Process-structure-property relationships in cyclically loaded adhesive layers with heterogeneous property profiles

• Applicants: Professor Dr.-Ing. Wilko Flügge; Professor Dr.-Ing. Maik Gude
• Subject Area: Joining and Separation Technology ; Polymeric and Biogenic Materials and Derived Composites
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558868093

In industries such as wind turbine, boat, rail and commercial vehicle construction, bonded joints in large, tolerance-prone FRP structures are widespread. According to the current state transferability of material and structural characteristics determined on a laboratory scale to real structures with adhesive layer thicknesses of more than 10 mm is limited. In particular, crack initiation and subsequent crack growth occurring during cyclic loading of the adhesive layer have not yet been adequately taken into account in the design, as the complex dependencies on the bonding partners, their surface preparation, the manufacturing and cross-linking conditions in the bonding process and the bonded layer thickness are not yet sufficiently understood. The joint project therefore aims to clarify the process-structure-property relationships of real-scale bonds and to understand the thickness-dependent influences on the material and structural properties. To this end, production methods are being developed on the basis of numerical process simulations, which allow the property profiles occurring in the large-scale structure to be reproduced in laboratory-scale test specimens. Particular attention is paid to reproducing the property profiles in the thickness direction of the adhesive layer, which are formed, for example, by an uneven cross-linking reaction during processing or the locally heterogeneous residual stress state of the large structure. The test specimens with defined property profiles are then examined in vibration tests using an automated method for crack detection and evaluation. In order to access the fracture mechanics parameters relevant for the subsequent design and to ensure that the test results can be transferred to any adhesive layer thickness and property profile, the tests are accompanied by a sub-model-based simulation approach based on the Virtual Crack Closure Technique and inspired by Finite Fracture Mechanics. In this way, energy release rates at the moment of crack formation and during crack growth can be calculated as a basis for determining characteristic value curves that can be transferred to real structural bonds.

• DFG Programme: Research Grants