Health monitoring of adhesively bonded glass elements by microcapsules

The building industry, where safety related joints are increasingly connected by adhesion technology, demand new quality assurance measures. Therefore, an appropriate monitoring technique for critical stresses and cracks in adhesive joints is essential. In this presentation, a new approach for in situ monitoring of transparent adhesives will be described. Microcapsules, filled with dyes or other marker substances, are embedded in structural adhesives like epoxies. Upon breakage by exceedance of critical stresses and arise of cracks within the joint, the capsules release their liquid cores. This causes a detectable color change resp. fluorescence and thus enable a condition monitoring of the bond. The applied approach is based on the idea of self-healing polymers presented by White et al. in 2001. For the preparation of microcapsules with defined release behavior, the principle of interfacial polymerisation is applied. After preparation, optical and electron microscopy allow a study of capsule morphology and dimensions (size and wall thickness). These characteristics are dependent on the used chemicals and preparation parameters and have a noticeable effect on the capsule strength and release properties. After the prepared microcapsules are divided into different strength and size fractions, they are dispersed within a structural adhesive. To determine the influences of capsule inclusions on the mechanical properties of the adhesive, tensile and shear specimen are prepared and tested quasi-statically. Furthermore, tapered double-cantilever beam specimen are produced to investigate the fracture toughness as a function of capsule parameters. The results of the mechanical tests is juxtaposed with microscopy analyses of the fracture surfaces.