HORK Enterprises
3221 Quick Road
Holly, MI 48442, USA
 +1 (248) 328-0231
 consult@hork.com

Side Impact

After frontal impact, lateral and oblique side impact collisions are the second most common cause of serious motor vehicle accidents. In vehicle to vehicle side impacts, those with a forward bias are most prevelent, which is reflected in the FMVSS-214 side impact protection regulation. Side impact collisions differ from frontal impacts in that there is direct interaction of the occupant with the vehicle structure. Before regulations for side impact protection were drawn up, it was not uncommon for the door of a car to be pushed more than two feet (0.5 m) into the vehicle interior under serious crash conditions.
With the adoption of side impact protection standards in both the United States and Europe, vehicle side structures have been beefed up quite a bit compared to older generations of vehicles. High strength steel is used in those areas that take the brunt of the impact, such as the B-Pillar between the doors. To further cushion the blow of a side impact collision more and more car makers are installing side impact airbags. These deploy in between the occupant and the vehicle structure to increase the duration of the contact and lower the peak forces acting on the occupants.

The industry makes extensive use of computer analysis during the development of a new vehicle for both structural improvements as well as for the development of side airbags. These models range from coarse beam models in the early stages of development to detailed Finite Element models in the latter stages. The coarser model allows analysis of the overall kinematics of the impact as well as the relative velocities of the door and the occupant. This gives design guidelines for the structure of the vehicle and padding needed to protect the occupant. This then helps to build the model for the more detailed study. Finite element models of both the dummy and the door structure are used to study the interaction. These analyses are of a complex nature as they involve large plastic deformations as well as foam paddings that have complex non-linear material properties. The effects of the various changes are studied by looking at the resulting dummy injury measures. Rib acceleration, thoracic viscous injury level, and pelvis acceleration are all indicators of the risk of serious injury. If the car maker is not satisfied with the injury mitigation level that can be achieved with only structural improvements and paddings, they may decide to add a side airbag to the standard equipment. The development of the side airbag starts on the computer as well and then propagates to sub system testing on a sled. Different types of side airbags can be analyzed for their protective characteristics. Some provide only cushioning in the area of the vulnerable ribs, others include head protection as well. If the side airbag is combined with a side curtain airbag, then the latter can be counted on to provide the head protection. The side airbag of course has its own crash sensor. This often comprises an inertial sensor, located at the bottom of the B-Pillar. However, other types of sensors exist. Some measure the change of pressure in the door cavity, which would change if the door is dented at a high rate. Another type of sensor, that we have had involvement in, generates a magnetic field in the door cavity and then measures changes in that field to predict that an impact is imminent Speed is of the essence, because an impact speed of 30mph (50 km/h) is equivalent to about 55 inches per second or 1.4 m/s and there typically are only 10 inches or 25 cm between the outside of the door and the occupant

To reduce the risk that the deploying side airbag induces injuries to the occupant, the deployment is in general aimed away from the normal seating position. Side airbags are not nearly as large and as frontal airbags, but they do deploy very fast. Therefore, extensive testing with out-of-position adult and child dummies is necessary to ensure minimal risk of induced injuries. For this a special instrumented arm was developed to gauge the forces from a deploying side airbag on an arm that would be casually resting on the arm rest at the time of deployment. Moreover certification tests are required, involving child dummies resting against the seat back and the door. Before a side airbag is developed, built into a seat, and installed in the vehicle, so it can be tested, many simulation runs will have already established that the test will be a pass.