NECK INJURY PREVENTION IN REAR-IMPACT CRASHES C.Dippel Institute for Lightweight Structures, Swiss Federal Institute of Technology Zürich (ETH)

R.Kaeser Winterthur Engineering School Although much progress has been made in the field of passive car safety in the last years, there is still a strong need for safety improvements in rear-end impacts. While rear-end collisions seldom cause fatal injuries, lesions of the cervical spine are known to cause a high societal cost associated with their often long-term consequences. The critical body region in rear impact cases is the cervical column. The biomechanics are not yet understood sufficiently to define tolerance l imits for soft tissue injuries that could be used for the design of restraint systems. Nevertheless, evidence is high that lesions of the cervical column are caused mostly by shear forces during the first phase of the impact. Therefore, the best strategy to reduce the risk of injury is to control the kinematics of the head-neck­ thorax system during impact in such a way that relative displacements within this system are minimised. A concept for a car seat offering enhanced safety has been developed and test models have been bui lt. Although this seat has been designed for the h'arder conditions in low mass vehicle collisions, the concept can easily be adapted to conventional cars. The seat features an automatically controlled head restraint adjustment mechanism which guarantees a small and relatively well defined distance between the occupant's head and the head restraint. Based on computer simulations, paddings for the d ifferent regions of the seat back and an energy absorbing yielding mechanism of the seat back have been designed in a way that only l ittle relative displacements in the head-neck-thorax system are to be expected even during a severe rear-end impact. Different occupant sizes ranging from the 5th percentile female to the 95th percentile male have been considered. A series of rear impact sied tests with velocity changes from 1 2.2 to 38.6 km/h and acceleration levels from 6 to 30 g has been performed using a 50 percentile Hybrid I I I dummy equipped with the new TRIO neck. The IRCOBI Conference Hannover, September 1997 239 measurements show a good correlation with the simulation and the risk of cer­ vical column injuries is considered to be substantially lower than in conventional seats. A SAFE SEAT FOR LOW MASS VEHICLES The Working Group on Accident Mechanics has developed a low mass vehicle (LMV) with a curb weight of 650 kg, called "Cratch". This experimental vehicle demonstrates that a high level of passive safety for the occupants of low mass vehicles is achievable in frontal collisions (Frei 97). The development of a car seat suited for use in LMVs has been a part of this project. The seat is an important element of the restraint system: In the case of a frontal crash, the initial position of the occupant is defined by the contour and position of the seat, and, during the crash, a part of the occupant's kinetic energy is absorbed through deformation of the seat base. In rear-end impacts the seat represents the entire restraint system. During a collision against a conventional car, the low mass vehicle, due to the fundamental laws of motion, is exposed to higher accelerations and a larger change in velocity than its counterpart (Niederer 93). The seat presented here was specially adapted to these severe conditions. Nevertheless, almest every feature of the concept could easily be adapted for use in conventional cars. The main focus of the development was on the improvement of the rear-end impact safety, which represents a substantial problem, also for conventional cars. Compared to the considerable improvements of crash safety in frontal and side impacts accomplished during the last years, progress concerning the rear­ end impact safety has somewhat stagnated. This may be related to the fact that rear-end crashes are often considered to be less dangerous, since there is a very high surviving probabil ity for the occupants. I n spite of this, it is very worthwhile to invest in rear-end impact safety since injuries caused by this coll ision type do not only cause high amounts of compensation costs but also can have very unpleasant consequences to the occupants involved. THE ADJUSTMENT CONCEPT I n order to provide an adequate safety level and an ergonomically correct driving environment to occupants of a wide range of size and weight, a seat needs various adjustment capacities. The seat must be adjustable in the longitudinal axis. The seat base should be adjustable in height and angle. The horizontal distance between backrest and front edge of the seat base (seat base length) should be variable in order to achieve a sufficient support for the thighs. Correct horizontal and vertical adjustment of the head restraint is indispensable for an efficient injury prevention in rear-end impacts. 240 IRCOBI Conference Hannover, September 1997 Figure 1: A djustment parameters of the seat. There is no seat back recliner adjustment. This is not considered necessary in combination with the other adjustment parameters and the inferior geometry of the Cratch vehic/e. The problem with such adjustment possibilities is that they only make sense if they are applied correctly by the occupants. lt has been shown (Parkin 93) that, especially in the case of head restraint adjustment, this cannot be taken for granted. Therefore, in the seat presented here, occupants are only given control over one single parameter: the position of the seat on the longitudinal axis. All other adjustments are performed automatically. The adjustment of the seat base and a part of the vertical adjustment of the head restraint are mechanically l inked to the horizontal movement of the seat in such a way that suitable seating positions result for all person sizes ranging from the 5th percentile female to 95th percentile male. Figure 2: Assemb/y of the adjustment system, different positions: 95-, 50and 5-percentile with head restraint in rearmost and in foremost position. IRCOBI Conference Hannover, September 1997 241 The horizontal and the final vertical adjustment of the head restraint are con­ trolled electronically. The horizontal adjustment reduces the initial head-to­ headrest distance in a rear-end impact. A smaller in itial distance reduces the . time during which the thorax of the occupant is accelerated while the head still remains more or less in its original position . In this early phase, relative dis­ placement between head and thorax takes place. Shear forces are effective and the cervical column is forced into a S-shaped bending (Penning 92). There is evidence that these shear forces are responsible for lesions in the upper neck region (Walz 95). Beside the reduction of the · initial distance between head and head support, an automatic horizontal head restraint adjustment has the advantage that it can be used to obtain a comparatively weil defined initial dis­ tance (Muser 94). This helps to implement additional measures to reduce the horizontal displacement of the head relative to the torso during rear-end impact. An automatic vertical adjustment is necessary for two reasons: first, it guarantees that the head is sufficiently supported and hyperextension is pre­ vented, and second, an exact vertical adjustment is required to enable the horizontal adaptation process and a continuous support of the head-neck contour of the occu pant. The seat back and the neck-head support are designed as a single unit . Therefore, vertical adjustment of the head restraint requires translation of the whole seat back. Since the shoulder belt passes through a d iversion clamp in the seat back, this leads to an automatic adjustment of the belt geometry as well. The geometry of the horizontal adjustment mechanism is based on a pa­ rallelogram, with the head restraint performing a translational movement and the neck restraint rotating with its lower end fixed on the seat back. A head-neck restraint based on this principle has the advantage that, with only one degree of freedom, a very good support of the occupant's spinal column can be obtained for a wide variety of seating postures. I' 1 0 cm,1