Local Weakening of Honeycomb Core for Improved Surface Quality and Bonding in Co-cured Sandwich Panels
To be published on September 16, 2019 by SAE International in United States
Event: AeroTech Europe
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 such strengthened panels without further processing of the core leads to higher flatness tolerances due to the added thickness 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 automated process for the reduction of the residual stresses in strengthened areas is presented. The process is based on a local reduction of the compressive strength in the surface area of the honeycomb core, which allows for a plastic deformation in thickened areas at curing pressure. For the reduction of compressive strength, a concept based on a robot guided tool with multiple parallel blades is presented. A model based analysis of the process parameters shows that the remaining compressive strength depends on the cutting pattern, the blade distance, the cutting depth and the angle of the blades. A testing tool was built and a series of tests was conducted in order to quantify the correlation and select suitable process parameters. 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 improved flatness it has been found that, the bonding strength increases up to 30% and telegraphing effects get reduced due to increased bonding surface of the honeycomb core.