We’re told that we should replace our bike helmets every couple of years or so, because minuscule cracks can develop over time, rendering them structurally unsound. For the same reason, we’re supposed to replace a helmet that has withstood a direct impact immediately, no questions asked. The problem is... it’s so hard to get yourself to throw away what looks like a perfectly good helmet, just because it might no longer be effective. New technology developed at the Fraunhofer Institute for Mechanics of Materials should eliminate this situation. When your helmet is getting past its prime, it will start to smell. If it develops any large cracks... well, you’d better plug your nose.

The secret lies in odoriferous oils, enclosed in formaldehyde resin microcapsules. These microcapsules are in turn added to liquid polypropylene, which is injection-moulded to form the shell of the helmet. When the shell breaks, the capsules rupture, and stinking ensues. The Fraunhofer researchers used a computer simulation to calculate the number of capsules needed, then used mechanical testing to check that they would rupture shortly before complete structural failure.

“Smell detection is already in use for coated metal components. We are applying the process for the first time to polymer materials,” stated Fraunhofer’s Dr.-Ing. Christof Koplin. “The cycle helmet is being used as a demonstrator. Work on the capsules has finished and we are now completing characterizing tests on individual configurations.” Possible other applications could include pressure hoses, and water and gas pipes.