A suitable procedure for determining the fatigue threshold of human cortical bone tissue under mode I loading

This work analyses the characterisation of hydrated/fresh human cortical bone tissue under Mode I fatigue-fracture. High-cycle fatigue tests under displacement control were performed using the double cantilever beam test. This procedure enables the identification of the coefficients of the modified Paris law and the fatigue threshold, i.e., the strain energy release rate below which the material theoretically has infinite life. To avoid the cumbersome and inaccurate monitoring of crack length during fatigue tests, an equivalent crack-length-based procedure was adopted for data reduction. A cohesive zone model for high-cycle fatigue was formulated specifically to account for the fatigue threshold locally, i.e., at the integration point, and was used to validate the experimental procedure. The model proved effective and was applied to five samples, and to overall results, thereby leading to the proposal of a modified Paris law that accurately represents the fatigue behaviour of this set of human femoral cortical bone, as well as the identification of its strain energy release rate at the threshold.