A study of the effects of current and proposed restraint concepts on the child occupants of vehicles
This research evaluates the performance of automotive child restraint systems (CRS) that conform to international proposals for a universal restraint concept to be adopted by both restraint and vehicle manufacturers. The concept is known as Isofix (International Standards Organisation FIXing), and is intended to ensure optimum compatibility and coupling between vehicle and CRS. In order to quantify the benefits of the proposed Isofix concept it has been necessary to establish the performance limits and benefits offered by current commercially available adult belt retained CRS. A considerable body of knowledge existed on the performance and limitations of the then current CRS. However, during 1995 a significant amendment was introduced affecting ECE R44, the compliance standards applicable to CRS in Europe to which most rnanufacturers require their products to conform (separate national standards also exist). In 1995 amendment 03 was added to ECE R44 and became a catalyst for considerable development activity by manufacturers of child restraints, that resulted in new or revised product ranges. These new products, in particular forward facing Group 1 (9-15 kg) child restraints have significantly improved dynamic performance in frontal impacts, notably in modern vehicles whose seat belt anchorage positions have been optimised for restraint of adults, but are commonly less effective in restraining framed CRS. It was important therefore to re-assess existing systems as the baseline for a realistic evaluation of the proposed Isofix concepts (chapter 10). It was evident that, of the different Isofix concepts being proposed, no overall evaluation of their relative performance had, to that date, been undertaken. A programme involving the design and manufacture of not only suitable test equipment but, in a number of cases, prototype devices, was undertaken. The resulting data have formed the basis of input to the ISO Working Group 1, the body responsible for the evolution of the Isofix concept. This programme also highlighted a number of shortfalls in the proposed concepts. The major results of this test programme have been published at international level, and were used to inform the Isofix discussions. During the programme of comparative evaluation of not only the Isofix but the current belt retained devices, it became clear to the writer that in a frontal impact the orientation of the occupant with respect to the direction of travel had significance. A literature survey produced evidence of minimal research in this area. Hence it was decided by the writer to include a programme of parametric tests to investigate the significance of occupant orientation, given that commercially available CRS often include a feature to vary the recline angle of the seat. The Isofix set-up was particularly suitable for this exercise in that it eliminated many of the variables associated with belt retained devices. The results of this work have been published at intemationallevel. A review of the available accident data indicates that side impacts are potentially more life threatening than the more common frontal impacts due to the proximity of the occupant to an intruding vehicle or object. However, current European certification standards do not require the evaluation of CRS in a side impact. This is considered to be an area where improvements, particularly aided by an Isofix type attachment concept, can be made. Therefore the final area of research undertaken by the writer was to develop and propose a test to evaluate CRS in a realistic side impact scenario. This involved the simulation of not only the acceleration imparted to the target vehicle occupant but in addition the intrusion component. This work, which again has been presented at international level, contributed towards a proposal to amend the European certification standard for CRS to include a side impact evaluation. This thesis commences with a review of the accident data currently available, and looks at how the physiological and anatomical properties of the child, vehicle design, and the inherent potential for misuse and mis-installation of the current generation of CRS, impact upon child safety. This is followed by an overview of the lsofix proposal before the results of the writer's detailed testing of both current belt retained and proposed Isofix CRS concepts (chapters 10 and 11) are reported. The subsequent chapters (12 and 13) detail the results of the writer's investigation into CRS orientation in a frontal impact and the development of a representative side impact test, based on a single sled, for inclusion in the European certification procedure. The document concludes with discussion and conclusions relating to the future of CRS design and evaluation. The major findings of this research were: • contrary to initial expectations, significant CRS recline angle in a forward facing device has been proven to be undesirable; • Isofix CRS with rigid lower anchors have been shown to be beneficial, particularly in side impacts, their efficacy in a forward impact being compromised by rotation in devices that do not incorporate an anti-rotation device; • a side impact test has been developed which more accurately represents the input to a CRS seen in a rear vehicle incident. Such a test is not only desirable but essential to drive CRS manufacturers into improving side impact protection for occupants.