Pedestrian protection

German engineers have developed a concept for steel car bonnets which will allow manufacturers to meet the strict standards laid down in the EC Directive on pedestrian protection.

Engineers at ThyssenKrupp Stahl have developed a concept for steel bonnets which will allow car makers to meet the strict standards laid down in the European Commission’s Directive on pedestrian protection.

Whether a bonnet fulfils the new European Commission’s pedestrian protection Directive is decided by what’s called its HIC, or head injury criteria value. This value describes how quickly and how severely a dummy head is decelerated when it hits the bonnet of a car.

From October, two thirds of the surface area of the bonnets 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 at an impact speed of 35mph.

In 2010, the requirements are likely to get tougher – car makers may need to deliver an HIC value of 1,000 for the entire surface area of the bonnet at an impact speed of 40mph.

And it was that 2010 target that the ThyssenKrupp engineers managed to exceed with their new design.

For the outer panel of their conceptual bonnet, the ThyssenKrupp team used a cold-rolled dual phase steel DP 30/50 with a thickness of 0.60 millimetres and a strength of 500 megapascals.

But it was the inner panel of the bonnet that held the secret to its success.

After evaluating ten different designs for the inner panels, modelling the variants on a cluster computer with ten parallel processors, the German engineers eventually decided upon a interstitial-free (IF) steel design that comprised evenly distributed octagonal recesses.

The number, distribution, size and shape of the recesses were designed so that the bonnet provided both high stiffness and good local energy absorption.

When they tested it out at 40 kilometres per hour, the entire structure showed an HIC value less than 1000. At an impact speed of 35 kilometres per hour, the values were significantly lower.