A Fast Running Model for Skeletal Impact Biomechanics Analysis

Skeletal trauma occurs in many blunt, ballistic and blast impact events. Even though the personal body armors and protective equipment were effective in stopping the penetration of bullets or fragments, the resulting impact loading could lead to the significant injuries and fractures to the thoracic skeleton and extremities. The finite element (FEM) method, with its capability to handle complex geometries and nonlinear materials, are commonly used to analyze the tissue biomechanical responses and correlate the simulation results with the injury outcomes. However, it is very difficult to construct the three-dimensional (3D) FEM model for the skeletal biomechanics analysis because of the complex geometry and different materials involved. Moreover the simulation of 3D FEM model is computationally expensive because both small element size and high speed of sound in materials lead to very small time step in an explicit transient analysis. The simulation process is often not robust enough when the model experiences the large deformation. To shorten modeling and simulation times, we have developed a fast running model based on a novel nonlinear beam element for the skeletal impact biomechanics analysis. In contrast to the conventional beam elements, the kinematics of the developed beam element is free of rotational degrees of freedom (DOFs). The current beam element offers the desired constant lumped mass matrix for the large deformable explicit transient analysis. The realistic treatment of junctions and surface intersections among beams becomes straightforward. Furthermore the model can account for the irregular shape and different materials at beam cross sections by using the numerical integration. The sophisticated material models such as elastoplasticity can also be incorporated directly in the integration points. Thus the fast running model is suitable for the analysis of complex nonlinear composite structures such as the loading-carrying thoracic skeleton and extremities. The stereolithograph (STL)-based anatomical geometry of skeletal structure is used to extract the one-dimensional (1D) curved beam model and the associated beam cross sections. The anatomical surface of skeleton is also utilized for the calculation of transferred loads to the underlined beams. The 3D responses such as displacements and stresses from the fast running model are subsequently reconstructed on the anatomical surface for the visualization and skeletal trauma analysis. We demonstrate the efficiency of such modeling technique by simulating the rib cage and the lower extremity under the impact loadings. As compared to the 3D FEM model, the developed model runs fast and robust, and achieves good results without the need of laborious 3D meshing process.