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Products and solutions, 2005-09-12, 11:03 AM

Advanced Closures: Pedestrian protection with steel

The Auto Division of ThyssenKrupp Stahl AG has developed a concept for steel auto hoods which will allow car manufacturers to meet the strict standards of the European Commission's directive on pedestrian protection that enters into force from 2010. The solution developed by ThyssenKrupp Stahl is based on a completely new geometry for the inner panels of the hoods. In many cases it can be used as a purely passive pedestrian protection solution. This means that complex devices to lift the hood when an accident occurs to cushion the impact can be dispensed with.

In a collision, pedestrians are most at risk in the leg and head areas. The measures demanded by the European Commission relate to the front end of vehicles and the engine hood. The most serious injuries occur when the head of the accident victim hits the hood. In most cases it penetrates so deep that it hits the engine block 20 to 30 millimeters underneath. Car manufacturers are therefore being forced to completely revise the design of their cars front ends: external parts must be shaped so that no load peaks occur at hard points during an impact. The aim is to find designs which dissipate kinetic energy effectively using as little space as possible. If the necessary space cannot be created between the hood and the engine block, sensor-controlled active systems have to be used to raise the hood shortly before impact.

Whether a hood fulfils the European Commission's pedestrian protection directive is decided by the so-called HIC value. HIC stands for head injury criteria. In physical terms, the value describes the acceleration of a body over time. In concrete terms it determines how quickly and how severely the head of a pedestrian is decelerated when it hits the hood of a car. From October 2005 two thirds of the surface area of the hoods of new cars entering the market must have an HIC value no higher than 1,000, while the final third is subject to an HIC requirement of 2,000 maximum. This is tested using a standardized test body called an impactor which is hurled against the engine hood at a speed of 35 kilometers per hour. The impactor weighs 3.5 kilograms. This year's IAA Motor Show in Frankfurt will show how far car manufacturers have come in meeting this challenge.

From 2010, phase 2 of the directive will come into force, introducing stricter standards. The original intention was to use two different impactor models, a higher impact speed of 40 kilometers per hour and a maximum HIC value of 1,000 for the entire surface area of the hood. At present, several proposals for amendments have been tabled and the final form of the directive has not yet been established. It is expected that a test involving 3.5 and 4.5 kilogram impactors will be specified, representing a child's head and an adult's head. Impact speed and HIC standards will possibly be coordinated with Japanese regulations. What is already clear is that the measurements with two different size impactors will further complicate engine hood design.

To support its OEM customers in the search for new solutions for pedestrian protection, the Auto Division of ThyssenKrupp Stahl has carried out its own development work into correspondingly designed engine hoods. The basis for the investigations was a representative production closure used on a high-volume automobile with good ratings in the Euro NCAP crash test. The aims were to achieve a significant improvement in the pedestrian protection properties of the part, reduce weight and match the other properties of the benchmark structure such as stiffness and dent resistance.

For the outer panel, the developers from the Auto Division used the cold-rolled dual phase steel DP 30/50, with a thickness of 0.60 millimeters, a strength of 500 megapascals and an outer panel quality finish. For the inner panel, an interstitial-free (IF) steel displaying good formability was used. The inner panel offers outstanding pedestrian protection thanks to its special geometry. During development, the project team from the Auto Division developed 10 different brace designs for the inner panels and analyzed their suitability for pedestrian protection. Certified impactor models and the simulation software LS-Dyna were used. A cluster computer with 10 parallel processors provided the necessary computing power to run through numerous variants and optimization loops.

The inner panel geometry which proved most successful was one with evenly distributed octagonal recesses. The number, distribution, size and shape of these recesses are so designed that the hood has high global stiffness as well as good local energy absorption capacity. In tests at 40 kilometers per hour impact speeds, the structure displays an HIC value of 883 for the adult impactor in the center of the panel. Tests with the smaller impactor showed an HIC value of 669 in the center of the panel. For both impactors, HIC values of just under 1,000 were also achieved in the particularly difficult, less yielding areas around the side gaps and hinges. At an impact speed of 35 kilometers per hour the load values were significantly lower. The global stiffness of the new structure is slightly higher than that of the benchmark hood. These reserves can be turned into weight advantages.

ThyssenKrupp Stahl is exhibiting at IAA in hall 4.1, booth C50.

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