Occupant and Seat Responses in Various Rear Impact Conditions: Focus on Head-to-Head Restraint Interactions

Head restraint requirements and designs have evolved to minimize the delay in head support and reduce differential loading in the neck. As a result, they have become bigger, closer to the occupant’s head, and angled forward relative to the seat back. Head restraints have been found missing or detached in the field; they may be removed pre-crash due to occupant comfort issues, or post-crash for better accessibility during extrication. Head restraints may also become detached during occupant loading in rear impacts, To better understand occupant-to-head restraint dynamic interactions, nine rear sled tests were conducted. The test conditions were selected to represent worst case severe loading scenarios. An instrumented 50th Hybrid III ATD (Anthropometric Test Device) was lap-shoulder belted on a right-front seat. The neck was equipped with a bracket and lower neck load cell designed for rear impacts. Occupant postures included seating nominally and leaning forward to simulate hard braking and/or an initial frontal impact. The sled pulse included a moderate speed (24 km/h delta V pulse based on EuroNCAP) and a very high-speed (49 km/h delta V) condition. Three types of modern seats were used including a conventional seat, a rigidized seat and an ABTS (all-belts-to seat). Each test was matched by severity and initial posture. The first series (Match #1) was conducted at 24 km/h with a leaned occupant. All biomechanical responses were below IARVs (Injury Assessment Reference Values). The highest responses relative to IARV were for upper and lower neck tension and extension. The Nij was greatest with the ABTS seat for upper neck and with the rigidized seat for lower neck, highlighting the importance of using both the upper neck and lower neck instrumentation. The second series (Match #2) was at 49 km/h with the nominally seated ATD, and the third (Match #3), leaning forward. The biomechanical responses were below IARV when nominally seated. They were greater in Match #2 than in Match #3 overall, highlighting the benefits of early energy absorption during the ride-down. For example, the relative upper neck Nij ratio was 2.4 in the conventional seat, 4.2 in the rigidized seat and 5.1 in the ABTS. The corresponding lower neck Nij was 4.2, 5.5 and 2.3. The chest 3 ms response was greatest in the rigidized seat, followed by the ABTS, irrespective of sitting posture. The neck was equipped with a bracket and lower neck load cell designed for rear impacts. This load cell improves artificial head restraint to load cell interaction sometimes observed with the standard Hybrid III load cell. There are numerous reasons that may cause an occupant to be out of position prior to a rear impact. In this study, the test conditions were selected to assess head-to-head restraint interactions in severe conditions, including leaning forward. The results provide insight in the seat and head restraint performance in some non-nominal postures.