The so-called lower bumper stiffener (LBS) is a functional part made of Ultramid B3WG6 CR, the glassfibre-reinforced polyamide 6 developed by BASF for crash applications. The LBS weighs about one kilogram, is one meter long and is installed behind the front bumper so as to diminish the risk of serious knee injury in the event of a collision with a pedestrian. En route to developing the LBS, BASF deployed its new method of integrative simulation.
Statutory regulations have been an integral part of the type-approval certification in Europe and consumer protection organizations such as EuroNCAP (European New Car Assessment Programme) also conduct pedestrian protection analyses of new vehicles. Moreover, the stipulations made in the rating tests of underwriters (the Research Council for Automobile Repairs - RCAR, the German Insurance Association - GDV) also have to be met.
In the quest to fulfil all of these conditions, the entire design concept of the front end of the vehicle is put to the test. At the same time, automobile manufacturers voice the classic demands for optimal space utilization, ease of assembly, sufficient sturdiness upon contact with obstructions, the lowest possible weight, a small number of parts as well as low costs that are acceptable to customers. This wide array of requirements has to be met by using innovative parts made of thermoplastic engineering plastics.
When designing the LBS on the computer BASF turned to its newly developed numerical material model which not only takes into consideration the non-linear viscoplastic behaviour, but also the anisotropic, that is to say, directionally dependent, behaviour of glassfibre-reinforced thermoplastics. Along with the material parameters of the pure plastic, the content, geometry and orientation distribution density of the fibres in the finished part all enter into the computation. This method, which is referred to as integrative simulation, is fed, on the one hand, with the findings from a classic mould-fill simulation and, on the other hand, with the experimental data obtained from a special high-speed measuring device made by BASF.
This yields the part shape that complies with the requirements as well as the optimum mould design. - Once the BASF method had been seamlessly integrated into the development process and into the simulation software at Opel, it became possible to model the LBS in detail and to describe its crash behaviour with an unprecedented degree of precision - explains Dr. Steffen Frik, group leader for Simulation Passive Safety at Opel in Rüsselheim.
Without having to perform all too many costly tests, this refined simulation method succeeded in designing the LBS in such a way that it fulfils the requirements made by the pedestrian protection directive. At the same time, if the vehicle crashes against a stationary obstacle where the load is much higher than in the case of impact against a leg, damage to other parts in the front end of the vehicle is prevented in that the LBS systematically fails. This lowers repair costs and translates into more favourable insurance ratings. The method of integrative simulation can image short glassfibre-reinforced plastic parts much more realistically than all other known methods. The integrative description of the material behaviour considerably improves virtual vehicle development.