Mechanisms of short crack growth at constant stress in bone.

This paper describes an experimental study of the growth of small (i.e. sub-millimetre) cracks in samples of cortical bone subjected to a constant tensile stress. Slow, stable crack growth occurred at a rate and angle which were dependent on the orientation of the sample: tests were conducted with the loading axis both parallel and perpendicular to the longitudinal axis of the bone. All cracks showed intermittent growth in which periods of relatively rapid propagation alternated with periods of temporary crack arrest or relatively slow growth. In some cases crack arrest could be clearly linked to microstructural features such as osteons or Volkmann's canals, which acted as barriers to crack growth. Crack-opening displacement increased over time during the arrest periods. These observations suggest a mechanism for the growth of small cracks in bone at constant stress, involving microstructural barriers, time-dependent deformation of material near the crack tip and strain-controlled propagation.