The phenomena of fracture formation were observed in relation to collagen fiber pattern and the microscopic structure of compact bone. The studies were carried out on ox bone specimens, thin enough (120–170 μm) to allow the microscopic examination before and after the mechanical test, without additional treatment. Longitudinal and transverse specimens of several histological structures were tested in slow and high speed uniaxial tension. The fractured surfaces were examined under ordinary and polarized light microscopes.

The tensile stress at fracture and the mode of fracture were highly dependent on the direction and organization of the collagen fiber bundles within the specimen and on the strain rate. Significant differences in the stress were found, not only between the longitudinal and transverse specimens, but within these groups, among primary bone with predominant longitudinal fibers, primary bone with fibrous structure, secondary bone and the transition zone between primary and secondary bone. The typical fracture shapes for each structure are described and discussed, as well as other phenomena of fracture propagation observed during the study such as microcracks forming ahead of a slowly advancing fracture line and secondary fractures branching off a fast advancing crack.