Novel mechanism indicates when cracks start in plastics

Paul Boughton
Cycle helmets are available in a wide range of types, including foldable models, models fitted with a flashing rear light or featuring an iPhone display. In future, they could even start to smell distinctively if they need to be replaced, thanks to a new process that causes odoriferous oils to exude from the helmet's shell material if it is cracked.
Only damage-free helmets can protect the wearer as designed, so it is recommended that a new helmet is purchased periodically. Nobody wants to throw away a perfectly good helmet, so it would be better to know for certain that this is really necessary. A new process developed by research scientists at the Fraunhofer Institute for Mechanics of Materials (IWM) in Freiburg, Germany, in co-operation with the Fraunhofer Institute for Environmental, Safety and Energy Technology (UMSICHT) in Oberhausen, makes this possible. The polymer materials produced by the new process start to smell if they develop small cracks. Large cracks smell very bad! The smell comes from odoriferous oils enclosed in microcapsules.
Dr-Ing Christof Koplin, a research scientist at the IWM, says: "Cyclists often replace their helmets unnecessarily after dropping them on the ground, because they cannot tell whether they are damaged or not. The capsules eliminate this problem. If cracks form, smelly substances are released." The capsules are added to a polypropylene mass, which is injection-moulded to form the final component. In the case of the bicycle helmet, the microcapsules are inserted in a thick foil made of polypropylene, which is fastened to the head gear.
A layer of melamine formaldehyde resin encloses the capsules so that they are completely airtight and mechanically sealed. It also protects the tiny pods, which are subjected to temperatures of 200 to 300 degrees C during injection moulding, as well as static pressures of up to 100bar. Koplin explains: "Melamine formaldehyde resin proved to be the most suitable encapsulation material in the comparison we conducted of the material systems. Inside the capsule there is a porous, hardly deformable silicon oxide core which absorbs the odoriferous substance. This core produced the best results."
To determine the loads at which the miniscule capsules (measuring just 1 to 50 micrometres) break open, the scientists test them at the IWM with a Vickers indenter. The engineers calculate the number of capsules required by means of numerical computer simulation. The finished component is then subjected to bending and drawing tests. The tests are only deemed to be successfully completed if the capsules are found to open and exude the odoriferous substances just before the component fails. Koplin comments: "Our method of detection by smell offers several advantages. It not only indicates when safety-critical polymer components need to be replaced; the exuding smells also enable damage outside the safety range to be detected."
The process is therefore said to be suitable for all products that are difficult to test for defects, such as cycle, motorbike and construction helmets. But it can also be used to check pressure hoses - in washing machines, for example, which are difficult to access. Smell sensors could also monitor plastic water and gas supply pipes to detect any cracks, because the odoriferous substances emitted are noticeable over long distances. Koplin concludes: "Smell detection is already in use for coated metal components. We are applying the process for the first time to polymer materials. 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."
For more information, visit

Recent Issues