The number of traffic fatalities is decreasing due to improvements of vehicle crash safety performance, which is promoted by crash safety regulations and consumer information assessments such as the New Car Assessment Program (NCAP). On the other hand, according to Japanese accident statistics, the reduction of fatality rates of those involved in oblique impact crashes is less than those in frontal and side impact crashes. This is considered because the risk of secondary impacts with interior structures increases as the seat belt slips, or occupants do not properly interact with the airbag due to lateral occupant motion induced by oblique vehicle movement. If restraint force is increased, thereby reducing the amount of occupant movement, it is possible to increase the injury risk of vulnerable people, such as the elderly. Therefore, applying high restraint force while distributing such loading to mitigate occupant injury should be the focus. The objective of this study is to develop restraint approaches that achieve effective occupant restraint and a reduction of injury risks by restraining body regions that have relatively high stiffness.

The stiffness sensitivity of a rib cage was investigated by using the thoracic impactor simulation with the Human Body Model (HBM) and it was found that the stiffness of the shoulder region, which includes upper rib cage and clavicle, has relatively higher stiffness than other thoracic regions. Therefore, it was determined to restrain around the shoulders.

Subsequently, the influences of restraint around the shoulders with respect to occupant kinematics and injury reduction were compared with those of conventional restraint systems under oblique impact simulation with the Test device for Human Occupant Restraint Anthropometric Test Dummy (THOR ATD). In addition, the influences of each restraint system on the number of fractured ribs caused by thoracic loading were investigated with the HBM under the same loading conditions as the THOR ATD simulation.

When the shoulder region was restrained, THOR ATD movement and chest deflection were reduced compared to the conventional restraint systems. The HBM results indicated that shoulder restraint reduces the chest deflection, similar to the THOR ATD, and the number of fractured ribs also reduced.

It was found that the load generated by the shoulder restraints was broadly distributed on the whole thorax, compared to loads of the conventional restraint systems. The restraint force was higher than that of conventional restraint systems and it reduced amount of occupant movement however the concentrated force on the thorax decreased. Based on these findings, an investigation of the mechanisms of load distribution is necessary considering the structural differences between the THOR ATD and the HBM.

This study described an approach to reduce thoracic injury while distributing loads over the whole thorax and keeping high restraint forces by restraining the shoulders, which have a relatively higher stiffness than other thoracic regions. In order to reduce the number of traffic fatalities, it is desirable to develop restraint systems based on the approach in this study.