Bone microdamage characterization as well as its involvement in bone metabolism or bone fragility remains a challenge, especially because no existing techniques are well suited to its measurement. Non invasive techniques for detecting and monitoring bone microcracks accumulation and propagation are thus highly desirable. The objective of this thesis was to evaluate the sensitivity of nonlinear resonant ultrasound spectroscopy (NRUS) measurements to the accumulation of damage in cortical bone by fatigue or by controlled crack propagation. First, NRUS method was optimized to achieve highly sensitive and reproducible bone nonlinearity measurements. Then, two groups of human cortical bone specimens were machined from the femoral mid-diaphysis. The first group was taken through a progressive fatigue protocol consisting of four steps of cyclic four-point bending. The second group was taken through a toughness protocol consisting of initiation and controlled propagation of a stable crack induced by 4-point bending mechanical loading. Our results evidenced a progressive increase of the nonlinear elastic parameter during fatigue testing or during toughness experiments. Moreover, the relative variation of nonlinear elasticity of the fatigued specimens was significantly related to the relative variation of the number density of small cracks assessed with micro-computed tomography, whereas in crack propagation experiments a significant relationship was found between the level of nonlinearity and total crack length. These results strongly suggest that NRUS measurements are sensitive to damage accumulation and bone non-linearity can be used as a marker of bone damage.
