Local Weakening of Honeycomb Core for Improved Surface Quality and Bonding in Co-Cured Sandwich Panels
Sandwich panels made of Nomex honeycomb core and fiber reinforced face sheets are a major component of aircraft interior parts. A common way to locally increase the strength of such panels, e.g. for load introduction, is the local thickening of the face sheets with additional prepreg layers. Curing of strengthened panels without further processing of the core leads to higher flatness tolerances as well as residual stresses. Machining of the core in the strengthened areas is possible, but expensive due to high machine costs and additional cleaning processes. In this paper a new process for the reduction of the residual stresses in strengthened areas, as well as improved bonding between core and face sheets is presented. The process is based on local reduction of the compressive strength in the surface area of the honeycomb core, which allows for controlled, irreversible deformation at curing pressure. For the reduction of compressive strength, a concept based on a robot guided tool with multiple blades is presented. The process parameters impacting the compressive strength after weakening are derived based on a model based analysis. A testing tool was built, and a series of tests was performed, in order to quantify the impact and to select suitable parameters. It was found that the remaining compressive strength depends mainly on the cutting pattern, the blade distance and the cutting depth. A Further reduction of compressive strength was achieved by bending the remaining structures after the cutting process. In order to validate the process, surface flatness, as well as bonding strength between core and face sheets have been measured. In addition to the flat integration of a large amount of additional prepreg layers it has been found that the bonding strength increases up to 30% due to increased bonding surface of the honeycomb core.