Inner glow to indicate damage in industrial bonded joints

Adhesive bonding is playing an increasing role in industries such as automotive, mechanical engineering and construction. However, testing methods for detecting cracks in such joints caused by operation or aging are very complex - or the components and adhesive joints are even destroyed during testing. A method that engineering scientists at the University of Kassel have refined could provide a remedy here in the future. Tiny capsules, only 20 to 100 micrometers in size, containing a dye are introduced into structural industrial adhesives. If microcracks occur in the cured adhesive, these also destroy the capsules. The dye leaks out and begins to fluoresce within the adhesive. Since the adhesive is transparent, this internal glow (in the case of transparent joining partners) is visible from the outside under UV light.

For the project, Dipl.-Ing. Martin Kahlmeyer (Department of Separating and Joining Manufacturing Processes, headed by Prof. Dr.-Ing. Stefan Böhm) experimented with different adhesives as well as with different sphere sizes and dyes. M.Sc. Johannes Scheel (Department of Engineering Mechanics/Continuum Mechanics, headed by Prof. Dr.-Ing. Andreas Ricoeur) supported the experiments with numerical simulations and fracture mechanics investigations.

The scientists also optimized the manufacturing process of the paint balls. The decisive factor here was to produce the so-called microcapsules as densely as possible by interfacial polymerization and to place them in the adhesive in such a way that they also crack even at the finest cracks in the adhesive. In doing so, the research group also built on preliminary work by other scientists.
"In the laboratory, we were already able to produce capsules suitable for fluorescence detection and distribute them in adhesives," Kahlmeyer explains. The method is still some way from being used in industrial production. But it opens up prospects for improving testing methods and increasing the safety of optically transparent joints such as bonded glass structures.

The project ran for two years and was funded by the German Research Foundation. Follow-up projects are expected to further develop the method to make it suitable for practical use. "Our goal is to one day develop capsules that not only reveal micro-cracks but can also heal them immediately with their liquid," Scheel looks ahead.
 

Contact:
Martin Kahlmeyer
University of Kassel
Department of Separating and Joining Manufacturing Processes
E-mail: m.kahlmeyer[at]uni-kassel[dot]de

Johannes Scheel
University of Kassel
Department of Engineering Mechanics/Continuum Mechanics
E-mail: j.scheel[at]uni-kassel[dot]de

Sebastian Mense
University of Kassel
Communication, Press and Public Relations
Tel.: +49 561 804-1961
E-Mail: presse[at]uni-kassel[dot]de
www.uni-kassel.de