The bone fragments originating from a blunt thoracic impact pose a risk of penetration thereby causing complex medical conditions, for example, pneumothorax, hemothorax or sometimes even a fail chest leading to eventual death. In order to mitigate the effects of these injury mechanisms, it is necessary to understand the process of rib fractures under similar mechanical environments. The objective of this study is to characterize mechanical behavior until the rupture of isolated porcine ribs, subjected to high-speed dynamic loads.
Dynamic three-point bending tests were carried out on fresh porcine ribs using a modified Split Hopkinson Pressure Bar (SHPB). The experimental responses were classified into three categories based on the fracture time with respect to time required to establish an equilibrium between input and reaction forces as (i) No fracture, (ii) fracture in equilibrium and (iii) fracture before equilibrium. The experimental studies showed that the impact velocity played an important role in the type of response observed i.e. as striker velocity increased, the response tends towards type (iii). Moreover, the experiments also show evidence of the influence of strain rate on the time for fracture. This proves that under dynamic environment, strain rate has an influence on the post-yield behavior of the rib.
The 3D geometrical model was generated through Simpleware platform of Synopsys. The 3D model of the rib was generated from DICOM images of High-Resolution peripheral Quantitative Computed Tomography (HR-pQCT) scans of dried and defatted ribs. This geometrical model was meshed using 8-node solid hexahedral elements for the commercial LS-Dyna FE explicit solver. The different greyscale values of the DICOM images were used to correlate corresponding dry apparent density using a linear equation. This apparent density was then considered as an independent variable to interpolate corresponding mechanical properties (Young's modulus and tensile yield strength) through power law regression equations. The striker, input and output bars were also meshed with 8-node solid hexahedral elements. Penalty base surface-to-surface contact was assigned at sample-bar(s) interface, in addition to an interior contact between cortical and trabecular bone elements of the rib.
The evidence of strain rate on post-yield behavior led us to use a modified elastic-plastic law using the Cowper-Symmonds yield scaling. Therefore resulting in an elastoviscoplastic constitutive law. This constitutive law was coupled with an incremental and stress-state dependent damage law, capable of considering nonlinear damage accumulation, progressive reduction in stiffness with crack propagation and influence of strain rate on fracture strain.
The proposed FE model was able to predict satisfactory force-displacement response and coherent fracture patterns. Thereby indicating that this model can be applied to numerical human ribs or thorax model under impact loads.