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Vehicle compatibility in car-to-car collisions

Literature review in the framework of the European research project ‘Improvement of crash compatibility between cars', Workpackage 1

Auteur(s)

Sluis, Jan van der

Jaar

2000

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Within the scope of the project ‘Improvement of crash compatibility between cars' scientific literature from the last 15 years was reviewed in order to obtain state-of-the-art knowledge about car compatibility in car-to-car accidents. In the first place, current definitions of (in)compatibility were reviewed. There appears to be a variety of definitions and descriptions. They have in common that, while improving occupant safety is (traditionally) the main purpose of improving crash safety of cars, currently (improving) the safety of occupants of opponent cars is of equal concern. The first property (the level of occupant safety) is often called the level of self-protection, while the second item (the level of opponent safety) is often described as the level of aggressiveness of a particular car. Therefore, combining these different purposes in one, compatibility may be described as ‘the capability of cars to protect their occupants in crashes, while at the same time produce as less harm as possible to occupants of opponent cars'. Literature was reviewed from three points of view: statistical, mechanical and geometrical. These views have been constructed to cover the whole range of scientific literature, including proceedings of international conferences as ESV, STAPP, IRCOBI and SAE and specialized journals. From the statistical point of view, literature focussing on the influence of car mass (including mass ratio) and car size on the injury severity of occupants is reviewed; also several existing crash rating systems are discussed. In almost all reports, the main data sources used for analysis are statistical accident data. The influence of car mass (and mass difference) appears to be very well documented, the main conclusion being that the fatality or injury risk is inversely proportional to car mass or mass difference: the heavier the car the lower the fatality or injury risk (and the higher the aggressiveness). Some authors also described the fact that in (frontal) collisions between cars of equal mass, the outcome was better for heavier than for lighter cars. The question, whether the influences described above should be solely attributed tot car mass or to car size as well, was addressed by at least one author (Evans), who concludes that car mass appears the stronger influence of the two, though in most instances size and mass are interchangeable. Crash rating systems are used to inform consumers about the level of occupant safety of different car models. Some of these systems are very sophisticated, using control variables such as exposure in traffic and driver sex and age. In general, ranking systems based on accident data reflect the important influence of car mass (and size) as described above. From the mechanical point of view, literature is reviewed concerning the classical theory of mechanics of collisions (Newton mechanics). Conservation of momentum, deformation energy, and the difference between plastic and elastic deformation are discussed and used by several authors to propose solutions to the problem of incompatibility. An example is the so-called bulkhead concept (Zobel): a maximum force level is defined for the front end of cars which should prevent intrusion into the compartments, after the crush zones of both or either car is fully deformed. Other solutions propose to change the force level of the crush zones of both cars to a more or less comparable (relatively low) level and also propose methods to measure these levels during crash testing (Steyer). Crash testing is an important instrument for both measuring and improving the level of (occupant) safety of cars, as is currently successfully demonstrated all over the world by the existing (consumer) programmes. These programmes test (new) cars according to specific test protocols, in which the test speed is higher than the level of current legal requirements. They tend to work two ways: 1) the public awareness of the importance of crash safety of cars is increasing, and 2) improvement of the design by manufacturers is often reached without changing legal requirements. In the chapter on the mechanical view, the importance of developing and applying mathematical modelling to analyse and ultimately improve crash safety of cars is reviewed, based on various types of models: lumped mass models en finite element method (FEM) models. These models are often used to develop new individual car design, but they will also be used to analyse the problem of incompatibility between different car types, or even models of the same car type in different collision modes. By using these instruments, numerous design and parameter changes may be analysed, without the need to build and crash test every separate item. From the geometrical point of view, literature is reviewed in which authors describe influence on outcome of (differences of) car geometry and stiffness geometry that cause incompatibility. Regardless of mass and size differences, geometrical differences may also cause considerable harm. This is clearly demonstrated in case of collisions between ‘normal' cars and specific four-wheel-drive cars (and vans) having far stiffer longitudinals and greater structural height than these normal cars; but geometrical differences may also exist between cars of the same category, dependent on the type of collision. Typical effects of geometrical differences are mismatches such as override/underride and fork effect of structures designed to (inter)act in case of collisions. Various solutions for this problem have been tested, such as increasing or lowering sill height (in side collisions) and increasing or lowering longitudinal height, bumper height and frontal stiffness. The difficulty of influencing (frontal or side) stiffness is that this property is almost never as homogeneous as it ideally should be, because of various local parts that contribute to the overall stiffness design. In view of the importance of geometrical properties, especially concerning stiffness, stiffness distribution, and alignment, the challenge for the coming years is to develop a test procedure of (new) car design that forces the future car fleet to converge to a far more compatible design than the current one. In view of the various interacting influences as described in this literature review, it may be doubted that one single test may be developed for this purpose, though some proposals already have been made, based on the current deformable barrier test, used for frontal impacts. Probably, a second test is needed to better represent the opponent car when testing and monitoring the level of aggressiveness.

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