direction. At this critical strain a sudden 50% decrease in the fibrillar and mineral strains was observed. This suggests the presence of partial local damage that leads to inhomogeneous strain distributions within the probed gauge volume. This also gives rise to diffraction peak broadening in the mineral phase, as well as strain relaxation at the nanoscale. Above the critical strain the longitudinally oriented strains below the nanoscale remained constant at a reduced level until failure. This suggests that the lateral orientation of the nanostructures toughens the bone, while a higher material level dominated the subsequent deformation process, either by sliding between the lamellar layers or by the growth of microcracks. Analysis of the diffraction data also shows that the bone has compressive residual stress in the crystal phase. A better understanding of the basic mechanics of the hierarchical bone structure could be the basis to enhance research in biomimetics, developing new advanced materials, and to find solutions for orthopedic problems.

Simultaneous small- and wide-angle scattering at high X-ray energies.

Combined small- and wide-angle X-ray scattering (SAXS/WAXS) is a powerful technique for the study of materials at length scales ranging from atomic/molecular sizes (a few angstroms) to the mesoscopic regime ( approximately 1 nm to approximately 1 microm). A set-up to apply this technique at high X-ray energies (E > 50 keV) has been developed. Hard X-rays permit the execution of at least three classes of investigations that are significantly more difficult to perform at standard X-ray energies (8-20 keV): (i) in situ strain analysis revealing anisotropic strain behaviour both at the atomic (WAXS) as well as at the mesoscopic (SAXS) length scales, (ii) acquisition of WAXS patterns to very large q (>20 A(-1)) thus allowing atomic pair distribution function analysis (SAXS/PDF) of micro- and nano-structured materials, and (iii) utilization of complex sample environments involving thick X-ray windows and/or samples that can be penetrated only by high-energy X-rays. Using the reported set-up a time resolution of approximately two seconds was demonstrated. It is planned to further improve this time resolution in the near future.