Experimental validation of honeycomb core forming limits

Abstract

The objective of this thesis is to understand the deformation and potential node bond failures of fiberglass/phenolic honeycomb core during forming. This data will be used to provide experimental validation of a hyper elastic finite element model developed by Wichita State University to provide a predictive failure envelope for this core. Experimental core forming was conducted on 15 specimens at temperatures of 350°F, 450°F and 575°F, and core was formed to inner mold line radii of 75 inches (spherical), 77 inches (cylindrical), and 150 inches (spherical). These temperatures and radii were chosen to test both predicted success and failure conditions by the finite element model. Cool down rates were calculated and analyzed for each specimen and the average initial rate of cooling was determined to be used in the validation exercise for the finite element model. After forming, digital image correlation was used to measure the radii in both the ribbon and transverse planes to quantify the springback. It was confirmed that for RIML=75 inches, node bond failures occurred at all temperatures, while at RIML=77 inches and RIML=150 inches no node bond failures occurred. This is consistent with the FEA model predictions. The cooling rates for all specimens using both the infrared oven and conventional oven showed that as the final core temperature increased, the cooling rate also increased. It was determined that the core that was heated in the infrared oven had higher cooling rates than the core heated in the conventional oven. This was due to environmental conditions and difference in testing temperatures. The springback data showed that the specimens sprung back more in the ribbon plane than in the transverse plane orthogonal to it, which was anticipated based on the higher stiffness of the core double cell walls in the ribbon direction. A final check of test control method found that using displacement control to apply the forming pressure was equivalent to using force control.