Strains and pressures in the brain are known to be influenced by rotation of the head in response to loading. This brain rotation is governed by the motion of the head, as permitted by the neck, due to loading conditions. In order to better understand the effect neck characteristics have on pressures and strains in the brain, a human head finite element model (HHFEM) was attached to two neck FEMs: a standard, well characterized Hybrid III Anthropometric Test Device neck FEM; and a high fidelity parametric probabilistic human FEM neck that has been hierarchically validated. The Hybrid III neck is well-established in automotive injury prevention studies, but is known to be much stiffer than in vivo human necks. The parametric FEM is based on CT scans and anatomic data, and the components of the model are validated against biomechanical tests at the component and system level. Both integrated head-neck models were loaded using pressure histories based on shock tube exposures. The shock tube loading applied to these head models were obtained using a computational fluid dynamics (CFD) model of the HHFEM surface in front of a 6 inch diameter shock tube. The calculated pressure-time histories were then applied to the head-neck models. The global head rotations, pressures, brain displacements, and brain strains of both head-neck models were compared for shock tube driver pressures from 517 to 862 kPa. The intracranial pressure response occurred in the first 1 to 5 msec, after blast impact, prior to a significant kinematic response, and was very similar between the two models. The global head rotations and the strains in the brain occurred at 20 to 100 msec after blast impact, and both were approximately two times higher in the model using the head parametric probabilistic neck FEM (H2PN), as compared to the model using the head Hybrid III neck FEM (H3N). It was also discovered that the H2PN exhibited an initial backward and small downward motion in the first 10 ms not seen in the H3N. The increased displacements and strains were the primary difference between the two combined models, indicating that neck constraints are a significant factor in the strains induced by blast loading to the head. Therefore neck constraints should be carefully controlled in studies of brain strain due to blast, but neck constraints are less important if pressure response is the only response parameter of primary interest.
Response of an Impact Test Apparatus for Fall Protective Headgear Testing Using a Hybrid-III Head/Neck Assembly