Nonlinear ultrasound monitoring of single crack propagation in cortical bone

Accumulation of bone microdamage is suspected to lead to severe impairment of mechanical properties with an increase in skeletal fragility and fracture risk. The objective of the study was to evaluate the sensitivity of nonlinear resonant ultrasound spectroscopy (NRUS) measurements to the propagation in cortical bone of a single microcrack induced by 4-point bending mechanical loading. Twelve human cortical bone specimens were machined as parallelepiped beams (50*2*2mm) to unambiguously identify resonant modes for NRUS measurements. A central notch of 600{lower case mu}m was made to control crack initiation and propagation during four-point bending loading. The nonlinear hysteretic elastic coefficient ({lower case alpha} f ) was derived from NRUS measurements achieved in dry and wet conditions. Each bone specimen was probed by a swept-sine around its first compression mode, applying progressively increasing drive levels. Moreover, the buried crack length was assessed by synchrotron radiation micro-computed tomography with a spatial resolution of 1.4{lower case mu}m. Despite between-sample variability, {lower case alpha} f increased significantly in the damaged state (44.9±85.4) compared to the initial value (5.5±1.5) in the control undamaged state. Crack length was significantly correlated to the nonlinear elastic parameter {lower case alpha} f (r ² =0.78, p