End milling as a surface pretreatment for adhesive bonding of CFRP - new approaches for root cause identification of reduced bond line performance

摘要

End milling is an interesting technology for the surface preparation of bonded joints of carbon fiber reinforced polymer (CFRP) for manufacturing and repair. It can be automated and therefore allows, in theory, a bondline pretreatment with reproducible surface characteristics. However, the mechanical cutting process of milling in microscale is not fully understood yet. Adhesive joints from milled CFRP surfaces often entail lower strength compared to sanding. In the work presented, the authors propose a new approach for investigations of drivers for the known, negative phenomena of end milling CFRP surface pretreatment, such as hard-to-remove micro particle layers on the surface or micro damages in the surface-near material. The approach uses a combination of cutting simulation in combination with a newly developed chip root investigation technique. The simulation focuses on microscale modeling of the material and the incorporation of strain rate-dependent behavior of the matrix material. The chip root investigations do not rely on simplification of the cutting mechanism, but use the actual end milling process. Chip roots are analyzed using field emission scanning electron microscopy (FE-SEM) in combination with a special cross-section preparation technique. The combined results lead to a better understanding of the material behavior during mechanical pretreatment. It can be used for process optimization.