Development and Calibration of the Large Omnidirectional Child ATD Head Finite Element Model 2021-01-0922

To improve the biofidelity of the currently available Hybrid III 10-year-old (HIII-10C) Anthropomorphic Test Device (ATD), the National Highway Traffic Safety Administration (NHTSA) has developed the Large Omnidirectional Child (LODC) ATD. The LODC head is a redesigned HIII-10C head with mass properties and modified skin material required to match pediatric biomechanical impact response targets from the literature. A dynamic, nonlinear finite element (FE) model of the LODC head has been developed using the mesh generating tool Hypermesh based on the three-dimensional CAD model. The material data, contact definitions, and initial conditions are defined in LS-PrePost and converted to LS-Dyna solver input format. The aluminum head skull is stiff relative to head flesh material and was thus modeled as a rigid material. For the actual LODC, the head flesh is form fit onto the skull and held in place through contact friction. In an attempt to identify the matching flesh-skull contact definition in the FE model, a comparative assessment was conducted under four different boundary conditions between head flesh and skull: completely unconstrained, fully constrained, partial constraint at the head cap boundary, and partial constraint at the jaw. The boundary conditions under consideration influenced how the head flesh separated from the skull during the impact event. It was clear that the nose of the ATD contacted the impact plate during all drop tests leading to varying levels of contact area. For the most suitable boundary condition, the viscoelastic material parameters of the flesh were identified using an inverse method that minimizes the difference between measured and predicted acceleration impulse of the head form center of gravity under impact loading. This inverse method resulted in a reasonable match between physical test data and model-simulated data for head impacts from drop heights of 150, 300, and 450 mm at an angle of 60 degrees. Additional model predictions were then compared to head drop tests from the same heights at a modified angle of 62 degrees. The FE model with the skull-flesh contact definition of the partial constraint at the head cap boundary predicts the peak accelerations within 4% error with good agreement across the full acceleration time signal and the phase shift for all drop heights between the 60 and 62 degree drop angles.