We demonstrate the effectiveness of a new method for expressing the load transfer in passenger car bodies to improve structural performance in order to protect occupants in side impact. For vehicle structures, one of the most important goals is the reduction of compartment deformation. For this purpose, indicating the load transfer paths in a vehicle compartment is fundamentally significant. The present authors previously developed an index Ustar (U*) to express the load paths in structures. Our purpose in the present study is to express the load transfer using U* in a vehicle compartment in side impact.

The index U* is defined as U* = 1－U/U′, where U is the work done at the loading point and U′ is the work done when an arbitrary point is constrained. We can say that U* shows the connectivity between the loading point and an arbitrary point. It is natural to think that the force is transferred along the highest part of the U* distribution. The index U* can realize a way to obtain the overall view of load transfer in the vehicle compartment during collisions.

We introduce the extended U* in which the effect of inertial force is included for the calculation of vehicle collision. The calculated distribution of U* for a sample passenger car shows that the impact force is transferred mainly to the lower structure of the compartment. However, the load is not transferred to the opposite body side, because of the separation caused by the center tunnel structure. The U* distribution shows that among the several transverse cross-members, the cross-member under the B-pillar plays a key role in load transfer. In contrast, the cross-member under the front seat has a small effect for load transfer. These results of load transfer are demonstrated by the colored U* contour lines in the entire compartment for any specified instant during impact. The calculated results are expected to improve the side impact crashworthiness to reduce the risk of injury to occupants.

As an example, to increase the load transfer of the cross-member under the front seat, we locate the stiffener member between the side sill and the tunnel structure. The designation of the stiffener location is pinpointed by the distribution of U*. A crash simulation of a sample vehicle equipped with the stiffened cross-member reveals that the side sill intrusion deformation decreases by more than 30%. The value of the decrease rate itself is not a key point of the result. The point of importance is the effectiveness of the deduction process by U* for the strict determination of structural improvement.