An energy-absorbent structure which typically comprises a blow-molded hollow body of thermoplastic material which encloses a foam filling on all sides serves to absorb impact energy, the impact energy being dissipated by work done upon deformation of the structure. One consideration in that respect is that the impact energy which occurs in the event of the vehicle to which the energy-absorbent component is mounted being involved in a collision is dissipated or diverted in such a way that the vehicle itself does not suffer from permanent deformation, while a further consideration is that a vehicle bumper should also guarantee sufficient pedestrian protection, in other words, in the event of a collision with a pedestrian, the bumper should guarantee a low level of energy absorption with a long deformation travel to minimise pedestrian injury.
In the case of the motor vehicle to which the energy-absorbent component is fitted colliding with a harder obstacle at a somewhat higher speed, it is necessary to ensure that, depending on the respective impact energy involved, that energy is dissipated by way of the elastic deformation of other components of the vehicle, for example by way of deformation of flexural cross-bearers, crash boxes or impact dampers. That involves the need to provide structures which, as viewed in the direction of travel of the vehicle, exhibit a differing energy absorption capability. In addition however it is also desirable to provide different impact zones involving differing energy absorption capability in parallel relationship with the direction of travel as, in the event of an impact with a pedestrian, the position of the first impact determines the further movement of the body of the pedestrian. In that respect reference may be directed for example to EP 1 046 546 which indicates that it is desirable for the point of first impact in the case of a pedestrian accident to be as low as possible in order to prevent the risk of knee injuries. In order to guarantee that without additional installation fitments on the bumper, EP 1 046 546 sets forth a bumper whose region which is the lower region in the mounted condition thereof is of higher strength than the upper region in which then a comparatively low level of energy absorption takes place, with a relatively great deformation travel.
The bumper described in EP 1 046 546 comprises a shell-like cover with an opening which in the mounted condition of the bumper faces towards the passenger compartment and which is filled with a plastic foam. Foams of differing strengths are introduced over the cross-section of the arrangement into the cover. It is optionally provided that a structure for stiffening the cover is to be disposed within that structure.
It will be noted however that the arrangement described in EP 1 046 546 suffers from the disadvantage that the foam body which is not closed by the cover on all sides does not afford adequate stability in relation to a relatively severe impact. A reinforcing structure within the cover is required in order to divert a relatively high level of impact energy to the deformable component parts of the vehicle body, which are disposed behind the bumper. Finally, it is not impossible that, after a light impact against a firm solid obstacle, for example in a parking bump situation, the bumper may suffer permanent damage so that complete replacement of the bumper would then be necessary after such a crash. That is also to be attributed to the fact that the foam structure in the interior of the bumper is not enclosed and held in place on all sides.
Reference may also be made to DE 32 09 464 describing an energy-absorbent component in the form of a vehicle bumper which, while being of low weight, is intended to be substantially insensitive and resistant to impacts and bumps at the visible surface thereof. For that purpose it is proposed therein that a blow-molded bearer is provided at its outside with a cover of polyurethane foam. That admittedly achieved a differing energy absorption capability in the direction of travel of the vehicle to which the bumper is mounted, but it will be appreciated that it is difficult to achieve differing impact zones transversely with respect to that direction. Furthermore the manufacture of such a bumper is relatively complicated and expensive as the blow-molded bearer has to be placed in a molding tool, within which the remaining space is then filled with foam. For that purpose the bearer has to be previously treated with a bonding agent.
Attention may be directed to EP 0 947 727 A1 with a bumper system having a foam structure comprising three foam layers of differing energy absorption capabilities, the layers being arranged in parallel relationship with the direction in which forces are applied to the system. For that purpose it is proposed therein that a transverse bearer is arranged on the longitudinal frame bearer of the motor vehicle, with the foam structure being mounted to the transverse bearer. The different foam layers are glued together while a cladding system is mounted to the foam structure on the outside thereof. That arrangement is also comparatively complicated both in respect of structure and also in its manner of manufacture.