Occupant protection arrangements for motor vehicles which comprise an inflatable air-bag are now very well known and serve to reduce, and ideally eliminate, injury to an occupant of a motor vehicle in the event of an accident involving a crash. Air-bags of this general type are inflated upon receipt of a signal indicative of a crash situation, the signal being generated by a crash sensor located elsewhere in the motor vehicle, so as to inflate, thereby providing a relatively soft restraint for an occupant of the motor vehicle.
Research has shown that as an occupant of a motor vehicle impacts with an inflated air-bag, which is typically inflated by a very high pressure of gas, two principal forces serve to act on the occupant. One of these forces results from the pressure of gas within the air-bag, and the other arises from so-called “membrane forces”. These membrane forces result from frictional resistance to the relative movement of the warp and weft yarns of the air-bag fabric, as the fabric is subjected to tension across the front surface of the inflate air-bag.
It has also been found that whilst the chest/sternum area of the human body is relatively resilient to these forces created in an inflating air-bag, due to the natural stiffness of the human body in this area, the head and neck region of the human body is significantly more vulnerable to injury as a result of these forces in an air-bag. It has therefore been found to be desirable to reduce the aforementioned membrane forces in the region of an air-bag which is likely to be impacted by the head of a vehicle occupant so as to reduce the overall force applied to the head and neck region of the occupant, whilst providing a lower region of the air-bag with a configuration susceptible to relatively large membrane forces, the lower region of the air-bag being provided to provide restraint to the chest/sternum region of a vehicle occupant.
EP1439096A1 discusses the concept of membrane forces in more detail, and in order to address the problem of potential neck injuries as a result of the membrane forces in an inflating air-bag, proposes an air-bag using seams, straps or ribbons to create a relatively “soft” wavy front surface on the air-bag. However, it has been found that a problem with the arrangement of EP1439096A1 is that the indentations provided between the adjacent waves of the relatively soft front surface, do not have a constant depth, with the result that the wavy area of the front surface is deeper in the middle portion than at its edges. The effect of this is that the membrane force-reducing effects of this prior art arrangement are different in different areas of the front surface of the air-bag, being better in the middle of the front surface than at its periphery.