Tensile material properties of human rib cortical bone under quasi-static and dynamic failure loading and influence of the bone microstucture on failure characteristics

Finite element models of the thorax are under development to assist vehicle safety researchers with the design of countermeasures such as advanced restrain systems. Computational models have become more refined with increasing geometrical complexity as element size decreases. These finite element models can now capture small geometrical features with an attempt to predict fracture. However, the bone material properties currently available, and in particular the rate sensitivity, have been mainly determined from compression tests or tests on long bones. There is a need for a new set of material properties for the human rib cortical bone. With this objective, a new clamping technique was developed to test small bone coupons under tensile loading. Ten coupons were harvested from the cortical shell of the sixth and seventh left ribs from three cadavers. The coupons were tested to fracture under quasi-static (target strain rate of 0.07 %/s) and dynamic loading (target strain rate of 170 %/s). Prior to testing, each coupon was imaged with a computed micro-tomograph to document the bone microstructure. An optical method was used to determine the strain field in the coupon for the quasi-static tests. The rib bone coupons were found to be elastic, with brittle fracture. No plastic behavior was observed in this test series. The bone coupons were assumed isotropic, homogeneous and elastic linear, and the average Young's modulus for the quasi-static tests (13.5 GPa) and the failure stresses (quasi-static: 112 MPa, dynamic: 124.6 MPa) were in line with published data. Fracture however did not always occur in the gage area where the cross-sectional area was the smallest, which contradicted the assumption of isotropy and homogeneity. The comparison with material properties obtained for long bones suggests that the effective cross-section has an effect on the calculated material properties.