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Nomex honeycomb cores have been widely used in composite sandwich panels. To accomplish meso-scale finite element modeling of these cores, cell wall mechanical properties are required, for which limited data are available. In this work, tensile testing was performed on Nomex paper, phenolic resin, and Nomex paper coated with phenolic resin. Flatwise tension and compression tests were also performed on two types of Nomex honeycomb cores. Test results were calibrated in finite element modeling to account for strain gage local stiffening effects and thickness normalization. Identified cell wall material properties were implemented in a honeycomb core finite element model and further calibrated by matching simulation results to manufacturer test data. The cells' double-wall thickness was also adjusted. These calibrations and adjustments led to an exact simulation match with test data. Numerically matching cell wall material properties depends on modeling sophistication and is subject to core test result variability and core construction differences among manufacturers. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an experimental and analytical study into the strengthening and toughening of sandwich composite joints by z-pinning. Cleats connecting the vertical stiffener and horizontal base panel to T-shaped sandwich joints were reinforced in the through-thickness direction with pins. Tensile (stiffener pull-off) tests revealed that pinning increased the ultimate fracture load and fracture energy by resisting crack growth along the cleat–skin and skin–core interfaces, which were the weakest points in the unpinned joint. The peak fracture load and fracture energy increased with the volume content of z-pins. The strengthening and toughening effect of the pins was analysed using multiple pin pull-out tests performed on the sandwich composite material. It is shown that elastic deformation, debonding and pull-out of the pins from the face skins to the sandwich composite is the primary toughening mechanism of the pinned T-joints. The pin pull-out process, which is the cause for the high strengthening and toughening of the T-joints, is analysed using bridging traction modelling. [Copyright &y& Elsevier]