Modeling of cortical bone response and failure is critical for the prediction of Crash Induced Injuries (CII) using advanced finite element (FE) Human Body Models (HBM). Although cortical bone is anisotropic and asymmetric in tension and compression, current HBM often utilize simple isotropic, symmetric, elastic-plastic constitutive models. In this study, a 50th percentile male femur FE model was used to quantify the effect of asymmetry and anisotropy in three-point bending and axial torsion. A complete set of cortical bone mechanical properties was identified from a literature review, and the femur model was used to investigate the importance of material asymmetry and anisotropy on the failure load/moment, failure displacement/rotation and fracture pattern. All models were able to predict failure load in bending, since this was dominated by the cortical bone material tensile response. However, only the orthotropic model was able to predict the torsional response and failure moment. Only the orthotropic model predicted the fracture initiation location and fracture pattern in bending, and the fracture initiation location in torsion; however, the anticipated spiral fracture pattern was not predicted by the models for torsional loading. The results demonstrated that asymmetry did not significantly improve the prediction capability, and that orthotropic material model with the identified material properties was able to predict the kinetics and kinematics for both three-point bending and axial torsion. This will help to provide an improved method for modeling hard tissue response and failure in full HBM.
In vehicle crash events there is the potential for fracture to occur at the processed edges of structural components. The ability to avoid these types of fractures is desired in order to minimize intrusion and optimize energy absorption. However, the prediction of edge cracking is complicated by the fact that conventional tensile testing can provide insufficient data in regards to the local fracture behavior of advanced high strength steels. Fracture prediction is also made difficult because there can be inadequate data on how the cutting processes used for hole piercing and blanking affect the edge condition. In order to address these challenges, research was undertaken to analyze edge fracture in simple test pieces configured with side notches and center holes. Test specimens were made from a number of advanced high strength steels including 590R (C-Mn), 780T (TRIP), 980Y (dual phase) and hot stamp 1500 (martensitic). Edges were prepared by three different cutting processes: shearing, laser, and water jet ablation. The specimens were pulled to failure and local fracture strains were measured by digital image correlation. Component level tests were also done on simple hat sections that featured a notch cut into the flange and side wall by either water jet or punching. These hat sections were made from select steel grades and were deformed in a three-point bend crush mode to initiate failure at the notch. The results indicate that edge fracture in high strength steels is highly influenced by both edge condition and specimen geometry. In addition, it was concluded that certain material grades can be more notch or punch sensitive than others depending on their metallurgical structure.A prime example of this type of complication is the possibility for AHSS components to fracture at cut edges during crash deformation. This susceptibility can evolve from the limited ductility of the steel, the pre-existing damage from the edge cutting process, and the geometrical stress effect of the feature itself. The potential for edge fracture can be a consideration when developing body structures. If the cracking is not accounted for in the overall design, the load paths through the vehicle frame can be misdirected and the resultant intrusion levels can exceed target levels. It is beneficial if fracture prone areas can be identified in early design layouts through FEM modeling, as opposed to expensive and time-consuming crash tests. However, establishing the proper material data and analytic methods needed for edge fracture prediction has been one of the challenges in the advent of advanced high strength steel. The basic aim of this research was to broaden the understanding of edge cracking by analyzing how different material compositions, geometrical stress states, and edge process conditions affect fracture limits. The investigation promotes the use of new optical measurement techniques, such as digital image correlation (DIC), as an innovative method to acquire strain data in a localized area.
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