Multi-scale investigation of orientation-dependent mechanical properties and fracture propagation in cortical bone using digital volume correlation and atomic force microscopy

This study investigates the orientation-dependent mechanical properties and fracture propagation behavior in bovine cortical bone, focusing on the role of lamellar plane orientation and its internal hierarchical structures—both of which are critical to bone mechanical strength and fracture resistance. By combining mechanical testing, micro-computed tomography (micro-CT), digital volume correlation (DVC), and atomic force microscopy (AFM), we examined region- and orentation-dependent mechanical properties of cortical bone, as well as the crack propagation influenced by underlying microstructures. At sub-millimeter scale, three-dimensional (3D) displacement and strain captured through in situ three-point bending and DVC provided detailed insights into internal deformation patterns during crack propagation. Distinct crack propagation behaviors were observed in bone samples with lamellar planes oriented parallel and perpendicular to the loading plane. AFM nanomechanical mapping was performed on cracked cross-sections, revealing the heterogeneous mechanical properties within the hierarchical lamellar structure. These micrometer-scale measurements, obtained from orthogonal cracked cross-sections, help explain the different crack propagation mechanisms observed during the bending experiments. Our findings demonstrate that the orientation of lamellar plane— the fundamental structural component of both plexiform and osteonal bone—relative to the loading plane plays a critical role in determining crack paths and local strain distribution. The integrated application of DVC and AFM provides a multiscale perspective on fracture resistance in cortical bone. These insights have important implications for the design of biomimetic materials in bone implants and for improving clinical assessments of fracture risk.