Further reduction of brain injuries is crucial to diminish traffic fatalities. Past studies suggest that strain of incompressible brain tissue is generated mainly due to head rotation. Accident statistics show a higher rate of pedestrian fatalities resulting from strain-induced brain injuries in accidents with AIS 2+ brain injuries than that of car occupants. One factor for this difference would be larger translation and rotation of an unrestrained pedestrian body than those of a restrained car occupant. This study aimed to clarify the influence of whole body kinematics on the brain strain in pedestrians and occupants.
Time histories of the head translational and rotational accelerations were taken from the NHTSA crash test database for full frontal and MDB side impacts. Pedestrian crash simulations were conducted for frontal and side impacts using a human, small-sedan and SUV FE models to obtain head acceleration time histories. These time histories were applied to the skull of the GHBMC head/brain model. The time histories of the maximum principal strain from the GHBMC model were compared between occupants and pedestrians in the same impact direction. The body kinematics and the rotational velocity of the head were also compared to identify factors for the difference in the time history patterns of the maximum principal strain. In addition, these time histories were compared to that of the CIBIC (Convolution of Impulse response for Brain Injury Criterion) criterion developed in a previous study under each of the four conditions.
Peaks of brain strain were identified in both head pre-impact and impact phase for pedestrian while that was identified only in head impact phase for occupant, regardless of the impact direction. The flip of the rotational direction of the head in the head pre-impact phase was found only in pedestrian, likely resulting in the peak of brain strain prior to the head impact. This trend applied regardless of the direction of impact. The time history of the CIBIC criterion provided waveform patterns similar to the maximum principal strain time history in all impact conditions.
Peaks of brain strain in both head pre-impact and impact phase in pedestrian identified in this study would require reduction of peaks in both phases. A criterion predicting time history of brain strain, such as CIBIC, was found to be an effective tool to address reduction of peaks in multiple phases seen in pedestrian. These findings would lead to novel pedestrian safety technologies that control pedestrian kinematics to reduce the primary peak.
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