Global and local characterization explains the different mechanisms of failure of the human ribs

Knowledge of the mechanics and mechanistic reasons inducing rib fracture is fundamental for forensic investigations and for the design of implants and cardiopulmonary resuscitation devices. A mechanical rationale to explain the different rib mechanisms of failure is still a challenge. The aim of this work was to experimentally characterize human ribs to test the hypothesis that a correlation exists between the ribs properties and the mechanism of failure. 89 ribs were tested in antero-posterior compression. The full-field strain distribution was measured through Digital Image Correlation. The fracture load ranged 7-132 N. Two main different mechanisms of failure were observed: brittle and buckling. The strain analysis showed that the direction of principal strains was either aligned with the ribs, or oblique, around 45°, with a rather uniform direction in the most strained area. The maximum principal strains were in the range between 1000 and 30000 microstrain and the minimum principal strain between -30000 and -800 microstrain. The ribs undergoing brittle fracture had significantly thicker cortical bone than those undergoing buckling. Also, larger tensile strains were observed in the specimens with brittle fracture than in the buckling ones. These findings support the focus of cortical thickness modelling which could help in sharpening computational models for the aforesaid purposes.